Cracking the AP Biology Exam

6

Molecular Genetics

MUTATIONS

There are two types of mutations: base substitutions and gene rearrangements.

BASE SUBSTITUTION

Base substitution (point) mutations, results when one base is substituted for another. Point mutations within a gene can be nonsense mutations (early termination of protein synthesis), missense mutations (a codon is altered and produces a different amino acid), or silent mutations (those that cause no detectable change in the corresponding protein sequence).

GENE REARRANGEMENTS

Gene rearrangements involve DNA sequences that have deletions, duplications, inversions, and translocations which are often caused by chemical and radioactive agents.

1. Deletions result in the loss of DNA or a gene. Deletions can involve either the loss of a single base or the loss of a larger portion of DNA. Insertions can have devastating consequences to the gene because translation of the gene causes a frameshift.

2. Duplications can result in an extra copy of genes, and are usually caused by unequal crossing over during meiosis or chromosome rearrangements.

3. Inversions can result when changes occur in the orientation of chromosomal regions. This may cause harmful effects if the inversion involves a gene or an important sequence involved in the regulation of gene expression.

4. Translocations occur when a portion of two different chromosomes (or a single chromosome in two different places) breaks and rejoins in a way that the DNA sequence or gene is lost, repeated, or interrupted.

Either type of mutation, if not repaired, will be kept in subsequent rounds of replication. Mutations in somatic cells may damage the cell, make it cancerous or kill it. Mutations in a germ cell will be passed down to the next generation.