Mutations: Changing DNA

SBI4U Biology

Mutation:

A change in the DNA sequence that is inherited as the DNA is transmitted through cell division.

Changes in number or structure of chromosomes Spontaneous Induced Mutagens = UV, X Rays, other radiation; chemicals; heavy metals

Categorising mutations

1. By effect on structure

2. By consequences on resulting proteins

1. Mutations by effect on structurea) Point mutations

Mutations that occur to a specific base pair in the genome.

b) Chromosomal mutations Mutations that involve large segments of DNA.

Point mutationsSubstitution: One base pair is replaced with another

Point mutations (cont`d)Deletion: One or more base pairs is eliminated from

the DNA sequence

Point mutations (cont`d)Insertion: One or more base pairs is inserted into the

DNA sequence

Frameshift mutationsDeletions and insertions of 1-2 base pairs will result in

a shift in the reading frame. “frameshift mutations”

Deleting or inserting 3 base pairs is not considered a frameshift mutation... why??

Chromosomal mutationsLarge scale changes to chromosomesInversion :The reversal of a segment of DNA within a chromosome.Results from breaking and rejoining (upside down)

Chromosomal mutations (cont`d)

Translocation:A fragment of DNA moves from one part of the genome to another.

Can result in a “fusion protein”

Chromosomal mutations (cont`d)

Gene duplication: Duplication of a coding region of DNA along a chromosome

Results from crossing over of misaligned homologues during meiosis I

Chromosomal mutations (cont`d)

Deletion:Part of a chromosome is deleted and becomes “missing”

Chromosomal mutations (cont`d)Insertion:

The complement of deletion. Part of another chromosome is inserted.

Gross Chromosomal

Gross Chromosomal

Aneuploidy = incorrect number of chromosomes

Trisomy = extra chromosome (2n +1)

Monosomy = missing chromosome (2n -1)

Triploid = 3n, Tetraploid = 4n

Trisomy & Monosomy

2. Mutations by consequence on resulting protein

a) Silent mutationsb) Missense mutationsc) Nonsense mutations

Silent mutationsDoes not result in a change in amino acid sequence

no phenotypic change; no effect on the cell

How?May occur within

an intronRedundancy of

genetic code

ACA and ACU are both codons for threonine

Missense mutationsResults in a change to a codon

The wrong amino acid is incorporatedDoes not necessarily drastically alter protein function

Checkpoint:Can you classify this mutation by its effect on structure?

Nonsense mutationsA codon is converted to a stop codon

truncated protein is produced

Checkpoint:Can you classify this mutation by its effect on structure?

Mutation Examples:

I LOVE HER SO MUCH I COULD MARRY HER.

Silent:I LOVE HER SO MUCH I COULD WED HER.

Missense:I LOVE HER SO MUCH I COULD PUNCH HER.

Mutation Examples:

I LOVE HER SO MUCH I COULD MARRY HER.

Chain Termination:I LOVE HER SO MUCH I COULD.

Nonsense:I LOVE HER SO MUCH I COULD MARY HERE THIRST PINK UTAH MONGOOSE LABEL WINE FLIRT LAVA

LAMP TOOL BARGE MAPLE NOSE HAIR RITZ CRACKER CONSPIRACY GONG SHOPPING SPREE WIT’S END NEVADA WONDERFUL SQUID RICE KANGAROO

CELEBRATE WART…

Mutation Examples:

I LOVE HER SO MUCH I COULD MARRY HER.

Deletion Frameshift:I LOVH ERS OM UCHI C OULDM ARRYH ER

Insertion Framshift:I LOXV EHE RS OMUC H ICOUL DMARR YHE

R

Consolidation

Causes of mutationsInnateSpontaneous: Due to errors in replication

EnvironmentalExposure to mutagenic agents

UV radiation, X rays, chemicals