Cell cycle and cell division
Made By- Vanie Singh
Department of Botany
Himachal Pradesh University
Cell Cycle And Cell Division
·
It is a
universally accepted fact that the cell is the most basic, structural and
functional unit of life. A cell could be an individual organism by itself (such
as an amoeba) or the cells could be a part of a larger organism (such as a
cat). But how does this fundamental unit multiply for growth or repair? And
what happens when the mechanisms that control the division of cells fail?
What is the Cell Cycle?
·
Essentially,
the cell cycle is an orderly sequence of events that occur before the cell
actually divides and gives rise to new cells. This is a very important process
and has varying implications from growth and development to repair of the
organism.
Mitosis
·
This is one
of the two types of cell division where a single cell divides into two
genetically identical daughter cells. In unicellular organisms, this type of
cell division typically contributes to asexual reproduction. In multicellular
organisms, mitosis is helpful for the growth and repair of damaged cells.
Mitosis is also called equational division.
Meiosis
Mitosis
Definition
“Mitosis is that step in the cell
cycle where the newly formed DNA is separated and two new cells are formed with
the same number and kind of chromosomes as the parent nucleus.”
Mitosis is a process
of asexual reproduction observed in unicellular organisms. Read on to explore
what is mitosis, and the different stages of mitosis.
What
is Mitosis?
Cell division is the
driving process of reproduction at the cellular level. Most eukaryotic cells
divide in a manner where the ploidy or the number of chromosomes remains the
same, except in the case of germ cells where the number of chromosomes is
halved.
Mitosis Diagram showing the different stages of mitosis
·
Mitosis is the phase of the cell cycle where the nucleus
of a cell is divided into two nuclei with an equal amount of genetic material
in both the daughter nuclei. It succeeds the G2 phase and is succeeded by
cytoplasmic division after the separation of the nucleus.
·
Mitosis is essential for the growth of the cells and the
replacement of worn-out cells. Abnormalities during mitosis may alter the DNA,
resulting in genetic disorders.
Features
of Mitosis
·
In each cycle of cell division,
two daughter cells are formed from the parent cell.
·
The cell is also known as
equational cell division because the chromosome number in the parent
cell and daughter cell is the same.
·
In plants, mitosis leads to the
growth of vegetative parts of the plant like root tip, stem tip, etc.
·
Segregation and combination do
not occur in this process.
The processes occurring during mitosis have
been divided into different stages.
Stages
of Mitosis
Right before
prophase, the cell spends most of its life in the interphase, where
preparations are made before the beginning of mitosis (the DNA is copied).
However, since the actual process involves the division of the nucleus,
prophase is technically the first stage of this process.
The different stages
of mitosis occurring during cell
division are given as follows-
Interphase
Before entering
mitosis, a cell spends a period of its growth under interphase. It undergoes
the following phases when in interphase:
- G1
Phase: This is the period before the synthesis of
DNA.
- S
Phase: This is the phase during which DNA synthesis
takes place.
- G2
Phase: This is the phase between the end of DNA
synthesis and the beginning of prophase.
Prophase
Prophase immediately follows S and G2 phase of the cycle
and is marked by condensation of the genetic material to form compact mitotic
chromosomes composed of two chromatids attached at the centromere.
The completion of prophase is characterised by the initiation
of the assembly of the mitotic spindle, the microtubules, and the proteinaceous
components of cytoplasm that help in the process.
The nuclear envelope starts disintegrating.
Prophase
Prometaphase
ü In the prometaphase,
the nuclear envelop disintegrates. Now the microtubules are allowed to extend
from the centromere to
the chromosome. The microtubules attach to the kinetochores which allow the
cell to move the chromosome around.
Metaphase
ü At this stage, the
microtubules start pulling the chromosomes with equal force, and the chromosome
ends up in the middle of the cell. This region is known as the metaphase plate.
Thus, each cell gets an entire functioning genome.
Metaphase
Anaphase
The splitting of the sister chromatids marks the onset of
anaphase. These sister chromatids become the chromosome of the daughter nuclei.
The chromosomes are then pulled towards the pole by the fibres attached to the
kinetochores of each chromosome. The centromere of each chromosome leads at the
edge while the arms trail behind it.
Anaphase
Telophase
·
The chromosomes that cluster at the two poles start coalescing into
an undifferentiated mass, as the nuclear envelope starts forming around it. The
nucleolus, Golgi bodies and ER complex, which had disappeared after prophase
start to reappear.
Telophase
Telophase is followed by cytokinesis, which denotes the division of the
cytoplasm to form two daughter cells. Thus, it marks the completion of cell
division.
Functions of Mitosis
Following are the two
important functions of mitosis:
- Mitosis helps in the development of an organism. In
single-celled organisms, mitosis is the process of asexual reproduction.
- Mitosis helps in the replacement of damaged tissues.
The cells near the damaged cells begin mitosis when they do not sense the
neighbouring cells. The dividing cells reach each other and cover the
damaged cells.
Significance of Mitosis
- Mitosis is responsible for the development of the
zygote into an adult.
- The chromosomes are distributed equally to the
daughter cells after each cycle.
- It is responsible for a definite shape, and proper
growth and development of an individual.
- It maintains the constant number of chromosomes in
all body cells of an organism.
- In plants, mitosis helps in the formation
of new parts and the repairing of damaged parts. Mitosis helps in
vegetative propagation of crops also.
- Since no recombination and segregation occurs in the
process, it helps in maintaining the purity of types.
- It helps in maintaining a balance between the DNA and
RNA contents as well the nuclear and cytoplasmic contents of the cell.
- It is responsible for replacing dead and old cells in
the animals: Eg., gut epithelium, and blood cells.
Meiosis I : Reductional Cell Division
·
Sexual reproduction in organisms takes place
through the fusion of male and female gametes, the sperm and the egg
respectively. Gametes are haploid in nature, i.e., they contain only half the
number of chromosomes. This genetic content makes them different from
other body cells.
·
Meiosis leads to the formation of haploid
cells.
Let us have a detailed look at meiosis 1 and the different stages
and phases of meiosis 1.
Meiosis 1
ü
Mitotic cell division is equational in nature
while meiosis is a reduction division. The salient features of meiotic
division that make it different from mitosis are
as follows:-
ü
It occurs in two stages of the nuclear and
cellular division as Meiosis I and Meiosis II. DNA replication occurs, however,
only once.
ü
It involves the pairing of homologous
chromosomes and recombination between them.
ü
Four haploid daughter cells are produced at
the end, unlike two diploid daughter cells in mitosis.
ü
Meiosis 1 separates the pair of homologous
chromosomes and reduces the diploid cell to haploid. It is divided into several
stages that include, prophase, metaphase, anaphase and telophase.
Meiosis 1 Stages
The different stages of meiosis 1 can be explained by the
following phases :
ü
Prophase 1
ü
Metaphase 1
ü
Anaphase 1
ü
Telophase
Phases of Meiosis 1
Meiosis 1 Prophase 1
Prophase I is longer than the mitotic prophase and is further
subdivided into 5 substages,
1.
Leptotene
2.
Zygotene
3.
Pachytene
4.
Diplotene
5.
diakinesis
Ø
The chromosomes begin to condense and attain a
compact structure during leptotene.
Ø
In zygotene, the pairing of homologous
chromosomes starts a process known as chromosomal synapsis, accompanied by the
formation of a complex structure called synaptonemal complex. A pair of
synapsed homologous chromosome forms a complex known as bivalent or tetrad.
Ø
At pachytene stage, crossing over of
non-sister chromatids of homologous chromosomes occurs at the recombination
nodules. The chromosomes remain linked at the sites of crossing over.
Ø
Diplotene marks the dissolution of the
synaptonemal complex and separation of the homologous chromosomes of the
bivalents except at the sites of cross-over. The X-shaped structures formed
during separation are known as chiasmata.
Ø
Diakinesis is marked by the termination of
chiasmata and assembly of the meiotic spindle to separate the homologous
chromosomes. The nucleolus disappears and the nuclear envelope breaks down.
Meiosis 1 Metaphase 1
The bivalents align at the equatorial plate and microtubules from
the opposite poles attach to the pairs of homologous chromosomes.
Meiosis 1 Anaphase
1
The two chromosomes of each bivalent separate and move to the
opposite ends of the cells. The sister chromatids are attached to each other.
Meiosis 1
Telophase 1
The nuclear
membrane reappears and is followed by cytokinesis. This gives rise to
a dyad of cells.
In the previous article Meiosis I, we have seen phase 1 of the meiotic
cell division. Meiosis II also comprises of the four stages and are relatively
simple as compared to Meiosis I. Meiosis II relates the mitotic cell division.
Stages of Meiosis II
The four stages of meiosis II are as follows:-
- Prophase II – It
immediately sets off after the cytokinesis when the daughter cells are
formed. The chromosomes begin to condense accompanied by the dissolution
of the nuclear membrane and the disappearance of the Golgi
apparatus and ER complex.
- Metaphase II – The
chromosomes are connected to the centriole poles at the kinetochores of
sister chromatids through the microtubules. They also get aligned at the
equator to form the metaphase plate.
- Anaphase II – In this
phase of meiosis II there is a simultaneous splitting of the
centromere of each chromosome and the sister chromatids are pulled away
towards the opposite poles. As the chromatids move towards the poles, the
kinetochore is at the leading edge with the chromosomal arms trailing.
- Telophase II – The
chromosomes dissolve again into an undifferentiated lump and a nuclear
envelope develops around it. Followed by cytokinesis, telophase II marks
the end of meiosis. Four haploid daughter cells are formed as a result.
Significance of Meiosis
Ø Reproduction in animals
takes place through the fusion of gametes i.e. two cells fuse together with
their genetic material to develop a zygote. If germ cells, which give rise to
gametes, also maintains their ploidy during division like the somatic cells,
the zygote will have an accumulation of chromosomes in its nucleus. This
accumulation will keep on increasing with every subsequent generation. Meiosis
offers a very smart solution to this problem as it reduces the number of
chromosomes in the gametes to half of their parent germ cells. Moreover,
prophase I of meiosis allows recombination of homologous chromosomes.
Ø This recombination is
essential for the variation to be introduced in the genetic makeup of the
gametes as this variation only holds the key to evolution through sexual
reproduction.
Refrences:
Willis X.Li
Springer
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http.cell
division.com
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