CELL CYCLE AND CELL DIVISION
PHASES OF
CELL CYCLE AND ITS SIGNIFICANCE
CELL CYCLE
AND ITS PHASES
Cell
Division
Cell division is the process where a mature cell divides
into two nearly equal daughter cells that share most characteristics with the parental
cell. In multicellular organisms, cell division is the development of new
individuals from a single cell while in unicellular organisms, it is the
division of a cell (parent cell) into two or more new cells (daughter cells.)
Cell division is necessary for the survival of a species.
Important
Rudolf Virchow proposed that new cells are formed by the
division of pre-existing cells (Omnis-cellula-e-cellula).
Cell Cycle
The cell cycle is the orderly and sequential changes by
which a cell duplicates its genome, synthesises other constituents, grows and
divides into two daughter cells. All these events occur in a coordinated manner
and are genetically controlled.
Phases of Cell Cycle
The time interval between two cell cycles is called
generation time. Generation time varies from a few minutes to a few days
depending upon the type of cell and its environmental conditions. For example,
Yeast cells divide once in 90 minutes. Human cells divide once approximately
every 24 hours. Bacterial cell divides in 20 minutes.
Cell cycle consists of two basic phases, stages or periods.
There is a long non-dividing growing, I phase, Le. Interphase and a short
dividing phase, M phase, also called as Mitosis phase.
Important
In all living organisms cell division Is a very Important
and necessary process. DNA replication and cell growth occur simultaneously
during cell division.
Interphase
It is the phase between two M phases. It is a series of
changes that takes place in a newly formed cell and its nucleus before it becomes
capable to divide again. The interphase lasts more than 95% of the duration of
cell cycle. Earlier it was known as the resting stage because there is no
apparent activity related to cell division. Although during this stage, cell
becomes metabolically very active. During this phase, cells prepare for cell
division.
Interphase of a dividing cell has three phasesG1 phase, S
phase and G2 phase.
The interphase lasts for more than 95% of the total cell
cycle span (around 23 hours) whereas about 5% of the total span (around 1 hour)
is required for the cell division, i.e. M-phase. The table below depicts the
different phases of cell cycle and their average time duration for the better
understanding.
G1 phase
Also known as Post mitotic, Pre-DNA synthetic phase or gap
I. It is the longest phase of interphase. During this phase, cell size
increases and cell synthesises all required elements including rRNA mRNA,
ribosomes and proteins. Carbohydrates, proteins and lipids are also synthesised
in this phase.
This phase also synthesise amino acids, enzymes,
nucleotides, and other compounds, however, there is no change in the DNA
amount. Due to synthesis of all these components, the cell is metabolically
active and grows continuously.
Nucleus, however, grows only to a small extent RNA and
proteins are synthesised. A large number of nucleotides, amino acids for
histone synthesis and energy-rich compounds are formed. Cell organelles also
increase in number.
When the cell Is in G1 phase, it has 3 options,
and they are:
(1) To continue cell cycle and move to S phase of the cell
cycle.
(2) Get arrested in G1 phase cycle and then,
enter Go phase for undergoing differentiation.
(3) Get arrested in G₁ phase when it may enter Go
phase or re-enter cell cycle.
The factors that decide the above three situations are
availability of mitogens and storage of energy rich compounds at the deciding
point called checkpoint.
S phase
Also known as Synthetic phase. In this phase, DNA amount
becomes double due to DNA replication. Along with DNA synthesis of histone
proteins and NHC (non-histone chromosomal proteins) takes place.
Each chromosome bears two chromatids and euchromatin replicates
earlier than heterochromatin. Centriole duplication also occurs in this phase.
In this phase, the DNA content doubles, i.e. 1C to 2C for haploid cells and 2C
to 4C for diploid cells. But the number of chromosomes remains same.
It is also called the invisible phase of M phase. Since it
is in this phase that the chromosomes prepare themselves for equal distribution
later on. Subunits of kinetochores are synthesised.
G2 phase
Also known as Pre-mitotic, Post synthetic phase, or gap-II.
In this phase, the synthesis of DNA stops.
The formation of RNAs and proteins continues. They are
required for duplication of cell organelles (like mitochondria, plastid, etc.),
spindle formation and cell growth.
It prepares the cell to undergo division. The synthesis of tubulin
protein occuring in this phase and the damaged DNA Is also repaired in this
phase.
Important
Go phase (Quiescent stage): Go phase
is the stage of inactivation of cell cycle due to non-availability of mitogens
and energy-rich compounds. After the cell has finished division it enters this
phase When an organism needs to grow, cells exit the Go phase and
enter mitosis.
Cell undergoes differentiation to perform a particular
function. Cell in this stage remain metabolically active but no longer proliferate
unless called on to do so depending upon the requirement of the organisms.
MITOSIS
M PHASE
Mitotic phase is also known as the dividing phase/ M phase.
It is also known as equational division, somatic cell division, or indirect
cell division. In this case, mature cells multiply in a way that the number of
chromosomes in daughter cells remains the same as in the parent cell Thus, it
is known as equational division. The most typical mechanism of cell division is
mitosis. M phase represents the phase of actual division. Prior to it (in
Interphase), the cell components have undergone replication. Therefore, the M
phase is a stage of separation of already duplicated components. Mitosis takes
place in somatic cells Mitosis occurs in meristematic cells in plants, such as
the root apex and shoot apex.
Mitosis is further divided into two stages Karyokinesis and
Cytokinesis.
Karyokinesis
Karyokinesis is simply the division of the nucleus.
Prophase, Metaphase, Anaphase and Telophase are four different phases of
karyokinesis.
Prophase
(Longest phase of karyokinesis)
(1) In the early prophase, condensation of chromosomal
material starts and chromosomal material becomes untangled during this process.
Chromatin fibres get thicker and shorter, forming chromosomes that may overlap
and resemble a ball of wool.
(2) Each chromosome contains two chromatids that remain
connected to the centromere.
(3) In the late prophase, the nuclear membrane along with
nucleolus begins to dissolve.
(4) Duplicated centrosomes begin to move towards opposite
poles of the cell.
(5) Both the centriole pair and centrosome radiate out fine
microtubular fibrils called astral rays.
(6) Each group of astral rays along with its centriole pair
is called an aster.
Metaphase
(1) Begins with the complete disappearance of the nuclear
membrane.
(2) No differentiation between cytoplasm and nucleoplasm.
(3) In this phase, condensation of a chromosome is
completed. Chromosomes are composed of two sister chromatids connected by kinetochores.
(Kinetochores are little disc-shaped structures on the surface of centromeres.)
(4) Each chromosome is attached to both the spindle poles by
distinct chromosome fibres, one for each chromatid.
(5) Kinetochores are the sites of attachment of spindle
fibres to the chromosome. Chromosomal fibres (spindle fibres) tighten. This
tightening brings the chromosomes on the equator of the spindle. The phenomenon
of bringing the chromosomes on the equator of the spindle is called
congression.
(6) Metaphasic plate or equatorial plate (the plane on which
chromosomes align themselves during metaphase) is formed at the centre.
(7) It is the best stage to count the number and study the
chromosome morphology.
Important
Colchicine, which Inhibits the assembly of microtubules and
stops cell division during metaphase, is widely used in plant breeding for
doubling the number of chromosomes. This is called polyploidy.
Anaphase
(1) During anaphase, the centromere of each chromosome
divides into two so that each chromatid comes to have its own centromere.
(2) Due to a repulsive force known as anaphasic movement,
both chromatids travel towards opposite poles.
(3) As the result, the anaphase chromosomes appear in
different shapes like V, L, J and I. The shapes are formed respectively in
metacentric, submetacentric, acrocentric and telocentric.
(4) Microtubule contraction is involved in the chromosomes'
formation by separating chromatids. Each separated chromatid is known as the
daughter chromosome.
(5) Centromere of the daughter chromosome faces towards
their poles and their arms trailing behind.
(6) Anaphase is the best phase to study the shapes of
chromosomes.
Telophase
(1) It is the reverse of prophase.
(2) The mitotic spindle disappears; the chromosomes arrive
at the poles of the cell and vesicles containing fragments of the earlier
nuclear membrane gather around the two sets of chromosomes.
(3) New nuclear membrane is formed around each chromosomal
group.
(4) Chromosomes decondensed and lost their identity.
(5) Two daughter nuclei are formed.
(6) Nucleolus, Endoplasmic reticulum and Golgi complex are
reappeared.
Cytokinesis
(1) It is the division of protoplast of a cell into two
daughter cells after the nuclear division or karyokinesis, so that each
daughter cell can have its own nucleus.
(2) Cell organelles (mitochondria, plastids, Golgi bodies,
lysosomes, endoplasmic reticulum, ribosomes) are distributed between the two
daughter cells.
(3) Mitochondria, plastids undergo division by cleavage or
fission mode.
(4) In some organisms, karyokinesis is not followed by
cytokinesis due to which a multinucleated condition arises called coenocyte or
syncytium. E.g. Liquid endosperm in coconut.
(5) Cytokinesis is different in animal and plant cells.
Difference between
plant and animal cytokinesis:
|
PLANT CYTOKINESIS |
ANIMAL CYTOKINESIS |
|
If usually occurs by the cell plate method. |
It takes place by cell furrow method. |
|
Central part of spindle grows in size and forms an interdigitated
complex called phragmoplast. |
A mid body of dense fibrous and vesicular material is formed in the
middle. |
|
Cell plate grows centrifugally. |
The cleavage or furrow formation progresses centripetally. |
|
The new cell membrane is derived from vesicle of Golgi apparatus. |
The new cell membrane is usually derived from endoplasmic reticulum. |
(1) It is essential for growth and development of
multicellular organisms. For example, all organisms developed from a zygote
which is a single cell. The zygote forms a multicellular organism by undergoing
repeated mitosis. Plants are able to grow throughout their life due to mitotic
divisions in their apical and lateral meristem.
(2) Mitosis usually results in the production of diploid
daughter cells with identical genetic complements.
(3) It generates new cells to aid in the repair and
regeneration of damaged body parts as well as wound healing.
(4) Asexual reproduction such as fragmentation, budding and
stem cutting is done by mitosis.
(5) Somatic variations can play a major role in speciation
when they are maintained by vegetative propagation.
(6) Plants develop continuously due to meristematic division
(mitotic) in the apical and lateral cambium.
(7) Mitosis is essential to maintain nucleocytoplasmic
ratio.
(8) The cells of the upper layer of the epidermis, cells of
the lining of the gut, and blood cells are being constantly replaced by
mitosis.
Important
There are so many significances of mitosis but there is also
a drawback or hazardous effect of uncontrolled mitosis Le Cancer. As we all
know uncontrolled mitotic division can lead to cancer.
MEIOSIS
The term melodies was coined by Farmer and Moore in the year
1905.
Meiosis is slower than mitosis. In meiosis, a mature diploid
reproductive cell undergoes a double division in which the nucleus divides
twice but the chromosome (DNA) replicates only once, resulting in four haploid
cells with half the number of chromosomes as the parent cell It is also called
reductional division because it reduces the number of chromosomes. It has been
found in diploid germ cells (reproductive cells) of sex organs (e.g.
Spermatozoa and ova in animals).
Interphase occurs prior to meiosis. It is generally similar
to interphase of mitosis except that S phase is prolonged. DNA replication
occurs during S phase. A distinct G2 phase is either short or absent. At this time,
each chromosome comes to have two chromatids. Chromosome replication occurs
once but meiosis has two M-phases each with its own Karyokinesis and
Cytokinesis. As a result, chromosome number is halved. The transition period
between M phase I (meiosis I) and M phase II (meiosis II) Is short without DNA
replication. It is called interkinesis.
Following are the details about the two stages of meiosis:
Meiosis I
In meiosis I, the number of chromosomes is reduced to half.
As a result, it is also known as heterotypic division or reductional division.
It produces two haploid cells from a diploid cell It is the process of nucleus
division. Prophase I, metaphase I, anaphase I, and telophase I are the four
phases of Meiosis I.
Prophase I
It is the longest phase of meiosis I It is further divided
into five sub-phases.
|
Subphase of prophase l |
Events taking place |
|
Leptotene |
Chromosomal compacting continues and the chromosomes become gradually
visible under the light microscope. |
|
Zygotene |
Pairing of homologous chromosomes called synapsis. Synapsis is
accompanied by the formation of a complex structure called synaptonemal
complex. Homologous chromosomes are called bivalent or tetrad. The number of
bivalent chromosomes is half the number of the total chromosomes. |
|
Pachytene |
Four chromatids of each bivalent are clearly visible and appear as
tetrad. Formation of recombination nodules at which crossing over occurs.
Le., exchange of genetic material |
|
Diplotene |
Dissolution of synaptonemal complexes. Homologous chromosomes
separate from each other except at the site of crossover. The X-shaped
structure formed is called Chiasmata. In oocytes of some vertebrates,
diplotene can last for months or years. |
|
Diakinesis |
Movement of chiasmata towards the chromosomal end. Chromosomes are
fully condensed. Meiotic spindle is assembled to prepare the homologous
chromosomes for separation. By the end of this phase, nucleolus disappears
and nuclear membrane disintegrates. Diakinesis represents the transition to
metaphase. |
Important:
➡ Synapsis: The steady pairing of
homologous chromosomes is known as synapsis.
➡ Crossing over: The exchange of
genetic material between two homologous chromosomes is called crossing over.
➡ Chiasmata: The chiasmata is a structure that arises between two homologous chromosomes by crossing over. recombination and physical links between them.
Metaphase I
On the equatorial plate of the spindle fibre. homologous
pairs of chromosomes align and form a metaphase plate.
Each chromosome of bivalent gets attached to the spindle
pole of its side by means of a chromosomal fibre or tractile fibril which
arises in the region of the centromere.
Two metaphasic plates are formed.
Anaphase I
The homologous chromosome pairs separate and move to the
spindle's opposite poles. But their sister chromatids remain associated at
their centromere. The process of separation of homologous pairs is called a
disjunction.
The separate chromosomes or univalents are also called dyads
(dyads - two) because each of them consists of two chromatids which lie at an
angle to each other.
Every tetrad has two daughter dyads.
Telophase I
Each daughter cell has the same number of haploid
chromosomes as the mother cell.
Genetic material is redistributed as a result of crossover.
Nuclear membrane and nucleolus reappear.
Like long chromatin fibres. chromosomes decondensed into the
thread.
Interkinesis
or Intermitotic Phase
It is a metabolic stage between telophase of meiosis I and
prophase of meiosis II. Chromosomes are elongated but chromatin reticulum is
not formed. Protein and RNA synthesis may occur. Centrosomes or centriole pairs
undergo replication in animal cells. However, there is no DNA synthesis. It is
important for bringing true haploidy (haploidy of DNA) in the daughter cell.
Meiosis II
It is also known as equational division/ homotypic division.
Meiosis II is similar to mitosis but it is shorter than mitosis. Though the meiosis
Il is similar to mitosis but meiosis II is not mitosis because it always occurs
in haploid cells. It is not preceded by DNA replication. The two chromatids of
the chromosome are often dissimilar. The daughter cell formed after meiosis Il
is neither similar to each other nor similar to parent cell After meiosis I
each daughter cell enters prophase II of meiosis II.
Prophase II
The chromosomes which were decondensed in telophase I are
then re-condensed.
The spindle is fully formed and the nuclear envelope gets
totally broken down.
Metaphase
II
Chromosomes align in the equatorial plane of the spindle to
form a metaphase plate.
The microtubules from opposite poles of the spindle get
attached to the kinetochores of sister chromatids.
Anaphase II
The centromere divides and the chromatids migrate to
opposite poles of the spindle to forming new chromosomes.
At the end of anaphase II, four groups of chromosomes are produced, each group having a haploid number.
Telophase
II
The four groups of chromosomes arrange themselves into
haploid nuclei. For this, chromosomes elongated very much to form chromatin.
A nucleolus is also produced followed by the formation of
nucleoplasm and a nuclear envelope.
The spindle fibres usually degenerate during telophase II.
Significance
of Meiosis
(1) Formation of
gametes: Meiosis forms gametes that are essential for sexual reproduction.
(2) Genetic
information: It switches on the genetic information for the development of
gametes or gametophytes and switches off the sporophytic information.
(3) Maintenance of
chromosome numbers: Meiosis maintains the fixed number of chromosomes in
sexually reproducing organisms by halving the same. It is essential since the
chromosomes number becomes double after fertilisation.
(4) Assortment of
chromosomes: In meiosis, parental and maternal chromosomes assort
independently. It causes reshuffling of chromosomes and the trait controlled by
them. The variation helps the breeder in improving the races of useful plants
and animals.
(5) Crossing over:
It introduces new combination of traits or variation.
(6) Mutations: Chromosomal and genomatic
mutations can take place by irregularities of meiotic division. Some of these
mutations are useful to the organism and are perpetuated by natural selection.
(7) Evidence of
Phylogenetic Relationship: Details of meiosis are essentially similar in
the majority of organisms showing their basic similarity and relationship.
Types of
Meiosis
The cells in which meiosis takes place is called meiocytes.
Depending upon the stage when meiosis occurs, It is of three types:
(1) Gametic meiosis:
Meiosis in most of animal takes place during the formation of gametes, Le.
during gametogenesis that's why termed as gametic meiosis. When two gametes
fuse in fertilisation, a diploid zygote is formed. Gametic meiosis result in
diplontic life cycle.
(2) Zygotic meiosis:
In some lower plants, meiosis takes place in the zygote and the resulting
organisms are haploid. It is called zygotic meiosis. Organism have zygotic
meiosis have haplontic life cycle.
(3) Sporic meiosis:
In plants, meiosis generally occurs at the time of sporogenesis (formation of
spores or microspores and megaspores). It is called sporic or intermediate
meiosis. Spores produce a new gametophytic phase in the life cycle. Gametes are
formed by gametophytes. Because of the presence of two distinct multicellular
phases, diploid and haploid, Life cycle of plants is diplohaplontic.
Important
→Need for meiosis: Meiosis is essential for all sexually
reproducing organisms It occurs in reproductive cells so that the gametes
formed are haploid or have half no. of chromosomes of those cells which are
directly formed from zygote. Meiosis by halving the number of chromosomes
maintains a fixed number of chromosomes of a species.