20.1 - Gene Mutations

Question 1

Define mutation

Answer 1

Mutations are changes to the base sequence in the DNA. Mutations can be caused by addition, deletion and substitution of bases.

Question 2

Define gene mutations

Answer 2

A gene mutation is a change in the sequence of base pairs in a DNA molecule that may result in an altered polypeptide. Mutations occur continuously and spontaneously

Question 3

What part of the cell cycle are mutations most likely to occur

Answer 3

Interphase

Question 4

What happens normally during interphase

Answer 4

• DNA is unwound as chromatin so the genes are accessible for transcription
• Cell growth and DNA replication take place
• this takes up around 90% of the cell cycle

Question 5

Describe how a mutation occurs during interphase

Answer 5

• DNA is proofread by specialist proteins to stop mistakes being made during the cell cycle when the DNA is replicated (S phase)
• if this doesn’t work and change happens, then we call this a mutation

Question 6

Why might not all mutations that make it past interphase continue to exist

Answer 6

• during S phase, before DNA replication the amount of DNA doubles
• and the DNA is once again checked for errors (mutations)

Question 7

How can changes to DNA triplets affect a protein

Answer 7

• change in DNA effects change in amino acid sequence for polypeptide coded
• this change in primary structure of a protein will affect the tertiary structure
• this is because it affects the bonding of r-groups in the tertiary structure which determines the shape and therefore functionality of the protein

Question 8

Why might a mutation have no effect on the amino acid sequence?

Answer 8

Due to the degeneracy of the genetic code, some amino acids are coded by more than one codon.

Question 9

What is a substitution mutation

Answer 9

One or more nucleotides in a DNA molecule are replaced by another nucleotide with a different base

Question 10

What are the three potential outcomes of substitution mutations?

Answer 10

1) Formation of a stop codon, prematurely terminating the polypeptide.
2) Substitution of a codon for a different amino acid, possibly altering the protein’s function.
3) Substitution of a codon for the same amino acid (no effect due to the genetic code’s degeneracy)

Question 11

Give an example of a condition caused by substitution mutation

Answer 11

Sickle cell anaemia

Question 12

What is a deletion mutation

Answer 12

One or more nucleotides are lost from the DNA sequence

Question 13

What major change might a deletion mutation cause

Answer 13

Frame shift - altering to the reading of all subsequent triplets after the mutation

Question 14

Why is the position of the deleted base important?

Answer 14

A deletion near the start of a sequence impacts all subsequent triplets, while one near the end has less severe effects.

Question 15

What is an addition/insertion mutation?

Answer 15

An extra one or more nucleotides are inserted into the DNA sequence
—> just like deletion, might cause a frame shift

Question 16

Why might an addition mutation not cause a frame shift?

Answer 16

If three or a multiple of three bases are added, the reading frame remains intact.

Question 17

What is a duplication mutation

Answer 17

One or more bases are repeated
—> can result in a frame shift

Question 18

What is an inversion mutation?

Answer 18

• A sequence of bases is reversed
• an example of a chromosomal mutation as a segment of DNA is reversed within the sequence, altering the amino acid sequence.

Question 19

What is a translocation mutation

Answer 19

• A segment of DNA is moved to a different chromosome, potentially disrupting gene expression and causing abnormal phenotypes.
• happens during meiosis between non-homologous chromosomes
• an example of chromosomal mutation

Question 20

What are spontaneous mutations?

Answer 20

Permanent DNA changes occurring during replication without external influence.

Question 21

What is the typical mutation rate in organisms?

Answer 21

Around 1–2 mutations per 100,000 genes per generation.

Question 22

Name two types of mutagenic agents and their effects.

Answer 22

1) High-energy ionising radiation (e.g., X-rays, UV light) disrupts DNA structure.
2) Chemical mutagens (e.g., benzopyrene) alter DNA structure or interfere with transcription.

Question 23

What are the costs and benefits of mutations?

Answer 23

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- Mutations provide genetic diversity essential for natural selection and speciation.
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- Mutations are often harmful, reducing fitness, and may disrupt cellular activities, leading to conditions such as cancer.

Question 24

Why are mutations in somatic cells usually harmful?

Answer 24

They can disrupt normal cellular functions like cell division, contributing to cancer.

Question 25

What is a nonsense mutation

Answer 25

- Occurs when the base substitution results in a stop codon being transcribed on to mRNA - when this occurs, the polypeptide chain is stopped prematurely and will often not function

Question 26

What is a missense mutation

Answer 26

- Occurs when the base substitution results in a different amino acid being coded for - since there’s a different amino acid in the polypeptide, it may not function correctly as the intermolecular bonds that give the unique shape of the tertiary structure may be changed and hence the whole shape of the protein will be different

Question 27

What is a silent mutation

Answer 27

- Occurs when the substitution does not result in a different amino acid being coded for - the polypeptide will therefore contain the same sequence of amino acids and so will still function correctly

Question 28

When talking about how mutations relate to functionality of enzymes, what MUST you talk about

Answer 28

How it effects the likelihood of forming an enzyme-substrate complex

Question 29

Atranslocation mutation is, in effect, a combination of two other different types of gene mutation. Deduce which two types of mutation these are and explain your answer.

Answer 29

deletion and addition because the bases are deleted from one chromosome and added to a different one.

Question 30

Amutation 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 30

- Where the duplicated bases are consecutive, the frame shift is three bases long and so the subsequent codons are not affected. - The polypeptide will have an additional amino acid but otherwise be 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 the duplications will be changed and the polypeptide will have many different amino acids (but not necessarily all — degenerate code). - After the third duplicate base the codons will be unchanged.

Question 31

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

Answer 31

- Some codons will be changed to ones that code for the same amino acid (degenerate code). - The frame shift might not alter some codons because the replacement bases are the same as the originals. (e.g. GCT TTT CGA —a single base frame shift to the right does not alter the TTT codon).