Micro-Mutations

Redundancy

minimizes effects from mistakes by mutations

Types of mutations

conditional or biochemical mutations occurred from spontaneous or induced

Mutation

changes to DNA sequence which affects the type of proteins synthesized

Conditional mutation

mutation expressed only under certain environmental conditions (high temp, less nutrients)

Biochemical mutation

change to biochemistry of cell (prototroph, auxotroph)

Prototroph

grow on minimal medium (CHNOPS)

Auxotroph

requires a nutrient supplement, can't synthesize if the molecule is missing

Spontaneous mutation

natural mutation, errors in replication, transposons, changes in bases, addition/deletion of DNA

Transposons

insertion of pieces of DNA from other parts of the genome

Induced mutation

use of agents that increase mutation frequency (UV light, sun rays, chemicals

Mutagen

agent (chemical or physical) that causes mutation bu changing the properties of nucleotide or increasing frequency

Types of mutagens

transition, base analogs, changes in base structure, intercalating agent, damage H-bonding

Transition mutagen

purine/purine or pyridine/pyridine substitution

Base analogs mutagen

structurally similar to bases but binding properties are different

Intercalating agent (mutagen)

distort DNA to introduce nucleotide pair insertions/deletions

UV light (mutagen)

damages H-bonding so pairing does not occur, induces thymine dimerization

Forward mutation

mutation of wild type (most common gene) leading to a mutant

Reverse mutation

2nd mutation is at same site as original mutation, true reversion, suppressor mutation, intragenic, extragenic

True reversion (back mutation)

same nucleotide change, A -> C -> A

Suppressor mutation

wild type phenotype restored by 2nd mutation with similar amino acid, intragenic (same) or extragenic (different gene)

Intragenic suppression

same gene, base substitution or frameshift restore original character

Extragenic suppression

somewhere else, nonsense or physiological

Nonsense suppression

occurs in tRNA-encoding gene, allows tRNA anticodon to pair with nonsense codon

Physiological suppression

defect in one pathway overcome by mutation in another pathway

Point mutation

affect protein structure, silent, missense, nonsense, frameshift

Silent mutation

no visible effect on phenotype because mutated codon still codes for the same amino acid

Missense mutation

change in nucleotide = change in amino acid, can result in total loss of protein function (in active site) or protein is still functional

Nonsense mutation

changes of sens codon to nonsense codon (stop), results in early termination of synthesis or little/no change if later in amino acid sequence

Ames test

detect mutagens by testing for reversions

In Ames test if there's lots of growth

revertants induced by mutagen

In Ames test if there is little growth

spontaneous revertants not caused by mutagen, complete medium

DNA repair mechanisms

mismatch, excision (nucleotide, base)

Mismatch repair

proofreading ability (DNA pol 3), DNA methylation

How does methylation work with mismatch repair

mut proteins look for mismatches in DNA, they cut the nonmethylated/new strand where mistake is and cleave portion of DNA, DNA pol 1 synthesizes new bases, DNA ligase seals nicks

Excision repair

corrects damages that causes distortions in double helix, nucleotide or base

Nucleotide excision repair

removes distortions caused by thymine dimers - general repair system (uvrABC) correct distortion by making nick around dimer, removing damaged region, DNA pol 1 synthesizes new bases, DNA ligase seals nick

Base excision repair

removal of damaged or unnatural bases yielding AP endonuclease by DNA glycosylase- DNA glycosylase recognizes and binds to abnormal base, cleaves DNA, DNA pol 1 synthesizes new bases, DNA ligase seals nick

Direct Repair

removal of thymine dimers by photoreactivation, excision enzyme, and alkyltransferase

Photoreactivation

photolyase splits thymine dimers by taking energy from light to cleave covalent bond between dimers

Alkyl transferase

removal of methyl or alkyl groups

Excision enzymes

cut out region with thymine dimer

Recombinational repair

corrects damaged DNA when both bases of a pair are missing, no template, use RecA protein

Recombinational repair mechanism

RecA protein aligns damaged DNA with 2nd copy of a gene and uses template copy to replace damaged strand, swaps out damaged area with a gap, DNA pol 1 synthesizes and fills gap, DNA ligase seals nick, thymine dimer on other strand still exists (but no distortion)

How are repair mechanisms regulated

SOS response- no damage = lexA active, damage = recA destroys lexA, major damage occurs = bacteria will use any kind of repair mechanism