Genetics 3 Quiz

Mutation

Heritable change in genetic material

3 types of mutations: Gene Mutations

Relatively small change in DNA that affects a single or few genes

3 types of mutations: Chromosome mutations

Changes in chromosome structure

3 types of mutations: Genome mutations

Changes in chromosome number

De novo spontaneous mutations are

more likely to be harmful than beneficial to the individual and more likely yet to be neutral

Mutation rate

is the likelihood that a gene will be altered by a new mutation

Mutation rates vary…

substantially between species and even within different strains, groups, or variants of the same species.

Hot spots

Locations within the chromosome that are more susceptible to mutation

Somatic hypermutation (SHM)

Mutation affect regions of immunoglobulin genes which diversifies B cell receptors to expand their recognition range.

Mutation Frequency

Number of mutant alleles divided by the total number of that allele in a population.

Mutation frequency depends on:

mutation rate

Timing of mutation

Likelihood that the mutation will be passed on to future generations

Point Mutation

Change in a single base pair.

Transition

Change of a pyrimidine (C,T) to another pyrimidine (C,T) or a purine (A,G) to another purine (A,G).

Transversion

change of a pyrimidine to a purine or vice versa

Silent mutations

do not alter amino acid sequence of polypeptide

Missense mutation

base substitutions in which an amino acid change occurs

nonsense mutation

base substitutions that change a normal codon to a stop codon

Frameshift mutation

The addition or deletion of bases not divisible by three.

Forward mutation

changes the wild-type genotype into a new variation

Reverse mutation

Changes mutant allele back to the wild-type allele.

Deleterious mutation

Decrease the chances of survival

Beneficial mutation

Enhance the survival or reproductive success of an organism

Suppressor mutations

a second mutation that counteracts the effects of a first mutation.

Suppressor mutation types: Intragenic

Second site is within the same gene as the first mutation

Suppressor mutation types: Intergenic

Second site is in a different gene from the first mutation.

Chromosomal rearrangement can affect a gene by:

Chromosomal breakpoint occurring within a gene

Position Effect

A gene is left intact but the expression is altered because of its new location.

Two common reasons for position effects:

Movement to a position next to regulatory sequences

movement to a heterochromatic region.

Germ-Line

Mutation can be passed on to future generations because it occurs in sperm or egg cells.

Somatic cell

Mutation can’t be passed on to future generations because occurs directly in a body cell.

Spontaneous mutations

Result from abnormalities in cellular/biological processes

Induced mutations

Caused by environmental agents, like chemical or physical agents.

Spontaneous mutation arise by three types of chemical changes

Depurination

Deamination

Tautomeric shift

Spontaneous mutations can occur by mispairings or “slipping” during DNA replication

Simple mispairing not corrected by proofreading

Wobble Pairing

Additions/deletions due to slippage and looping out

Depurination

Removal of a purine (guanine or adenine)

Apurinic site

Covalent bond between deoxyribose and purine base is somewhat unstable and it undergoes hydrolysis (beta-N-glycosidic link) that releases the base from the sugar.

Readily repaired and happens very frequently

Deamination

Removal of amino group from cytosine (C) resulting in uracil (U)

Tautomeric shift

Temporary change in base structure

Stable form of thymine and guanine is

Keto form

T and G can interconvert to an enol form

Stable form of adenine and cytosine is

Amino form

A and C can interconvert to an imino form

If DNA loops out on template strand

Pol III skips bases

If DNA loops out on daughter strand

Pol III adds random bases

Slipped Strand Mispairing

DNA strand loops out and becomes displaced or if DNA polymerase slips then small insertions and deletions can occur resulting in mutations during replication.

Trinucleotide repeat expansion (TNRE)

Number of 3 nucleotide repeat sequences can increase from one generation to the next

IF TNRE located in noncoding regions then:

May cause abnormal changes in RNA structure

Produce methylated CpG islands which may silence the gene

TNRE disorders

Severity of disease tends to worsen in future generations, called anticipation.

Chemical mutagens are categorized as:

Base analog

Base modifiers

Intercalating agents

Base Analog

May change pairing

Base modifiers

Change one base into another

Intercalating agents

Alter shape of DNA and cause deletion or addition

Physical mutagen types

Ionizing radiation

Nonionizing radiation

ionizing radiation

X-rays and gamma rays

Short wavelength and high energy

Penetrate deeply into biological molecules

Create chemically reactive molecules (free radicals)

Ionizing radition can cause

Base deletions

Oxidized bases

Single nicks in DNA strands

Cross-linking

Chromosomal breaks

Nonionizing radiation (includes UV light)

Lower energy than ionizing

Cannot penetrate deeply into biological molecules

Causes formation of cross-linked pyrimidine dimers (C or T)

Pyrimidine dimers may cause mutations when that DNA strand is replicated

Common test for mutagen

Ames test

Ames test

Uses a strain of Salmonella typhimurium that cannot synthesize the amino acid histidine and has a point mutation in a gene involved in histidine biosynthesis. Second mutation may occur resotring ability to synthesize histidine. This test monitors rate at which second mutation occurs.