Genetics: Chapter 15 - Gene Mutation, DNA Repair and Transposition

Chromosomal Mutations

Loss of entire chromosome (non-disjunction) or a large region from a chromosome (break)

Gain of whole chromosome (non-disjunction)

Duplication of chromosome region on same chromosome or during transfer (recombination) to another chromosome

Unequal (non-reciprocal) recombination ebvents cause loss or duplication

Affect many genes

Gene Mutations

Usually point mutations

Change identity of one or a few base pairs; add or delete a few base pairs

Segregates as a genetic point: can destroy gene function, may have an intermediate effect, may have no effect

In one gene but may affect others (domino effects possible if regulatory gene)

Somatic

Affects host organism

Only daughter cells within that organ/individual will be affected

Somatic mosaicism: how early in development the mutation occurred

Gamete (germ line)

Affects progeny

Altered gene may be recessive or dominant; inherited inborn error

All cells in new organism will contain the altered chromosome

Impact depends on other allele

Exon

Coding region; premature termination; alter reading frame, functionality, stability

Intron

Splice acceptor, donor, invariant internal A; generate new splicing signals

Null

Complete loss of function

Most common

Recessive

Most common

Masked by unaffected allele (sufficient amount)

Carriers appear normal = wild type

Dominant Negative

Lack sufficient gene product

Altered gene product interferes with function of normal gene product(s)

Haploinsufficiency

Inactivation of one allele by mutation exerts a dominant effect

Remaining allele cannot produce enough

Not enough fibrillin protein to maintain wild type phenotype

Gain of interfering function

Creates gene product with novel properties

Alter ability to sense negative regulation

Typically dominant: oncogene mutations

Point Mutation

Alternation of one or very few bases in one location

Substitution: change in identity

Insertion

Deletion

Consequences of Nucleotide Substitution

Silent: no change to affected amino acid codon (degeneracy of genetic code)

Missense: altered codon specifies different amino acid; conservative: amino acid with side change having similar properties; nonconservative: amino acid with dramatically different side chain

Nonsense: generates one of three stop codons; prematurely terminates translation; shortened polypeptide often non-functional

Insertions or Deletions

Noncoding regions: recognition site(s) for a DNA-binding regulatory protein; distance separating binding sites for interacting DNA-binding regulators; remove critical signal

Coding region (ORF): frameshift mutation - codons beyond in/del do not encode original protein, frameshift early in coding region means most of original protein not made

Spontaneous Mutations

Natural, random errors during DNA replication

SNPs = single-nucleotide polymorphisms

Induced Mutations

External/environmental chemical or radiation

Mutation Hot Spots

Some sequences (genes) experience more errors

Luria-Delbruck Fluctuation Test (1943)

Utilized E. coli and bacteriophage T1 system: E. coli cells infected with phage T1 lyse (die) rapidly, mutations make E. coli cells resistant to T1 infection

Result: mutants occurred at different frequencies in the smaller, independent cultures

Conclusion: mutations did not occur as a response to environmental pressure; happened spontaneously, randomly and the environment only determined the benefit

Transition

Base ring of similar size occupies position after change

Transversion

Larger or smaller base ring occupies position after change

Result only from SOS DNA repair

Base Analogues

Substitute for normal bases during synthesis

BrdU = 5’ bromodeoxyuridine: replaces thymidine, ring can shift electrons to base pair as C; causes A-T to G-C transition

2-AP = 2-amino purine: analogue of adenine, can pair with T or C; C pairing causes A-T to G-C transition

Alkylating Agents

Add alkyl groups to the base rings

EMS = ethylmethane sulfate: alkylates keto groups; 6 of G, 4 of T

Causes transitions

DNA Intercalating Agents

Bind/wedge between base pairs

Interfere with DNA replication, DNA repair, transcription

Ethidium bromide (DNA gel stain)

Cancer chemotherapy drug: doxorubicin, dactinomycin

Adduct-forming Agents

Covalently attach to base rings

Interfere with DNA replication and DNA repair

Acetaldehyde (cigarette smoke)

Heterocyclic amines (HACs): burned/grilled meats, covalently alter guanine rings

Both have been linked to cancer

Ultraviolet light

Non-ionizing radiation

Thymine dimers

Ionizing Radiation

X, Gamma, and Cosmic Rays

Generate free radicals

Free radicals break phosphodiester bonds

Double-stranded chromosome breaks especially problematic

Repair Mechanisms

During S phase: proofreading —> mismatch repair —> post-replication repair —> SOS repair

Outside of S phase: photo reactivation repair to reverse UV damage, base excision repair (BER), nucleotide excision repair (NER), double strand break repair