Cell Cycle
Cell
is the structural and functional unit of life. New cells arise from the
pre-existing ones.
The
process by which new cells are formed from the pre existing cells is called cell division.
In
unicellular organisms, the cell division directly produces two individuals and
thus represents a type of reproduction (multiplication) commonly called binary fission.
In
multicellular organisms, there are two types of cells; the somatic cells or the body cells (which form the body of the
organism) and the reproductive cells
(such as gamete producing cells and spore producing cells).
The
somatic cells divide by mitosis
(equational division) and the reproductive cells divide by meiosis (reduction division). Mitosis helps in growth and
development of an organism. Meiosis produces gametes in sexual reproduction and spores in asexual reproduction.
All
eukaryotic organisms, plants as well as animals, show great regularity as well
as similarly in the cell divisions. Generally
cell increases in size before dividing. This is mainly due to the
synthesis of proteins, RNA and DNA. This is followed by division of the cell
nucleus (karyokinesis) and finally
the division of the cell cytoplasm (cytokinesis).
All these events collectively form a cell
cycle.
The Cell cycle
“Is
the sequence of events in the life of a cell which starts immediately after one
cell division and ends with the completion of the next division is known as the
cell cycle, also referred as generation time.”
The
cell cycle of eukaryotic cells is classified into (1) Interphase (2) Karyokinesis
and (3) Cytokinesis (Fig. 4.1).
1) Interphase:
It is the preparatory phase during which cell is metabolically very active and
prepares itself for the division.
Three important processes occur in interphase, viz
a) replaction of chromosomal DNA, synthesis of RNA and the basic nuclear
proteins histones, b) synthesis of energy rich compounds which provide energy
for mitosis and c) in animal cells, division of the centriole.
On the basis of DNA synthesis, interphase is
sub-divided into following three stages –
i)
G1
(Gap 1): It starts immediately after the previous division
.therefore G1 is called post-division gap phase or first growth phase. synthesis
of proteins and RNA takes place. The cell grows in volume.
ii)
S
Phase (Synthesis Phase): It is the period during which DNA
synthesis occurs, that is, replication of chromosomal DNA takes place. This
results in doubling of the chromosomal threads.
iii)
G2 (Gap 2):
It is the last part of interphase and occurs just before the new cell division.
Hence G2 is called Pre-division gap phase or second growth phase. It begins
after completion of DNA synthesis in S phase and ends when new division
(karyokinesis) commences. During G2 synthesis of proteins and RNA takes place
and the nuclear volume increases.
2) Karyokinesis:
It is the division of the parent nucleus into daughter nuclei .
3) Cytokinesis:
This is the division of the cytoplasm. It occurs after karyokinesis and divides
the parent cell into daughter cells.
Karyokinesis and
cytokinesis together form the M phase, that is, cell division.
The
total duration of a cell cycle
varies greatly in different organisms and under different conditions, example,
it may be as short as 20 – 30 minutes in the bacterium Escherichia coli or may take 1 – 24 hours as in most
higher plants and animals.
The
time required for completion of each phase in the cell cycle varies greatly. In
general, actual cell division (M-Phase) occupies only a short span of the total
cycle while major span is occupied by the interphase. Normally, time duration
of S and G2 phases are more or less
equal. The duration of G1 is longer in cells which do not divide frequently and
is very short in cells which divide repeatedly in close succession.
G0
stage: It is a stage during which cell
cycle is arrested for an indefinite period.
Significance
of cell cycle
1) In
multicellular organisms, the cycling type of cells (dividing cells) help in
reproduction, growth and replacement of dead cells, healing of wounds etc.
2) The
interphase allows time for synthesis and growth of dividing cell
3) Properly
controlled and regulated cell cycle results in normal and proportionate growth
of organism
4) Loss
of control over the cell cycle can lead to cancerous growth
Cancerous
cells: For some reasons, in some cells, if the control
over the cell cycle is lost, then they start behaving abnormally. These cells
divide repeatedly in an uncontrolled manner at abnormally high rates. As a
result, they do not get enough time for growth and differentiation. Such cells
mass together and form tumors in the body which may lead to cancer.
The
cancerous or malignant cells are those which show continued and uncontrolled
growth through repeated cell divisions at abnormally high rates.
In the cancerous tissue, the
metabolism of cells is disturbed and abnormal. The cells continue to move and
crawl (grow) over one another, that is,
they do not show contact inhibition. The cancerous growth may remain
localized or may spread to other parts of the body.
Mitosis
Mitosis
is the characteristics division of the body cells, hence called somatic division. It can be studied in
the meristematic cells in root and stem tips of plants.
“Mitosis
is an equational division, dividing
the mother cell into two daughter cells which are identified to one another and
also to the original mother cell in every respect. In mitosis, the chromosome
of the mother cell are duplicated and distributed equally to the two daughter
cells”
The
mitotic cell cycle consists of interphase and M-hase. Interphase is sub-divided into G1, S and G2 phases. The
M-phase consists of karyokinesis
followed by cytokinesis.
Interphase
nucleus: The nucleus increases in volume during interphase.
At this stage, the nuclear membrane and nucleolus are prominently visible. The
chromosomes appear to form a continuous network (nuclear reticulum or chromatin
network) of very fine threads. DNA replication has taken place (S-Phase)
and chromosome have doubled. Interphase ends as karyokinesis begins.
Karyokinesis:
It involves a series of changes in the nucleus which are visible under the
microscope. This is a continuous process but, for convenience, it has been
divided into four phases. These are,
i) Prophase, ii)
Metaphase iii) Anaphase and iv) Telophase (Fig. 4.2).
The main
features of the nuclear division during each phase are summarized below.
i)
Prophase:
During early prophase, the chromatin network becomes visible as separate
threads or chromosomes. At this
stage, each chromosome appears as a very fine, long single thread, the chromonema and is described as the monad. The nuclear envelope and nucleolus
are prominently visible.
As
the prophase progresses, chromosomes become shorter and thicker (due to
condensing of their coils). In each chromosome, the chromonema splits
lengthwise into two identical threads or chromonemata
(dyads). These are coiled round one another. A substance called nuclear matrix accumulates around each
chromonema surrounded by the matrix is called a chromatid. At this stage, each chromosome is shorter, thicker and
consists of two identical sister
chromatids joined together by a spherical body called centromere (kinetochore).
By
the end of prophase, nuclear envelope and nucleolus disappear completely. The
chromosomes remain distributed in the nucleoplasm.
ii)
Metaphase:
Metaphase begins with the formation of a bipolar spindle body in the region of
nucleoplasm, that is, centre of the cell. It consists of numerous spindle fibers. These are fine thread
like structures formed by the organization of proteins called tubulin into microtubules. There are two types of fibers in the spindle; (a) Continuous fibers which extend from pole to pole and (b) Chromosomal fibers which extend from pole to the centre (equator of
the spindle).
The
chromosomes move and get arranged in a plane along the equator of the spindle
in such a way that in each chromosome, the two chromatids are facing the
opposite poles. This results in the formation of equatorial plate (metaphasic
plate). The centomere of each chromosome in the plate is connected with both the poles by the
chromosomal fibers.
iii)
Anaphase:
During early anaphase, the centromere
of each chromosome divide longitudinally
into two. As a result, each chromosome is now completely divided into two identical halves (sister chromatids) called daughter chromosomes. The centromere of
each daughter chromosome remains connected to the pole on its respective side
by a chromosomal fiber.
During
late anaphase, the two groups of daughter chromosomes are pulled away from each
other and start moving towards the opposite poles. Most probably, this movement
is caused by the shortening of the chromosomal fibers. In each group,
chromosomes appear “V” or “L” shaped as the centromeres are
pulled towards the poles and the arms trail behind. During late anaphase, in
both the groups, each chromosome has one chromatid and one centromere. The
chromosomes start becoming longer and thinner as they move towards the poles.
iv)
Telophase:
This is the last phase in karyokinesis.
The two sets of daughter chromosomes reach the opposite poles. The chromosomal
matrix disappears. The chromosomes again become long and thin. A new nucleolus
is organized. Nuclear envelope is formed around each set of chromosomes. In
other words, a daughter nucleus is organized at each pole in the parent cell.
Each daughter nucleus has the same number of chromosomes as that of the mother
cell. The original structure of each chromosome is also retained, unchanged in
both the daughter nuclei. The spindle fibers dissolve and disappear gradually.
The two daughter
nuclei are identical in
structure and characters. They are also exact
copies of the original parent nucleus.
Cytokinesis:
The division of the cell cytoplasm is called cytokinesis. It starts towards the
end of telophase.
In
plant cells, cytokinesis usually begins with centrifugal formation of cell plate along the equatorial plane
and is followed by new wall formation. This divides the mother cell into two
equal daughter cells.
In
animal cells, cytokinesis takes place by the cleavage constriction of the cell cytoplasm. It begins peripherally
and progresses centripetally.
Astral
and anastral mitosis: In the cells of higher animals and
lower plants, centriole is present
just outside the nucleus. It plays definite role in mitosis. During interphase,
centriole divides into two. The two daughter centrioles then move to the bipolar spindle body.
From
each centriole at the pole, radiating fibers extend into the cytoplasm. These
are called astral rays and form the aster (fig. 4.3).
Astral mitosis:
“The mitosis in which asters are formed from the centrioles is called astral mitosis”
Anastral
mitosis: The centriole is absent in the cells of the higher
plants and some animals. In such cells, the astral rays and the asters are not
formed at the poles of the spindle body during the metaphase. “The mitosis in
which asters are not formed at the poles of the spindle body is called anastral
mitosis.” It is common in the higher plants.
Mitotic
apparatus: It is called achromatic
figure and is jointly formed by the centrioles, asters and the spindle
body.
Significance
of Mitosis
i)
It is an equational division which
maintains equal distribution of chromosomes after each cell cycle.
ii)
The resulting daughter cells inherit
identical chromosomal material (hereditary material) both in quantity, that is,
number and quality, that is, genetic makeup or characters.
iii)
Mitosis maintains constant number of
chromosomes in all body cells of an organism
iv)
It helps to maintain the equilibrium in
the amount of DNA and RNA contents of a cell as well as the nuclear and
cytoplasmic balance in the cell
v)
Dead cells are replaced by newly formed
cells through mitosis. It thus helps in the repair of the body
vi)
It helps asexual reproduction, growth
and development of organism.
Meiosis
Introduction: In
the sexually reproducing organisms, two important phenomena regulate the number
of chromosomes in the life cycle. These
are meiosis and fertilization. Meiosis is the reduction division in which the
diploid (2n) number of chromosomes is reduced to haploid (n) during gamete
formation or spore formation. Whereas, in fertilization, the two haploid
gametes fuse to form diploid zygote. In this way, the diploid condition is
restored again in the life cycle.
Definition:
“Meiosis
is a special type of division characteristic of reproductive cells only in which,
the diploid number of chromosomes is reduced to haploid I the daughter cells,
In meiosis, chromosomes divide once while the nucleus ( and in some cases the
cytoplasm also) divides twice. Four haploid daughter cells result from one
diploid mother cell. These differ from each other as well as from the mother
cell”.
Homologous
chromosomes: Sexually reproducing diploid organism
develops from a diploid zygote (2n). The zygote is formed when a haploid (n)
male and a haploid (n) female gametes fuse at the time of fertilization. Thus,
the diploid individual receives two sets of chromosomes: one through the male
gamete (paternal set) and the other through the female gamete (maternal set).
For every chromosome in the paternal set, there is a similar looking chromosome
present I the maternal set. Such similar chromosomes from paternal and maternal
sets have identical gene loci and are called homologous chromosomes. In this
way, every chromosome in the paternal set has its homologue in the maternal set
and vice versa.
The
meiotic cell cycle: It consists of the interphase,
karyokinesis and cytokinesis.
Interphase:
It
consists of G1, S and G2 phases and involves changes as described earlier in
The
interphase nucleus: The nucleus enlarges during interphase.
The chromosomes are not clearly visible being very thin and long threads.
However, nuclear envelope and nucleolus are prominent.
Karyokinesis
in meiosis: It
consists of two complete nuclear
divisions: Meiosis- I and Meiosis- II. The time interval between M-I and M-II
is called interkinesis.
Dia
Pg 65
Meiosis-I
( M-I, First meiotic division) (Fig. 4.4-A)
i) The karyokinesis of meiosis-Iinvolves
division of the diploid (2n) nucleus of the mother cell.
ii) The
diploid nucleus divides to form two daughter nuclei.
iii) Each
daughter nucleus is haploid (n) I,e, it receives only one set of chromosomes
from the mother nucleus.
iv) Reduction
in the number (sets) of chromosomes occurs due to the separation of the
homologous chromosomes during the nuclear division of M-I.
v) As the
original diploid (2n) number of chromosomes (present in the mother nucleus) is
reduced to haploid (n) in each daughter nucleus, Meiosis-I is described as the
reduction division.
vi) Chromosomes
do not divide in M-I.
vii) Segregation of alleles take place due to the
separation of homologous chromosomes during Meiosis-I.
The
various events in M-I are classified into Prophase-I, Metaphase-I, Anaphase-I
and Telophase-I. The important features in each phase are as follow:
Prophase-I:
This
is the longest phase in meiosis and involves some very important events.
Prophase-I is sub-divided into five sub-phases a) Leptotene b) Zygotene c)
Pachytene d) Diplotene and e) Diakinesis.
a)
Leptotene/Leptonema:
Chromosomes
become visible as long slender threads. Each thread like chromosome shows
presence of numerous bead like nucleosomes (chromomeres) arranged in a linear
fashion along its length. The nuclear envelop aand the nucleolus are
prominently visible. The thin chromosomes are scattered in the nucleus.
b) Zygotene/ Zygonema: This
phase is characterized by pairing of homologous chromosomes.
The homologous chromosomes (one paternal and one
maternal) from the two sets are attracted towards
each other and form pairs. In each pair, the two homologous lie parallel to
each other all along their lengths. This pairing is called synapsis. The paired
chromosomes are called bivalent. At this stage, each chromosome appears to have
only one chromid. Thus, each pair has in all two chromatids. Hence, each pair
is in the dyad stage. During zygotene, chromosomes become shorter, thicker and
more distinct.
c) Pachytene/Pachynema: This is the most
important stage in meiosis in which recombination of characters (genes) takes place
through a phenomenon called crossing over.
The
chromosomes become shorter, thicker and more distinct. Each chromosome has two
sister chromatids joined by a centromere. Thus, each pair of homologous
chromosomes (bivalents) at this stage consists of four chromatids (tetrad). The
paternal and maternal chromatids in each homologous pair are non-sister to one
another. The non-sister chromatids are twisted round each other in relational
coiling and may take part in crossing over.
Crossing
over: Crossing over is
an important genetic phenomenon. It takes place between any two non-sister
chromatids of a homologous pair. Crossing over consists of a mutual exchange of
equal quantity of chromosomal material between two non-sister chromatids, which
are taking events (Fig.
4.4-B).
a) The
relationally coiled non-sister chromatids, which are taking part in crossing
over, break simultaneously at the identical points.
b) The broken
segments are of equal lengths and carry alleles of same genes at identical gene
loci.
c) The
segments again join with the chromatids.
d) However,
there may be an exchange of the segments between the non-sister chromatids i.e.
maternal segments may join with the paternal chromatid and the paternal segment
may join with the maternal chromatid. This is called crossing over.
e) In this
process, the alleles of the genes located on the segments are exchanged between
the two non-sister chromatids.
f) Thus,
crossing over results in the recombination of genes on the concerned
chromatids.
g) Crossing
over never takes place between sister chromatids.
Significance of recombination
(crossing over): i) The gametes produced through meiosis
receive now combination of characters. ii) Therefore, when the gametes fuse,
individuals with new combination of characters are produced in each generation.
iii) This forms the genetic basis for variations and plays important role in
evolution.
d)
Diplotene/Diplonema: Two important events begin during
diplotene i) repulsion of homologous chromosomes and ii) terminalization.
i)
Repulsion: In each pair, the homologous chromosomes start repelling each other.
As a result, they begin to separate and uncoil. The non-sister chromatids
involved in the cross-over form a cross-like (x) figure at the point of
crossing over. Each such point is called a chiasma. A homologous pair can show
one or more chaismata.
e)
Diakinesis: This is the last phase of prophase-I.
Chromosomes are still in pairs and in contact with each other by terminal
chaisma. The chromosomes become shorter, thicker and more prominent.
Right
from the beginning of prophase-I, the nuclear envelop and the nucleous start
disappearing gradually. By the end of prophase-I, these two nuclear organelles
disappear completely and the pairs of chromosomes are seen scattered in the
nucleoplasm.
Metaphase-I: There is
formation of bipolar spindle fibers, a) Continuous fibers extending from pole
to pole and b) Chromosomal fibers extending from pole to equator of the spindle
body. The homologous chromosomes, still in pairs, move towards the centre of
the spindle. These are arranged along the equatorial plane in such a way that
in each pair, the two homologous are facing the opposite poles. In every pair,
the centromere of each chromosome is connected to the pole on its respective
side only. Each chromosome has one centomere and two sister chromatids.
As
paired chromosomes are arranging themselves alone the equatorial plane, the
base is being laid down naturally and automatically for an important phenomenon
of free and independent assortment of chromosomes.
At the
equatorial plane, the pairs lie randomly in such a way that some maternal and some
paternal homologous are facing each pole. From the total number of pairs
present, this assortment (mixing) of maternal and paternal chromosomes facing
each pole takes place randomly in any and every possible combinations. This
arrangement (involving fee assortment of maternal and paternal chromosomes)
during metaphase-I is ultimately responsible for the assortment of the alleles
(Mendel’s law of independent assortment).
Centriole
and aster: In
animals, a centriole is present outside the nucleus. It divides into two during
prophase-I and later on helps in the organization of bipolar spindle as well as
the asters.
Anaphase-I: The homologous
chromosomes are pulled away from each other and finally separate completely (terminalization
is completed). In other words, the two sets of homologous chromosomes separate
(segregate) and start moving towards opposite poles. This is due to the
shorting of the chromosomal fibers.
Each set has
haploid number (n) 0f chromosomes. Similarly, each set consists of a random
mixture of chromosomes from the original paternal and maternal sets.
Significance of
anaphase - I:
i)
As the two homologous in each pair
separate, it automatically brings about segregation of alleles of all the genes
located in those chromosomes (Mendel’s law of segregation).
ii)
In metaphase-I, during the arrangement
of the paired homologous along the equator, free and random assortment (mixing)
of chromosomes from the original maternal and paternal sets had already taken
place. Anaphase-I brings about separation of these assorted groups. Free
assortment of maternal and paternal chromosomes brings into effect the
assortment of alleles. (Mendel’s law of
independent assortment).
Telophase-I:
The two sets reach the opposite poles. The chromosomes, each with two
chromatids and one centromere, become thin and long. A nucleolus is organized
and a new nuclear membrane appears
around each set. Thus, two daughter nuclei are organized. Each nucleus is
haploid as it has received only one set of chromosome. Hence M-I is called
reduction division.
The haploid set
in each daughter nucleus is a random assortment (mixture) of chromosomes from
the original paternal and maternal sets.
Interkinesis:
The time interval between M-I and M-II is called interkinesis.
Meiosis-II
(M-II or Second meiotic division) (Fig. 4.4-C):
Second
meiotic division is similar to mitosis i.e. it is an equational division in
which there is division of the chromosomes. The two haploid daughter nuclei
formed at the end of M-I divide during
M-II and produce in all four haploid nuclei.
The various events in M-II are classified into
Prophase-II, Metaphase-II, Anaphase-II and Telophase-II. Both nuclei divide
simultaneously and all the changes during each phase are similar in both.
Prophase-II:
The chromosomes again become shorter, thicker and distinct. Each chromosomes
has two sister chromatids joined by a centromere. The nuclear envelop and the
nucleolus disappear by the end of prophase-II.
Metaphase-II:
The bipolar spindle body is formed. Chromosomes are arranged along the
equatorial plane in such a way that their sister chromatids are facing the
opposite poles. The centromere of each chromosomes is connected with both the
poles by chromosomal fibers.
In animals,
centriole is present outside the nucleus and participates in the formation of
bipolar spindle and asters.
Anaphase-II:
During early anaphase-II, the centromere of each chromosome divides
longitudinally into two. Therefore, each chromosome is divided into two halves
(chromatids) or daughter chromosomes.
During late
anaphase-II, the two sets of daughter chromosomes are pulled away from each
other and move toward opposite poles.
In
each set, some chromatids (daughter chromosomes) show original unchanged
structure while some others may show changes due to the crossing over. Thus,
out of the total four daughter sets, no two sets are exactly identical to each
other in the quality (characters) of chromosomes.
Telophase-II:
The sets of chromosomes reach the opposite poles and new nucleus is organized
at each pole. In all four daughter nuclei are formed. Each nucleus has half the
number of chromosomes (n) as compared to the original mother nucleus (2n).
These nuclei also differ from each other in the structure and characters of
chromosomes. This is because of the crossing over during prophase-I.
Cytokinesis:
This is the division of cell cytoplasm. It follows the nuclear division and may
be successive or simultaneous.
Sucessive
cytokinesis: In this type, each nuclear division
(M-I and M-II) is immediately followed by cytokinesis. Thus, cytokinesis occurs
twice. It may take place either by cell plate formation or by cleavage
constriction.
Simultaneous cytokinesis: In this type, cytokinesis takes place only once i.e. at
the end of meiosis-II and all the four daughter cells are formed simultaneously
(Fig. 4.5). Cytokinesis usually takes place by cleavage constriction.
Highlights
of meiosis
The important
features of the overall process of meiosis may be summarized as follows:
i)
Complete nuclear division occurs twise
(M-I and M-II).
ii)
Chromosomes divide only once
(Anaphase-II of M-II).
iii)
Separation of homologous chromosomes
occurs once (Anaphase- I of M-I).
iv)
This brings about the reduction in the original diploid number
(2n) of chromosomes. It (reduction) occurs only once. ( Anaphase-I of M-I).
v)
Segregation of alleles occurs once
(Anaphase-I of M-I) (Mendel’s law of segregation).
vi)
Random assortment of chromosomes from
the original maternal and paternal sets occurs once (Metaphase-I of M-I). This
effectively results in the assortment of characters (their alleles) (Mendel’s
law of independent assortment) Anaphase-I of M-I).
vii)
Cytikinesis may occur once (simultaneous
type) or twice (successive type).
Role
of kinetochores in M-I and M-II: The kinetochore is also
called centromere.
Role
in M-I: Kinetochores help in the separation (segregation)
of homologous chromosomes during M-I. (anaphase-I) This causes i) segregation
of alleles and ii) reduction in the number of chromosomes from diploid (2n) to
haploid (n).
Role
in M-II: In anaphase-II of M-II, the kinetochore
(centromere) of each chromosome divides into two. Hence, division of
chromosomes occurs in M-II.
Significance
of meiosis
1) Meiosis
and fertilization regulate the chromosomes number in the life cycle of an
organism.
2)
Crossing over during prophase-I
results in recombination of genetic material.
3)
This causes variations in
offspring.
4)
Variation play an important role in
organic evolution.
Table:
4.1 Comparison between mitosis and meiosis
|
Mitosis
|
Meiosis
|
|
1. Occurs
in somatic cells.
|
1. Occurs
in reproductive cells.
|
|
2. Consists
of only one nuclear division.
|
2. Consists
of two nuclear divisions M-I and M-II.
|
|
3. Cytokinesis
takes place only once.
|
3. May
take place only once or twice
|
|
4. Involves
division of chromosomes.
|
4. Involves
separation of homologous chromosomes in M-I and division of chromosomes in
M-II.
|
|
5. Dividing
cell can be haploid or diploid.
|
5. Dividing
cell is diploid.
|
|
6. Dose
not involves either pairing of
homologous chromosomes or crossing over.
|
6. Pairing
of homologous chromosomes and crossing over occur during prophase-I.
|
|
7. Two
daughter cells are formed.
|
7. Four
daughter cells are formed.
|
|
8. Number
of chromosomes present in mother cell is maintained in both the daughter
cells. Therefore it is an equational division.
|
8. Diploid
number of chromosomes is reduced to haploid in each daughter cell. Therefore
it is a reductional division.
|
|
9. Original
character of chromosomes are maintained in the daughter cells.
|
9. Chromosomal
characters are altered due to crossing over causing recombination of genes.
|
|
10. Daughter
cells are similar to each other and also to the mother cell.
|
10. Daughter
cells differ from each other as well as from the original mother cell.
|
|
11. Helps
in growth and body repairs.
|
11. Helps
in reproduction and regulation of chromosomes number in the life cyle of
sexually reproducing organisms.
|
|
12. Helps
in the perpetuation of the same chromosome number.
|
12. Helps
in the changeover of the chromosome number from diploid to haploid and thus,
brings about the change over from diploid to haploid phase in the life cycle.
|
Exercise
Q.1 Multiple choice questions (each 2
marks)
1. In cell cycle
- - - - - - - - - phase known as synthetic phase.
a) S phase
b) G1 phase c)
G2 phase. d) M phase
2. Mitosis is
also called as ------------
a) Reductional b) Equational c) Longitudinal d)
None
3. Meiosis is
also called as ------------- division.
a) Reductional b) Equational c) Longitudinal d)
None
4. In cell cycle
- - - - - - - - - phase longest phase.
a) S phase b) G1 phase c) G2 phase. d)
M phase
5. In cell cycle
DNA replication occurs in------------
a) S phase b) G1 phase c) G2 phase.
d)
M phase
6. Mitosis occur
in---------------
a) Roots b)
Shoots c) Germ cells
d) Somatic cells
Q.2 Define or
explain (each 2 marks)
a) Cell cycle b) M phase c)
S phase d) Meiosis
e) G1 phase. f) G2 phase. g) Mitosis
Q.3 Write short notes on (each 4 marks)
1. Cell cycle.
2. Regulation of
cell cycle
3. Sketch and
label cell cycle
4. Write the
significance of mitosis
Q.4 Questions for 6 marks
1. Explain the
various phases involved in the mitotic division of an animal cell.
2. Give an account of
the meiotic type of cell division.
*(these notes are compilation of the study material for the students)
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