3.1.7 DNA, GENES AND CHROMOSOMES
Q: Explain why complementary base-pairing is important in DNA replication (2 MARKS)
- DNA can be replicated without error - Allows reformation of hydrogen / H bonds
How is DNA stored in prokaryotic cells?
- DNA is Shorter, circular / non-linear (loop) - Not associated with proteins / histones - No nucleus - No introns / no non-coding DNA (only exons) - Have plasmids
What is the differences between RNA and DNA molecules:
- DNA molecules are double stranded (double helix), whereas RNA molecules are single stranded - DNA is longer whereas RNA is shorter
Q: Describe how the structure of RNA differs from that of DNA. (3 MARKS)
- Has ribose (DNA had deoxyribose) - Uracil instead of thymine - Single stranded / doesn't have double helix OR RNA shorter/smaller than DNA
EQ: Scientists have discovered that within species genomes regions of introns do not change in frequency at the same rate as exons. Explain why. (2 MARKS)
- Introns do not code for proteins / amino acids - So there is no selection pressure
Q: Mikanolide is a drug that inhibits the enzyme DNA polymerase. Explain why this drug may be effective against some types of cancer. (2 MARKS)
- Prevents / slows DNA replication - New strand not formed / nucleotides (of new strand) not joined together
EQ: Other than for use in replication, give one advantage of DNA molecules having two strands. (1 MARK)
- Stability (of the molecule) - Protects bases (from mutagens / digestive enzymes)
From the DNA base codes, name which are purines and which are pyrimidines.
- purine: adenine, guanine - pyrimidine: cytosine, thymine
Q: Cytarabine is a drug used to treat certain cancers. It prevents DNA replication. The diagram shows the structures of cytarabine and the DNA base cytosine. Cytarabine has a greater effect on cancer cells than on healthy cells. Explain why.
(Cancer cells / DNA) divide / replicate fast(er) / uncontrollably;
EQ: A small amount of DNA can be extracted from mitochondria and DNA sequencing used to try to find a mutation (lines 18-19). From this sample: • how would enough DNA be obtained for sequencing? • how would sequencing allow the identification of a mutation? (2 MARKS)
1. Enough DNA using PCR; 2. Compare DNA sequence with 'normal' DNA.
What is a gene?
A section of DNA bases that codes for: - The amino acid sequence of a polypeptide - The functional RNA e.g. ribosomal RNA and tRNAS
EQ: Use information from Table 1 to tick one box that shows which single base mutation below would result in the amino acid Leu being replaced with the amino acid Arg. (1 MARK)
CUA --> CGA
EQ: A scientist replicated DNA in a test tube. To do this, he mixed an enzyme with identical single-stranded DNA fragments and a solution containing DNA nucleotides. Name the enzyme used in this DNA replication. (1 MARK)
DNA polymerase
What does DNA stand for?
Deoxyribonucleic acid
EQ: Some DNA nucleotides have the organic base thymine, but RNA nucleotides do not have thymine. RNA nucleotides have uracil instead of thymine. Give one other difference between the structure of a DNA nucleotide and the structure of an RNA nucleotide. (1 MARK)
Deoxyribose in DNA and ribose in RNA;
Q: Read through the following passage on the structure of DNA, then fill in the blanks with the most appropriate word or words to complete the passage. A DNA molecule consists of two strands of mononucleotides. Each of these strands is twisted around the other, forming a _____. Each mononucleotide consists of a pentose sugar called ___, a base and a _____. In each strand, the mononucleotides are held together by _____ bonds. The two strands are held together by complementary base pairing. Adenine bonds with ___ and cytosine bonds with ___. The name of the bond that forms between these bases is a ____ bond. A DNA molecule that is composed of 34% adenine will be composed of _____% cytosine. (8 MARKS)
Double helix, deoxyribose, phosphate group, phosphodiester, thymine, guanine, hydrogen, 16,
Q: Explain why DNA replicates semi-conservatively. (1 MARK)
Ensure genetic continuity / preserves the genetic code between generations of cells
EXAM TIP:
Examiners can be picky when it comes to comparison questions. To get full marks, try and include 'wheeras' to ensure you have covered both sides.
EXAM TIP!
INtrons INterrupt the exons, which code for protein
State the purpose of a phosphodiester bond in DNA and RNA
Join nucleotides together in a single strand (of DNA / RNA) / sugar phosphate backbone
Why was nitrogen used in the experiment to look for evidence for semi-conservative replication?
Nitrogen is used because it is found in the nitrogenous bases (A, C, T, G).
What does RNA stand for?
Ribonucleic acid
What is the type of pentose (sugar that has 5 carbon atoms) in DNA?
deoxyribose
Q: The sequence of bases on one strand of DNA is important for protein synthesis. What is its role?
determines (sequence of) amino acids / specific protein produced / mRNA formation;
Q: Explain the difference between pre-mRNA and mRNA.
pre-mRNA contains introns, mRNA contains only exons
Q: The diagram shows the process of DNA replication. The horizontal lines represent the positions of bases. What is represented by the part of the DNA molecule labelled W?
sugar or phosphate / S-P / nucleotide chain / backbone / original / parent DNA;
What is a genome?
the complete set of genes (all) in a cell / organism, including those in mitochondria and / or chloroplasts
EQ: The synthesis of cytochrome c takes place on ribosomes (line 12). In human cells, most ribosomes occupy a fixed position. In bacterial cells, the ribosomes are not in a fixed position. What causes this difference? (1 MARK)
(Most) ribosomes attached to endoplasmic reticulum / to membranes in human;
EQ: Figure 1 shows part of a DNA molecule. (a) How many nucleotides are shown in figure 1? (1 MARK) (b) Name the type of bond labelled X in Figure 1. (1 MARK)
(a) 8 (b) Phosphodiester bond
Q: Figure 1 shows a short section of a DNA molecule. (a) Name part R (b) Name part Q (c) Name the bonds that join A and B. (3 MARKS)
(a) Deoxyribose (b) Phosphate / phosphoric acid (c) Hydrogen bonds
Q: The black mamba is a poisonous snake. Its poison contains a toxin. The base sequence of mRNA that codes for the first two amino acids of this toxin is ACGAUG. (a) What is the base sequence of DNA from which this mRNA was transcribed? (1 MARK) (b) What is the base sequence of the anticodon on the first tRNA molecule that would bind to this mRNA sequence? (1 MARK)
(a) TGCTAC (b) UGC
Q: The diagram shows part of a DNA molecule. (a) Name two components of the part of the DNA molecule labelled. M. (2 MARKS) (b) What is the maximum number of amino acids for which this piece of DNA could code? (1 MARK)
(a) phosphate, deoxyribose (b) 4
EQ: CFTR is a transmembrane regulator protein. Its molecules have 1480 amino acids. People with cystic fibrosis produce defective CFTR protein which is missing one amino acid from its structure. Which type of gene mutation produced the cystic fibrosis allele? Explain your answer. (2 MARKS)
(deletion) of three bases; because substitution / addition would change amino acid(s);
Describe how to find the frequency of bases on DNA strands.
- % of adenine in strand 1 = % of thymine in strand 2 (vice versa) - % of guanine in strand 1 = % of cytosine in strand 2 (vise versa) Because of specific complementary base pairing between 2 strands
Q: Calculate the fraction of a cell's original DNA (not copies) that will be present in a granddaughter cell (i.e. after 3 full cycles of replication. (2 MARKS)
- 1/2 x 1/2 x 1/2 = 1/8 = 12.5 %
Q: In a section of DNA, 17% of the nucleotides were found to contain cytosine. Calculate the percentage of thymine in this DNA. Give one reason why the calculation could not be performed for a piece of RNA.
- 33% - RNA doesnt contain thymine / contains uracil instead of thmine / answer always be 0% - RNA is usually single-stranded so doesnt form base pairs
EQ: Cytosine and guanine can be bonded together in two ways, in a dinucleotide or in a base pair. Explain how these bonds differ (2 MARKS)
- A dinucleotide contains a phosphodiester bond / covalent bond and is formed by condensation - A base pair between cytosine and guanine forms with three hydrogen bonds
EQ: The DNA of the virus is different from that of the human, caterpillar and mouse. using data from Table 1 and your knowledge of DNA suggest what this difference is. Explain your answer. (2 MARKS)
- A doesnt equal T OR G doesnt equal C - This shows that there is no base pairing - So DNA is single-stranded
EQ: Tumour suppressor genes code for the proteins that control cell division in cells. Describe and explain how a mutation in a tumour suppressor gene could lead to a malignant tumour. (4 MARKS)
- A mutation is a change in the DNA base sequence Or a mutation leads to a different triplet code - This leads to a different sequence of amino acids - The protein formed may have a different 3D / tertiary structure that prevents it from carrying out its function (to control cell division) - This allows a malignant tumour to form as there is no control of cell division / cells can divide uncontrollably
Q: A rare form of diabetes, maternally inherited diabetes and deafness (MIDD), affects approximately 1% of people with diabetes. - During fertilisation the nuclear DNA of the sperm (spermatozoon) only, enters the egg cell (ovum) - MIDD is not caused by a mutation of a gene in nuclear DNA, but by a mutation of a gene in mitochondrial DNA known as A3242G - The expression of the mutation is dominant. Fig. 5.1 shows the pattern of inheritance of the A3242G gene. Analyse the information given and explain the pattern of inheritance of the A4342G gene shown in Fig. 5.1 (3 MARKS)
- A3242G / gene contained in DNA of mitochondria - Mitochondria are in ovum cytoplasm / come from mother / mitochondria don't come from sperm - Only mother can / father cannot pass on A3242G / gene
EQ: During replication of DNA, nucleotides are activated prior to incorporation into a nascent strand of DNA. Describe what is meant by 'activated' nucleotides and their role in replication. (2 MARKS)
- Activated nucleotides: nucleotides that contain two extra phosphate groups / nucleoside triphosphates - Role: source of energy / energy is released from breaking the two phosphate groups / to form phosphodiester bonds
EQ: State where telomeres are found within a chromosome as well as their function. (2 MARKS)
- At the end / end regions of chromosomes - To protect DNA from damage
Describe the structure of DNA & RNA nucleotides and polymers
- BOTH DNA and RNA are polymers of nucleotides. - Nucleotides are formed from a pentose sugar, a nitrogenous base and a phosphate group - Nucleotides (monomers) make up DNA or RNA which are nucleic acids (polymers) - DNA or RNA nucleotides are joined together by condensation reactions forming phosphodiester bonds
EQ: The ends of a DNA strand are referred to as the 3' ("3-prime") end and the 5' ("5-prime") end. Describe the aspects of DNA structure that give rise to this naming system. (3 MARKS)
- Carbon 3 and 5 (carbon atoms on the deoxyribose sugar) - Number 1 is the carbon bounded to the nitrogenous base / number 5 is the carbon not included in the ring structure - C5 of one nucleotide is bonded by phosphodiester bond to C3 adjacent nucleotide - There will always be one free / unbounded carbon atom C3 and one free unbounded carbon at C5 at each end of the strand
EQ: Explain how a change in the DNA base sequence for a protein may result in a change in the structure of the protein. (3 MARKS)
- Change in amino acids / primary structure - Change in hydrogen / ionic . disulphide bonds - Alters tertiary structure
EQ: Explain how a change in a sequence of DNA bases could result in a non-functional enzyme. (3 MARKS)
- Change in sequence of amino acids / primary structure - Change in hydrogen / ionic / disulphide bonds - Alters tertiary structure / active site of enzyme - Substrate cannot bind / no enzyme-substrate complexes can form
DNA polymerase moves in opposite directions along the DNA strands, describe this.
- DNA has antiparallel strands - The nucleotides on each strand are arranged differently - DNA polymerase is an enzyme with a specific shaped active site with a specific tertiary structure which can only bind to substrate with a complementary shape and orientation and form an enzyme-substrate complex - This means DNA replication can be continuous on one strand and on the other side DNA replication is discontinuous - DNA polymerase can only bind to and add nucleotides to the phosphate (3') end of the developing strand (so works in a 5' to 3' direction for new strand) - DNA polymerase in lagging strand keeps going back up because the DNA helicase is still unwinding the DNA - This is why it only does fragments which are joined together by DNA ligase - Note - 5' ("5 prime") and 3'("3 prime") indicate the carbon numbers in DNA's sugar backbone; 5' carbon has a phosphate group attached and a 3' carbon has a hydroxyl (-OH) group
EQ: State where the DNA molecule in Figure 1 would be found and explain why. (3 MARKS)
- DNA is located in nucleus - This DNA molecule is located in eukaryotic cells - The presence of the proteins / histones associated with the DNA are only in eukaryotic cells - The linear constructution of the DNA molecule is a characteristic only of eukaryotes.
What is the difference between DNA and RNA nucleotides?
- DNA nucleotides have the pentose sugar deoxyribose, whereas RNA nucleotides have the pentose sugar ribose - DNA nucleotides can have the base thymine, whereas RNA nucleotides have the uracil instead.
Q: Name three enzymes involved in DNA replication. (3 MARKS)
- DNA polymerase - DNA helicase - DNA ligase; has a role in joining the breaks in the sugar-phosphate backbone of DNA.
What is the function of DNA & RNA?
- Deoxyribonucleic ecid (DNA) holds genetic information / code; Instructions for making proteins - Ribonucleic acid (RNA) transfers genetic information from DNA to ribosomes
Q: Suggest why a variety of different protein structures could be formed from the polypeptides synthesised using the mRNa molecules from a single gene. (4 MARKS)
- Difference mRNA - Different primary structure / sequence of amino acids - Secondary / tertiary structure of proteins depends on primary structure / sequence - Due to change in / different bonds or hydrogen / ionic / disulphide bonds
Q: Scientists calculated the percentage of different bases in the DNA from a species of bacterium. They found that 14% of the bases were guanine. The scientists found that, in a second species of bacterium, 29% of the bases were guanine. Explain the difference in the percentage of guanine bases in the two species of bacterium. (2 MARKS)
- Different genes; - Different (DNA) base sequences;
Q: Suggest how changing the sequence of DNA nucleotides could affect the final product the DNA codes for. (3 MARKS)
- Different sequence of amino acids / primary structure - Different proteins / protein folds up differently / different tertiary structure - Product no longer functions / different functions
Q: Explain how the structure of DNA allows replication. (4 MARKS)
- Double stranded - Each / both strands act as template - Hydrogen bonds easily break / form between bases - Complementary (specified) base pairing / 3 H bonds between C & G and 2 H bonds between A & T
Q: Describe the structure of DNA. (4 MARKS)
- Double stranded → double helix → 2 antiparallel polynucleotide chains are held together by hydrogen bonds formed between specific complementary base pairs - long polymer / polynucleotide chain - carries the genetic information which codes for proteins - relatively simple structure which meant many scientists didn't believe it carried the genetic code
Watson and Crick first proposed the double-helical structure of DNA and, in later work, the semi-conservative replication of DNA. Meselson and Stahl's experiment proved Watson and Crick's theory of semi-conservative replication. Describe how evidence for semi-conservative DNA occured (the experiment).
- Experiment done by meselson + stahl - Replication of bacteria cultured in growth medium containing only heavy nitrogen (15N) only for several generations - Nitrogen incorporated into bacterial DNA bases and synthesisied - After DNA is extracted and spun in centrifuge only one heavy band is observed near the bottom of the tube - Bacteria / Cells transferred to a medium / nutrient solution with only light nitrogen (14N) and allowed to grow and divide twice - During this process, DNA from different samples of bacteria was extracted, suspended in a solution in separate tubes and spun in a centrifuge - After one replication the DNA band was intermediate → twice the thickness between positions of 14N and 15N - After two replication in light nitrogen → intermediate and light bands were observed - Proving DNA replication is semi-conservative
EQ: DNA holds the genetic information of the cell and is kept protected in the nucleus. Within the nucleus, DNA is replicated semi-conservatively in order to produce new cells. State two features of DNA and explain how these features are important in the process of semi-conservative replication of a cell's DNA. (2 MARKS)
- Feature 1: weak / easily broken hydrogen bonds between bases - Explanation: allow the two strands to separate / unzip - Feature 2: two strands - Explanation: both can act as templates - Feature 3: complementary base pairing - Explanation: allows accurate replication
EQ: The structure of DNA has many characteristics that give the molecule excellent functionality. Describe and explain three such structural features (6 MARKS)
- Feature: Double helix, with base pairs on inside, sugar phosphate backbone on outside - Explanation: Gives excellent stability / 360 protection from chemical attack - Feature: Strength of hydrogen bonds between pairs - Explanation: Bonds are strong enough to hold strands together / weak enough to be split when needed for replication / transcription - Feature: Supercoiled around histones / into chromosomes - Explanation: provides compact structure
EQ: In a length of DNA 1,000 nucleotides long, there are 382 guanine nucleotides in one strand of the DNA, Using this information, explain why it is not possible to calculate the number of guanine nucleotides in the opposite strand. (2 MARKS)
- Guanine base-pairs with cytosine / C (not with itself) - We cant tell the number of cytosines in the first strand; we only know there are 382 cytosines in the opposite strand
EQ: Humans and caterpillars have very similar percentages of each base in their DNA but are not the same class of organism. Use your knowledge of DNA structure and function to explain how this is possible. (3 MARKS)
- Have different genes - So base triplets are in a different sequence - So the DNA codes for a different amino acid sequence / protein / polypeptide
How is DNA stored in mitochondria and chloroplasts in eukaryotic cells?
- Have their own DNA - Similar to prokaryotic DNA - short, circular, not associated with proteins / histones - No plasmids
EQ: Adenine / thymine and guanine / cytosine are well known to hydrogen-bond to each other in complementary base-pairs within the DNA double helix. These bases can also form hydrogen bonds to other molecules in order to carry out their function. Suggest how other such molecules can bind to the bases and explain how this adds functionality. (4 MARKS)
- Helicase / DNA polymerase can bind to regions at the edges of the bases - To intiate / carry out DNA replication - Binidng site for other proteins / restriction enzymes / ligases / gene promoters / transcription factors / gene blockers - For control gene expression
Q: A husband and wife wanted to know whether they were carriers of the mutated form of a gene. This mutation is a deletion that causes a serious inherited genetic disorder in people who are homozygous. A geneticist took samples of DNA from the husband and wife. HE use a DNA probe to look for the deletion mutation. The DNA probe was specific to a particular base sequence in an exon in the gene. Exons are the coding sequences in a gene. The geneticist compared the couple's DNA with that of a person known not to carry this mutation. The DNA probe the geneticist used was for an exon in the DNA, not an intron. Explain why. (3 MARKS)
- Introns not translated / not in mRNA - Exons code for amino acids / introns dont code for amino acids - Mutations of these exons affect amino acid sequences
EQ: Describe and explain the structure and function of a ribosome. (4 MARKS)
- Is a small organelle, free / attached to RER - Made from RNA and protein (2 subunits) - Contains ribosomal RNA which catalyses peptide bond formation / condensation reactions between amino acids - Eukaryotic ribosomes (80S) are larger than prokaryotic ribosomes (70S) - Ribosomes are the site of translation / protein synthesis
EQ: Hypoxanthine is a drug used to treat certain cancers as it prevents DNA replication and therefore inhibits tumour growth. Figure 1 shows the structures of Hypoxanthine and the DNA base guanine. Use the information in Figure 1 to suggest how hypoxanthine prevents DNA replication. (2 MARKS)
- It has a similar shape / structure to guanine - It prevents (complemetary) base pairing OR prevents H-bonds forming OR prevents the formation of new strand OR inhibits / binds to DNA polymerase
EQ: There are several different types of gene mutation that can occur within a sequence of DNA. Figure 1 shows a section of a gene with two different substitutions. Further along the DNA strand, beyond point B in the coding region, there was a further substitution of a base. This also didn't lead to any changes in the protein structure and function. Suggest and explain two reasons why. (4 MARKS)
- It may have been a silent mutation - This is when the triplet code still codes for the same amino acid due to the DNA code being degenerate - the substitution was from an amino acid with a similarly charged R group - So the same bonds formed between R groups in the tertiary structure.
How is DNA stored in eukaryotic cells?
- Long, linear + double helix - Found in nucleus - Folded around / associated with proteins called histones - Tightly coiled / folded into chromosomes (DNA molecule + its associated with proteins)
EQ: Account for the long period of time (approximately 80 years) between the identification of a molecule of (what turned out to be) DNA and the proof that DNA carried genetic information. (2 MARKS)
- Many separate discoveries were made in separate research groups / over a long period that culminated in Watson and Crick work - DNA's relatively simple repeated structure was regarded as too simple to code for complex proteins with 20 amino acids
EQ: Describe the difference between mitochondrial DNA and nuclear DNA. (3 MARKS)
- Mitochondrial DNA is shorter, nuclear DNA is longer - Mitochondrial DNA is circular, nuclear DNA is linear - Mitochondrial DNA is not associated with proteins, nuclear DNA has histones - Mitochondrial DNA has no introns / has exons only
EQ: a scientist investigated the structure of an enzyme containing 647 amino acids, She looked at mutations that commonly occurred and their effects on the protein structure. The mutations were all single base substitutions. One of the changes she discovered was at position 389 where Asp was substituted for Glu. Use table 1 to suggest the likely effect of this mutation on the protein structure. (3 MARKS)
- No effect - because Asp and Glu are both negatively charged - They will likely form the same bonds / interactions in the tertiary structure
Cell division occurs in two main stages, these are...
- Nuclear division is the process by which the nucleus divides. There are two types of nuclear division, mitosis and meiosis - Cell division follows nuclear division and is the process by which the whole cell divides
Q: DNA replication takes place during interphase of the cell cycle. It occurs by a semi-conservative mechanism. Explain why DNA replication is considered to be semi-conservative. (2 MARKS)
- One strand from original DNA and one strand newly formed - An original strand / polynucleotide acts as a template (for new strand)
EQ: There are several different types of gene mutation that can occur within a sequence of DNA. Figure 1 shows a section of a gene with two different substitutions. Explain why a substitution at point A will not lead to a change in the protein structure but a substitution at point B can. (3 MARKS)
- Point A represents a region of non-coding DNA - This region doesnt code for amino acids - So the tertiary structure of the protein wont be affected
EQ: Compare the sizes and structures of prokaryotic and eukaryotic chromosomes. (4 MARKS)
- Prokaryotic chromosomes are generally smaller than eukaryotic nuclear chromosomes - Prokaryotic chromosomes are circular, whereas eukaryotic nuclear chromosomes are linear - prokaryotic chromosomes have a comparatively simple composition: DNA and some RNA whereas eukaryotic nuclear chromosomes are more complex: DNA, some RNA, and a lot of protein/histones - DNA in prokaryotic chromosomes dont have non-coding regions whereas the DNA in eukaryotic chromosomes have introns (non-coding DNA within genes) and non-coding multiple repeats between genes - Prokaryotic chromosomes and eukaryotic mitochondrial / chloroplast chromosomes are similar structures as both are short / circular / not associated with proteins
EQ: In living organisms the ratio (C + A) / (G + T) is constant but the ratio of (A + T) / (C + G) can vary between organisms. Using your knowledge of the structure and function of DNA, explain this information. (3 MARKS)
- Purines will always pair with pyrimidines - So A + T and C + G always pair - So the number of A will always be the same as the number of T, vise versa
EQ: Name the pyrimidine and purine bases in DNA and RNA and explain their base-pairing characteristics. (3 MARKS)
- Pyrimidines are T, C and U - Purines are A and G - A-T or A-U with two hydrogen bonds and C-G with three hydrogen bonds
EQ: Identify and explain the structural difference between a single strand of DNA and an RNA strand that makes DNA a better storage molecule than RNA. (2 MARKS)
- Ribose sugar in RNA contains hydroxyl OR deoxyribose sugar in DNA contains a hydrogen - Which makes ribose more unstable / more easily hydrolysed OR the H atom in deoxyribose makes DNA more stable / less easily hydrolysed
EQ: Explain which organelle would contain the greatest percentage of RNA. (2 MARKS)
- Ribosomes / RER - Ribosomes are made up of rRNA / RER contains many ribosomes which are made up of rRNA
Q: DNA helicase is important in DNA replication. Explain why. (2 MARKS)
- Separates / unwinds / unzips strands / helix / breaks H-bonds (unzips bases) - So nucleotides can attach / are attracted / strands can act as templates
EQ: Table 2 shows the position of three different single-base substitutions. The mutation at position 531 didn't affect the enzymes structure but the mutation at 109 did. Using your knowledge of protein structure and information from Table 2, explain this. (2 MARKS)
- The mutation at position 531 represents a part of the primary structure / sequence of amino acid that doesn't make up the active site SO there is no change to the shape of the active site - The mutation at position 109 represents a part of the primary structure / sequence of amino acids that does make up the active site SO there is a change to the shape of the active site
EQ: Explain why it is incorrect to state that RNA is always single-stranded. (2 MARKS)
- There are regions of double-stranded RNA in tRNA - Certain viruses contain double-stranded RNA in their genome
EQ: Ultraviolet exposure can cause guanine to be oxidised to 8-oxyguanine, which is unable to hydrogen bond to cytosine. Instead, during replication, 8-oxyguanine can form hydrogen bonds with adenine, resulting in a base pair. Outline the possible consequences of this change. (3 MARKS)
- Thymine is copied onto the first strand during replication / replaces G in the original sequence - A mutation / substitution mutation occurs - An incorrect / non-functional protein is synthesised from this gene
EQ: Describe the main role of the molecule DNA. (2 MARKS)
- To carry the genetic code - Instructions for making proteins
EQ: The technique of DNA sequencing is used to determine the exact nucleotide sequence of an individual gene, chromosome or even an entire organism's genome. One of the applications of this technique is in forensic science, to examine samples of DNA left at crime scenes. DNA sequencing is preceded by a technique called Polymerase Chain Reaction (PCR) which employs DNA polymerase. Suggest an explain two advantages of using PCR to produce samples for forensic analysis. (3 MARKS)
- Uses DNA polymerase enzyme so the DNA is accurately replicated - Fast copying / produces large numbers of copies quickly - This allows data to be gather from very small deposits of biomaterial
EQ: The Figure below illustrates a small section of a DNA molecule from the nucleus or a eukaryotic cells. Name the structures labelled X and Y. (2 MARKS)
- X = non-coding multiple repeats - Y = introns The non-coding regions that occur within genes are called introns and the non-coding regions between genes are called multiple repeats
What is the DNA sense strand?
- contains the genes - doesnt act as the template for mRNA
EQ: Having the base pairs on the inside of the sugar-phosphate backbone helps to protect the important code-carrying parts of the DNA molecule. Suggest one additional structural feature of DNA that boosts its protection from chemical attack. (1 MARK)
- double helix - DNA is compact / wound around histones
EQ: Scientists sequenced the gene of hormone X in order to understand more about why some individuals stop producing the hormone. The scientists determined that the gene consisted of 1500 base pairs, 30% of which were cytosine. How many nucleotides of adenine and guanine in this sample of DNA? (2 MARKS)
- guanine = 900 nucleotides - adenine = 600 nucleotides
EQ: There are two forms of nitrogen. These different forms are called isotopes. 15N is a heavier isotope than the normal isotope 14N. In an investigation, a culture of bacteria was obtained in which all the nitrogen in the DNA was of the 15N form. The bacteria (generation 0) were transferred to a medium containing only the normal isotope, 14N, and allowed to divide once. A sample of these bacteria (generation 1) was then removed. The DNA in the bacteria of generation 1 was extracted and spun in a high-speed centrifuge. The bacteria in the 14N medium were allowed to divide one more time. The DNA was also extracted from these bacteria (generation 2) and spun in a high speed centrifuge. The diagram shows the results of this investigation. Explain why the DNA from generation 1 is found in the position shown. (2 MARKS)
- it has been produced by semi-conservative replication / one old strand and one new; - One strand has 15N bases and the other 14N;
Name three main components of all nucleotides.
- pentose sugar - phosphate - nitrogenous base
Describe the structure of RNA.
- single stranded - short polynucleotides chain - transfer genetic code from DNA (nucleus) to ribosomes in the cytoplasm - ribosome formed from RNA and proteins
What is the DNA antisense strand?
- template strand - acts as the template for mRNA - complemetary to genes on the sense strand
EQ: The two strands of DNA are described as 'antiparallel'. Explain the meaning of 'antiparallel' in this context. (2 MARKS)
- two DNA strands run alongside each other - but in opposite directions
EQ: When HIV infects a human cell, the following events occur. • A single-stranded length of HIV DNA is made. • The human cell then makes a complementary strand to the HIV DNA. The complementary strand is made in the same way as a new complementary strand is made during semi-conservative replication of human DNA. Describe how the complementary strand of HIV DNA is made. (3 MARKS)
1. (Complementary) nucleotides/bases pair OR A to T and C to G; 2. DNA polymerase; 3. Nucleotides join together (to form new strand)/phosphodiester bonds form;
Q: Describe the role of two named enzymes in the process of semi-conservative replication of DNA. (3 MARKS)
1. (DNA) helicase causes breaking of hydrogen/H bonds (between DNA strands); 2. DNA polymerase joins the (DNA) nucleotides; 3. Forming phosphodiester bonds;
Q: Scientists investigated the function of a eukaryotic cell protein called cyclin A. This protein is thought to be involved with the binding of one of the enzymes required at the start of DNA replication. The scientists treated cultures of cells in the following ways. C - Control cells, untreated D - Added antibody that binds specifically to cyclin A E - Added RNA that prevents translation of cyclin A F - Added RNA that prevents translation of cyclin A and added cyclin A protein They then determined the percentage of cells in each culture in which DNA was replicating. Their results are shown in the table. Suggest explanations for the results in the table. (3 MARKS)
1. (Treatment D Antibody binds to cyclin A so) it cannot bind to DNA/enzyme/initiate DNA replication; 2. (Treatment E) RNA interferes with mRNA/tRNA/ribosome/polypeptide formation (so cyclin A not made); 3. In Treatment F added cyclin A can bind to DNA/enzyme (to initiate DNA replication)
EQ: Adenosine triphosphate (ATP) is a nucleotide derivative. Contrast the structures of ATP and a nucleotide found in DNA to give two differences. (2 MARKS)
1. ATP has ribose and DNA nucleotide had deoxyribose 2. ATP has 3 phosphate (groups) and DNA nucleotide has 1 phosphate (group) 3. ATP - base always adenine and in DNA nucleotide base can be different / varies
EQ: Suggest how the production of a protein with one amino acid missing may lead to a genetic disorder such as Ellis-van Creveld syndrome.
1. Change in tertiary structure / active site; 2. (So) faulty / non-functional protein / enzyme;
EQ: A large and growing number of disorders are now known to be due to types of mitochondrial disease (MD). MD often affects skeletal muscles, causing muscle weakness. We get our mitochondria from our mothers, via the fertilised egg cell. Fathers do not pass on mitochondria via their sperm. Some mitochondrial diseases are caused by mutations of mitochondrial genes inside the mitochondria. Most mitochondrial diseases are caused by mutations of genes in the cell nucleus that are involved in the functioning of mitochondria. These mutations of nuclear DNA produce recessive alleles. One form of mitochondrial disease is caused by a mutation of a mitochondrial gene that codes for a tRNA. The mutation involves substitution of guanine for adenine in the DNA base sequence. This changes the anticodon on the tRNA. This results in the formation of a non-functional protein in the mitochondrion. There are a number of ways to try to diagnose whether someone has a mitochondrial disease. One test involves measuring the concentration of lactate in a person's blood after exercise. In someone with MD, the concentration is usually much higher than normal. If the lactate test suggests MD, a small amount of DNA can be extracted from mitochondria and DNA sequencing used to try to find a mutation. Suggest how the change in the anticodon of a tRNA leads to MD (lines 10-13). (3 MARKS)
1. Change to tRNA leads to wrong amino acid being incorporated into protein; 2. Tertiary structure (of protein) changed; 3. Protein required for oxidative phosphorylation / the Krebs cycle, so less / no ATP made.
Q: Sometimes, damage occurs during DNA replication. One enzyme involved in repairing damage to DNA is called ATR. ATR works as follows. • ATR phosphorylates other enzymes involved in repairing DNA. • ATR also phosphorylates substrates required to repair DNA. When ATR phosphorylates other enzymes, these enzymes become able to bind to their substrates. Use your knowledge of enzyme structure to suggest why. (2 MARKS)
1. Changes tertiary structure of the enzyme; 2. (Enzyme) active site formed / able to be formed / active site becomes complementary;
Describe the process of Semi-conservative DNA replication.
1. DNA Helicase (enzyme) unwinds / unzips DNA double helix by breaking hydrogen bonds between the complementary base pairs → strands separate 2. = both two strands act as templates 3. Free floating DNA nucleotides attracted to exposed bases on template strands and attach via specific complementary base pairing, (adenine-t; guanine-cytosine) 4. DNA polymerase catalyses / joins adjacent nucleotides (free DNA) on new strand by condensation, forming phosphodiester bonds = sugar phosphate backbone 5. New complementary strand on DNA is formed 6. Hydrogen bonds reform 7. DNA Replication is semi-conservative; each new DNA strand formed contains one original / template strand and one new strand 8. Ensures genetic continuity between generations of cells. 9. process occurs on both strands of original DNA simultaneously
EQ: Contrast the structures of DNA and mRNA molecules to give three differences (3 MARKS)
1. DNA double stranded/double helix and mRNA single-stranded; 2. DNA (very) long and RNA short; 3. Thymine/T in DNA and uracil/U in RNA; 4. Deoxyribose in DNA and ribose in RNA; 5. DNA has base pairing and mRNA doesn't/ DNA has hydrogen bonding and mRNA doesn't; 6. DNA has introns/non-coding sequences and mRNA doesn't;
EQ: During replication, the two strands of a DNA molecule separate and each acts as a template for the production of a new strand. Figure 2 represents DNA replication. The arrows in Figure 2 show the directions in which each new DNA strand is being produced. Use Figure 1, Figure 2 and your knowledge of enzyme action to explain why the arrows point in opposite directions. (4 MARKS)
1. DNA has antiparallel strands 2. nucleotides aligned differently; 3. Enzymes have active sites with specific shape; 4. Only substrates with complementary shape / only the 3' end can bind with active site of enzyme / active site of DNA polymerase.
EQ: The enzymes DNA helicase and DNA polymerase are involved in DNA replication. Describe the function of each of these enzymes. (2 MARKS)
1. DNA helicase - unwinding DNA and breaking hydrogen bonds / bonds between chains / bases / strands 2. DNA polymerase - catalyse the joining of (adjacent) free DNA nucleotides in condensation reactions OR forms phosphodiester bond / sugar-phosphate backbone, to form a new complementary DNA strand
EQ: During replication, the two DNA strands separate and each acts as a template for the production of a new strand. As new DNA strands are produced, nucleotides can only be added in the 5' to 3' direction. Use the figure in part (a) and your knowledge of enzyme action and DNA replication to explain why new nucleotides can only be added in a 5' to 3' direction. (4 MARKS)
1. DNA polymerase; 2. (is) specific; 3. Only complementary with / binds to 5' end (of strand); 4. Shapes of 5' end and 3' end are different as DNA is antiparallel
EQ: The nucleus and a chloroplast of a plant cell both contain DNA. Give three ways in which the DNA in a chloroplast is different from DNA in the nucleus. (3 MARKS)
1. DNA shorter; 2. Fewer genes; 3. DNA circular not linear; 4. Not associated with protein/histones, unlike nuclear DNA; 5. Introns absent but present in nuclear DNA;
EQ: E. coli bacteria were allowed to grow and divide in a solution that contained the four DNA nucleotides. These were called group A bacteria. Some of the group A bacteria were then transferred to a second solution, where they were allowed to grow and divide once. These were called group B bacteria. The second solution was the same as the first, except that the nucleotides containing thymine were radioactive. DNA from each group was tested for radioactivity. Table 4 shows these results. Explain why all the DNA molecules in group B bacteria were radioactive. Use your knowledge of DNA replication. (4 MARKS)
1. DNA strands separate; 2. Each/one strand of old DNA acts as template; 3. New (DNA) stand consists of one old and one new strand; 4. Suitable ref. to H-bonding between bases; 5. New (DNA) chain will incorporate *T(-nucleotides) (from solution); 6. Correct reference to DNA polymerase in joining (adjacent) nucleotides together (to form new strand);
EQ: Deletion of the sixth base (G) in the sequence shown in the diagram above would change the nature of the protein produced but substitution of the same base would not. Use the information in the table and your own knowledge to explain why. (3 MARKS)
1. Substitution would result in CCA / CCC / CCU; 2. (All) code for same amino acid / proline; 3. Deletion would cause frame shift / change in all following codons / change next codon from UAC to ACC.
EQ: Explain how the structure of DNA is related to its functions (6 MARKS)
1. Sugar phosphate (backbone) / double helix so provides strength / stability / protects bases / protects hydrogen bonds 2. Long / large molecule so can store lots of information 3. Helix / coiled so compact 4. Base sequence allows information to be stored / base sequence codes for amino acids 5. Double stranded so replication can occur semi-conservatively / strands act as templates 6. Complementary base pairing / A-T and C-G so accurate replication / identical copies can be made 7. Weak hydrogen bonds for replication / unzipping / strand separation 8. Many hydrogen bonds so stable / strong
EQ: There are three main theories for the mechanism of DNA replication. Figure 11 shows what the three main different theories predict about how new strands of DNA form. Each theory, A, B or C, predicts that the DNA double-helix strands produced by the bacteria after one cycle of DNA replication would contain different amounts of radioactive DNA. The scientists then allowed enough time for the bacteria to complete another cycle of DNA replication in the medium containing non-radioactive phosphate ions. This time they found that half of the DNA molecules were non-radioactive and half were 50% radioactive. Which theory did the scientists conclude was the only correct one? Explain your answer. Use the information above. (2 MARKS)
1. Theory C, because it is the only theory which is correct for both results / not been disproven by either result; 2. Theory A rejected because it would predict that all of the DNA molecules contain 25% radioactivity; OR Theory B would predict this, but was already excluded by the results after one generation;
EQ: Not all mutations in the nucleotide sequence of a gene cause a change in the structure of a polypeptide. Give two reasons why. (2 MARKS)
1. Triplets code for same amino acid 2. Occurs in introns /non-coding sequence;
EQ: Give two features of DNA and explain how each one is important in the semi-conservative replication of DNA. (2 MARKS)
1. Weak / easily broken hydrogen bonds between bases allow two strands to separate / unzip; 2. Two strands, so both can act as templates; 3. Complementary base pairing allows accurate replication;
Q: Describe and explain how the structure of DNA results in accurate replication.
1. two strands therefore semi-conservative replication (possible); 2. base pairing / hydrogen bonds holds strands together 3. hydrogen bonds weak / easily broken, allow strands to separate; 4. bases (sequence) (exposed so) act as template / can be copied; 5. A with T, C with G / complementary copy; 6. DNA one parent and one new strand;
EQ: Phosphorus has two forms, one non-radioactive (31P) and one radioactive (32P). Scientists can use a Geiger detector to measure the level of radioactivity emitted from the phosphorus atoms. They can use this information to calculate the proportion of radioactive to non-radioactive phosphorus atoms in a DNA polymer. Scientists investigate DNA replication. This is the method the scientists use: • grow bacteria for several generations in a nutrient solution containing the radioactive form of phosphorus (32P) • extract DNA from a sample of the bacteria • transfer the bacteria to a nutrient solution containing only the non-radioactive form of the phosphorus (31P) • allow the bacteria to grow and divide once • extract DNA from a sample of the bacteria • use a Geiger detector to measure the amount of radioactivity emitted from the DNA • use a conversion table to relate the reading from the Geiger detector to the percentage of radioactive DNA in the molecule. Table 9 shows the conversion table. A DNA sample measured 1.500 MeV on the Geiger detector. Calculate what percentage of radioactive DNA the DNA sample would contain. Use information from Table 9. Give your answer to two significant figures. (2 MARKS)
1.5 ÷ 1.275 × 75 = 88% OR 1.5 ÷ 1.7 × 100 = 88%;;
What is a pentose sugar?
5 carbon atom sugar
EQ: Replication of mitochondrial DNA (mtDNA) is different from that of nuclear DNA. The replication of the second strand of mtDNA only starts after two-thirds of the first strand of mtDNA has been copied. A piece of mtDNA is 16 500 base pairs long and is replicated at a rate of 50 nucleotides per second. Tick the box that shows how long it would take to copy this mtDNA. (1 MARK)
550 seconds
EQ: Mitochondrial DNA (mtDNA) replicates differently to nuclear DNA. Scientists believe replication of the the first mtDNA strand begins after two-thirds of the first strand has been replicated. A piece of mtDNA contains 33140 nucleotides and it takes 0.02 seconds to replicate a nucleotide. How long would it take to copy this mtDNA to the nearest whole second? (2 MARKS)
552 seconds
EQ: Crossing over increases genetic variation in the gametes. Figure 4 shows some stages in gamete production. G / g and H / h are two pairs of alleles. • Label structures A and B in Stage 1.
A = centromere; B = chromatid;
What is the genetic code in DNA?
Adenine Thymine Cytosine Guanine
What is the genetic code in RNA?
Adenine Uracil Cytosine Guanine
EQ: There are three main theories for the mechanism of DNA replication. Figure 11 shows what the three main different theories predict about how new strands of DNA form. Each theory, A, B or C, predicts that the DNA double-helix strands produced by the bacteria after one cycle of DNA replication would contain different amounts of radioactive DNA. After the experiment, the actual results show that 50% of the nucleotides in all the DNA molecules are radioactive. State which theory, A, B or C, is shown to be incorrect by these results. (1 MARK)
B
Q: The length of DNA has the base sequence AATCGCGGTCGCTCA. Select the row that shows the correct complementary DNA strand and the sequence of mRNA made during transcription of the DNA sequence above. (1 MARK)
B
Not all DNA codes for proteins / polypeptides / functional RNA - In eukaryotes, much of the DNA does not code for polypeptides. Explain where this happens.
Between genes... - Non-coding multiple repeats (or Variable Number Tandem Repeats (VNTRs) in second year) With genes.. - Only exons code for amino acid sequences, which are separated by one or more non-coding sequences, called introns
Q: Describe the role of DNA polymerase in DNA replication. (1 MARK)
Catalyse the joining of adjacent free DNA nucleotides in condensation reactions to form a new complementary DNA strand
EQ: Compare the structures and functions of DNA and RNA. (5 MARKS)
Comparison: 1. DNA holds genetic information whereas RNA leaves the nucleus to transfer the genetic information to the ribosomes in protein synthesis Similarities: 2. both are polymers of nucleotides 3. Both have phosphodiester bonds / sugar-phosphate back-bone 4. Both have 4 complimetary bases Differences: 5. DNA contains deoxyribose sugars, RNA contains ribose sugars 6. RNA molecules are smaller than DNA 7. DNA contains thymine bases, RNA contains uracil bases
EQ: Compare and contrast the DNA in eukaryotic cells with the DNA in prokaryotic cells. (5 MARKS)
Comparisons 1. Nucleotide structure is identical; 2. Nucleotides joined by phosphodiester bond; 3. DNA in mitochondria / chloroplasts same / similar (structure) to DNA in prokaryotes; Contrasts 4. Eukaryotic DNA is longer; 5. Eukaryotic DNA contain introns, prokaryotic DNA does not; 6. Eukaryotic DNA is linear, prokaryotic DNA is circular; 7. Eukaryotic DNA is associated with / bound to protein / histones, prokaryotic DNA is not;
EQ: State the purpose of DNA replication. (1 MARK)
Copy / make new DNA for new cells / daughter cells OR ensure that daughter cells after mitosis are genetically identical to parent cell
EQ: A large and growing number of disorders are now known to be due to types of mitochondrial disease (MD). MD often affects skeletal muscles, causing muscle weakness. We get our mitochondria from our mothers, via the fertilised egg cell. Fathers do not pass on mitochondria via their sperm. Some mitochondrial diseases are caused by mutations of mitochondrial genes inside the mitochondria. Most mitochondrial diseases are caused by mutations of genes in the cell nucleus that are involved in the functioning of mitochondria. These mutations of nuclear DNA produce recessive alleles. One form of mitochondrial disease is caused by a mutation of a mitochondrial gene that codes for a tRNA. The mutation involves substitution of guanine for adenine in the DNA base sequence. This changes the anticodon on the tRNA. This results in the formation of a non-functional protein in the mitochondrion. There are a number of ways to try to diagnose whether someone has a mitochondrial disease. One test involves measuring the concentration of lactate in a person's blood after exercise. In someone with MD, the concentration is usually much higher than normal. If the lactate test suggests MD, a small amount of DNA can be extracted from mitochondria and DNA sequencing used to try to find a mutation. Two couples, couple A and couple B, had one or more children affected by a mitochondrial disease. The type of mitochondrial disease was different for each couple. None of the parents showed signs or symptoms of MD. • Couple A had four children who were all affected by an MD. • Couple B had four children and only one was affected by an MD. Use the information in lines 5-9 and your knowledge of inheritance to suggest why: • all of couple A's children had an MD • only one of couple B's children had an MD. (4 MARKS)
Couple A, 1. Mutation in mitochondrial DNA / DNA of mitochondrion affected; 2. All children got affected mitochondria from mother; 3. (Probably mutation) during formation of mother's ovary / eggs; Couple B, 4. Mutation in nuclear gene / DNA in nucleus affected; 5. Parents heterozygous; 6. Expect 1 in 4 homozygous affected.
Q: Fill the gaps in the following passage using the most appropriate term: Much of the _____ in cells contains sequences called genes. Many genes code for ______ that fold to make enzymes. Often, enzymes are kept in an inactive form until needed. These enzymes may then be activated by cAMP, which involves changes in their ______. (3 MARKS)
DNA Polypeptide(s) Tertiary, structure / shape
Q: A particular gene is 652 base pairs long. The mRNa produced from this gene is only 441 nucleotides long. Explain this difference. (1 MARK)
DNA contains introns / pre mRNA is edited
Fill in the blanks. The _________ can continuously join nucleotides together in the leading strand because it's moving in the ______ direction as the _______ is unravelling the strands of DNA and in the other lagging strand it has to join them in ________ as the polymerase still needs to be inline with orientation of the _________ but because it's the anti-parallel strand DNA is being unravelled in the opposite direction to the one that the nucleotides are being joined meaning it has to go back and do it again?. This also explains why it only moves in a __________ direction.
DNA polymerase, same, helicase, fragments, nucleotide, 5' to 3'
What is an allele?
Different version (sequence of bases / triplets) of the same gene
Q: What is meant by a non-coding base sequence? (1 MARK)
Does not code for amino acid / tRNA / rNA
Ribosomes are surrounded by a membrane, true or false?
FALSE; they arent membrane bound
Describe the structure / important features of DNA related to its functions:
For replication: - Double stranded → both strands can act as templates for semi-conservative replication - Weak hydrogen bonds between bases → can be unzipped for replication - Complementary base pairing / A-T and C-G → accurate replication For protection / stability: - Many hydrogen bonds between bases → stable / strong molecule - Double helix with sugar-phosphate backbone → so provides strength / stability / protects bases / H bonds Other: - Long molecule → stores a lot of (genetic) information (that codes for polypeptides) - Double helix (coiled) → compact - Base sequence allows information to be stored / base sequence codes for amino acids - Feature: Supercoiled around histones / into chromosomes - Explanation: provides compact structure
How are ribosomes formed?
From RNA (ribonucleic acid) and proteins
Q: A piece of DNA consists of 74 base pairs. The 2 strands of DNA, strands Aa and B, were analysed to find the number of bases of each type that were present. Some of the results are shown in the table. Fill in the gaps.
G in strand B = 26; G in strand A = 19; T in strand A = 9; A in strand A = 74 (total) - 19 - 26 - 9 = 20; T in strand B = 20
Compare the structure of DNA and RNA
Look at image.
Draw a DNA molecule (polynucleotide chain)
Look at image.
Draw a diagram of a DNA molecule.
Look at image.
Draw a diagram of a RNA molecule
Look at image.
Draw the DNA polynucleotide
Look at image.
Draw the structure of a chromosome.
Look at image.
EQ: A section fo DNA contains 1,200 base pairs. The number of guanine molecules on one strand is counted as 156. Complete Table 1 below. (2 MARKS)
Look at image.
EQ: Annotate Figure 2 to illustrate the following terms: - Locus - Gene - Centromere. (3 MARKS)
Look at image.
EQ: Both DNA and mRNA are polynucleotides. Give two differences between the structure of DNA and the structure of mRNA. (2 MARKS)
Look at image.
EQ: Draw and label a DNA nucleotide. (2 MARKS)
Look at image.
EQ: Table 1 shows features of a bacterium and the human immunodeficiency virus (HIV) particle. Complete Table 1 by putting a tick where a feature is present. (2 MARKS)
Look at image.
EQ: Table 4 shows the types of DNA molecule that could be present in samples 1 to 3. Use your knowledge of semi-conservative replication to complete the table with a tick if the DNA molecule is present in the sample. (3 MARKS)
Look at image.
EQ: The structures of the four bases of DNA are shown below. Draw the positions of the hydrogen bonds that form during base-pairing.
Look at image.
EQ: Suggest how a mutation can lead to the production of a protein that has one amino acid missing. (2 MARKS)
Loss of 3 bases / triplet
DNA replication is semi-conservative, what does this mean?
New DNA is made of one original strand and one new stand.
EQ: The cytochrome c molecule contains one polypeptide chain. Cytochrome c has a primary, a secondary and a tertiary structure. Give the reason why cytochrome c cannot have a quaternary structure. (1 MARK)
Only 1 polypeptide chain;
Q: The diagram below shows an organic molecule. What bond is formed when the molecule is polymerised? (1 MARK)
Phosphodiester
Q: During DNA replication, DNA polymerase can only work in one direction - from the 3' end to the 5' end. This means that the lagging strand has small gaps left in the backbone. DNA ligase works to seal these gaps. What is the bond formed?
Phosphodiester bond The formation of phosphodiester bonds will occur in the same direction on each strand during DNA replication.
Q: Complete the following paragraph by using the most appropriate term(s). A gene is a section of DNA that codes for the production of a _____________. The molecule that copies a gene and carries the information to a _______ is called RNA. (2 MARKS)
Polypeptide, ribosome.
EQ: Cytochrome c is a protein. The amino acid sequence of cytochrome c can be used to measure how closely related two species are. Human cytochrome c contains 104 amino acids. Scientists compared 22 of the amino acids found in cytochrome c in a range of organisms and recorded the number of differences from human cytochrome c. The results are shown in Table 4. A student looked at these data and concluded that organisms A and B are the most closely related. Evaluate this conclusion. (3 MARKS)
Pro: 1. (because) amino acid (sequence) most similar; Con: 2. only compared 22 amino acids / did not compare all (104) amino acids; 3. might not be the same 3 amino acids that are different; 4. (Amino acid sequence) compared to human cytochrome c not each other; 5. Degeneracy of genetic code so different number of base differences (more or less) 6. Cytochrome c is only one protein so other proteins might vary more or less;
What are introns?
Regions within a gene that don't code for polypeptides; non-coding regions
EQ: The diagram below shows DNA. DNA is a polymer. What evidence from the diagram shows this?
Repeating units / nucleotides / monomers
EQ: Use your knowledge of semi-conservative replication of DNA to suggest: 1. the role of the single-stranded DNA fragments 2. the role of the DNA nucleotides. (3 MARKS)
Role of single-stranded DNA fragments 1. Act as template; 2. Determines order of nucleotides/bases; Role of DNA nucleotides 3. Forms complementary pairs / A - T, G - C OR Forms complementary (DNA) strand;
What are the test tube observations from meselson and stahl's experiment to prove DNA replication is conservative?
Sample 1. DNA from bacteria grown for several generations in a nutrient solution containing 15N → DNA molecules contain 2 'heavy' strands Sample 2. DNA from bacteria grown orginall in a nutrient solution containing 15N, then transferred for one division to a solution containing 14N → DNA molecules contain 1 original 'heavy' and 1 new 'light strand' Sample 3. DNA from bacteria grown originally in a nutrient solution 15N, then transferred for two divisions to a solution containing 14N. → 50% DNa molecules contain 1 original 'heavy' and 1 new 'light' strand, 50% contain both 'light' strands
One advantage of DNA having two complementary strands is that...
Semi-conservative replication is possible.
EQ: Compare and contrast the structures of DNA and RNA. (6 MARKS)
Similarities: - Both are polynucleotides / contain many nucleotides - Both contain, A, C and G bases - Both contain a pentose sugar and a phosphate group - Both linked by phosphodiester bonds between 3' and 5' carbon atoms Differences: - DNA is double stranded / RNA is single-stranded - DNA is long / RNA is short - DNA contains deoxyribose sugars / RNA contains ribose sugars - DNa contains thymine, RNA contains uracil
Q: A polypeptide has 51 amino acids in its primary structure. The gene for this polypeptide contains more than this number of bases. Explain why. (1 MARK)
Some regions of the gene are non-coding / introns
EQ: Explain why the DNA which codes for the human protein, alpha globin, has 850 base pairs but there are only 141 amino acids in this protein. (1 MARK)
The DNA contains introns / non-coding DNA or not all of the base pairs code for amino acids
What is a locus / loci?
The fixed location of a gene on a chromosome on a particular DNA molecules
What is a homologous pair of chromosomes?
The maternal and paternal pairs of chromosomes in offspring - same size chromosomes with same genes, but different alleles; the total number is the diploid number = 46
EQ: Name the structures labelled W and X in Figure 1 (1 MARK)
W = amino acid binding site and X = anticodon;
EQ: Name the two scientists who proposed models of the chemical structure of DNA and of DNA replication. (1 MARK)
Watson and Crick
Q: Fig 7.1 represents part of a DNA molecule. State the precise name of each of the parts of the DNA molecule labelled X, Y and Z. (3 MARKS)
X - phosphate Y - deoxyribose Z - thymine
EQ: State the cells that chromosomes X and Y would be sources from.
X = Prokaryotes Y = Eukaryotes
EQ: The image below show maternal and paternal chromosomes with the alleles of four different genes show. Identify the structure labelled X and annotate the diagram to show the locus of gene B. (2 MARKS)
X = centromere Labelled line to either B letters.
What is a DNA triplet?
a sequence of 3 DNA bases that code for a specific amino acid
EQ: The table shows the percentage of different bases in the DNA of some organisms. The virus has single-stranded DNA as its genetic material. Explain the evidence from the table which suggests that the DNA is single-stranded. (2 MARKS)
amounts of A and T / C and G / complementary bases different; therefore no base-pairing;
Centrioles are present in __________ cells only.
animal
How many hydrogen bonds form between the bases A and T?
double (2) hydrogen bonds
DNA is a double stranded molecule. The two DNA strands are held together by...
hydrogen bonds
In eukaryotes, much of the nuclear DNA does not code for polypeptides. There are ______ ________ repeats of base sequences between genes. Even within a gene, only some sequences, called _______, code for amino acid sequences. Within the gene, these ______ are separated by non-coding sequences called ________.
non-coding multiple, exons, exons, introns
DNA consists of mononucleotides joined together by bonds between...
one deoxyribose sugar and one phosphate group
What is the type of pentose (sugar that has 5 carbon atoms) in RNA?
ribose
Q: Give one advantage of DNA molecules having two strands.
stability / protects bases / replication;
What is a proteome ?
the full range of proteins that a cell / genome is able to produce
What are nucleotides?
the monomers that makeup of polynucleotides (DNA + RNA)
How many hydrogen bonds form between the bases C and G?
triple (3) hydrogen bonds
EQ: Name two other biological molecules that always contain phosphorus. (2 MARKS)
• ATP; • ADP; • Phospholipids; • RNA/mRNA/tRNA/rRNA;
