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MEIOSIS (GRAPHICAL (mitosis (DNA goes from 4DNA to 2DNA, chromosomes go…
MEIOSIS
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
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
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)
2 divisions with 1 round of DNA replication
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)
possible combinations of 2^n maternal or paternal chromosomes
crossing over
exchange of new genetic materials
independent assortment of homologous chromosomes
orientation of homologous chromosomes during metaphase I is independent of other bivalents
random fusion of male and female gametes during fertilisation
so (2^n)2 combinations
MUTATIONS
change in chromosome structure
during crossing over
inversion
chromosomes detaches and reattaches at an inverted position
translocation
chromosome detaches and rejoins at a different chromosome or different point in chromosome (gene loci changed)
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
2 euploids, 2 aneuploidy (1 trisomy, 1 monosomy)
homologous chromosomes don't separate during anaphase I
all cells are mutated
forms aneuploidy (number of chromosomes is not euploid - appropriate)
when gametes fuse during fertilisation
polyploidy can be formed (zygote has 2n/3n chromosomes)
sister chromatids don't separate during anaphase of mitosis
PROCESS
S PHASE (DNA REPLICATION)
MEIOSIS I
MEIOSIS II
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)
METAPHASE II
chromosomes arrange themselves along the equator
centromere is attached to spindle fibre via kinetochore
ANAPHASE II
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
1 more item...
PROPHASE 1
METAPHASE 1
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)
homologous chromosomes reach opposite poles
spindle fibres disintegrate
nucleolus and nuclear envelope reforms
nucleus
CYTOKINESIS
ref. mitosis
results in 2 daughter cells, each with n chromosomes
1 more item...
chromatin condenses into chromosomes
homologous chromosomes pair up (synapsis)
forming a bivalent
crossing over occurs
non-sister chromatids of homologous chromosomes physically break and rejoin at several points (chiasma)
exchange of genetic material
variation due to new combination of alleles
portion of non-sister chromatids exchanges places with an equivalent portion of another non-sister chromatid
nucleolus and nuclear envelope disintegrates
no nucleus
spindle fibres form and centrioles move to opposite poles :
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)