Gene Mutations

Question 1

What is a mutation?

Answer 1

Any change to the quantity or structure of the DNA of an organism

Question 2

What is a gene mutation?

Answer 2

Any change to one or more nucleotide bases, or any rearrangement of the bases in the DNA of an organism

Question 3

Gene mutations might arise during

Answer 3

Replication of DNA

Question 4

What is substitution of bases (substitution)?

Answer 4

Type of gene mutation in which a nucleotide in a section of a DNA molecule is replaced by another nucleotide that has a different base

Question 5

What are 3 possible consequences of base substitution?

Answer 5

1- formation of 1 of 3 stop codons marks end of polypeptide chain = production of polypeptide coded for by the section of DNA would be stopped prematurely = final protein would almost certainly be significantly different and the protein could not perform its final function

2- formation of codon for a different amino acid = structure of polypeptide produced would differ in a single amino acid- the protein of which this polypeptide is a part may differ in shape and not function properly e.g. if enzyme, its active site may no longer fit the substrate and so enzyme-substrate complex cannot be formed and enzyme won’t catalyse reaction

3- the formation of a different codon but codes for the same amino acid as before due to genetic code’s degenerative nature = mutation has no effect on polypeptide produced = mutation has no effect

Question 6

What is deletion of bases (deletion)?

Answer 6

Loss of a nucleotide base from a DNA sequence

Question 7

Why is the effect of deletion on the phenotype often enormous?

Answer 7

• one deleted base causes frame shift
• because the reading frame that contains each three letters of the code has been shifted to the left by one letter
• gene is now read in the wrong three-base groups and so the coded information is altered
• mist triplets will then be different, as will the amino acids they code for
• the polypeptides will be different and lead to the production of a non-functional protein that could considerably alter the phenotype
• so ultimately, one deleted base at the very start of the sequence for example could alter every other triplet from that point in the sequence

Question 8

What are other types of gene mutation?

Answer 8

• addition of bases
• duplication of bases
• inversion of bases
• translocation of bases

Question 9

What is addition of bases?

Answer 9

Extra base becomes inserted into the sequence

Question 10

What is usually the effect of base addition?

Answer 10

• Usually similar effect as base deletion in that there is usually a frame shift and the whole sequence of triplets from the point of mutation becomes altered
• if three extra bases are added, or any multiple of three bases, there will not be a frame shift- the resulting polypeptide will be different from the one produced from a non-mutant gene, but not to the same extent as if there were a frame shift

Question 11

What is duplication of bases?

Answer 11

One or more bases are repeated

Question 12

What is the effect of duplication of bases?

Answer 12

Frame shift to the right

Question 13

What is inversion of bases?

Answer 13

A group of bases become separated from the DNA sequence and rejoin at the same position but in the inverse order (back to front) = effects amino acid sequence that results

Question 14

What is translocation of bases?

Answer 14

A group of bases become separated from the DNA sequence on one chromosome and become inserted into the DNA sequence of a different chromosome

Question 15

What is often the effect of translocation of bases?

Answer 15

Significant effects on gene expression leading to an abnormal phenotype- these effects include the development of certain forms of cancer and also reduced fertility

Question 16

Gene mutations can arise spontaneously during DNA replication- spontaneous mutations are

Answer 16

Permanent changes in DNA that occur without any external influence

Question 17

Despite gene mutations being random occurrences, mutations occur with predictable frequency- the natural mutation rate varies from species to species, but is typically around

Answer 17

One or two mutations per 100,000 genes per generation

Question 18

The basic mutation rate can be increased by

Answer 18

Mutagenic agents

Question 19

Give some examples of mutagenic agents

Answer 19

1- high energy ionising radiation such as x-rays and UV light (these forms of radiation can disrupt the structure of DNA

2- chemicals such as Benzopyrene, a constituent of tobacco smoke, is a powerful mutagenic agent that inactivates a tumour-suppressor gene TP53 leading to cancer

Question 20

Mutations have both cost and benefits- what are some benefits?

Answer 20

provide genetic diversity necessary for natural selection and speciation

Question 21

Mutations have both cost and benefits- what are some costs?

Answer 21

• almost always harmful and produce an organism that is less suited to its environment
• mutations that occur in body cells rather than in gametes leads to the disruption of normal cellular activities such as cell division, for example, cancer

Question 22

A translocation mutation is, in effect, a combination of two other different types of gene mutation- deduce which two types of mutation these are

Answer 22

Deletion and addition as the bases have been deleted from one chromosome and added to a different one

Question 23

Explain why the effects of a single additional base in a sequence of DNA may have a considerable effect on the polypeptide produced

Answer 23

• will cause a frame shift causing codons in a sequence to be read differently as each has been shifted to the right by one base
• if the additional base is inserted early in the sequence, most codons will be changed and so will the amino acids they code for
• resultant polypeptide will be very different from normal

Question 24

Explain why the effects of a single additional base in a sequence of DNA may have little effect on the polypeptide produced

Answer 24

• if the additional base is inserted at the end of the sequence, few, if any codons, will be changed
• few, if any, amino acids they code for will differ
• resultant polypeptide will be normal or near normal

Question 25

A mutation causes three bases in the DNA of a gene to become duplicated. Explain how the effects of this mutation might differ if the duplicated bases are consecutive rather than in three separate locations on the DNA molecule

Answer 25

- where the duplicated bases are consecutive, the frame shift is three bases long so the subsequent codons are not affected - polypeptide will have an additional amino acid but otherwise unchanged - if the bases are separate, the frame shift will initially be one base long, becoming two bases long after the second duplicate base is added - codons after both duplications will be changed and the polypeptide will have many different amino acids not necessarily all (degenerate code) - after the third duplicate base, the codons will be unchanged

Question 26

Suggest 2 reasons why the addition of a single base into a DNA sequence may not alter the amino acid sequence in the resultant polypeptide

Answer 26

- some codons will be changed to ones that code for same amino acid (degenerate code) - frame shift might not alter some codons because the replacement bases are the same as the originals