most organisms are diploid (2n) with 2 copies of each
chromosome (homologous pair) one from each parent
humans have 23 homologous pairs
giving total of 46 chromosomes
gametes haploid with only alf number of
chromosomes/ DNA present
when gametes fuse in sexual reproduction
the diploid number is restored
Meiotic stages I
INTERPHASE I
dna content doubled and new
cell organelles formed
PROPHASE I
1) chromosomes become shorter and thicker 2) homologous
chromosomes pair up as a bivalent 3) each chromosome is two
chromatids and crossing over between homologous
chromosomes with formation of chiasma 4) centrioles move to
opposite ends of nucleus, microtubules form asters and then the
spindle 5) nuclear envelope breaks and nucleolus disappears
METAPHASE I
bivalents align on the equator attached to spindle. Maternal
and paternal chromosomes are arranged randomly
ANAPHASE I
whole chromosome (centromere do not
split) move to the poles. each one of the
random paternal or maternal homologous
pair moves to opposite pole by
contraction of spindle fibres
TELOPHASE I
animal cell : cytokinesis may
occur or plants: may go
straight onto meiosis II
Meiotic stages II
PROPHASE II
new spindles form at right
angles to the old spindle
METAPHASE II
chromosomes line up at
spindle equator attached
to centromere
ANAPHASE II
centromeres divide and sister
chromatids pulled to opposite poles
TELOPHASE II
1) chromosomes lengthen and become indistinct
2) spindle disappears 3) nuclear membrane
reforms 4) cytokinesis
four haploid cells
produced (eg.
sperm/eggs)
meiosis can generate variation
RANDOM ASSORTMENT
independent segregation of maternal
and paternal homologous
chromosomes in metaphase I
CROSSING OVER
occurs in prophase I with the formation of
chiasmata (so get different combinations of alleles
when genetic information is exchanges)
RANDOM
FERTILISATION
of many genetically different haploid
gametes
Meiosis is the reduction division producing gametes in sexually reproducing organisms (diploid 2n to haploid n cells)