Gene Mutations and DNA Repair

3 types of gene mutations

Base substitutions

Insertions and deletions

Expanding nucleotide repeats

Base substitutions include what 3 categories

Missense

Nonsense

Silent

Insertions and deletions cause

frameshift mutations

Insertions and deletions may also introduce what

Nonsense

Can insertions and deletions be in frame

Yes

Base substitution

Exchanges one base for another

2 types of substitutions

transitions and transversions

transition mutation

purine to purine or pyrimidine to pyrimidine(T:A pair becomes G:C pair or vice versa)

Transversion mutation

purine to pyrimidine or vice versa

Insertions and deletions

Adding or removing 1 or more nucleotides

Frameshift mutation

Change in the reading frame of the ribosome due to the uneven adding or removal of nucleotides

Frameshift mutations do what

Drastically change the encoded polypeptide

Insertions or deletion of multiples of 3 nucleotides

Do not affect reading frame(insertion/deletion but not Frameshift)

Expanding nucleotide repeats

Short DNA sequences(CNG) repeated several times in a row

Expansion of trinucleotide repeats occur via

Slippage of template during DNA replication

Trinucleotide repeat disorders generally show

Genetic anticipation

Genetic anticipation

Genetic trait becomes more strongly expressed at an earlier stage as it is passed from generation to generation

Suppressor mutation

Hides or suppresses the effect of another mutation

forward vs reverse mutation

forward: WT --> mutant

reverse: mutant --> WT

Suppressor mutations may be

intragenic or intergenic

Intragenic vs Intergenic

Intragenic: within same gene as original mutation

Intergenic: within different gene

Conditional mutation

Mutation only expressed under certain conditions

3 mutations that involve effects in protein function

Neutral mutation

Loss of function

Gain of function

Missense mutation

Point mutation which changes an amino acid

nonsense mutation

changes a normal codon into a stop codon

Silent mutation

Alters the base but does not change the amino acid encoded(synonymous codon)

Neutral mutation

missense mutation that does not affect the function of the protein

Loss of function mutation

causes a complete or partial loss of function in gene product

Gain of function mutation

Gene product gains new activity or is expressed at an inappropriate time/place

Mutations can be

spontaneous or induced

Spontaneous mutations arise via what 2 things

Replication errors

Chemical changes in DNA

Chemical changes in DNA(2)

Depurination

Deamination

Induced mutations can arise via what 2 things

Chemicals

Radiation

What can lead to a replicated error(spont)

Mispairing through wobble

Strand slippage causes

Small insertions and deletions

Depurination

Spontaneous hydrolysis of N-glycoside bond that can remove base from deoxyribose

depurination causes

Loss of a purine base from DNA

Deamination

converts cytosine to uracil

Converts 5 methyl C into T

Mutagen

Chemical or physical agent that significantly increases rate of DNA mutations above the spontaneous rate of

3 chemical mutagens

Base analogs

Alkylating agents

Intercalating agents

2 radiation based mutagens

X-ray

Base analogs

Structurally similar to normal bases which incorporate themselves into DNA

Base analogs often pair with

The wrong base

Most common base analog and what it replaces

5 bromouracil, replaces thymine

When paired with the wrong base, base analogs induce

Transition mutation

Certain chemicals modify the structure of DNA bases so that they do what

Base pair differently than unmodified bases

This difference in base pairing induces

Transition

Intercalating agents

Bulky molecules which insert themselves into the DNA and induce Frameshift mutations

Carcinogens

Agents that increase the likelihood of developing cancer

90-95 percent of human cancers are related to

Environment and lifestyle(exposure to carcinogens)

Most carcinogens are mutagens which do what

Introduce genetic changes in somatic cells

These mutations can have a negative impact on

Gene expression

Ionizing radiation (X rays and gamma rays)

Causes breaks in the phosphodiester backbone

Non-ionizing radiation (UV light)

Pyrimidine dimers

3 types of dimers in UV radiation

Thymine dimers

Cytosine dimers

T-C dimers

These dimers cause what

Inhibition of replication and transcription

This inhibition leads to

Cell death

Benzo[a]pyrene

Component of tobacco smoke which is metabolized into BPDE

This BPDE then attaches to what which forms what

Attaches to base(guanine) which forms an adduct

Auxotroph

Nutritional mutant that cannot synthesize an essential compound(nutrient must be added in medium to grow)

Amino acid auxotroph

cannot make one of the 20 amino acids required for protein synthesis

Histidine mutant

Cannot make histidine(can only grow in medium supplemented with histidine)

His- mutants make a defective enzyme in what pathway

Bio synthetic pathway

His- salmonella

Mutant strain unable to grow without histidine

Ames test procedure

Control contains liver extract on plate lacking histidine

Experimental plate contains suspected mutagen, along with liver extract. If bacteria grows on salmonella lacking plate FINISH CARD

4 mechanisms of DNA repair

Mismatch repair

Direct repair

Base excision repair

Nucleotide excision repair

Mismatch repair

MMR enzymes

DNA pol accuracy

Less than 1 mistake per 100 thousand nucleotides

TP53 gene

Codes for transcription factor that binds DNA and activates target genes

TP53 is expressed when

DNA is damaged

P53 gene works at what cell cycle checkpoint

G1 checkpoint

What does p53 do?(2)

Prevents progression from G1 to S until DNA damage is repaired

Activate transcription of DNA repair genes

If damage is repaired a cell may do what

Proceed through cell cycle

If DNA damage is too severe, p53 activates other genes that promote

Apoptosis

About 50 percent of all human cancers are associated with

TP53 gene

Mutated p53 is unable to do what 3 things

Activate DNA repair genes

Arrest growth at G1 checkpoint

Initiate apoptosis of damage persists

Due to these mutations, what occurs

Loss of control over division

This loss of control over division causes

Tumor formation