3.4.3 Genetic Variation - Mutation + Meiosis

mutation

any change to the base sequence or quantity of DNA

gene mutation

any change to one or more nucleotide base sequences of DNA which results in the formation of a new allele

-these arise spontaneously during DNA replication

TYPES of gene mutation -substitution

a nucleotide is replaced by another nucleotide with a different base

-change in sequence of amino acids, change in hydrogen bonds, alters tertiary structure

TYPES of gene mutation -deletion

a nucleotide is lost from the normal DNA sequence

-amino acid sequence is usually entirely different because bases are read in triplets so removal of one shifts each base one to the left forming new codons = frameshift

different effects of mutations

-no change = some triplets code for the same amino acid (degenerate), occurs in introns, changes amino acid but not tertiary structure

-translation stops early = shorter polypeptide

-changes polypeptide = codes for different amino acid, changes properties of protein (positive - can lead to increased reproductive success)

CHROMOSOME mutations -polyploidy

has 3 or more sets of chromosomes rather than 2

-changes in whole sets of chromosomes

CHROMOSOME mutations -non-disjunction

individual chromosomes do not separate in meiosis so results in 1 fewer or 1 more chromosome

-changes in number of individual chromosomes

mutagenic agent

a factor that increases the rate of mutations

EXAMPLES of mutagenic agents

-high energy radiation

-x-rays

-UV light

-carcinogens

meiosis 1

-homologous chromosomes pair up and their chromatids wrap around each other

-then go through crossing over

-cell divides into 2 where homologous pairs separate with 1 chromosome from each pair going into a separate daughter cell = independent segregation

meiosis 2

-chromatids move apart as the centromere is divided so each chromatid from one chromosome ends up in a different daughter cell as the cell divides again resulting in 4 genetically different daughter cells

3 ways meiosis produces genetic variation

1) crossing over

2) independent segregation

3) random fertilisation

crossing over

-homologous pairs associate

-chiasma form - chromatids twist together

-equal lengths of non-sister chromatids are exchanged and join other chromosome

-producing new different combinations of alleles but still the same genes

independent segregation

-during meiosis 1, each homologous pair is lined up and then separated where the arrangement is random

-so 1 chromosome from each pair goes into a different daughter cell

-so have different combinations of chromosomes = variation

random fertilisation

-produces new different allele combinations