Division of cells at the time of growth is the character of life. The cell cycle is essentially divided into two phases.
a) interphase b) division phase
A) Interphase
1. It is known as the non division phase or resting phase.
2. It lasts for more than 95% of the duration of cycle
3.Although it's called a resting phase, but the cell is biochemically very active. During this phase the cell prepares itself for cell division and synthesises various biochemicals including two similar copies of DNA.
4. It is divided in three phases G1 Phase (First gap), the S phase( synthesis) and G2 phase( second gap). in S phase DNA replication occurs
B) Division Phase
1. It is also known as M phase or mitosis phases
2.It lasts for only 5% of the duration of the cycle.
Cell division consists of three steps:
1 Replication of genome.
2.Karyokinesis(division of nucleus )
3.Cytokinesis(Division of cytoplasm)
Three different modes of cell division are amitosis,Mitosis and meiosis.
Amitosis
1. It was first observed by Robert remak( German scientist) in the RBC of chick embryos.
2.Is direct nuclear division, without formation of spindle and recognisable appearance of chromosomes.
3. It is a primitive type of cell division.
4. It occurs in prokaryotes, protozoans, yeast, foetal membranes of mammals and in degenerative and old tissue.
Mitosis
Mitosis was first described by E strasburgerin 1875 in plants and by w Fleming in 1879 in animals.
Mitosis term coined by w Fleming In 1882
It is usually occur in somatic cells hence called somatic division
In plants, it occurs in meristematic tissue,i.e,Shoot and root tip. Root tip is the most preferred material to observe Mitosis.
In this type of cell division two daughter cells are formed. the actual mitotic phase is completed in two steps
1) karyokinesis
It consists of following four stages:
i) Prophase: after interphase, professor comes, which is the first and longest stage of Mitosis. longitudinal splitting or division of each chromosome into two sister chromatids take place .Nuclear membranes break down and nucleolus disappear.A spindle is also formed at late prophase or prometaphase.
ii)Metaphase: due to attachment of spindle fibre at the centromere of chromosomes, the chromosomes are arranged in the centre or equator due to their active movement.
iii) Anaphase: this is the shortest stage. chromosomes divided at the point of centromeres and thus two sister chromatids are formed. these sister chromatids move towards the opposite poles of spindle
iv) Telophase: chromosomes reach opposite poles and nuclear membranes reappear around each group of chromosome,thus forming daughter nuclei.
2) Cytokinesis
Now two nuclei are followed by division of cytoplasm,thus forming two cells( daughter cell) by cell plate or cell furrow method.
Significance of Mitosis: it helps in the growth and development of multicellular organisms, in the healing and reappear of wounds. in maintaining the chromosome number and nucleocytoplasmic ratio.
Meiosis:
Farmer and Moore in 1905 proposed the name meiosis( reduction).
This type of cell division is found only in reproductive mother cells.
The nucleus divides twice but the chromosome divides only once.
The number of chromosomes in the daughter cell is half the number of chromosomes of the mother cell.
The best material to study the meiosis is unopened Flower Buds( onion buds).
A)meiosis-1 or first meiotic Division:
It is very important because it reduces the chromosome number to half. it comprises four substages i.e. prophase-1 ,metaphase-1, anaphase-1, and telophase-1
a)Prophase- 1:it is the longest phase and divided into four steps:-
i) Leptotene:Chromosomes appear as thin, uncoiled thread like structure. half chromosomes are found male and other half are from female parents.
ii)Zygotene:Pairing of homologous chromosomes (called synapsis) takes place. This condition of the chromosome shows bivalent.
iii)Pachytene: chromosome split and show tetravalent stage And crossing over take place here between non sister chromatid
iv)Diplotene: separation starts from centromere but it is not completed, but the homologous chromosomes remain attached at one or more points and this point is called chiasmata.
v)Diakinesis: terminalisation is almost complete. both nucleolus and nuclear membrane completely disappear
b)Metaphase-1 :chromosome arranged at equator and attached to the opposite pole
c)Anaphase-1: nuclear and type chromosome with its two chromatid move to the opposite pole
d)Telophase-1: nuclear membrane appears around the chromosome at each pole of the cell. the two daughter nuclei with the half the nuclear of chromosomes are formed .
B)meiosis-2 or Second meiotic Division:
After the completion of meiosis-1 second meiotic division start meiosis-2 is similar to mitosis.
a)Prophase-2:Chromosome become thick and short, nuclear membrane disappear
b)Metaphase-2: chromosome get arranged on equator and spindle apparatus is formed
c)Anaphase-2: centromere divided into two chromatids which move to opposite poles
d)Telophase-2: chromosome form a group at the opposite poles .nuclear membrane and nucleolus reappear.thus two daughter nuclei are formed
significance of meiosis:meiosis is significantly proven to be the important mechanism in living organisms Because this process brings stability in the number of chromosomes in an organism. It also increases genetic variability in the population of organisms from one generation to next generation. variations are important to the process of evolution news is at as a source of new genetic variation
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