Meiotic cell division occurs in reproductive cells of plants and animals.Meiotic cell division is also known as “Reduction division” where diploid number of chromosomes are reduced to haploid(number of chromosomes reduced to half).Meiosis consists of two complete division.
- Meiosis I or heterotypic division(no.of.chromosomes reduced to half)
- Meiosis II or homotypic division (mitotic or equational)
During sexual reproduction two gametes(male and female) each having same set of chromosomes(n) fuse to form a zygote.Thus zygote consist of twice the number of chromosomes as gamete(n+n=>2n one derived from male parent and other from female parent).Note: This is the reason why diploids posses two set of identical chromosomes called “Homologous chromosome“
Events in plant cells when mitosis occurs:
At the time of ,
- spore formation.
- gamete formation
- zygote formation
It consist of four stages
Meiosis I by Boumphreyfr
Its a longest phase of meiotic cell division.It includes five sub-stages
- Leptotene: It means “thin thread”.chromosomes uncoil and become large and thin.Each chromosome consist of two chromatids.
- Zygotene: Pairing or synapsis is the process where homologous chromosomes come together lies side by side through their length.Paired chromosomes are called “Bivalent”.Adjacent non-sister chromatids are joined together at some points called “chiasmata”.
- Pachytene: Chromosomes condense further, becomes very shorter and thicker(very distinct).Two sister chromatids of homologous chromosomes clearly visible.Now bivalent become tetrad with four chromatids.Segments of non-sister chromatids are exchanged at egion of chiasmata and the process is called “Crossing over”.
- Diplotene: Homologous chromosomes condense further and begin to separate each other except chiasmata.Due to this, dual nature of bivalent becomes apparent and the name “diplotene”.
- Diakinesis: chromosome contraction continues.separation of chromosomes becomes complete due to “terminalisation”.Nucleolus and nuclear membrane starts disappearing and spindle fibres are formed.
Spindle fibres becomes prominent(bivalent align on equatorial axis).spindle fibres from opposite poles get attached to the centromere of homologous chromosomes.
Two chromosomes of each bivalent(with chromatids attached to the centromere still) separate from each other and move to the opposite poles.Because of this only one chromosome of each homologous pair reaches each pole.Thus only half the number of chromosomes(haploid) received at each pole.However these chromosomes are not same as existed at the beginning of prophase,each chromosome consist of one original chromatid and one with mixture of chromatid segments(as a result of crossing over).
Reorganisation of chromosomes occur.to form two haploid nuclei.Nucleolus reappear and spindle fibres disappear.No cytokinesis.
Second meiotic division is very much similar to mitotic division.
Meiosis II by Boumphreyfr
Nucleolus and nuclear membrane disappears.spindle fibres formed at each pole.
chromosome move to equatorial axis and attached to to spindle fibres through centromere.
sister chromatids separate from one another and pulled to opposite poles because of spindle fibre contraction.
chromosome uncoils and become thin.Reorganise to form nucleus as nucleolus and nuclear membrane reappears.Its followed by cytokinesis as a result four haploid daughter cells are formed,meiotic cell division comes to an end.
Significance of meiotic cell division:
- Helps to maintain chromosome number constant.
- Recombination of genes through crossing over
- Recombination of genes results in genetic variation
- Genetic variation is key to “Evolution” of species.