10. Mutation and its Effects

Mutation

A heritable change in the DNA sequence.

What causes mutations?

Errors during DNA replication or unrepaired DNA damage.

Is DNA damage the same as a mutation?

No. DNA damage can lead to mutation if it is not repaired.

Why do mutations occur despite proofreading?

DNA polymerases are highly accurate but not perfect.

Point mutation

A change affecting one or a small number of nucleotides.

Transition mutation

A mutation in which a purine is replaced by another purine or a pyrimidine by another pyrimidine.

Examples of transition mutations

A→G or C→T.

Why are transitions common?

Many forms of DNA damage preferentially cause transitions.

Transversion mutation

A mutation in which a purine is replaced by a pyrimidine or vice versa.

Examples of transversion mutations

A→T, A→C, G→T or G→C.

Difference between transitions and transversions

Transitions preserve the base class whereas transversions change the base class.

Mutagen

An agent that increases the mutation rate.

What are the main classes of mutagen?

Base analogues, chemical mutagens and radiation.

Base analogue

A chemical that resembles a normal DNA base and can be incorporated into DNA.

5-bromouracil

A base analogue of thymine that increases mutation rates.

Why is 5-bromouracil mutagenic?

It can adopt alternative forms that pair with different bases.

What type of mutation is commonly caused by 5-bromouracil?

Transitions.

Chemical mutagen

A chemical that damages DNA and increases mutation frequency.

Why do chemical mutagens cause mutations?

They alter the structure or pairing properties of DNA bases.

Oxidative DNA damage

Damage caused by reactive oxygen species.

What is a common product of oxidative DNA damage?

8-oxo-guanine.

Why is 8-oxo-guanine mutagenic?

It can pair incorrectly during replication.

What mutation is commonly associated with oxidised guanine?

G→T transversions can occur if 8-oxoG mispairs during replication.

How do cells protect themselves against oxidative DNA damage?

DNA repair enzymes remove damaged bases.

What are antioxidants?

Molecules that reduce oxidative damage by neutralising reactive oxygen species.

Can oxidative damage still cause mutations despite repair systems?

Yes, if the damage overwhelms repair capacity.

Nitrous acid

A chemical mutagen that deaminates DNA bases.

How does nitrous acid cause mutations?

It changes base-pairing properties and causes misincorporation during replication.

Radiation mutagen

An agent such as UV light, X-rays or gamma rays that damages DNA.

What type of DNA damage is produced by UV radiation?

Thymine dimers.

Thymine dimer

A covalent linkage between adjacent thymine residues in DNA.

Why are thymine dimers harmful?

They distort DNA and interfere with replication.

What can happen when DNA polymerase encounters a thymine dimer?

Error-prone replication may introduce mutations.

Why is radiation highly mutagenic?

It produces large amounts of DNA damage simultaneously.

Silent mutation

A mutation that does not alter the amino acid sequence.

Why can silent mutations occur?

The genetic code is degenerate.

Degenerate genetic code

Multiple codons can encode the same amino acid.

Missense mutation

A mutation that changes one amino acid into another.

Conservative missense mutation

Amino acid substitution with similar chemical properties.

Example of a conservative mutation

One negatively charged amino acid replaced by another negatively charged amino acid.

Non-conservative missense mutation

Amino acid substitution with very different chemical properties.

Why are non-conservative mutations often more damaging?

They are more likely to disrupt protein structure or function.

Nonsense mutation

A mutation that converts a codon into a stop codon.

Effect of a nonsense mutation

Produces a truncated protein.

Why are nonsense mutations often harmful?

The protein is shortened and often non-functional.

Can mutations outside coding sequences be important?

Yes, they can affect promoters and regulatory sequences.

What can happen if a mutation affects a promoter?

Gene expression may increase, decrease or stop entirely.

Neutral mutation

A mutation with little or no effect on fitness.

Advantageous mutation

A mutation that increases fitness under particular conditions.

Deleterious mutation

A mutation that reduces fitness or damages function.

Insertion mutation

Addition of one or more nucleotides into DNA.

Deletion mutation

Removal of one or more nucleotides from DNA.

Strand slippage

A replication error in which DNA strands misalign.

How does strand slippage cause insertions or deletions?

Loops form during replication leading to nucleotide gain or loss.

Frameshift mutation

An insertion or deletion that changes the reading frame.

What causes a frameshift mutation?

Addition or deletion of nucleotides not divisible by three.

Why are frameshift mutations often severe?

All downstream codons are altered.

Common consequence of a frameshift mutation

Premature stop codons and loss of protein function.

What happens if three nucleotides are inserted or deleted?

The reading frame is maintained.

Loss-of-function mutation

A mutation that reduces or eliminates protein activity.

Why are loss-of-function mutations common?

There are many ways to damage a protein but fewer ways to improve it.

What happens if a non-essential bacterial function is lost?

The bacterium may survive.

What happens if an essential function is lost?

The cell dies.

Gene duplication

The creation of an additional copy of a gene.

Why is gene duplication important for evolution?

One copy can maintain the original function while the other evolves a new function.

Gene family

A group of related genes that evolved from duplication events.

Heterotroph

An organism able to synthesise all essential metabolites needed for growth.

Auxotroph

An organism that requires a nutrient supplied externally because it cannot synthesise it.

How can an auxotroph arise?

Loss-of-function mutation in a biosynthetic pathway.

Reversion mutation

A mutation that restores a previous phenotype.

True reversion

A mutation that restores the original DNA sequence.

Equivalent reversion

A different DNA sequence restores the original protein function.

Why can equivalent reversion occur?

The genetic code is degenerate.

Pseudoreversion

A second mutation compensates for the first mutation.

How does pseudoreversion work?

The second mutation restores protein function without restoring the original sequence.

Suppressor mutation

A second mutation that compensates for the effect of an earlier mutation.

Why are reversion mutations useful experimentally?

They can be used to measure mutation rates.

Ames test

A test used to determine whether a chemical is mutagenic.

What organism is commonly used in the Ames test?

Salmonella typhimurium.

What type of mutant is used in the Ames test?

A histidine auxotroph.

Why is histidine auxotrophy useful in the Ames test?

Only revertants can grow without histidine.

What is being measured in the Ames test?

The frequency of reversion mutations.

How is mutagenicity assessed in the Ames test?

By comparing the number of revertant colonies to a control.

What does an increased number of revertant colonies indicate?

The test compound is mutagenic.

Why is there always a small number of revertant colonies?

Spontaneous mutations occur naturally.

Why are liver extracts often included in the Ames test?

Some chemicals become mutagenic only after liver metabolism.

Pro-mutagen

A substance that becomes mutagenic after metabolic activation.

Why is the Ames test important?

It provides a rapid screen for potential carcinogens and mutagens.

What are the three main learning outcomes of the mutation lecture?

DNA damage-induced mutation and mutagens, mutation types and effects, and reversion mutations as the basis of the Ames test.