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)