6.3 DNA Mutations

SBI4U

Ms. Ho-Lau

DNA Mutations

Gene expression can be affected by

errors that occur during DNA replication.

Some errors are repaired, but others can become mutations (changes in the nucleotide sequence of a cell’s DNA)

All cells undergo spontaneous mutation

Mutagens: substances or events that increase the rate of mutation

DNA Mutations

in reproductive cells can affect the next generation

in somatic cells can affect daughter cells

can be neutral or harmful

in rarer cases, they are beneficial

important in terms of species change and adaptation

DNA Mutations

2 categories:

• single-gene mutations

• chromosome mutations

Single-gene Mutations

involve changes in the nucleotide sequence of one gene

categorized according to how they affect the amino acid sequence:

1. Silent mutations

2. Missense mutations

3. Nonsense mutation

categorized according to how they affect the nucleotide sequence:

1. Point mutations

2. Frameshift mutations

1. Silent Mutations

one nucleotide is changed

But does not change the

amino acid sequence of

the protein

2. Missense Mutations

one nucleotide is changed

changes the amino acid sequence of the protein

usually harmful

Occasionally, produce

a protein that helps with

survival

3. Nonsense Mutations

Introduced a premature stop codon

Shorten the mRNA

Shorten the protein

Usually harmful

Single-gene Mutations

involve changes in the nucleotide sequence of one gene

categorized according to how they affect the amino acid sequence:

1. Silent mutations

2. Missense mutations

3. Nonsense mutation

categorized according to how they affect the nucleotide sequence:

1. Point mutations

2. Frameshift mutations

1. Point Mutations

result from a change in a single base pair within a

DNA sequence

can involve substitution, addition, or deletion of one

nucleotide

A substitution may not affect the protein due to the

redundancy built into the genetic code.

2. Frameshift Mutations

result from the insertion or

deletion of nucleotides not

divisible by three

causes a change in the

reading frame

DNA Mutations

2 categories:

• single-gene mutations

• chromosome mutations

Chromosome Mutations

Changes in chromosomes and many genes

1. Deletion: a series of nucleotides are deleted from the chromosome

2. Duplication: a series of nucleotides are duplicated on the same

chromosome

3. Inversion: a chromosome segment is broken and re-inserted in the

opposite direction

4. Reciprocal Translocation: a group of nucleotides from one

chromosome are exchanged with the

nucleotides of a different chromosome

Chromosome Mutations

Causes of Mutations

Mutations may be spontaneous or induced.

Causes of Spontaneous Mutations

• the result of normal molecular interactions

• incorrect base pairing by DNA polymerase during replication

• transposition, in which specific DNA sequences (called

transposons) move within and between chromosomes

Causes of Induced Mutations

1. physical mutagens

• an event or substance such as high-energy radiation (X rays

and UV rays) that physically change the structure of DNA

• example, UV radiation distorts the DNA molecule at adjacent

C and T bases, causing replication interference.

Causes of Induced Mutations

2. chemical mutagens

• molecules that can enter the nucleus of a cell and induce

mutations by reacting with DNA

• Example: nitrites and chemicals in cigarette smoke

• Some mutagens have a structure similar to DNA nucleotides

and can incorporate into a DNA strand, causing incorrect

base pairing during replication.

DNA Repair

DNA polymerases and mismatch repair mechanisms

help to repair replication errors

Two additional types of DNA repair mechanisms are:

• specific repair mechanisms: tailored to fix certain

types of damage (eg. photorepair)

• non-specific repair mechanisms: can correct different

forms of damage (eg. excision repair)

Specific Repair

Photorepair is a specific

mechanism to repair damage to

DNA caused by exposure to UV

radiation.

A photolyase enzyme recognizes

the damage, binds to the dimer,

and uses visible light to cleave

the dimer.

Non-specific Repair

Excision repair is a non-specific

mechanism of DNA repair

because it can fix a variety of

damage.