Download Genetic Mutations: Types, Sources, and Causes and more Study notes Genetics in PDF only on Docsity! 1 Chpt 17 Gene Mutations & DNA Repair • Mutation: – Heritable change in expression of a gene (heritable is key word) – Change in base sequence or physical structure of chromosome – Primary source of new forms of genes that give rise to genetic variation • Natural or artificial selection Source of Mutations Spontaneous: - random occurrence - naturally produced Induced: - artificial agent of some sort - chemicals or radiation 2 Source of Mutations Regardless of source, mutations are rare Mutation frequency varies: - among species - among genes 5 Types of Mutations 2. Insertions/deletions: addition or loss of one or more bases - often called frameshift mutations because they shift the reading frame during translation (protein synthesis) 6 Types of Mutations 3. Expanding trinucleotide repeats: - extra copies of a 3 base sequence - the norm may have 10 copies but affected individuals may have up to or more than a hundred copies of the trinucleotide 7 Cause of Mutations • Spontaneous replication mistakes – tautomeric shifts: proton change that results in bases mispairing – wobble pairing: normal or structurally altered bases wobble and form bonds – strand slippage: small loop forms during replication and results in insertions/deletions – unequal crossing-over: misalignment between homologous chromosomes results in insertions/deletions Tautomeric Shifts 10 Causes of Mutations • Spontaneous chemical changes – depurination: loss of a purine from the nucelotide causes mistake to be made during replication – deamination: loss of amino group from the base which causes a mistake during replication Depurination
Deamination
(a)
°
H H LH
N Deamination N
H N ° H N oO
| INH, |
Cytosine Uracil
Deamination
(b)
°
H
N Deamination "a
H N oO \ H N °
| NHS |
5-Methylcytosine Thymine
(5mC)
11
12 Induced Mutations • Mutagen: increases the mutation rate above the spontaneous rate – Chemical mutagens: alter structure of base and causes mistakes during replication – Radiation mutagens: • x-rays, gamma rays, etc., cause structural damage (alter atomic structure and break DNA strands) • UV light causes bonding between neighbors on the same strand (pyrimidine dimers) that can prevent replication causing cell death Pyrimidine Dimers
