Mutations - Additional Chapter

Q: Figure 1 shows a DNA base sequence. It also shows the effect of two mutations on this base sequence. Figure 2 shows DNA triplets that code for different amino acids. (a) Complete Figure 1 to show the sequence of amino acids coded for by the original DNA base sequence. (1 mark) (b) Some gene mutations affect the amino acid sequence. Some mutations do not. Use the information from Figure 1 and Figure 2 to explain... (i) Whether mutation 1 affects the amino acid sequence (2 marks) (ii) How mutation 2 could lead to the formation of a non-functional enzyme. (3 marks)

(a) ✓ Ile Gly Val Ser (b) (i) ✓ Has no effect / same amino acid (sequence) / same primary structure ✓ Glycine named as same amino acid (b) (ii) ✓ Leu replaces 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: Figure 1 shows the effects of two different mutations of the DNA on the base sequence of the mRNA. The table shows the mRNA codons for three amino acids. Use the information in the table to (i) explain how each mutation may affect the polypeptide for which this section of DNA is part of the code. (4 MARKS)

(mutation 1): - no change(to sequence of amino acids); - codon for alanine / degenerate codon / same amino acid coded for; (mutation 2): - (change in sequence) valine replaced by alanine / codon for alanine; folding / shape / tertiary structure / position of bonds may change; (reject peptide bonds)

What is a gene mutation?

- A change in the base sequence of DNA (on chromosomes) - Can arise spontaneously during DNA replication (interphase) - Involves base deletion / substitution

Q: Mutation results in the generation of new alleles and contributes to genetic diversity or the size of the gene pool. Define the term gene pool. (2 MARKS)

- All the (different) alleles (of all the genes); [1 mark] - in a population at a given time; [1 mark]

A mutation can lead to the production of a non-functional protein / enzyme

- Change in base / triplet sequence of DNA / gene during replication - Changes sequence of codons on mRNA - Different anticodon --> Different amino acid - Changes sequence of amino acids in the primary structure of the polypeptide - Changes position of hydrogen / ionic / disulphide bonds in tertiary structure of protein and different bonding - Changes tertiary structure / shape of the protein (and active site if enzyme) - (if enzyme) substrate can't bind to active site and form an enzyme-substrate complex

A mutation doesn't lead to any change in the primary structure:

- DNA is degenerate - Each amino acid is coded for by more than one triplet codon - Change to DNA base (subsitution) might not cause a change to amino acid in primary structure (e.g. GGT --> GGC) - No change to bonding

Mutagenic agents

- Increase the rate of gene mutation (above the rate of naturally occurring mutations) as they interfere with base pairing - E.g. ultraviolet light or alpha particles or carcinogens (e.g. benzene)

Q: In protein synthesis, what effect might a mutation in an intron section of the genetic code, involving a single base substitution, have on the protein produced? (2 MARKS)

- Intron non-coding DNA / only exons coding - So not translated / no change in mRNA produced / no effect (on protein) no effect on amino acid

Q: Suggest how a mutation can lead to the production of a protein that has one amino acid missing. (2 MARKS)

- Loss of bases - triplet / 3 bases / codon forms an intron

Q: In the genetic code, a mutation involving a single base deletion can lead to the production fo a non-functional protein. Explain why. (4 MARKS)

- Mutation changes triplets / codons after that point (causes frameshift) - Changes amino acid sequence after this / codes for different amino acids - Change in primary structure - Affects / changes hydrogen / ionic / sulfur bond position (not peptide bond) - Changes tertiary structure of protein (so nonfunctional)

EQ: Suggest why most mutations in eukaryotic cells are regarded as 'neutral'. (3 MARKS)

- Mutation has no effect on phenotype of organism - They occur in genes that are not expressed in the cell - They occur in introns - they are 'silent mutations'; base substitutions that do not affect the amino acid sequence of the protein being coded for

Base substitution

- One nucleotide / base in DNA replaced with another nucleotide / base - Change in one base→changes one triplet 1. Changes one mRNA codon and one amino acid→sequence of amino acids in primary structure of polypeptide changes etc. OR 2. Due to the degenerate nature of the genetic code, the new triplet may still code for the same amino acid so the sequence f amino acids in the primary structure of the polypeptide remains unchanged

Base deletion

- One nucleotide / base removed from DNA sequence - Changes triplet / codon sequence from the point of mutation (causes a frameshift) - Changes sequence of codons on mRNA after point of mutation - Changes sequence of amino acids in primary structure of polypeptide - Changes position of hydrogen / ionic / disulphide bonds in tertiary structure of protein - Changes tertiary structure / shape of protein i.e. non-functional or new and superior

Q: New alleles arise as a result of mutations in existing genes. These mutations may occur during DNA replication. Explain why a mutation involving the deletion of a base may have a greater effect than one involving substitution of one base for another. (3 MARKS)

- deletion causes frame shift / alters base sequence (from point of mutation); - changes many amino acids / sequence of amino acids (from this point); - substitution alters one codon / triplet / one amino acid altered / code degenerate / same amino acid coded for;

Q: New alleles arise as a result of mutations in existing genes. These mutations may occur during DNA replication. Explain why a mutation involving the deletion of a base may have a greater effect than one involving substitution of one base for another. (3 MARKS)

- deletion causes frame shift / alters base sequence (from point of mutation); - changes many amino acids / sequence of amino acids (from this point); - substitution alters one codon / triplet / one amino acid altered / code degenerate / same amino acid coded for;

Q: A rare form of diabetes, maternally inherited diabetes and deafness (MIDD), affects approximately 1% of people with diabetes. The mutated gene is known as A3242G. The convention for naming mutated forms of a gene is to show: - correct nucleotide - nucleotide position in the gene - incorrect nucleotides Use this information to describe the change in the DNA caused by this mutation. (3 MARKS)

- substitution - adenine changed to guanine - at base / nucleotide / position 3 242

EQ: Table 2 shows a sequence of DNA bases and the mutated version of the same sequence. Table 3 shows the DNA triplets that code for some amino acids. Two single base mutations have occurred. Use the information provided in table 2 and Table 3 to state and explain the mutations and their effect on the amino acids that will be produced from the mutated DNA sequence(5 MARKS)

- there has been a base deletion at A - which has caused a frameshift - resulting in the two middle amino acids becoming Asp and Gly (rather than Lys and Arg) - there has been a base addition of T (at position 9) - Meaning the last two amino acids remains as Tyr Explanation: The original sequence of amino acids would have been: Thr, Lys, arg, Tyr. The new sequence is: Thr, Asp, Gly, Tyr. we can see that only the middle two amino acids have changes. The triplet CGG remains in both sequences but in different positions. This means there has been a frameshift

Q: Some mutations can cause a 'frameshift'. Describe and explain what is meant by a frameshift and show how this may have affected the protein made. (2 MARKS)

- there is a deletion or addition of bases in the DNA - That occurs during DNA replication - So this causes the bases to shift in position / change any subsequent triplet codes - As a result, the amino acids coded can change which leads to a different 3D shape change in the tertiary structure of the protein created - Likely loss of function of that gene OR possible new function arising from the new protein - If mutation occurs in an intron, it is possible that there is no effect / neutral mutation

EQ: Scientists have discovered five new species, A, B, C, D and E, of Chinese giant salamander. In their natural habitat all of these species are in danger of extinction. Large populations of species B are kept in captive breeding farms.The Chinese government has taken action to conserve salamanders by releasing individual salamanders from the breeding farms into rivers. Figure 3 shows how species A-E may have evolved. Suggest two reasons why scientists are concerned about the release of individual salamanders from the captive breeding farms. (2 MARKS)

1. May release individuals with a disease / cause release of pathogen OR Species B may increase (interspecific) competition / outcompete other species for resources e.g. food / disrupt food chains OR Cross-breeding / may form hybrids; 2. (Leads to) reduced biodiversity / loss of other species;

EQ: Haemoglobins are chemically similar molecules found in many different species. Differences in the primary structure of haemoglobin molecules can provide evidence of phylogenetic (evolutionary) relationships between species. Explain how. (5 MARKS)

1. Mutations change base / nucleotide (sequence); 2. (Causing) change in amino acid sequence; 3. Mutations build up over time; 4. More mutations / more differences (in amino acid / base / nucleotide sequence / primary structure) between distantly related species; OR Few(er) mutations / differences (in amino acid / base / nucleotide sequence / primary structure) in closely related species; 5. Distantly related species have earlier common ancestor; OR Closely related species have recent common ancestor;

EQ: Table 1 shows the base triplets that code for two amino acids 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 would not. Use the information in Table 1 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;

Q: The table shows the DNA base sequences that code for three amino acids Some substitution mutations would affect the sequence of amino acids in a polypeptide, and others would not. Using only the information in the table, explain why. (3 marks)

Met: ✓ Substitution always gives different amino acids ✓ Substitution of C gives isoleucine Gly/iso: ✓ Substitution of first two bases gives different amino acid ✓ In glycine, substitution of 3rd base still codes for glycine