genetic diversity

what different ways can genetic variation be introduced

1. meiosis

2. mutations

3. random fertilisation

meiosis

creates genetically different gametes

results in 4 haploid daughter cells

what are haploids and diploids

haploids (n) - one copy of each chromosome

diploids (2n) - 2 copies of each chromosome

2 key processes in meiosis

independent segregation of homologous chromosomes

crossing over of homologous chromosomes

what happens in interphase

DNA and organelles double

stages of meiosis

2 nuclear divisions; meiosis 1 and meiosis 2

both include prophase, metaphase, anaphase, telophase and cytokinesis

diagram

what is crossing over

occurs in meiosis 1 when homologous pairs line up opposite each other at the equator and parts of chromatids become twisted

puts tension on the bonds causing pairs of chromatids to break

broken parts chromatid recombine with another chromatid

results in a new combination alleles in the gametes

what is independent segregation

when chromosomes line up it is random which side the paternal and maternal chromosomes lie

pairs are seperated so one of each homologous pair ends up in each daughter cell

creates a large number of possible combinations of chromosomes in daughter cells produced

formula to calculate possible combinations of chromosomes

2n

n = number of homoglous pairs

process of meiosis

prophase 1 homologous chromosomes pair up and cross over (exchange of genetic material)

metaphase 1 paired homologous chromosomes line up along equator

anaphase 1 chromosomes separation and pulled to opposite poles

telophase 1 cell divides resulting in 2 daughter cells, each haploid set of chromosomes

metaphase 2 sister chromatids line along cell’s equator

anaphase 2 sister chromatids separated and pulled to opposite polices

telophase 2 cells divides again resuling in 4 haploid daughter cells

what is a gene mutation

change in the base sequence of DNA that randomly occur during DNA replication

what factors makes gene mutations more likely to occur

mutagenic agents;

○ high energy radiation

○ ionising radiation (X rays etc)

○ chemical carcinogens etc cigarette smoke

different gene mutations

substituted or deleted

silent mutation

when the new base/codon still codes for the same amino acid

deleted mutation

the removal of one base changes all of the codons

means that multiple amino acids may be incorrectly coded for

non overlapping

each base is only part of one triplet

universal

same sequence of bases codes for the same amino acids in all organisms

degenerate

each amino acid is coded for by more than one triplet

eg. proline coded for by; CCC, CCA, CCT, CCG

how do chromosome mutations occur

non disjunction during meiosis

when chromosomes or chromatids do not split equally during anaphase

2 forms of chromosome mutations

1. changes in the whole sets of chromosomes (polyploidy)

2. changes in the number of individual chromosomes (aneuploidy)

polyploidy

changing in whole set of chromosomes

occurs when organisms have three or more sets of chromosomes rather than usual 2

mainly occurs in plants

aneuploidy

change in the number of indivdiual chromosomes

when chromosomes fail to separate during meiosis - called non disjunction and results in gamete having one more or one less chromosome

EG. down syndrome caused by 3 copies of chromosome 21