Genetic mutations: Types, effects and mutagenic agents

In a nutshell

Mutations are changes to the DNA sequence. They can be harmful if they cause the primary structure of a protein to change. However, due to the degenerate nature of the genetic code, this isn't always the case. Mutagenic agents can increase the rate of DNA mutations.

Mutations

Definition

Mutations are changes to the base sequence of DNA. They can arise randomly during DNA replication or can be the result of a mutagenic agent.

Types of mutations

Type of mutation	Definition	Example
Substitution	This type of mutation is when one or more nucleotides are swapped and replaced with a different nucleotide.	GCTGAT becomes GCAGAT.
Deletion	This type of mutation occurs when one or more bases are removed.	GCTGAT becomes GTGAT.
Addition	This type of mutation involves the addition of one or more DNA bases.	GCTGAT becomes GCTTGAT.
Duplication	This type of mutation occurs when one or more bases are repeated.	GCTGAT becomes GCCCCTGAT.
Inversion	This type of mutation involves a sequence of bases reversing.	GCTGAT becomes GGTCAT.
Translocation	This type of mutation arises when a sequence of bases is moved and attaches to a different part of the same chromosome or a completely different chromosome.	Some cases of Down's syndrome are caused by translocation of a piece of chromosome $14$ to chromosome $21$ .

Example

Part of chromosome $4$ and $20$ have broken off and attached to one another. This is a translocation mutation.

Biology; Gene expression; KS5 Year 12; Genetic mutations: Types, effects and mutagenic agents

Genetic diversity

Mutations increase genetic diversity and are influenced by a variety of environmental factors. Some mutations do not result in a phenotypic change, some mutations positively impact an organism and some mutations can negatively impact an organism.

Examples

A mutation in the DNA of a bacterium might make it resistant to antibiotics, this is beneficial to the bacteria.
Mutations in the DNA of humans may lead to the formation of cancer.

Proteins

The base sequence of DNA codes for amino acids which form polypeptide and proteins. A change in the DNA base sequence may affect the amino acid sequence as the codon will have changed.

Example

A substitution mutation to the codon TTG to GTG will change the amino acid from leucine to valine.

Biology; Gene expression; KS5 Year 12; Genetic mutations: Types, effects and mutagenic agents

This may affect the tertiary structure ( $3D$ structure) of the protein as this is determined by the sequence of amino acids, or primary structure. This could lead to the production of non-functional or faulty proteins.

Example

A mutation in the CFTR gene can lead to cystic fibrosis.

Degenerate genetic code

The degenerate nature of the genetic code means that not all mutations will affect the protein function. Degenerate means that some amino acids are coded for by the same codon.

Example

A substitution mutation in the codon TAT to TAC will still lead to the amino acid tyrosine.

Frame shift

Additions, duplications and deletions frequently cause changes in the amino acid sequence. This is because they alter the number of bases in the genetic code. They cause a shift in bases known as a frame shift; this is because after the mutation, all of the subsequent bases (1.) are affected so each codon will be read in a different way.

Biology; Gene expression; KS5 Year 12; Genetic mutations: Types, effects and mutagenic agents

Mutagenic agents

Definition

Mutagenic agents are things that increase the rate of mutations in cells. They can act in different ways.

Mutagenic effect	Description	Example
Acting as a base	Base analogs are chemicals that can be substituted for a base during DNA replication. This is because they have a similar structure to existing bases. This changes the base sequence of the new DNA.	$5$ -bromouracil is a base analog for thymine. However, it pairs with guanine instead of adenine and therefore causes a substitution mutation.
Altering or deleting bases	Certain chemicals can alter or delete bases.	Alkylating agents add an alkyl group to guanine. This causes it to pair with thymine.
Changing DNA structure	Certain types of radiation can change the structure of DNA.	UV radiation causes thymine bases to pair up with each other.

Note: Some mutagenic agents can be biological, these include bacteria and viruses.