GENETIC MUTATIONS Section 5.6 Pg. 259
Mutation: A change in the DNA sequence that is inherited as the DNA is transmitted through cell division.
Categorising mutations • By effect on structure • By consequences on resulting proteins
1. Mutations by effect on structure • Point mutations • Mutations that occur to a specific base pair in the genome. • Chromosomal mutations • Mutations that involve large segments of DNA.
Point mutations • Substitution: One base pair is replaced with another
Point mutations (cont`d) • Deletion: One or more base pairs is eliminated from the DNA sequence
Point mutations (cont`d) • Insertion: One or more base pairs is inserted into the DNA sequence
Frameshift mutations • Deletions and insertions of 1-2 base pairs will result in a shift in the reading frame. • “frameshift mutations”
Deleting or inserting 3 base pairs is not considered a frameshift mutation... why??
Chromosomal mutations • Inversion :The reversal of a segment of DNA within a chromosome. • Results from breaking and rejoining
Chromosomal mutations (cont`d) • Translocation:A fragment of DNA moves from one part of the genome to another. • Can result in a “fusion protein”
Chromosomal mutations (cont`d) • Gene duplication: Duplication of a coding region of DNA along a chromosome • Results from crossing over of misaligned homologues during meiosis I
Chromosomal mutations (cont`d) • Deletion:Part of a chromosome is deleted and becomes “missing”
Chromosomal mutations (cont`d) • Insertion:The complement of deletion. Part of another chromosome is inserted.
2. Mutations by consequence on resulting protein • Silent mutations • Missense mutations • Nonsense mutations
Silent mutations • Does not result in a change in amino acid sequence • no phenotypic change; no effect on the cell How? • May occur within an intron • Redundancy of genetic code ACA and ACU are both codons for threonine
Missense mutations • Results in a change to a codon • The wrong amino acid is incorporated • Does not necessarily drastically alter protein function Checkpoint: Can you classify this mutation by its effect on structure?
Nonsense mutations • A codon is converted to a stop codon • truncated protein is produced Checkpoint: Can you classify this mutation by its effect on structure?
Causes of mutations Innate • Spontaneous: Due to errors in replication Environmental • Exposure to mutagenic agents • UV radiation, X rays, chemicals
LEVELS OF GENETIC ORGANIZATION Section 5.8 Pg. 266
Nucleosome • dsDNA is coiled around stabilizing proteins called histones • DNA + histone = nucleosome
Chromatin • Fibrous complex of coiled nucleosomes • In interphase, DNA is in the form of chromatin
Chromosome • During mitosis/meiosis, chromatin supercoils • Supercoiling condenses the strands into chromosomes
DNA composition Coding regions: 5% of genome • 42 000 genes Noncoding regions: 95% of genome • Repetitive DNA • VNTRs (“microsatellites” – genetic markers used in forensics) • Telomeres • Centromeres • Pseudogenes
Homework Mutations: Pg. 263 #1-7