3.1.8 PROTEIN SYNTHESIS

EQ: Give one way in which the structure of an mRNA molecule is different from the structure of a tRNA molecule. (1 MARK)

- Doesnt contain hydrogen bonds / base pairs - Contains codons / doesnt contain anticodon - Straight / not folded - No amino acid binding site / longer

Q: Agrobacterium tumefaciens is a bacterium that is often used in recombinant DNA technology to produce transformed plants that benefit humans. A. tumefacuens contains a plasmid which can be used as a vector to transfer a desired gene into plant cells. The desired gene is from an insect. After the recombinant DNA technology, the plant containing this gene was able to use it to synthesise an insect protein. The plant is able to synthesise the insect protein. Explain why this is possible. (3 MARKS)

- Genetic code is universal / triplets in DNA always code for same amino acid - Insect DNA can be transcribed (basic transcription process/mechanism same in all organisms) - Can be translated (process/mechanism same in all organisms)

Q: State why it is possible for a gene from one organism to be introduced into and function in a different organism. (1 MARK)

The genetic code is universal

EQ: What is the proteome of a cell? (1 MARK)

(The proteome is the full) range of / number of different proteins that a cell is able to produce (at a given time); OR (The proteome is the full) range of / number of different proteins the genome / DNA is able to code for;

Q: To investigate how two new drugs affect nucleic acids and their role in protein synthesis, bacteria were grown in normal conditions for a few generations, then moved to media containing the drugs After a short period of time, the concentration of protein and complete strands of mRNA in the bacteria were analysed. The results are shown in figure 2. (a) Describe and explain the results for drug 1. (b) Describe and explain the results for drug 2.

(a) - Figure 2 shows both mRNA and protein concentration were lower in the presence of drug 1 compared to the no drug control - This suggest that drug 1 affects the production of full length mRNA, so there's no mRNA for protein synthesis during translation (b) - Figure 2 shows mRNA production in the presence of drug 2 was unaffected, but less protein was produced -3mg cm-3 compared to 8mg cm-3 (always use data!) - This suggests that drug 2 interferes with trans;ation - mRNA was still produced but less protein was translated from it - Drug 1 was found to be a ribonuclease (an enzyme that digest RNA) which could explain the results for the first experiment - most strands of mRNA produced would be digested by drug 1, so couldn't be used in translation to make proteins - Drug 2 was found to be a single stranded, clover shaped molecule capable of binding to the ribosome. This helps explain the results as drug 2 could work by binding to the ribosome, blocking tRNA from binding to it and preventing translation

EQ: Figure 2 shows a DNA base sequence. It also shows the effect of two mutations on this base sequence. Table 2 shows DNA triplets that code for different amino acids. Some gene mutations affect the amino acid sequence. Some mutations dont. Use the information from Figure 1 and Table 2 to explain: (a) Whether mutation 1 affects the amino acid sequence. (2 MARKS) (b) How mutation 2 could lead to the formation of a non-functional enzyme. (3 MARKS)

(a) - Has no effect / same amino acids sequence / same primary structure - Glycine named as same amino acid (b) - Leu replaced Val / change in amino acid sequence / primary structure - Change in hydrogen / ionic bonds - Alters tertiary structure / active site - Substrate cannot bind / no longer complementary / no enzyme-substrate complexes form

Q: A species of bacteria has a gene that codes for the production of a blue-coloured antibiotic. (a) Describe the role of RNA polymerase in the transcription of a gene sequence. (2 MARKS) (b) The mRNA for the gene coding for the antibiotic is the same length as its DNA. explain how and why this might be different for a eukaryotic gene. (3 MARKS)

(a) - RNA polymerase lines up free RNA nucleotides alongside exposed bases on the DNA template strand. - The enzyme then moves along the DNA strand, assembling a complementary mRNA sequence strand from the RNA nucleotides by joining them together (b) - Genes in eukaryotic DNA contain introns (sections that dont code for amino acids) - After transcription the introns are removed from pre-mRNA strands by splicing, leaving only the exons (parts of the gene that code for amino acids) which form mRNA - So eukaryotic mRNA would be shorter than the DNA it was transcribed from

Q: Researchers have been studying the genetic code of a gene with the aim of developing a treatment for a particular genetic disease. The genetic disease is caused by the production of a specific enzyme. Part of the enzyme's amino acid sequence is shown below: Glycine -- histidine -- alanine -- proline --- histidine Table 1 shows the DNA sequence for some amino acids. (a) Use Table 1 to give the tRNA anticodons for the amino acid sequence shown above (b) Describe how the structure of tRNA differs from mRNA (c) The researchers are exploring a possible treatment for the genetic disease that would involve disrupting the process of translation. Name the organelle that mRNA attaches to for translation to take place and give a detailed description of tRNA's role in translation. (4 MARKS)

(a) DNA sequence: CCT GTG CGT GCA GTG tRNA anticodons:CCU GUG CGU GCA GUG (b) - tRNA is folded into a clover shape and held together by hydrogen bonds whereas mRNA is not - Three adjacent bases in mRNA form a codon whereas tRNA has three specific bases called an anticodon - tRNA has an amino acid binding site whereas mRNA does not (c) - Ribosomes - tRNA molecules carry amino acids to the ribosome - A tRNA molecule with an anticodon that's complementary to the first codon on the mRNA attaches itself to the mRNa by complementary base pairing - A second tRNA molecule attaches itself to the next codon on the mRNA in the same way and the two amino acids are joined by a peptide bond - The first tRNA molecule moves away, leaving its amino acid behind and this process continues and produces a polypeptide chain

EQ: Read the following passage: The sequence of bases in a molecule of DNA codes for proteins. Different sequences of bases code for different proteins. The genetic code, however, is degenerate. Although the base sequence for AGT codes for serine, other sequences may also code for this same amino acid. There are four base sequences which code for the amino acid glycine. These are CCA, CCC, CCG and CCT. There are also four base sequences coding for the amino acid proline. These are GGA, GGC, GGG and GGT Pieces of DNA which have a sequence where the same base is repeated many times are called slippery. When slipper DNA is copied during replication, errors may occur in copying. Individual bases may be copied more than once. This may give rise to differences in the protein which is produced by the piece of DNA containing the errors. Use the information in the passage and your own knowledge to answer the following questions (a) The base sequence AGT codes for serine. Give the mRNA codon transcribed from this base sequence. (2 MARKS) (b) Glycine-proline-proline is a series of amino acids found in a particular protein. Give the sequence of DNA bases for these three amino acids which contain the longest slippery sequence. (2 MARKS)

(a) UCA (b) CCG GGG GGG

Q: Ricin is a protein produced by some plants. In animal cells, ricin acts as an enzyme. The enzyme removes the adenine molecule from one of the nucleotides in the RNA of ribosomes. As a result, the ribosome changes shape. Ricin is found in high concentrations in the seeds of some species of plant. Suggest and explain one advantage of this to the plant. (2 MARKS)

- Animals that can eat the plant get ill / killed / avoid plant - So seeds / plants / species more likely to survive

EQ: Suggest which group of molecules in Figure 2 (A or B) best represents the proteome of a cell. Give a reason for your answer. (2 MARKS)

- B - The diagrams represent proteins EXPLANATION: Diagram A shows all the chromosomes of a human male (46 chromosomes with XY as the last pair) which would be a good representation of the genome. However, diagram B has illustrations of different proteins (e.g. antibody, collagen, porin, haemoglobin) which represents the proteome (the full range of proteins that a cell can produce

Q: DNA codes for the structure of polypeptides. State the role of messenger RNA (mRNA). (3 MARKS)

- Carries / transfers the (complementary DNA) code / genetic information - Out of the nucleus to the ribosome / RER / site of translation - For protein / polypeptide synthesis

EQ: Suggest the effect of mutations on the protein structure coded by this mutated sequence of DNA. (2 MARKS)

- Change in 3D shape / tertiary structure of protein formed - Due to the R groups of amino acids being different OR due to the ionic bonds / disulphide bridges / hydrophobic interactions that the amino acids form being disrupted

Q: State what the chromosomes of fungi are made of. (2 MARKS)

- DNA - Proteins / histones fungi are eukaryotes therefore the nuclei of eukaryotic cells contain DNA molecules!

EQ: Give one difference between transcription in prokaryotic and eukaryotic cells. (1 MARK)

- DNA of eukaryotic cell has introns within gene, prokaryotic cell doesn't have introns in DNA - Therefore eukaryotic pre-mRNA contains introns - After transcription . during modification these regions are removed from the pre-mRNA - Prokaryotes directly produce mRNA molecules / eukaryotes produce pre-mRNA

Q: Describe how mRNA is synthesised. (6 MARKS)

- DNA unwinds / strands separate / hydrogen bonds broken - DNA helicase - RNA mononucleotides line up against / attach to one DNA strand / template - Complementary base pairing between DNA and mononucleotides - Mononucleotides joining together / formation of phosphodiester bonds (condensation reaction) - RNA polymerase

EQ: Figure 2 shows the stages that occur to produce a part of a polypeptide (found in a human). Explain the features of the genetic code that this diagram illustrates. (4 MARKS)

- Degenerate - The mRNA sequences UCU and AGU both code for the same amino acids (ser) - Non-overlapping - The 12 bases are coding for 4 amino acids

EQ: The synthesis of proteins is determined by the genetic code. The genetic code is provided by the sequence of nucleotides in DNA. As the code is the same for almost all organisms it is described as universal. Name and describe other features of the genetic code. (6 MARKS)

- Degenerate - There is more than one codon for most amino acids - Triple code - Three bases are needed to code for one amino acid - Non-overlapping - The sequence is read so that each base is only part of one codon / triplet

Describe the effect of R-groups on proteins.

- Disulphide bonds (eg in cystine with another cystine) - Acidic / basic R groups form ionic bonds - Hydrophobic R groups aggregate together / join other hydrophobic molecules → protein is mis-folded → non functional → build up of misfolded protein can cause disease eg alzheimer's disease

Describe post transcriptional modification.

- Eukaryotic genes = pre-mRNA (messenger) containing exons and introns. - pre-mRNA goes into splicing - Introns removed - Exons spliced together in different combos for different proteins and exported - Leaving just the exons and forming mRNA - mRNA now leaves nucleus via nuclear pore for translation in cytoplasm

EQ: In Figure 1, the first codon is CAU Give the base sequence of: i) The complementary DNA base sequence of the template strand ii) The missing region X

- GTA TAC CTG AA - GUA

EQ: Lactase is an enzyme that is used to produce lactose-free milk. A student was given the RNA sequence of the enzyme lactase in Figure 1 and was asked to use table 1 To identify the first four amino acids. Describe and explain how the mRNA nucleotide sequence worked out to correspond to the first four amino acids using Table 1 may not be the same as the mRNA nucleotide sequence for those amino acids present in a typical bacterial cell. (4 MARKS)

- Genetic code is degenerate; 64 codons for 20 amino acids - More than one codon / triplet species for an amino acid - e.g. Glu = GAG, Leu = UUA, UUG, CUU, CUA, CUG - Start codon is always AUG

What are the two types of chaperone proteins?

- HSP 70. Bind to hydrophobic regions on polypeptides and prevent incorrect hydrophobic interactions occurring before polypeptide chain is completed - HSP 60: chaperonins, large cylindrical proteins with central compartment. Polypeptide chain fits into central compartment and isolated from other proteins → prevented from interacting with them SO protein to fold correctly

Describe the role of ATP in translation

- Hydrolysis of ATP, to ADP + Pi, releases energy - For the bond between the amino acid and its corresponding tRNA molecule - Amino acid attaches at amino acid binding site - For peptide bond formation between amino acids

Q: Comparing the base sequence of a gene provides more information than comparing the amino acid sequence from which the gene codes. Explain why. (3 MARKS)

- More base triplets than amino acids - Introns / non-coding DNA - Same amino acid may be coded for / DNA code is degenerate

EQ: The figure below shows part of a pre-mRNA molecule. Geneticists identified two mutations that can affect this pre-mRNA, as shown in the figure. Mutation 1 leads to the production of a non-functional protein. Explain why. (3 MARKS) What effect might mutation 2 have on the protein produced? Explain your answer. (2 MARKS)

- Mutation changes triplets after that point / causes frameshift - Changes amino acid sequence after this / codes for different amino acid - affects hydrogen / ionic bond - Changes tertiary structure of protein - Intron non-coding DNA / only exons coding - So not translated / no change n mRNA produced / no effect on protein / no effect on amino acid sequence OR - Prevents / changes splicing - so faulty mRNA formed - Get different amino acid sequence

Describe how proteins are folded

- Primary structure of polypeptide is coiled / folded to produce secondary structure - Interactions between R-groups further coil / fold polypeptide to produce tertiary structure (3D shape) - Different polypeptide chains + non-protein groups link to form functional proteins - This is a quaternary structure

Q: Suggest two reasons why prokaryotes have a much greater percentage of protein-coding DNA than humans. (2 MARKS)

- Prokaryotes dont have introns within genes - Prokaryotes have shorter / fewer multiple repeat sequences between genes

Compare protein synthesis in prokaryotes and eukaryotes.

- Prokaryotes → transcription :DNA → mRNA - No introns - No splicing → mRNA produced directly from DNA - Eukaryotes: DNA → premRNA → mRNA - Introns are removed (splicing) to form mRNA

EQ: Different sequences of bases code for different proteins. Explain how. (2 MARKS)

- Protein made of chain of amino acids - Each amino acid has its own base / triplet codes

Q: RNA polymerase and DNA polymerase are both enzymes. RNA polymerase is involved in the action of some control elements, whereas DNA polymerase is not. Describe and explain the different between the functions of these two enzymes. (6 MARKS)

- RNA polymerase makes messenger / transfer / ribosomal RNA - In transcription - One strand DNA used / short section used / one strand formed - DNA polymerase - DNA replication - Semi conservative / both strands used / whole length used / 2 strands formed - Before nuclear / cell division

Q: Describe the differences between the processes of replication and transcription of DNA. (3 MARKS)

- Replication uses DNA nucleotides, transcription uses RNA nucleotides - Replication uses DNA polymerase, transcription uses RNA polymerase - Replication is semiconservative, transcription is not semi-conservative - whole DNA molecule unzipped for replication, with only part of transcription - all DNA copied in replication but only part in transcription

Describe protein synthesis: translation

- Ribosome attaches to mRNA - (Sequence of mRNA codons determines sequence of amino acids) - Ribosome moves to start codon: AUG - tRNAS carry specific amino acids to ribosome, (in relation to their anticodon) - Ribosome holds 2 tRNAs in place - At the ribosome, tRNA anticodon binds to mRNA codon at a start codon by complementary base pairing; Hydrogen bonds formed - Two amino acids joined by condensation, forming a peptide bond using energy from ATP - tRNA detaches (without its amino acid), ribosome moves along mRNA to next codon - Continues until stop codon (polypeptide released)

Q: Describe the process of protein synthesis that occurs in the cytoplasm. (4 MARKS)

- Ribosome attaches to mRNA - tRNA carries an amino acid - Anticodon and codon interaction / complementary base pairing between tRNA and mRNA / formation of hydrogen bonds between tRNA and mRNA - Peptide bond between amino acids formed by a condensation reaction

Where are chaperone proteins found?

- Some found in endoplasmic reticulum → facilitate folding and assembly of membrane and secreterory proteins - Cytoplasm

Q: A strand of pre-mRNA consists of exons and introns. Exons are sections that can be used during translation for polypeptide synthesis. Introns are lost during the modification of pre-mRNA and are not used during translation. During this modification, a variety of mRNA molecules is formed. Each molecule contains all or only some of the original exons in the pre-mRNA. However, the sequence of the exons in a strand of mrNA will always be the same as in the original pre-mRNA Explain the function of the codons at each end of a strand of mRNA, during the process of translation. (2 MARKS)

- Start / stop / nonsense codon - Start codon needed to being polypeptide synthesis / stop / nonsense codon needed to end polypeptide synthesis

Q: A gene codes for the alpha-polypeptide chain. There are 423 bases in this gene that code for amino acids. The total number of bases in the DNA of the alpha-polypeptide gene is more than 423. Give three reasons why there are more than 423 bases. (3 MARKS)

- Stop / start sequences - Non coding DNA in the gene / introns - Two chains / a non-coding strand / complementary base pairs OR addition of base by mutation

Q: Ricin is a protein produced by some plants. In animal cells, ricin acts as an enzyme. The enzyme removes the adenine molecule from one of the nucleotides in the RNA of ribosomes. As a result, the ribosome changes shape. Ricin causes the death of cells and is very poisonous to many animals. Suggest how the action of ricin on ribosomes could cause the death of cells. (2 MARKS)

- Stops translation / formation of a protein - Prevents mRNA / tRNA binding - Prevents formation of peptide bond - Consequence of loss of identified protein

EQ: Table 3 shows the base triplets that code for two amino acids. (a) Deletion of the sixth base (G) in the sequence shown in Figure 3 would change the nature of the protein produced but substitution of the same base wouldn't. Use the information in table 3 and your own knowledge to explain why. (3 MARKS)

- Substitution would result in CCA/CCC/CCU - All code for same amino acid / proline - Deletion would cause frame shift / change in all following codons

Q: Tetracycline is an antibiotic. The diagram shows how tetracycline bind to bacterial ribosome. Protein synthesis in bacteria is similar to that in eukaryotic cells. Explain how tetracycline stops protein synthesis in bacteria. (4 MARKS)

- Tetracycline binds to mRNA codon - tRNA Anticodon cannot pair with mRNA codon - Amino acid not added to polypeptide chain - Translation prevented

EQ: Use information in the passage below and your own knowledge to answer the questions below. In 2018 the US Food and Drug Administration and the European Medicine Agency approved two drugs that targeted mRNA molecules. These drugs were designed to treat a rare genetic disease that causes a blood protein produced by the liver to form clumps (amyloid deposits) in the cardiovascular system, digestive system and around nerve fibres. This disease called hereditary transthyretin (hATTR) amyloidosis affects about 50,000 people in the world. It is an autosomal dominant inherited condition that causes the mutation of the blood protein called transthyretin. The two drugs were designed to bind with the mRNA molecules. Suggest why the two drugs could be used as a treatment for hereditary transthyretin (hATTR) amyloidosis. (4 MARKS)

- The drug binding to mRNA prevents binding to ribosome - the drug binding to mRNA prevents binding to tRNA anticodon - As a result, translation cannot occur - So the polypeptide / protein is not produced

EQ: The Flavr Savr tomato plant was genetically engineered to ripen and soften more slowly. The inserted gene prevented the enzyme Beta polygalaturonase from breaking down pectine which softened the tomatoes. The matching parts of the base sequences for the mRNA produced from the transcription of the softening gene (in the normal tomato plant) and the mRNA base sequence of the inserted gene are shown in the diagram below. Suggest how the inserted gene reduces the production of the softening enzyme. (3 MARKS)

- The inserted gene / mRNA is complementary to normal gene - The inserted gene binds to the normal gene to prevent protein synthesis - This prevents mRNA binding to ribosomes - It doesnt stop all translation, as some mRNA reaches ribosomes - Not all mRNA is bound by the inserted gene mRNA

EQ: Some species of organisms can have the same percentage of bases as other organisms yet they can still be very different animals. Explain why. (3 MARKS)

- The sequence of bases will be different OR there will be different triplet codes - So they will have a different primary structure for their proteins / sequence of amino acids - The proteins produced by an organism will affect their appearance/phenotype

EQ: Explain why desert rat cells contain a large number of different mRNA nucleotides but only 64 different tRNA molecules. (3 MARKS)

- There are different mRNAs for every protein - tRNA are specific to one type of amino acid - The code is degenerate: there are some amino acids that can be coded by multiple codons

What is the role of ATP in translation?

- To provide energy to attach / bind a specific amino acid to tRNA - To provide energy to attach amino acids together.

Q: Describe how a nucleotide base sequence in a gene is used to synthesize a polypeptide. (7 MARKS)

- Transcription - DNA / gene copied / transcribed into mRNA - Free RNA nucleotides line up by complementary base pairing - To one template / DNA strand - Catalysed by RNA polymerase - mRNA moves to ribosomes - Translation - anticodons match / pair with / bind to codons / specific amino acid (attached to tRNA) - Formation of peptide bond between amino acids

Q: Describe the role of transfer RNA (tRNA) in protein synthesis. (3 MARKS)

- Translation - Binds to an amino acid / takes the amino acid to the ribosome / mRNA / holds the amino acid in place - tRNA is specific to the amino acid

What is a codon?

- Triplet of bases / sequence on mRNA - That codes for a specific amino acid

EQ: Scientists have genetically engineered maize plants to resist attack by insects known as stem borers. The scientists inserted a gene from Bacillus thuringiensis (Bt), a soil bacterium, that produces proteins that are highly toxic to the stem borers. These proteins bind to an embedded enzyme aminopeptidase (APN 1) on cell-surface membranes causing the membranes to become more permeable affecting the cells ability to maintain an optical osmotic potential. In Kenya, a study was carried out to see which types of Bt genes and their protein products would be most efficient against three types of stem borer. The stem borers were allowed to feed on nine types of maize (A-I), modified with Bt genes. Figure 2 below shows the leaf areas damaged by the stem borers after feeding on maize leaves for five days. Recently, scientists have seen insects like Busseola Fusca (maize stalk borer) developing resistance to genetically engineered maize plants. Cornell researchers have discovered that the insects with resistance have a different type of aminopeptidase (APN 6) which allows insects to digest their food and Bt without harm. Explain how two different aminopeptidases could evolve resulting in some insects developing resistance to the Bt toxin. (3 MARKS)

- Two different DNA base / nucleotide sequences - transcription results in two different mRNA strands - Translation of these strand result in two different sequences of amino acids

EQ: Figure 2 shows part of the mRNA and its corresponding amino acid sequence for cytochorme c, a protein involved in aerobic respiration, for Gorilla gorilla (gorilla) and Loxodonta africana (African elephant) Explain how Figure 2 illustrates the genetic code. (3 MARKS)

- Universal as both the gorilla and elephant have the same four bases (U, A, C, G) - Non-overlapping as skipping bases would create a difference amino acid sequence for the gorilla and elpehant - Degenerate as the same amino acid has different codon base sequence e.g. Phe is coded for by UUc and UUU

EQ: The diagram shows part of the process of making normal and defective CFTR in a cell. A normal CFTR protein molecule has sugar molecules attached to it which make it functional. Describe how the information on mRNA is translated into CFTR at the ribosome. (4 MARKS)

- codon on mRNA; - specific / complementary base pairing with; - anti-codon on tRNA; - specific tRNA for each amino acid; - protein formed by condensation reactions / peptide bonds formed;

EQ: Describe how the process of translation leads to the production of a polypeptide. (4 MARKS)

- mRNA attaches to ribosome - codon on mRNA - binds to an anti-codon on tRNA - each tRNA brings a specific amino acid - sequence of codons / bases on mRNA determines the order of amino acids - formation of peptide bonds OR amino acids joined by condensation reactions

What are the causes of a mutation?

- mistakes during DNA replication (internal) - mutagenic agents (external) e.g. high energy radiation, carcinogenic substances

Relating the base sequence of nucleic acids to the amino acid sequence of polypeptides, when provided with suitable data about the genetic code

- tRNA anticodons are complementary to mRNA codon - Eg mRNA codon = ACG; tRNA anticodon = UGC - mRNA sequence of bases / codons are complementary to sequence of bases / triplets on DNA template strand - Eg mRNA base sequence = ACG UAG AAC; DNA base sequence = TGC ATC TTG - In RNA, uracil replaces thymine - You may then have to relate this to amino acid sequences

Describe the role of tRNA in translation

- tRNA attaches to and transports a specific amino acid, in relation to its anticodon - tRNA anticodon complementary base pairs to mRNA codon, forming hydrogen bonds - Two tRNAs bring / carry specific amino acids together to ribosome for the formation of a peptide bonds - Correct sequence of amino acids along polypeptide - About 60 types of tRNAS to carry 20 different amino acids = Genetic code is degenerate

EQ: Explain the role of transfer RNA (tRNA) in the process of translation. (2 MARKS)

- tRNA attaches to specific amino acid - tRNA with amino acid moves to the ribosome - The anticodon of tRNA binds to / forms hydrogen bonds with a complementary codon of mRNA

Q: The diagram represents a tRNA molecule. Using the information shown in the diagram, describe two ways in which the structure of a tRNA molecule differs from the structure of a mRNA molecule. (3 MARKS)

- tRNA is folded (mRNA is straight / unfolded) - tRNA has hydrogen bonds holding the structure together but the mRNA does not - tRNA has a fixed size / length but mRNA depends on the size of the gene OR tRNA has an anticodon but mRNA has codons OR tRNA has an amino acid binding site

EQ: Describe the function of ribosomes in protein synthesis. (4 MARKS)

- translation; construction of polypeptides - provide a binding site for mRNA - provides binding site for two tRNA molecules - two amino acids are held close together - formation of peptide bonds - allows assembly of amino acids into the sequence / primary structure

EQ: A change from Glu to Lys at amino acid 300 had no effect on the rate of reaction catalysed by the enzyme. The same change at amino acid 279 significantly reduced the rate of reaction catalysed by the enzyme. Use all the information and your knowledge of protein structure to suggest reasons for the differences between the effects of these two changes. (3 MARKS)

1. (Both) negatively charged to positively charged change in amino acid; 2. Change at amino acid 300 does not change the shape of the active site / tertiary structure 3. Amino acid 279 may have been involved in a (ionic, disulfide or hydrogen) bond and so the shape of the active site / tertiary structure changes

Q: Using the sequence shown in the diagram, explain the meaning of each of the following terms. 1. Triplet code (2 MARKS) 2. Non-overlapping. (2 MARKS) 3. Degenerate. (2 MARKS)

1. - Each amino acid is coded for by three nucleotides / bases - Nucleotides / bases code for 4 amin 2. - Each triplet is discrete / each base is only used one in a triplet - Idea that AAT + AAC + CAG + TTT gives 4 distinct triplets / codes 3. - More than one code can be used for a particular amino acid / stop code - AAT and AAC code for leucine

EQ: A scientific magazine recently produced an article with the headline 'At least 75% of the DNA in our genome is useless junk after all.' Use your knowledge of DNA to suggest what the author of the article meant by 'useless junk'. In your answer you should clearly describe where this junk DNA is located within the human genome. (2 MARKS)

1. DNA that is non coding/does not code for a polypeptide protein; 2. Introns within a gene; 3. (Non-coding) multiple repeats (of base sequences) between genes;

EQ: Table 1 shows the percentage of different bases in two pre-mRNA molecule. The molecules were transcribed from the DNA in different parts of the chromosome. Explain why the percentages of bases from the middle part of the chromosome and the end part are different. (2 MARKS)

1. Different genes 2. Have different base sequences 2. pre-mRNA transcribed from different DNA / codes for different proteins

EQ: Messenger RNA (mRNA) is used during translation to form polypeptides. Describe how mRNA is produced in the nucleus of a cell. (6 MARKS)

1. Helicase; 2. Breaks hydrogen bonds; 3. Only one DNA strand acts as template; 4. RNA nucleotides attracted to exposed bases; 5. (Attraction) according to base pairing rule; 6. RNA polymerase joins (RNA) nucleotides together; 7. Pre-mRNA spliced to remove introns;

EQ: Comparing the base sequence of a gene provides more information than comparing the amino acid sequence of the protein for which the gene codes. Explain why. (2 MARKS)

1. More bases than amino acids / longer base sequence than amino acid sequence; 2. Idea that the genetic code is degenerate;

EQ: Multiple copies of the AMY1 gene is an adaptation to a high-starch diet. Use your knowledge of protein synthesis and enzyme action to explain the advantage of this adaptation. (3 MARKS)

1. More mRNA / more transcription; 2. More translation / enzyme; 3. So reaction faster;

EQ: Table 1 shows mRNA codons and the amino acids coded for by each codon. It also shows some properties of the R group of each amino acid. The genetic code is described as degenerate. What is meant by this? Use an example from Table 1 to illustrate your answer. (2 MARKS)

1. More than one codon codes for a single amino acid; 2. Suitable example selected from Table 1;

EQ: Herpes simplex virus (HSV) infects nerve cells in the face, including some near the lips. Like many other viruses, HSV can remain inactive inside the body for years. When HSV becomes active, it causes cold sores around the mouth. Human cells infected with a virus may undergo programmed cell death. While HSV is inactive inside the body, only one of its genes is transcribed. This gene is the latency-associated transcript (LAT) gene that prevents programmed cell death of an infected nerve cell. Scientists have found that transcription of the LAT gene produces a microRNA. This microRNA binds to some of the nerve cell's own mRNA molecules. These mRNA molecules are involved in programmed cell death of nerve cells. The scientists concluded that production of this microRNA allows HSV to remain in the body for years. Use information from the passage and your own knowledge to answer the following questions. HSV can remain inactive inside the body for years (lines 2-3). Explain why this virus can be described as inactive. (2 MARKS)

1. No more (nerve) cells infected / no more cold sores form; 2. (Because) virus is not replicating.

EQ: Herpes simplex virus (HSV) infects nerve cells in the face, including some near the lips. Like many other viruses, HSV can remain inactive inside the body for years. When HSV becomes active, it causes cold sores around the mouth. Human cells infected with a virus may undergo programmed cell death. While HSV is inactive inside the body, only one of its genes is transcribed. This gene is the latency-associated transcript (LAT) gene that prevents programmed cell death of an infected nerve cell. Scientists have found that transcription of the LAT gene produces a microRNA. This microRNA binds to some of the nerve cell's own mRNA molecules. These mRNA molecules are involved in programmed cell death of nerve cells. The scientists concluded that production of this microRNA allows HSV to remain in the body for years. Use information from the passage and your own knowledge to answer the following questions. HSV infects nerve cells in the face (line 1). Explain why it infects only nerve cells. (3 MARKS)

1. Outside of virus has antigens / proteins; 2. With complementary shape to receptor / protein in membrane of cells; 3. (Receptor / protein) found only on membrane of nerve cells.

EQ: Describe the structure of proteins. (5 MARKS)

1. Polymer of amino acids; 2. Joined by peptide bonds; 3. Formed by condensation; 4. Primary structure is order of amino acids; 5. Secondary structure is folding of polypeptide chain due to hydrogen bonding 6. Tertiary structure is 3-D folding due to hydrogen bonding and ionic / disulfide bonds; 7. Quaternary structure is two or more polypeptide chains.

Q: Triplets of bases in a DNA molecule code for the sequence of amino acids in a polypeptide. The genetic code is frequently written as the three bases on mRNA that are complementary to a triplet on DNA. Table 1 shows what different combinations of bases on mRNA code for. The names of amino acids are abbreviated. For example, 'Ala' stands for alanine. Suggest the role of the mRNA base triplets UGA, UAG and UAA. (2 MARKS)

1. Stop translation; 2. Result in detachment of polypeptide chain from ribosome

EQ: Describe how mRNA is produced in a plant cell. (5 MARKS)

1. The DNA strands separate by breaking the H bonds; OR H bonds broken between (complementary) (DNA) bases; 2. (Only) one of the strands/template strand is used (to make mRNA/is transcribed); 3. (Complementary) base pairing so A→U, T→A, C→G, G→C; 4. (RNA) nucleotides joined by RNA polymerase; 5. pre-mRNA formed; 6. Splicing / introns removed to form mRNA;

EQ: In this study, scientists used DNA to determine the genetic diversity between the species of flatworms. Explain one additional way in which genetic diversity could be measured. (2 MARKS)

1. The base sequence of mRNA of the same gene 2. The greater the number of differences in the mRNA base sequence, the greater the genetic diversity OR 3. The amino acid sequence of an encoded protein 4. The greater the number of differences in the amino acid sequence of a protein, the greater the genetic diversity

EQ: Starting with mRNA in the cytoplasm, describe how translation leads to the production of a polypeptide. Do not include descriptions of transcription and splicing in your answer. (5 MARKS)

1. mRNA associates with a ribosome / ribosome attaches to mRNA; 2. Ribosome moves to / finds the start codon / AUG; 3. tRNA brings / carries (appropriate / specific) amino acid; 4. Anticodon (on tRNA complementary) to codon (on mRNA); 5. Ribosome moves along to next codon; OR Ribosome 'fits' around two codons / can fit two tRNAs; 6. (Process repeated and) amino acids join by peptide bonds / condensation reaction (to form polypeptide); OR (Process repeated and) amino acids joined using (energy from) ATP (to form polypeptide);

EQ: Give two structural differences between a molecule of messenger RNA (mRNA) and a molecule of transfer RNA (tRNA). (2 MARKS)

1. mRNA does not have hydrogen bonds / base pairing, tRNA does; OR mRNA is linear / straight chain, tRNA is cloverleaf; 2. mRNA does not have an amino acid binding site, tRNA does; 3. mRNA has more nucleotides; 4. (Different) mRNAs have different lengths, all tRNAs are similar / same length; 5. mRNA has codons, tRNA has an anticodon;

EQ: Starting with mRNA in the nucleus of a cell, describe how a molecule of protein is synthesised.

1. mRNA leaves nucleus through nuclear pore 2. To ribosome 3. tRNA molecule bring amino acids to ribosome 4. Specific trNA molecule for specific amino acid 5. Anticodon of tRNA corresponds / complementary to codon on mRNA 6. Peptide bonds form between amino acids 7. tRNA detaches and collects another amino acid 8. Ribosome moves along mRNA

The genetic code used four different DNA bases. What is the maximum number of different DNA triplets that can be made using these four bases?

64 = 4^3

EQ: Determine the mRNA sequence that is coded by the following strand of DNA. (1 MARK)

AUG GGU UCA it is the antisense strand that forms mRNA

What is the function of chaperone proteins?

Ensure polypeptide chains synthesised on ribosomes fold correctly and transport of polypeptides across the ER, Golgi, and beyond.

EQ: Herpes simplex virus (HSV) infects nerve cells in the face, including some near the lips. Like many other viruses, HSV can remain inactive inside the body for years. When HSV becomes active, it causes cold sores around the mouth. Human cells infected with a virus may undergo programmed cell death. While HSV is inactive inside the body, only one of its genes is transcribed. This gene is the latency-associated transcript (LAT) gene that prevents programmed cell death of an infected nerve cell. Scientists have found that transcription of the LAT gene produces a microRNA. This microRNA binds to some of the nerve cell's own mRNA molecules. These mRNA molecules are involved in programmed cell death of nerve cells. The scientists concluded that production of this microRNA allows HSV to remain in the body for years. Use information from the passage and your own knowledge to answer the following questions. The scientists concluded that production of this microRNA allows HSV to remain in the body for years (lines 10-12). Explain how this microRNA allows HSV to remain in the body for years. (4 MARKS)

MicroRNA binds to cell's mRNA (no mark) 1. (Binds) by specific base pairing; 2. (So) prevents mRNA being read by ribosomes; 3. (So) prevents translation / production of proteins; 4. (Proteins) that cause cell death.

EQ: Figure 1 below represents a model for the production of a polypeptide. Name and outline what occurs in: 1) Process 1 2) Process 2

Process 1: Transcription - Production of a mRNA molecule that is complementary to a section of DNA - Occurs in nucleus - Involved RNA polymerase Process 2: Translation - Genetic code on the mRNA is converted into a polypeptide chain - Occurs in cytoplasm - Requires a ribosome, tRNA and amino acids

What is a DNA triplet?

Sequence of 3 bases coding for specific amino acid, E.g. UAU codes for tyrosine

Q: There is a maximum of 64 different base combinations in DNA that could each code for an amino acid. Twenty different amino acids are commonly used for protein synthesis. In theory, this would need only 20 different base combinations. Explain the uses of the remaining 44 combinations. (1 MARK)

Several triplets / codons code for one amino acid OR some are used as start / stop termination

What are the similarities / difference between structure of mRNA and tRNA molecules?

Similarities Both single polynucleotide strand Differences - mRNA straight, whereas tRNA folded into clover-leaf shape - mRNA is longer, variable length, whereas tRNA is shorter - mRNA contains no paired bases or hydrogen bonds, whereas tRNA has some paired bases and hydrogen bonds - tRNA has an amino acid attachment site

EQ: Figure 1 shows part of the mRNA and its corresponding amino acid sequence for cytochrome, c, a protein involved in aerobic respiration, for Mus musculus (house mouse) and Loxodonta africana (African elephant. Identify the DNA triplet that codes for the amino acid serine (Ser).

TCC

EXAM TIP!!

The DNA triplet that codes for the amino acid is on the sense strand or non-template strand so the base sequence will be the same as the mRNA sequence except uracil will be replaced with the DNA base thymine.

What is protein folding determined by?

The amino acid sequence of the polypeptide

Q: The codons ACU and ACG both code for the amino acid threonine. Identify the property of the genetic code that illustrates this.

The genetic code is degenerate

DNA is Degenerate, what does this mean?

The same amino acid can be coded for by more than one base triplet, e.g. tyrosine can be coded for by UAU or UAC

DNA is universal, what does this mean?

The same specific DNA base triplets codes for the same amino acids in all living organisms e.g. UAU codes for tyrosine in all organisms

What is the genetic code?

The sequence of DNA triplets bases (or mRNA codons) that code for a specific amino acid.

What is translation?

Translation (translating from one language to another): mRNA (nucleotides) → amino acids / protein / polypeptides (amino acids)

What is a anticodon?

Triplet of bases on tRNA that is complimentary to codon on mRA

EQ: A different base could replace cytosine in the mRNA sequence shown in Figure 4. The new code would produce a peptide with the same primary structure. Name this base. (1 MARK)

Uracil

Specialised proteins, called __________, assist in the folding of other proteins.

chaperones

What are exons?

coding regions on the DNA molecule

The genetic code is....

universal, non-overlapping and degenerate.

Q: Suggest one advantage of showing the genetic code as base sequences on mRNA, rather than triplets on DNA.

(Because) ribosomes assemble polypeptides using mRNA code OR DNA has two strands each with a different (complementary) base sequence;

EQ: Figure 1 shows a part of a pre-mRNA molecule. (a) Name two substances that make up part X. (1 MARK) (b) Give the sequence of bases on the DNA from which this pre-mRNA has been transcribed. (1 MARK)

(a) Phosphate and ribose (b) TAGGCA

Q: Table 2 shows some mRNA codons and the amino acids for which they code. (i) A tRNA molecule has the anticodon UAA. Which amino acid does the tRNA molecule carry? (ii) Give the DNA base sequence that codes for threonine. (2 MARKS)

(i) Isoleucine; (ii) TGG;

Q: Two single base mutations occurred in the DNA coding for this section of mRNA. These mutations caused an alteration in the sequence of amino acids in the enzyme. The diagram shows the original and altered sequences of amino acids. (i) Use the mRNA codons provided in the table to complete the altered mRNA base sequence in the diagram. (1 MARK) (ii) Use the information provided to determine the precise nature of the two single base mutations in the DNA. (3 MARKS)

(i) UAC UUA UGG; (ii) addition and deletion (of bases / nucleotides); thymine added; adenine deleted;

EQ: Figure 1 shows the expose bases (anticodons) or two tRNA molecules involved in the synthesis of a protein. Complete the boxes to show the codon sequence of bases found along the corresponding section of the mRNA strand. (2 MARKS)

- ACG - UUG

EQ: identify the anticodons of the tRNA when these are the first three triplets of the antisense strand.

- AUG GUG CAU As the anticodons on the tRNA molecules are complementary to the mRNA codons (which are complementary to the antisense/template strand), the anticodons will have the same base sequence as the antisense/template strand except thymine is replaced with uracil in the tRNA anticodons.

Describe the role of ribosomes in translation

- Attaches to mRNA and houses tRNA, allowing codon-anticodon complementary base pairing - Allows peptide bonds to form between amino acids

EQ: Cycloheximide binds to specific sites on the eukaryotic ribosomes. This is shown in Figure 1 below. Explain how cycloheximide stops protein synthesis. (2 MARKS)

- Binding the ribsome so tRNA cannot form complementary base pairs with mRNA or blocking the mRNA codon from forming a base-pair with a tRNA anticodon - As a result, the amino acids dont join - So translation will not occur

EQ: Explain how copying bases more than once may give rise to differences in the protein. (2 MARKS)

- Changes base sequence - Of later triplets / amino acid codes

EQ: Describe how a gene codes for a polypeptide. (2 MARKS)

- DNA / the gene has a specific base sequence - The DNA base sequence determines the base sequence on mRNA - The DNA base sequence, and therefore the mRNA base sequence determines the sequence of amino acids in the polypeptide - Three bases / a triplet on the DNA / mRNA code for one specific amino acid

EQ: Contrast the chemical structure of DNA and RNA nucleotides. (2 MARKS)

- DNA has deoxyribose sugar but RNA has ribose sugar - DNA has the base T but RNA has the base U

Q: Protein-encoding DNA leads to the production of mRNA. Other parts of the DNA encode functional RNA/ Give two examples of functional RNA. (2 MARKS)

- Genes encoding ribosomal RNA - Genes encoding tRNAs

Q: State one difference between the genome and proteome of a cell. (1 MARK)

- Genome is the complete set of genes in a cell - The proteome is the full range of proteins that a cell can produce - All cells with a nucleus have the same genome - Expression of proteome varies between tissues

EQ: Distinguish between the genome and proteome of a cell. (2 MARKS)

- Genome is the complete set of genes in a cell AND the proteome is the full range or proteins that a cell can produce - All cells with a nucleus have the same genome AND expression of proteome varies between tissues

EQ: Scientists studied two diptera animals Anopheles gambiae (mosquito) and drosophila melanogaster (fruit fly) who diverged 250 million years ago. Their study was to determine how similar their genes and proteins were. It was known that D. melanogaster has 17,864 genes which code for 30,717 proteins whereas A. gambiae has 13,247 genes coding for 14,102 proteins. The study showed that almost half of their genes were similar and that they had diverged more rapidly than vertebrates. Using examples from the passage explain the difference between genome and proteome. (2 MARKS)

- Genome is the complete set of genes in a cell OR the proteome is the full range of proteins that a cell can produce - All cells with a nucleus have the same genome OR expression of proteome varies between tissues - D. melanogaster has 17,864 genes and Aa. gambiae has 13,247 genes representing the genome - D. melanogaster has 30,717 proteins and A. gambiae has 14,102 proteins representing the proteome

Describe protein synthesis: transcription

- In nucleus - DNA double helix unzipped by DNA Helicase - Breaking the hydrogen bonds - Forming 2 DNA strands - Only one DNA strand acts as template (antisense strand) - Free RNA nucleotides attach / attracted to the template strand's exposed nucleotides - By complementary base pairing (U replaces T in RNA) - H-bonds reform - RNA polymerase joins adjacent mRNA nucleotides together in a condensation reaction built in the 5' --> 3' direction, forming phosphodiester bonds to form premRNA - When RNA polymerase reaches stop codon, mRNA (prokaryotes) and pre-mRNA (eukaryotes) detaches from DNA - pre-mRNA spliced to remove introns

EQ: Gene mutations occur spontaneously. During which part of the cell cycle are gene mutations most likely to occur? Suggest a reason for your answer. (2 MARKS)

- Interphase / synthesis phase - DNA / gene replication / synthesis occurs / longest stage

EQ: Alternative splicing occurs during protein synthesis. Describe what happens during this process. (2 MARKS)

- Introns / non-coding regions are removed from pre-mRNA strands - Exons / coding regions are joined/spliced together to form the final mRNA strand

Describe tRNA (transfer RNA)

- Involved in translation - Single stranded molecule / polynucleotide made up of RNA nucleotides - Uracil nitrogenous base instead of thymine - Carries an amino acid = Amino acid binding site at 3' end - Anticodon in the middle loop = 3 bases - Anticodon bases complementary to mRNA codon - Each tRNA specific to one amino acid, specific to its anticodon - Folded - 3 hairpin loops = three-leafed clover shape - Hydrogen between specific base pairs holds molecule in this shape

Describe mRNA (messenger RNA)

- Made by transcription in the nucleus - Acts as a template for translation in the cytoplasm - Sequence of bases on RNA determines sequence of amino acids in polypeptide chain - Straight chain molecule - Sequence of bases on RNA determined by sequence of bases on DNA, Triplet code = codon - Chemically unstable: So breaks down after a few days

EQ: Read the following passage. Herpes simplex virus (HSV) infects nerve cells in the face. HSV can remain inactive inside the body for years, but when it comes active, it causes cold sores around the mouth. Human cells infected with a virus may undergo programmed cell death. While HSV is inactive inside the body, only one of its genes is transcribed.This gene is the latency-associated transcript (LAT) gene that prevents programmed cell death of an infected nerve cell. Scientists have found that transcription of the LAT gene produces a microRNA. This microRNA binds to some of the nerve cell's own mRNA molecules. These mRNA molecules are involved in programmed cell death of nerve cells. Use the information from the passage and your own knowledge to answer the following question. Scientists have concluded that the production of microRNA allows HSV to remain in the body for years. Suggest how the presence of the microRNA allows HSV to remain inactive for years. (3 MARKS)

- MicroRNA binds, by complementary base pairing, to mRNA molecules - Preventing translation - Of the protein that causes programmed cell death of infected nerve cells

EQ: Table 1 below shows some mRNA codons and the amino acids for which they code. i) Give the DNA sense strand sequence for leucine ii) Name the amino acid carried by the tRNA with the anticodon CAU

- TTA - Valine

EQ: Explain why the mRNA transcripts leaving the nucleus have fewer nucleotides than the RNA initially produced by DNA transcription. (3 MARKS)

- The intial / primary RNA transcripts have exons / coding regions and introns / non coding regions - The introns / non coding regions are removed - Via exon splicing

EQ: A piece of mRNA is 972 nucleotides long but the DNA coding strand form which it was transcribed is 1215 nucleotides long. i) Explain why there is a difference in the numbers of nucleotides ii) State the maximum number of amino acids in the polypeptide translated from this piece of mRNA and explain your answer (3 MARKS)

- The mRNA was edited OR introns / non-coding regions present in DNA - maximum number = 324 amino acids - 972 / 3 = 324 because three bases / nucleotides code for one amino acid

EQ: The KRT35 gene codes for an acidic protein which is a type I hair keratin. Along with type II keratin proteins, it forms the hair and nails on our body. This gene is found on chromosome 17. Explain how a gene like KRT35 would code for a protein. (2 MARKS)

- There is a base sequence on DNA - which is transcribed onto an mRNA during transcription - The code is a triplet code / codon for an amino acid - mRNA attaches to a ribosome in the cytoplasm - Translation occurs producing the protein

EQ: Figure 1 is the template strand of a DNA molecule that codes for six amino acids. Identify the sequence of anticodons on the tRNAs that would synthesise the string of six amino acids. (3 MARKS)

- UAC GCG - UCA AUG - AUA CUG

What are the types of small scale mutations?

- point mutations aka SNPs, are considered small scale mutations; substation of a single base: - silent: has no effect on the protein sequences - missense: results in an amino acid substitution - nonsense: substitutes a stop codon for an amino acid - frameshift mutation: insertion or deletion of one or more bases: - insertion or deletion result in a shift in the reading frame

EQ: Ricin is a protein produced by castor beans. In animal cells, ricin acts as an enzyme. This enzyme removes the adenine molecule from one of the nucleotides in the RNA of ribosomes. As a result, the ribosome changes shape. Ricin causes the death of cells and is very poisonous to many animals. Suggest how the action of ricin on ribosomes could cause the death of cells. (3 MARKS)

- stopping translation - which prevents mRNA binding to the ribosome / tRNA binding to ribosome - preventing the formation of peptide bond - as a result, the cell no longer has the function of that protein

EQ: In the early 1990s a company in the united states attempted to market a genetically engineered tomato (Solanum lycopersicum) that has an extended shelf life. The gene they inserted that interfered in the production of the enzyme Beta polygalacturnosa was just one of the 31,760 genes that a tomato plant has. Explain what the 31,760 genes represent in a tomato. (2 MARKS)

- the genome - the complete set of genes of an organism

EXAM TIP!

- the tRNA anticodon is opposite the amino acid DNA on the tRNA - the mRNA triplet is the same as the amino acid DNA on the tRNA

How many polynucleotide strands are in the tRNA molecule?

1

EQ: Table 5 shows some of the mRNA codons for some amino acids. A different mRNA strand did not contain the base sequence GCC. The protein it translated contained the amino acid alanine. Explain how this is possible. (2 MARKS)

1. (Genetic) Code is degenerate / amino acids coded for by more than one triplet; 2. Different codon but same amino acid / different codon but still alanine;

EQ: Not all mutations result in a change to the amino acid sequence of the encoded polypeptide. Explain why. (1 MARK)

1. (Genetic) code degenerate; 2. Mutation in intron.

EQ: A chemical called oligomycin prevents respiration. Protein synthesis does not happen in cells treated with oligomycin. Suggest why. (3 MARKS)

1. (No respiration so) no ATP produced; 2. ATP needed to form peptide bond; 3. ATP needed to activate the amino acid / ATP needed to attach amino acid to tRNA;

EQ: A mutation of a tumour suppressor gene can result in the formation of a tumour. Explain how. (2 MARKS)

1. (Tumour suppressor) gene inactivated / not able to control / slow down cell division; 2. Rate of cell division too fast / out of control.

EQ: Some cancer cells have a receptor protein in their cell-surface membrane that binds to a hormone called growth factor. This stimulates the cancer cells to divide. Scientists have produced a monoclonal antibody that stops this stimulation. Use your knowledge of monoclonal antibodies to suggest how this antibody stops the growth of a tumour. (3 MARKS)

1. Antibody has specific tertiary structure / binding site / variable region; 2. Complementary (shape / fit) to receptor protein / GF / binds to receptor protein / to GF; 3. Prevents GF binding (to receptor).

Q: DNA is a polymer of nucleotides. Each nucleotide contains an organic base. Explain how the organic bases help to stabilise the structure of DNA. (2 MARKS)

1. Hydrogen bonds between the base pairs holds two strands together 2. Many hydrogen bonds provides strength

Q: In a eukaryotic cell, the base sequence of the mRNA might be different from the sequence of the pre-mRNA. Explain why. (2 MARKS)

1. Introns (in pre-mRNA); 2. Removal of sections of (pre-mRNA) / splicing;

State the overview of protein synthesis: 2 stages

1. Transcription; Production of mRNA from DNA --> occurs in Nucleus 2. Translation; Production of polypeptides form the sequence of codons carried by mRNA --> occurs in Cytoplasm on ribosomes

Q: Describe the role of a ribosome in the production of a polypeptide. Do not include transcription in your answer. (3 MARKS)

1. mRNA binds to ribosome; 2. Idea of two codons / binding sites; 3. (Allows) tRNA with anticodons to bind / associate; 4. (Catalyses) formation of peptide bond between amino acids (held by tRNA molecules); 5. Moves along (mRNA to the next codon) / translocation described;

EQ: Some types of cell contain special proteins called chaperones. Chaperone proteins are mostly found on the endoplasmic reticulum. Describe the role of chaperone proteins. (2 MARKS)

1.Chaperone proteins bind to hydrophobic groups (on the secretory proteins); 2. Prevent incorrect hydrophobic interactions; 3. Allows protein to be correctly folded;

EQ: A scientist investigated changes in the amino acid sequence of a human enzyme box resulting from mutations. All these amino acid changes result from single base substitution mutations. This enzyme is a polypeptide 465 amino acids long. Table 2 shows the result of three of the base substitutions. What is the minimum number of bases in the gene coding for this polypeptide?. (1 MARK)

1395;

EQ: The figure below shows part of a DNA molecule. State the maximum number of amino acids for which this piece of DNA could code. (1 MARK)

4

EQ: Figure 1 shows the sequence of bases in a section of the mRNA strand used to synthesise papain. Using Figure 1, deduce how many different types of tRNA molecules would attach to the section of mRNA shown in the diagram. (1 MARK)

5

Q: The diagram shows the sequence of bases in a section of the mRNA strand used to synthesise this enzyme. How many different types of tRNA molecule would attach to the section of mRNA shown in the diagram?

5;

Q: Table 1 shows a sequence of DNA. Complete Table 1.

ACCGUAUG

What is a mutation?

Any change in the sequence of bases in a molecule of DNA

EQ: The sequence with the deletion contains 14 nucleotides and so should code for 4 amino acids. Why are only 3 amino acids coded by this sequence? (1 MARK)

As the 4th codon forms a stop codon / 4th codon now UGA which is a stop code;

EQ: Table 3 shows some mRNA base sequences and what they code for. Complete Table 4 to show: • the original DNA that this mRNA sequence was transcribed from • the amino acids that would be coded for by this sequence. Use information from Table 3 to help you. (3 MARKS)

Asp, Gly, Ser, Leu, Ser

EQ: A protein in a eukaryotic cell may have a different shape from a protein in a bacterium even though they are coded for by an identical gene. Suggest one reason why. (1 MARK)

Bacteria do not have endoplasmic reticulum/chaperone proteins;

EQ: A scientist investigated changes in the amino acid sequence of a human enzyme box resulting from mutations. All these amino acid changes result from single base substitution mutations. This enzyme is a polypeptide 465 amino acids long. Table 2 shows the result of three of the base substitutions. Use information from Table 1 to tick one box that shows a single base substitution mutation in DNA that would result in a change from Val to Ala at amino acid number 203. (1 MARK)

CAA → CGA

What does triplet refer to?

DNA

DNA is Non-overlapping, what does this mean?

DNA is discrete; each base from any triplet / codon gene is part of only one triplet

EQ: Figure 1 shows the sequence of DNA bases coding for seven amino acids in the enzyme papain. Figure 2 shows the genetic code (mRNA codons) Use Figure 2 to determine the sequence of amino acids in this part of the enzyme. Write your answer in the boxes below.

Gln, Phe, Gln, Ser, Cys, Phe, Leu

EQ: Describe how two amino acids differ from one another. (1 MARK)

Have different R groups

EQ: Figure 1 below shows the exposed bases (anticodons) of two tRNA molecules involved in the synthesis of a protein. Complete the boxes to show the sequence of bases found along the corresponding section of the coding DNA strand. (2 MARKS)

Look at image.

EQ: Some individuals have a mutation which deletes one of the bases from the mRNA sequence shown in Figure 4. This results in a peptide with only the first three amino acids. Figure 5 shows the same mRNA sequence as Figure 4. Draw a circle on Figure 5 around the base that was deleted. (1 MARK)

Look at image.

Q: The diagram represents a tRNA molecule. What does part X bind to, what is part Y and what does part Z bind to?

Part X binds to: an amino acid for translation Part Y is a: hydrogen bond Part Z bind to: mRNA during translation

EQ: Explain the difference between pre-mRNA and mRNA. (1 MARK)

Pre mRNA contains introns, mRNA contains only exons

EQ: Herpes simplex virus (HSV) infects nerve cells in the face, including some near the lips. Like many other viruses, HSV can remain inactive inside the body for years. When HSV becomes active, it causes cold sores around the mouth. Human cells infected with a virus may undergo programmed cell death. While HSV is inactive inside the body, only one of its genes is transcribed. This gene is the latency-associated transcript (LAT) gene that prevents programmed cell death of an infected nerve cell. Scientists have found that transcription of the LAT gene produces a microRNA. This microRNA binds to some of the nerve cell's own mRNA molecules. These mRNA molecules are involved in programmed cell death of nerve cells. The scientists concluded that production of this microRNA allows HSV to remain in the body for years. Use information from the passage and your own knowledge to answer the following questions. Suggest one advantage of programmed cell death (line 4). (1 MARK)

Prevents replication of virus.

What is protein folding?

Process by which a polypeptide folds into its characteristic three-dimensional structure

The sequence of amino acids position of the ________ in the polypeptide determines how a protein is folded.

R groups

What does codon refer to?

RNA

EQ: Lactate dehydrogenase (LDH) has a key role in ensuring glycolysis continues. The enzyme catalyses the reversible conversion of pyruvate into lactate, oxidising the cofactor NAD. LDN is a tetrapeptide containing two types of polypeptides: polyppetide M (muscle), which is coded for by the LDH-A gene and polypeptide H (heart), which is coded for by the LDH-B gene. These two types of polypeptides in various combinations make up five isoenzymes. Isoenzymes are enzymes that differ in amino acid sequence but catalyse the same chemical reaction. The base sequence of the LDH-A and LDH-B genes and the sequences of the amino acids encoded by these genes were determined. Figure 1 shows the first ten amino acids of polypeptides M and H and the corresponding base sequences of one of the DNA strands on each gene. Table 1 shows the genetic code (mRNA codons). Compare and contrast both sequences shown in Figure 1, using the information given. (5 MARKS)

Similarities: - the base sequences are from the non-template strand - There is major similarity at the beginning of the nucleotide sequences; e.g. first 11 nucleotides / bases are identical - As a result there is major similarity at the beginning of the amino acid sequences e.g. first five amino acids are, met-ala-thr-leu-lys - Both have retained met/START amino acid Differences: - there are differences between the nucleotide and amino acid sequences e.g. same amino acid but different nucleotide sequence / triplet code e.g. first leu/fourth amino acid = CTA in LDH-A and CTT in LDH-B - Different amino acid sequences may allow for different polypeptide folding OR different amino acid sequences may lead to changes in the active site

EQ: The scientists extracted a gene from the mitochondrial DNA of tissue samples from individual salamanders of the five new species (A-E). The gene extracted from the mitochondrial DNA codes for a protein. Suggest one possible function of this protein. (1 MARK)

Suitable suggestion e.g. enzyme involved in (aerobic) respiration / transport protein (in mitochondrial membrane);

EQ: Schistosomiasis is a serious disease affecting millions of people each year. It is caused by infection with a parasitic flatworm. Scientists studied the genetic diversity of the three species of flatworm known to cause the disease. Samples of the flatworms were collected at random from different locations. Why was it important that the samples were collected at random? (1 MARK)

To reduce / prevent bias

What is transcription?

Transcription (like a scribe): DNA (nucleotides) → mRNA (nucleotides)

Some amino acids have neutral non-polar R groups that are _________, ________ parts of a molecule interact with each other

hydrophobic, hydrophobic

What are introns?

non-coding regions

EXAM TIP:

the anticodon of the tRNA codon is opposite the mRNA codon