IB Biology SL Topic D1.3

State the two causes of mutation

• Spontaneous mutations: Errors that occur naturally during DNA replication (e.g., DNA polymerase inserting the wrong base).

• Induced mutations: Changes caused by exposure to external environmental factors called mutagens.

State two types of mutagens and give an example of each

• Chemical mutagens: Chemicals that alter DNA structure (e.g., components of tobacco smoke, alkylating agents, or base analogs).

• Radiation (Physical) mutagens: High-energy radiation that damages DNA (e.g., UV radiation from the sun, X-rays, or gamma rays).

State what occurs with a base substitution mutation

A base substitution occurs when a single nucleotide (base) in the DNA sequence is replaced by a different base. This can lead to:

• Silent mutation: Codes for the same amino acid.

• Missense mutation: Codes for a different amino acid.

• Nonsense mutation: Codes for a premature stop codon.

State what occurs with an insertion and deletion mutation

• Insertion: One or more extra nucleotides are added into the DNA sequence.

• Deletion: One or more nucleotides are removed from the DNA sequence. Both typically result in a frameshift mutation (unless the total number of inserted or deleted nucleotides is a multiple of three).

State the difference between a base substitution mutation and insertion and deletion mutations with regards to the codons of a gene.

• Base substitution: Only affects a single codon (changing or keeping the amino acid) or has no effect, leaving the reading frame of all subsequent codons completely intact.

• Insertion and deletion: Alters the reading frame of the entire mRNA sequence from the point of mutation onward (frameshift), changing every downstream codon and usually resulting in a completely different, non-functional protein.

What is meant by “random” mutation when we know that some parts of the genome are more likely to mutate?

"Random" means that mutations do not occur because an organism needs them or in response to an environmental challenge (they are not goal-directed). Even though certain regions of DNA (hotspots) or repetitive sequences have higher statistical frequencies of mutation due to their chemical structure, which specific base pairs change and whether that change will be beneficial or harmful is entirely a matter of chance.

What is the difference in inheritance of mutations in germ cells and somatic cells?

• Somatic cells (body cells): Mutations affect only the individual organism (e.g., leading to cancer or aging) and are not passed on to offspring.

• Germ cells (gametes: sperm and egg cells): Mutations occur in the cells that give rise to gametes and can be inherited by the organism's offspring.

What do mutations mean for individual organisms?

• Harmful: Leading to genetic disorders, reduced health, or cancer.

• Neutral: Having no noticeable effect (e.g., occurring in non-coding regions or resulting in silent mutations).

• Beneficial: Providing an advantage that helps the organism survive and thrive.

What do mutations mean for populations?

Mutations are the ultimate source of genetic variation in a population. They introduce completely new alleles (gene variants) into the gene pool, providing the raw material necessary for natural selection and evolution over generations.