meiosis Flashcards
what is meiosis?
the term meiosis was coined by farmer and Moore, “Meio” means to lessen. this division was by Wenn Benidin Winiwarter and strassburger .it contains two sequential nuclear division but only one cycle of DNA replication, which forms 4 haploid cells.
where does meiosis occur?
in sexually reproducing animals. germs cells undergo meiosis to produce gametes
what are the two divisions of meiosis?
1) meiosis 1: it is a reductional division i.e DNA content reduces by half
2) meiosis 2: it is an equational division i.e DNA content remains the same
what are the divisions under meiosis 1?
1) prophase 1:
a) leptotene
b) zygotene
c) pachytene
d) diplotne
e) diakenisis
2) metaphase 1
3) anaphase 1
4) telophase 1
explain leptotene.
1) condensation and coiling of chromatin occurs
2) chromosomes gradually becomes visible under a microscope
3) bouquet stage
explain zygotene.
1) homologous chromosomes start to pair this is known as synapsis. and forms a bivalent
2) formation of the synaptonemal complex occurs
what are homologous chromosomes?
they are chromosomes identical in shape size and structure. one of the homeologous chromosomes is maternal and the other is paternal.
why does the formation of the synaptonemal complex occur?
help is stabilising the homologous chromosomes until the crossing is over.
explain pachytene.
1) the synapsed chromosomes continue to m become thick and short. the 4 chromatids form a tetrad
2) two non-sister chromatids start to cross over ie they start to exchange genetic matter, this done by breakage and reunion of chromosomal segments
3) the recombination of the chromosome is done with the help of recombinase enzyme.
explain diplotene.
1) synaptonemal complex disappears
2) x shaped structure named chiastama forms
3) in oocytes(eggs) of some vertebrates, this stage lasts for months or years. the longest phase of prophase
4) aka dictyotene stage
explain diakinesis.
1) criminalisation of chiasmata due to tight condensation of chromosomes,
2) spindle appears
3) nuclear membrane and nucleolus disappears, nuclear envelop disintegrates
explain metaphase 1.
1) two equatorial / metaphasic plates form
2) the centromere arrange themselves on the equatorial plates, forming a double metaphasic plate
3) there is no fixed direction in which the chromosomes should face
4) the distribution of the bivalent chromosomes are random
4) microtubules from the spindle fibres attach to the centromere facing towards it.
explain anaphasic 1.
1) the two homologous chromosomes, separate and moves toward their respective poles, centromere does not split
2) there is a reduction of genetic material in daughter cells and this occurs in anaphase 1
explain telophase 1.
1) the chromosomes reach their respective poles and uncoils
2) nuclear membrane, nuclear envelop and nucleolus reappears
3) spindle fibres disappears
explain cytokinesis 1.
1) 1 diploid cell forms 2 haploid cells
wha t is interkinesis?
interkinesis or intramieotic interphase is a gap between meiosis 1 and 2, present between telophase 1 and prophase 2. there is no DNA replication during this phase. RNA and protein required for meiosis 2 are synthesised in this stage, centriole replication occurs in animal cells.
explain prophase 2.
1) condensation of chromosomes
2) nuclear membrane, nucleolus and nuclear envelop disappears
3) initiation of spindle apparatus
explain metaphase 2.
1) one equotorial or metaphasic plate appears
2) chromosomes align themselves on equatorial plate
explain anaphase 2.
1) centromere splits
2) chromosmes moves towards thier respective poles
what is the terminalisation of chiasmata
movement of chiasmata toward the tip of the chromosomes due to condensation
explain telophase 2.
1) disintegration of spindle fibers
2) nuclear membrane , nucleolus and nuclear envelop reappears.
explain cytokinesis 2.
1) cleavage furrows and moves toward the center in animals , and cell plate forms in plant cells
2) forms 4 haploid daughter cells
explain the significance of meiosis.
1) formation of gametes
2) maintenance of chromosome number
3) introduction of variation
distinguish between meiosis and mitosis
study from TB
