MEIOSIS

nuclear division

producing 4 daughter cells

genetically different from parents

forming gametes (sex cells)

each gamete has half the number of chromosomes of original nucleus

PROCESS

S PHASE (DNA REPLICATION)

MEIOSIS I

MEIOSIS II

forms diploid cell

2n chromosomes

each chromosomes has 2 sister chromatids

forming n homologous pairs of chromosomes

have the same:

size and centromere location

gene loci (gene sequence)

but may have alternate alleles (gene forms) of each gene

DNA replicates in the form of chromatin

centrioles replicate (ref. Mitosis)

PROPHASE 1

METAPHASE 1

chromatin condenses into chromosomes

homologous chromosomes pair up (synapsis)

forming a bivalent

nucleolus and nuclear envelope disintegrates

spindle fibres form and centrioles move to opposite poles :

crossing over occurs

non-sister chromatids of homologous chromosomes physically break and rejoin at several points (chiasma)

exchange of genetic material

portion of non-sister chromatids exchanges places with an equivalent portion of another non-sister chromatid

variation due to new combination of alleles

homologous chromosomes are arrange along the equator of the spindle

spindle fibres attach to the centromeres via kinetochore

independent assortment occurs where the arrangement of each homologous chromosome in a bivalent is independent of other bivalents

so one homologous chromosome being on the left or right of the equator is independent of the other

variation as each cell gets a chromosome with different alleles

ANAPHASE I

homologous chromosomes separate

move to opposite poles due to shortening of spindle fibres

sister chromatids attached at centromere and move together

TELOPHASE I (not all meiotic cells)

no nucleus

homologous chromosomes reach opposite poles

spindle fibres disintegrate

nucleolus and nuclear envelope reforms

nucleus

similar to mitosis

PROPHASE II

nucleolus and nuclear envelope disintegrates

centriolesles move to opposite poles

spindle fibres form at right angles to spindle axis of meiosis I (so equator is perpendicular to meiosisI)

CYTOKINESIS

ref. mitosis

METAPHASE II

chromosomes arrange themselves along the equator

centromere is attached to spindle fibre via kinetochore

ANAPHASE II

results in 2 daughter cells, each with n chromosomes

each chromosome has 2 sister chromatids

centromeres divide and separate

non-identical sister chromatids (bc of crossing over) separate and move to opposite poles

TELOPHASE II

chromosomes reach opposite poles of spindle

chromosomes uncoils, lengthens and becomes indistinct

spindle fibres disintegrate

nucleolus and nuclear envelope reforms

CYTOKINESIS II

results in 4 total haploid cells

each chromosomes has 1 sister chromatid only

IMPORTANCE

prevents doubling of chromosomes in sexual reproduction

since gametes are haploid, when they fuse to form a zygote the zygote is diploid

genetic variation

new combination of alleles in gametes (random fertilisation)

crossing over

exchange of new genetic materials

independent assortment of homologous chromosomes

orientation of homologous chromosomes during metaphase I is independent of other bivalents

possible combinations of 2^n maternal or paternal chromosomes

random fusion of male and female gametes during fertilisation

so (2^n)2 combinations

GRAPHICAL

visible DNA during prophase I

mitosis

DNA goes from 4DNA to 2DNA

chromosomes go from 2n to 2n (or n to n)

meiosis

DNA goes from 4DNA to 2DNA to 1 DNA

chromosomes go from 2n to n to n

2 divisions with 1 round of DNA replication

gametes (male and female) fuse to form a zygote

zygote has diploid chromosomes

diploid = ploidy level

shows number of 'sets' of chromosomes (eg gamete has 1 set of 23 chromosomes - monoploidy)

MUTATIONS

change in chromosome structure

during crossing over

inversion

translocation

chromosome detaches and rejoins at a different chromosome or different point in chromosome (gene loci changed)

chromosomes detaches and reattaches at an inverted position

deletion of genes

duplication

replication of past of chromosome (gene duplication --> over production)

change in chromosome number

non-disjunction

sister chromatids don't separate during anaphase II

homologous chromosomes don't separate during anaphase I

forms aneuploidy (number of chromosomes is not euploid - appropriate)

sister chromatids don't separate during anaphase of mitosis

all cells are mutated

2 euploids, 2 aneuploidy (1 trisomy, 1 monosomy)

when gametes fuse during fertilisation

polyploidy can be formed (zygote has 2n/3n chromosomes)