2. There are two types of cell divisions: sisoieM dna sisotiM
3. Mitosis
This type of cell division occurs in somatic cells. The mitotic cell division is
a process in which one cell divides into two daughter cells that are
genetically identical to the parent cell. Each daughter cell receives the
complete complement of 46 chromosomes. The period between the two
mitotic division is called Interphase.
4. The various stages of mitosis are as follows:
Prophase: In this stage, nucleolus disappears.The chromosomes become
coiled. They condense, shorten and thicken. Each chromosomes now
consists of two parallel subunits called chromatids, denioj sniamer hcihw
dellac noiger nommoc worran a ta rehto hcae otcentromere. ,revewoH
egats siht ni desingocer eb tonnac sditamorhc eht
.
6. Metaphase: In this stage , nuclear membrane breaks The double structure
chromosomes line up in the equatorial plane of the spindle and get
attached to the microtubules of the spindle extending between two
centrioles, one at each pole.
13. Clinical Correlation
Significance of mitosis
1. Genetic stability: sllec lacitnedi fo noisseccus suounitnoc serusne tI
snoitareneg hguorht
.
2. Growth and development: It helps in growth and development of the
body.
3. Regeneration, replacement, and repair: It helps in regeneration of new
cells to replace the dead or damaged cells.
14. Meiosis
The meiosis is a special type of cell division that takes place only in the
germ cells to produce male and female gametes .The meiosis consists of
two cell divisions that take place one after the other.
1. First meiotic division (meiosis 1 or reductional division):In this division, the
number of chromosomes of the daughter cells is reduced to half of the
mother cell.
2. Second meiotic division (meiosis II):It is the mitotic division similar to the
one described above except that there is no duplication of DNA during
short interphases.
15. First Meiotic Division
Prophase:1 Prophase of the first Meiotic Division is very long and complicated. It is
therefore subdivided into five stages.
Leptotene: In this stage, the chromosomes, as in mitosis, appear as slender
threads. Although each chromosomes consists of two chromatids that are joined
at centromere, the chromatids are not visible at this stage.
Zygotene:In this stage, the lengthwise pairing of homologous chromosomes is
from the father and the other is from the mother. This event is called synapsis
and each synapsing pair is called bivalent.
16. Pachytene:This stage is very long and may extend even for years. It is characterized by
following changes: The chromatids of each chromosome become visible separately. Each
bivalent chromosomes this appears to have four chromatids and is called tetrad. Each
chromatid pair is United by a .erohcotenik
owt ehT .sditamorhc lartnec owt era erehT
.stniop fo rebmun a ta ssorc yeht taht os rehto hcae revo lioc dartet fo sditamorhc lartnec
dellac sI sihTcrossing over.Because of crossing over, the central chromatids present a
cross-like configuration called chiasmata.
Diplotene: ehT .gnitarapes strats dartet fo suogolomoh deriap eht ,ssecorp eht gniruD
etisoppo eht ot detinU dna revo gnissorc fo tniop eht ta kaerb sditamorhc lartnec
sditamorhc eseht neewteb lairetam citeneg fo egnahcxe ni stluser sihT .sditamorhc
17. Diakinesis: The chromosomes become more contracted and migrate
towards the nuclear membrane. At the end of prophase, the nuclear
membrane disappears.
18.
19. Metaphase 1
: The homologous pairs of chromosomes become arranged
on the equatorial plane of the spindle.
Anaphase 1
: In this stage, the homologous chromosomes migrate to the
opposite poles of the spindle, Unlike mitosis, the chromosomes move
randomly. The shorter chromosomes move earlier than the longer
chromosomes.
Telophase 1
: In this stage, the nuclear membrane is formed around the
polarized group of chromosomes. The cell membrane constricts and two
daughter cells are formed. Each daughter cell this formed contains only
half the number of chromosomes with exchanged genetic material.
20.
21. Second Meiotic Division
The second Meiotic Division is essential similar to mitosis. It, however,
differs from mitosis in that the DNA does not duplicate. In second Meiotic
Division, the two daughter cells of first Meiotic Division form four
daughters cells, each with haploid number of chromosomes.
22.
23.
24. Clinical Correlation
Significance of Meiosis
1. Maintenance of normal chromosomal number: As the chromosome number is reduced
to half during meiosis, each germ cell has haploid number of chromosomes. When two
germ cells United to form a Zygote, the chromosomes number is restored to normal.
Thus, because of Meiosis, the chromosome number is maintained for the species.
2. Genetic variation: Because of random assortment of paternal and maternal
chromosomes, and exchange of genetic material during crossing over in the meiosis, the
daughter cells have a new genetic configuration. This causes individual variation within
the species, which is essential for evolution.
3. Hybrid vigor: Meiosis helps to maintain vigor in progeny through sexual reproduction.
25. Distinguishing features between
mitosis and meiosis
Mitosis
Takes place in somatic cells.
Completes in one sequence.
Crossing over of chromatids does not take place.
Daughter cells have the same number of
chromosomes as parent cells.
Daughter cells are identical to each other and to
the parent cell.
Equational division.
Meiosis
Takes place in germ cells.
Completes in two sequences, ie, there are two
successive division, namely, meiosis 1 and
Meiosis 2
.
Crossing over of chromatids takes place.
Daughter cells have half the number of
chromosomes as parent cells.
Daughter cells are not identical to each other
and to the parent cell.
Reductional division.
