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MUTATIONS

What Are Mutations?

• Changes in the nucleotide sequence of DNA

• May occur in somatic cells (aren’t passed to offspring)

• May occur in gametes (eggs & sperm) and be passed to offspring

Are Mutations Helpful or Harmful?

• Some type of skin cancers and leukemia result from somatic mutations

• Some mutations may improve an organism’s survival (beneficial)

Types of Mutations– Chromosome mutations

• Changes in chromosome structure– Genome mutations

• Changes in chromosome number– Single-gene mutations

• Relatively small changes in DNA structure• Occur within a particular gene

Chromosome Mutations

• May Involve:– Changing the

structure of a chromosome

– The loss or gain of part of a chromosome

Chromosome Mutations

• Five types exist:–Deletion–Inversion–Translocation–Nondisjunction–Duplication

Deletion

• Due to breakage• A piece of a

chromosome is lost

Inversion

• Chromosome segment breaks off

• Segment flips around backwards

• Segment reattaches

Duplication

• Occurs when a gene sequence is repeated

Translocation

• Involves two chromosomes that aren’t homologous

• Part of one chromosome is transferred to another chromosomes

Nondisjunction• Failure of chromosomes to separate

during meiosis• Causes gamete to have too many or too

few chromosomes• Disorders:

– Down Syndrome – t(hree 21st chromosomes– Turner Syndrome – single X chromosome– Klinefelter’s Syndrome – XXY chromosomes

Chromosome Mutation Animation

Gene Mutations

• Change in the nucleotide sequence of a gene

• May only involve a single nucleotide

• May be due to copying errors, chemicals, viruses, etc.

Types of Gene Mutations• Include:

–Point Mutations–Substitutions–Insertions–Deletions–Frameshift

Point Mutation

• Change of a single nucleotide

• Includes the deletion, insertion, or substitution of ONE nucleotide in a gene

Point Mutation

• Sickle Cell disease is the result of one nucleotide substitution (GAA GTA )

• Valine is replaced with Glutamic Acid

• Occurs in the hemoglobin gene

Base Pair Substitutions :SilentMissense – new protein (Amino Acid Substitutions)Nonsense – stop codon

1. Silent mutation

5’ ATG GGA GCT CTA TTA ACC TAA 3’ met glyala leu leu thr stop

5’ ATG GGA GCT CTA TTG ACC TAA 3’ met gly ala leu leu thr stop

Silent mutation

2. Missense mutation

5’ ATG GGA GCT CTA TTA ACC TAA 3’ met glyala leu leu thr stop

5’ ATG GGA GCT CTA TTT ACC TAA 3’ met glyala leu phe thr stop

Missense mutation

3. Nonsense mutation

5’ ATG GGA GCT CTA TTA ACC TAA 3’ met glyala leu leu thr stop

5’ ATG GGA GCT CTA TGA ACC TAA 3’ met glyala leu stop

Nonsense mutation

Frameshift Mutation

• Inserting or deleting one or more nucleotides

• Changes the “reading frame” like changing a sentence

• Proteins built incorrectly

Frameshift Mutation

• Original:–The fat cat ate the wee rat.

• Frame Shift (“a” deleted):– The fat cta tet hew eer at.

Amino Acid Sequence Changed

Gene Mutation Animation

Normal Male

2n = 46

Normal Female

2n = 46

Male, Trisomy 21 (Down’s)

2n = 47

Female Down’s Syndrome

292n = 47

Klinefelter’s Syndrome

2n = 47

Turner’s Syndrome

312n = 45

CAUSES OF MUTATIONS• Chemical

agents and physical agents

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MECHANISM of MUTAGENs

• Deamination of DNA structure – Nitrous acid (HNO2) replaces amino groups of base

with keto groups • -NH2 =O• Can change cytosine

to uracil– Pairs with A, not G

• Can change adenine to hypoxanthine

– Pairs with C, not T

MECHANISM of MUTAGENs• Alkylation of DNA structure

– Alkylating agents covalently attach methyl or ethyl groups to bases

• e.g., Nitrogen mustards, ethyl methanesulfonate (EMS)

– Appropriate base pairing is disrupted

Effect of EMS

MECHANISM of MUTAGENs

• Interfere with the DNA replication process– e.g., Acridine dyes such as proflavin

• Flat, planar structures interchelate into the double helix– Sandwich between adjacent base pairs

• Helical structure is distorted• Single-nucleotide additions and deletions can result

MECHANISM of MUTAGENs

• Base analog – Some mutagens are base analogs

• e.g., 2-aminopurine• e.g., 5-bromouracil (5BU)• Become incorporated into

daughter strands during DNA replication

Effect of BU

MECHANISM of MUTAGENs

• Effect of Radiation– Ionizing radiation such as X rays and gamma rays

– Short wavelength and high energy– Can penetrate deeply into biological materials– Creates “free radicals”

» Chemically reactive molecules– Free radicals alter DNA structure in a variety of ways

» Deletions, single nicks, cross-linking, chromosomal breaks

MECHANISM of MUTAGENs

– Nonionizing radiation such as UV light

– Contains less energy– Penetrates only the surface of material

such as the skin– Causes the formation of thymine dimers– May be repaired through one of numerous

repair systems– May cause a mutation when that DNA

strand is replicated

DETERMINING OF MUTAGENS

• Ames test is commonly used• Developed by Bruce Ames

– Uses his- strains of Salmonella typhimurium• Mutation is due to a point mutation rendering an

enzyme inactive– Reversions can restore his+ phenotype

• Ames test monitors rate of reversion mutations

• Ames test– Suspected mutagen is mixed with rat liver extract

and his- Salmonella typhimurium• Rat liver extract provides cellular

enzymes that may be required to activate a mutagen

– Bacteria are plated on minimal media

• his+ revertants can be detected• Mutation frequency calculated

– Compared to control