Cell cycle and cell division
Are you aware that all organisms, even the largest, start their life from a
single cell? You may wonder how a single cell then goes on to form such
large organisms. Growth and reproduction are characteristics of cells,
indeed of all living organisms. All cells reproduce by dividing into two,
with each parental cell giving rise to two daughter cells each time they
divide. These newly formed daughter cells can themselves grow and divide,
giving rise to a new cell population that is formed by the growth and
division of a single parental cell and its progeny. In other words, such
cycles of growth and division allow a single cell to form a structure
consisting of millions of cells.
Cell cycle
Cell division is a very important process in all living organisms. During the division of a cell, DNA replication and cell growth also take place. All these processes, i.e., cell division, DNA replication, and cell growth, hence,
have to take place in a coordinated way to ensure correct division and formation of progeny cells containing intact genomes. The sequence of events by which a cell duplicates its genome, synthesises the other constituents of the cell and eventually divides into two daughter cells is termed cell cycle. Although cell growth (in terms of cytoplasmic increase) is a continuous process, DNA synthesis occurs only during one specific stage in the cell cycle. The replicated chromosomes (DNA) are then
distributed to daughter nuclei by a complex series of events during cell
division. These events are themselves under genetic control.
Phases of Cell Cycle
A typical eukaryotic cell cycle is illustrated by
human cells in culture. These cells divide once
in approximately every 24 hours (Figure).
However, this duration of cell cycle can vary from
organism to organism and also from cell type
to cell type. Yeast for example, can progress
through the cell cycle in only about 90 minutes.
The cell cycle is divided into two basic
phases:
-Interphase
-M Phase (Mitosis phase)
The M Phase represents the phase when the
actual cell division or mitosis occurs and the
interphase represents the phase between two
successive M phases. It is significant to note
that in the 24 hour average duration of cell
cycle of a human cell, cell division proper lasts
for only about an hour. The interphase lasts
more than 95% of the duration of cell cycle.
The M Phase starts with the nuclear division, corresponding to the
separation of daughter chromosomes (karyokinesis) and usually ends
with division of cytoplasm (cytokinesis). The interphase, though called
the resting phase, is the time during which the cell is preparing for division
by undergoing both cell growth and DNA replication in an orderly manner.
The interphase is divided into three further phases:
-G1 phase (Gap 1)
-S phase (Synthesis)
-G2 phase (Gap 2)
G1 phase
corresponds to the interval between mitosis and initiation
of DNA replication. During G1 phase the cell is metabolically active and
continuously grows but does not replicate its DNA. S or synthesis phase
marks the period during which DNA synthesis or replication takes place.
During this time the amount of DNA per cell doubles. If the initial amount
of DNA is denoted as 2C then it increases to 4C. However, there is no
increase in the chromosome number; if the cell had diploid or 2n number
of chromosomes at G1, even after S phase the number of chromosomes
remains the same, i.e., 2n.
In animal cells, during the S phase, DNA replication begins in the
nucleus, and the centriole duplicates in the cytoplasm. During the G2
phase, proteins are synthesised in preparation for mitosis while cell growth
continues.
How do plants and
animals continue to
grow all their lives?
Do all cells in a plant
divide all the time?
Do you think all cells
continue to divide in
all plants and
animals? Can you
tell the name and the
location of tissues
having cells that
divide all their life in
higher plants? Do
animals have similar meristemat c tissues?
Some cells in the adult animals do not appear to exhibit division (e.g.,
heart cells) and many other cells divide only occasionally, as needed to
replace cells that have been lost because of injury or cell death. These
cells that do not divide further exit G1 phase to enter an inactive stage
called quiescent stage (G0) of the cell cycle. Cells in this stage remain
metabolically active but no longer proliferate unless called on to do so
depending on the requirement of the organism.
In animals, mitotic cell division is only seen in the diploid somatic
cells. However, there are few exceptions to this where haploid cells divide
by mitosis, for example, male honey bees. Against this, the plants can
show mitotic divisions in both haploid and diploid cells. From your
recollection of examples of alternation of generations in plants (Chapter 3)
identify plant species and stages at which mitosis is seen in haploid cells.
M phase
This is the most dramatic period of the cell cycle, involving a major reorganisation of virtually all components of the cell. Since the number of
chromosomes in the parent and progeny cells is the same, it is also called as
equational division. Though for convenience mitosis has been divided
into four stages of nuclear division (karyokinesis), it is very essential to
understand that cell division is a progressive process and very clear-cut
lines cannot be drawn between various stages.
Karyokinesis involves following four stages:
-Prophase
-Metaphase
-Anaphase
-Telophase
Prophase
Prophase which is the first stage of karyokinesis of mitosis follows the
S and G2 phases of interphase. In the S and G2 phases the new DNA
molecules formed are not distinct but intertwined. Prophase is marked
by the initiation of condensation of chromosomal material. The
chromosomal material becomes untangled during the process of
chromatin condensation (Figure 10.2 a). The centrosome, which had
undergone duplication during S phase of interphase, now begins to move
towards opposite poles of the cell. The completion of prophase can thus
be marked by the following characteristic events:
-Chromosomal material condenses to form compact mitotic
chromosomes. Chromosomes are seen to be composed of two
chromatids attached together at the centromere.
- Centrosome which had undergone duplication during interphase,
begins to move towards opposite poles of the cell. Each centrosome
radiates out microtubules called asters. The two asters together
with spindle fibres forms mitotic apparatus.
You have studied mitosis in onion root tip cells. It has 16 chromosomes in each cell. Can you tell how many chromosomes will the cell have at G1 phase, after S phase, and after M phase? Also, what will be the DNA content of the cells at G1, after S and at G2, if the content after M phase is 2C?
Cells at the end of prophase, when viewed under the
microscope, do not show golgi complexes, endoplasmic
reticulum, nucleolus and the nuclear envelope.
Metaphase
The complete disintegration of the nuclear envelope marks
the start of the second phase of mitosis, hence the
chromosomes are spread through the cytoplasm of the cell.
By this stage, condensation of chromosomes is completed
and they can be observed clearly under the microscope. This
then, is the stage at which morphology of chromosomes is
most easily studied. At this stage, metaphase chromosome
is made up of two sister chromatids, which are held together
by the centromere (Figure 10.2 b). Small disc-shaped
structures at the surface of the centromeres are called
kinetochores. These structures serve as the sites of attachment
of spindle fibres (formed by the spindle fibres) to the
chromosomes that are moved into position at the centre of
the cell. Hence, the metaphase is characterised by all the
chromosomes coming to lie at the equator with one chromatid
of each chromosome connected by its kinetochore to spindle
fibres from one pole and its sister chromatid connected by
its kinetochore to spindle fibres from the opposite pole (Figure
10.2 b). The plane of alignment of the chromosomes at
metaphase is referred to as the metaphase plate. The key
features of metaphase are:
-Spindle fibres attach to kinetochores of
chromosomes.
-Chromosomes are moved to spindle equator and get
aligned along metaphase plate through spindle fibres
to both poles.
Anaphase
At the onset of anaphase, each chromosome arranged at the
metaphase plate is split simultaneously and the two daughter
chromatids, now referred to as daughter chromosomes of
the future daughter nuclei, begin their migration towards
the two opposite poles. As each chromosome moves away
from the equatorial plate, the centromere of each chromosome
remains directed towards the pole and hence at the leading
edge, with the arms of the chromosome trailing behind
(Figure 10.2 c). Thus, anaphase stage is characterised by the following key events:
-Centromeres split and chromatids separate.
-Chromatids move to opposite poles.
Telophase
At the beginning of the final stage of karyokinesis, i.e.,
telophase, the chromosomes that have reached their
respective poles decondense and lose their individuality. The
individual chromosomes can no longer be seen and each set
of chromatin material tends to collect at each of the two poles
(Figure 10.2 d). This is the stage which shows the following
key events:
-Chromosomes cluster at opposite spindle poles and their
identity is lost as discrete elements.
-Nuclear envelope develops around the chromosome
clusters at each pole forming two daughter nuclei.
-Nucleolus, golgi complex and ER reform.
Cytokinesis
Mitosis accomplishes not only the segregation of duplicated
chromosomes into daughter nuclei (karyokinesis), but the
cell itself is divided into two daughter cells by the separation
of cytoplasm called cytokinesis at the end of which cell
division gets completed (Figure 10.2 e). In an animal cell,
this is achieved by the appearance of a furrow in the plasma
membrane. The furrow gradually deepens and ultimately
joins in the centre dividing the cell cytoplasm into two. Plant
cells however, are enclosed by a relatively inextensible cell
wall, thererfore they undergo cytokinesis by a different
mechanism. In plant cells, wall formation starts in the centre
of the cell and grows outward to meet the existing lateral
walls. The formation of the new cell wall begins with the
formation of a simple precursor, called the cell-plate that
represents the middle lamella between the walls of two
adjacent cells. At the time of cytoplasmic division, organelles
like mitochondria and plastids get distributed between the
two daughter cells. In some organisms karyokinesis is not
followed by cytokinesis as a result of which multinucleate
condition arises leading to the formation of syncytium (e.g.,
liquid endosperm in coconut).
Significance of Mitosis
Mitosis or the equational division is usually restricted to the diploid cells
only. However, in some lower plants and in some social insects haploid
cells also divide by mitosis. It is very essential to understand the
significance of this division in the life of an organism. Are you aware of
some examples where you have studied about haploid and diploid insects?
Mitosis usually results in the production of diploid daughter cells
with identical genetic complement. The growth of multicellular organisms
is due to mitosis. Cell growth results in disturbing the ratio between the
nucleus and the cytoplasm. It therefore becomes essential for the cell to
divide to restore the nucleo-cytoplasmic ratio. A very significant
contribution of mitosis is cell repair. The cells of the upper layer of the
epidermis, cells of the lining of the gut, and blood cells are being constantly
replaced. Mitotic divisions in the meristematic tissues – the apical and
the lateral cambium, result in a continuous growth of plants throughout
their life.
Meiosis
The production of offspring by sexual reproduction includes the fusion
of two gametes, each with a complete haploid set of chromosomes. Gametes
are formed from specialised diploid cells. This specialised kind of cell
division that reduces the chromosome number by half results in the
production of haploid daughter cells. This kind of division is called
meiosis. Meiosis ensures the production of haploid phase in the life cycle
of sexually reproducing organisms whereas fertilisation restores the diploid
phase. We come across meiosis during gametogenesis in plants and
animals. This leads to the formation of haploid gametes. The key features
of meiosis are as follows:
-Meiosis involves two sequential cycles of nuclear and cell division called
meiosis I and meiosis II but only a single cycle of DNA replication.
-Meiosis I is initiated after the parental chromosomes have replicated
to produce identical sister chromatids at the S phase.
- Meiosis involves pairing of homologous chromosomes and
recombination between non-sister chromatids of homologous
chromosomes.
- Four haploid cells are formed at the end of meiosis II.
Meiotic events can be grouped under the following phases:
Meiosis I -
Prophase I,
Metaphase I,
Anaphase I,
Telophase I,
Meiosis II- Prophase II, Metaphase II, Anaphase II, Telophase II,
Meiosis I
Prophase I: Prophase of the first meiotic division is typically longer and
more complex when compared to prophase of mitosis. It has been further
subdivided into the following five phases based on chromosomal
behaviour, i.e.,
Leptotene, Zygotene, Pachytene, Diplotene and Diakinesis.
During leptotene stage the chromosomes become gradually visible
under the light microscope. The compaction of chromosomes continues
throughout leptotene. This is followed by the second stage of prophase
I called zygotene. During this stage chromosomes start pairing together
and this process of association is called synapsis. Such paired
chromosomes are called homologous chromosomes. Electron
micrographs of this stage indicate that chromosome synapsis is
accompanied by the formation of complex structure called
synaptonemal complex. The complex formed by a pair of synapsed
homologous chromosomes is called a bivalent or a tetrad. However,
these are more clearly visible at the next stage. The first two stages of
prophase I are relatively short-lived compared to the next stage that is
pachytene. During this stage, the four chromatids of each bivalent
chromosomes becomes distinct and clearly appears as tetrads. This stage
is characterised by the appearance of recombination nodules, the sites
at which crossing over occurs between non-sister chromatids of the
homologous chromosomes. Crossing over is the exchange of genetic
material between two homologous chromosomes. Crossing over is also
an enzyme-mediated process and the enzyme involved is called
recombinase. Crossing over leads to recombination of genetic material
on the two chromosomes. Recombination between homologous
chromosomes is completed by the end of pachytene, leaving the
chromosomes linked at the sites of crossing over.
The beginning of diplotene is recognised by the dissolution of the
synaptonemal complex and the tendency of the recombined
homologous chromosomes of the bivalents to separate from each other
except at the sites of crossovers.
These X-shaped structures, are called chiasmata. In oocytes of some vertebrates, diplotene can last for months or years.
The final stage of meiotic prophase I is diakinesis.
This is marked by
terminalisation of chiasmata. During this phase the chromosomes are
fully condensed and the meiotic spindle is assembled to prepare the
homologous chromosomes for separation. By the end of diakinesis, the
nucleolus disappears and the nuclear envelope also breaks down.
Diakinesis represents transition to metaphase.
Metaphase I:
-The bivalent chromosomes align on the equatorial plate
(Figure 10.3). -The microtubules from the opposite poles of the spindle
attach to the kinetochore of homologous chromosomes.
Anaphase I:
The homologous chromosomes separate, while sister
chromatids remain associated at their centromeres (Figure 10.3).
Telophase I: The nuclear membrane and nucleolus reappear, cytokinesis
follows and this is called as dyad of cells (Figure 10.3). Although in many
cases the chromosomes do undergo some dispersion, they do not reach
the extremely extended state of the interphase nucleus. The stage between
the two meiotic divisions is called interkinesis and is generally short lived.
There is no replication of DNA during interkinesis. Interkinesis is followed
by prophase II, a much simpler prophase than prophase I.
Meiosis II
Prophase II: Meiosis II is initiated immediately after cytokinesis, usually
before the chromosomes have fully elongated. In contrast to meiosis I,
meiosis II resembles a normal mitosis. The nuclear membrane disappears
by the end of prophase II (Figure 10.4). The chromosomes again become
compact.
Metaphase II: At this stage the chromosomes align at the equator and
the microtubules from opposite poles of the spindle get attached to the
kinetochores (Figure 10.4) of sister chromatids.
Anaphase II: It begins with the simultaneous splitting of the centromere
of each chromosome (which was holding the sister chromatids together),
allowing them to move toward opposite poles of the cell (Figure 10.4) by
shortening of microtubules attached to kinetochores.
Telophase II: Meiosis ends with telophase II, in which the two
groups of chromosomes once again get enclosed by a nuclear
envelope; cytokinesis follows resulting in the formation of tetrad
of cells i.e., four haploid daughter cells (Figure 10.4).
Significance of meiosis
Meiosis is the mechanism by which conservation of specific
chromosome number of each species is achieved across
generations in sexually reproducing organisms, even though the
process, per se, paradoxically, results in reduction of chromosome
number by half. It also increases the genetic variability in the
population of organisms from one generation to the next. Variations
are very important for the process of evolution.
Exercises
1. What is the average cell cycle span for a mammalian cell?
2. Distinguish cytokinesis from karyokinesis.
3. Describe the events taking place during interphase.
4. What is Go (quiescent phase) of cell cycle?
5. Why is mitosis called equational division?
6. Name the stage of cell cycle at which one of the following events occur:
(i) Chromosomes are moved to spindle equator.
(ii) Centromere splits and chromatids separate.
(iii) Pairing between homologous chromosomes takes place.
(iv) Crossing over between homologous chromosomes takes place.
7. Describe the following:
(a) synapsis (b) bivalent (c) chiasmata
Draw a diagram to illustrate your answer.
8. How does cytokinesis in plant cells differ from that in animal cells?
9. Find examples where the four daughter cells from meiosis are equal in size and
where they are found unequal in size.
10. Distinguish anaphase of mitosis from anaphase I of meiosis.
11. List the main differences between mitosis and meiosis.
12. What is the significance of meiosis?
13. Discuss with your teacher about
(i) haploid insects and lower plants where cell-division occurs, and
(ii) some haploid cells in higher plants where cell-division does not occur.
14. Can there be mitosis without DNA replication in ‘S’ phase?
15. Can there be DNA replication without cell division?
16. Analyse the events during every stage of cell cycle and notice how the following
two parameters change
(i) number of chromosomes (N) per cell
(ii) amount of DNA content (C) per cell
B
B
C
B
D
B
A
A
A
A
D
D
C
B
B
A
D
D
A
C
Cell Division
Cell cycle is a series of programmed cyclic changes by which the cell duplicates its contents and divides into two. The sequence of events is genetically controlled.
It is complex of changes that occurs in a newly formed cell before it is able to divide. Interphase is also called intermitosis, preparatory phase or energy phase.
Interphase also accomplishes three important processes which are preparatory to cell division:
Interphase has three stages.
(ii) S-Phase (Synthetic Phase). DNA of each chromosome replicates followed by synthesis of histones. As a result each chromosome undergoes replication producing two chromatids.
(iii) G2-Phase (Second Growth or Gap Phase, Post-Synthetic Phase, Pre-mitotic Phase).
M-Phase. It is the phase of cell division. Cell division consists of nuclear division or karyokinesis and protoplast division or cytokinesis. AMITOSIS (Direct Division; Robert Remak, 1855)
Mitosis
It is also called somatic division because it occurs during formation of somatic or body cells. Karyokinesis. It is the stage of nuclear division (indirect nuclear division) which is continuous but is divided into four stages for the sake of convenience –
1. Prophase. Prometaphase. It is considered to be second phase by certain cytologists. Metaphase is then called third stage of karyokinesis. Spindle apparatus or mitotic apparatus gets organiscd. It is spindle-shaped colourless fibrous body which can be observcd with the help of pol arising microscope.
2. Metaphase.
3. Anaphase. It is the phase of shortest duration.
4. Telophase. Nucleoplasm and nuclear envelope appear so that two daughter nuclei are formed.
Dinomitosis.
Cytokinesis Cleavage Cytokinesis. Importance. (i) Growth. Mitosis is essential for formation of new cells required for growth of all multicellular organisms. A human neonate has 6 X 1012 cells all derived from a single celled zygote. The adult human body has 1014 cells.
00
Meiosis I It is the actual reduction division which is also called heterotypic division because it brings about change from diploid to haploid state.
Prophase I. It is long, complex and divisible into five stages –
(i) Leptotene (Leptonema).
(ii) Zygotene (Zygonema).
(iii) Pachytene (= Pachynema). (iv) Diplotene (Diplonema). At most places synaptinemal complex dissolves. (v) Diakinesis. Chiasmata shift towards the chromosome ends (terminalisation). RNA synthesis stops. Nucleolus degenerates. Metaphase I. A bipolar fibrous spindle appears in the area of nucleus. It has asters at the two poles in animal cells (amphiaster) while the same are absent in plant cells (anastral). Anaphase I.
Telophase I. It is homotypic or equational division which is meant for '"maintaining the haploid number, converting dyad chromosome state into monad state and separating the two chromatids of a chromosome which have become different due to crossing over.
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MCQs
SV021110.0012020 Cell Cycle
021110.001. A cell cycle is :
(A) the time from the formation of a cell until its death.
(B) the series of events that takes place from the formation
of a cell until it divides again.
(C) the sequence of events that assures each daughter
cell of a set of chromosomes identical with that of
its parent cell (mitosis)
(D) the growth of a cell until it is large enough to divide
again.
(A) cannot undergo division again
(B) can undergo mitosis but not meiosis
(C) can undergo mitosis or meiosis
(D) can undergo meiosis but not mitosis.
(A) its chromosomes are visible as distinct thread-like
structures
(B) the nuclear membrane is not visible
(C) the chromosomes are lined up on the equator of the
spindle
(D)the chromosomes are separated into two distinct
groups attached to the spindle
(E) the chromosomes are found in two compact groups
in two small patches of cytoplasm that are in the
process of separating into two distinct cells.
(A)the presence of only half as many chromosomes in
the meiotic cell
(B) the formation of tetrads in the meiotic cell
(C) the presence of twice as many chromosomes
in the meiotic cell.
(D)None of the above.
(A) reproduction of identical individuals if the organism
is unicellular
(B) growth of an individual if the organism is multicellular
(C) repair of injured tissue
(D) All of the above.
(A)microtubules must be distributed equally to
the new cells
(B) microtubules are involved in the precise
separation of the chromosomes which ensures that
into each daughter cell
(C) without microtubules, cytokinesis cannot
take place and syncytium is formed
(D) microtubules are essential for the disappearance of
the nuclear membrane, and without them the
chromosomes have to stay too close together
within the nuclear membrane to be able to
separate into two new nuclei.
(A)ensures genetic homogeneity of the cells
(B) reduces the chromosome number to half
(C) causes transformation of DNA to RNA
(D) increases the water contents of the cell.
(A) dependent on association of spindle fibres
with kinetochore.
(B) independent of spindle fibres
(C) due to cytoplasmic streaming
(D) due to excess of ATP formed in mitochondria.
(A)once it has entered the S-period
(B) once it has entered the Grperiod
(C) at any time during cell division activity
(D)None of the above.
(A) 44 (B) 22
(C) 11 (D) 66.
(A) crossing over
(B) halfing the chromosome number between the two
daughter cells
(C) segregation of paternal and maternal chromosome
(D) formation of two daughter cells with identical DNA.
(A) Metaphase (B) Anaphase
(C) Telophase (D) S-phase of interphase.
(A) dicentric chromosome
(B) inversion
(C) acentric chromosome
(D) duplication of a gene.
(A)48 (B)36
(C) 24 (D) 12.
(A)diplotene through metaphase I
(B) zygotene through pachytene
(C) pachytene through diplotene
(D)leptotene through diplotene.
(A) 150 (B) 400
(C) 100 (D) 125.
(A) cell plate formed during cytokinesis of plant cells
(B) cell membrane formed during cytokinesis of animal cells
(C) plastid which helps in cytokinesis
(0) plastid capable of self-duplication
(A) mitochondria (B) centriole
(C) golgi complex (D) is not precise one.
B
021110.022. Each longitudinal half of a chromosome of late prophase is known as:
(A) chromatin (B) chromatin fibre
(C) chromatid (D) half chromatid.
C
021110.023. As a cell proceeds towards mitosis, it can be halted by depriving the cell of oxygen or by poisoning its oxidative
enzyme with carbon monoxide. What does this indicate?
(A) Mitosis is a type of cellular respiration
(B) Mitosis is an enzyme controlled event
(C) Mitosis involves an expenditure of energy
(D) Mitosis yields energy that the cell uses to make ATP molecules
C
021110.024. During cell division one of the cytoplasmic organelles plays an important role, it is :
(A) mitochondria (B) centriole
(C) golgi complex (D) is not precise one.
B
021110.025. Each longitudinal half of a chromosome of late prophase
is known as:
(A) chromatin (B) chromatin fibre
(C) chromatid (D) half chromatid.
C
021110.026. As a cell proceeds towards mitosis, it can be halted by depriving the cell of oxygen or by poisoning its oxidative
enzyme with carbon monoxide. What does this indicate?
(A) Mitosis is a type of cellular respiration
(B) Mitosis is an enzyme controlled event
(C) Mitosis involves an expenditure of energy
(D) Mitosis yields energy that the cell uses to make ATP molecules
C
021110.027. Each longitudinal half of a chromosome of late prophase
is known as:
(A) chromatin (B) chromatin fibre
(C) chromatid (D) half chromatid.
C
021110.028. The spindle fibre during cell division arises from:
(A) centromere (B) nucleus
(C) aster (D) mitochondria.
C
021110.029. Mitosis consists of :
(A) a series of nuclear division
(B) cleavage of cytoplasm
(C) karyokinesis followed by cytokinesis
(D) formation of daughter nuclei
C
021110.030. During metaphase :
(A) chromosomes occupy equatorial zone
(B) chromosomes are not yet ready for division
(C) chromosomes orient themselves into any place on the mitotic spindle
(D) chromosomes assemble at one of the poles of mitotic spindle.
A
021110.031. The points where the crossing over takes place between chromatids of two homologous chromosomes of a pair are:
(A) centromere (B) chiasmata (C) chromomere (D) chromocentre
B
021110.032. Which of the following is capable of self-reproduction
in cell division ?
(A) protein molecule
(B) Fat molecule
(C) Carbohydrate molecule
(D) DNA molecule.
D
021110.033. Mitosis and meiosis differ because during meiosis:
(A) homologous chromosomes pair and exchange parts
(B) chromosome number is halved
(C) four nuclei formed are not similar
(D) All the above.
D
021110.034. During cell division nuclear membrane reappears in :
(A) interphase (B) prophase
(C) telophase (D) S-phase.
C
021110.035. In which stage of mitosis, all the chromosomes
are arranged on equatorial plate ?
(A) Prophase (B) Metaphase
(C) Anaphase (D) Telophase.
B
021110.036. During prophase the chromosomes are :
(A) larger and coiled
(B) much contracted, thickened and coiled
(C) much contracted, thickened and uncoiled
(D) larger and straight.
B
021110.037. Cell division or mitosis is a normal process in
living cells but sudden and abnormal mitosis
in an organ results in :
(A) new organ (B) new organism
(C) zygote (D) cancer.
D
021110.038. Meiosis is a type of cell division in which:
(A) the chromosome number is reduced to half
(B) the chromosome number remains the same
(C) the chromosome number is doubled
(D) the chromosome number is reduced to one
fourth.
A
021110.039. In which one of the following stages chromosomes
appear as long thin threads in :
(A) leptotene (B) zygotene
(C) pachytene (D) diplotene.
A
021110.040. During Meiosis 1, in which stage synapsis takes place?
(A)Pachytene (B) Zygotene (C) Diplotene (D) Leptotene.
B
021110.041. One of the following types of cell division results in
reduction of chromosomes :
(A) mitosis (B) binary fission
(C) meiosis (D) amitosis.
C
021110.042. Chiasmata are formed by exchange between non-sister
chromatids of homologous chromosome was first
indicated by :
(A) Altman (B) Sax
(C) Janssen (D) T.H. Morgan.
C
021110.043. How many times the mitotic division must
occur in the cell of root tip to form 256 cells ?
(A) 128 (B) 255
(C) 64 (D) 8.
D
021110.044. When a diploid cell undergoes meiosis :
(A) Homologous chromosomes segregate
(B) All dominant genes segregate into daughter
Cells
(C) Homologous chromosomes pair but do not separate
(D) All linkage groups get distributed
A
021110.045. Amitosis is a usual process of cell division in :
(A) eukaryotic cells
(B) prokaryotic cells
(C) spore mother cells (D) meristematic cells.
B
Cell Cycle (Howard and Pelc, 1953)
Time interval between two successive divisions is called generation time. Cell cycle has two parts-long non dividing I-phase and short dividing M-phase.
cell cycle is defined as the period between successive divisions of a cell.
Interphase (I-phase).
Previously it was known as resting phase.
The nondividing state of mature cell/nucleus is also called interphase.
It lasts throughout the life of the cell.
For example, nerve cells do not divide after birth and, therefore, last throughout the life of the individual.
Inter mitotic interphase is the period of intense biosynthetic activity wherein the cell doubles its size and duplicates its chromosome complement along with the increase in number of various cell organelles.
(a) Replication of DNA and synthesis of nuclear proteins such as histones.
(b) Duplication of centriole in animal cells. Daughter centriole develops at right angle to parent centriole.
(c) Synthesis of energy rich components for providing energy.
(i) G1-Phase (First Growth or Gap Phase, Pre-Synthetic Phase, Post-Mitotic Phase). It is the longest phase of cell cycle. Cell and its nucleus (to a lesser extent) grow in size.
There is synthesis of biochemicals like RNAs, proteins, enzymes for DNA synthesis, amino acids for histone formation, nucleotides and ATP. Cell organelles also increase in number.
Duration of G1 phase is longer in cells dividing infrequently. It is shorter in frequently dividing cells.
In G1 phase a cell has three options.
The G0 phase is a period in the cell cycle in which cells exist in a quiescent state. G0 phase is viewed as either an extended G1 phase, where the cell is neither dividing nor preparing to divide, or a distinct quiescent stage that occurs outside of the cell cycle.
(a) Continue cycle and enter S-phase.
(b) Stop cell cycle and enter Go phase for undergoing differentiation.
(c) Get arrested in G1 phase from where it may enter cell cycle or pass into G0 phase.
(Internal Checkpoints During the Cell Cycle: The cell cycle is controlled at three checkpoints.
The integrity of the DNA is assessed at the G1 checkpoint. Proper chromosome duplication is assessed at the G2 checkpoint.
Attachment of each kinetochore to a spindle fiber is assessed at the M checkpoint. Cyclins are a family of proteins that control the progression of cells through the cell cycle by activating cyclin-dependent kinase (Cdk) enzymes.
Through phosphorylation, Cdks signal the cell that it is ready to pass into the next stage of the cell cycle.
As their name suggests, Cyclin-Dependent Protein Kinases are dependent on cyclins, another class of regulatory proteins.
Cyclins bind to Cdks, activating the Cdks to phosphorylate other molecules.
A cyclin-dependent kinase complex (CDKC, cyclin-CDK) is a protein complex formed by the association of an inactive catalytic subunit of a protein kinase, cyclin-dependent kinase (CDK), with a regulatory subunit, cyclin. Once cyclin-dependent kinases bind to cyclin, the formed complex is in an activated state.)
The deciding factor is storage of energy rich compounds and availability of mitogens. The stage where this decision is made can be called check point.
Once this check point is crossed, the cell reaches a state called ante phase (Bullough, 1952) whereby it will divide even under unfavourable conditions. There is abundant storage of ATP energy.
S-phase is also known as invisible stage of M-phase. Synthesis of other biochemicals also continues. Centrioles replicate towards the end of S-phase or beginning of G2-phase.
There is again intense synthesis of proteins and RNAs. Cell organelles increase in number. Protein tubulin is formed.
G0-Phase. It is phase of cell differentiation when cell cycle is stopped about the middle of G1 Phase due to activation of certain genes.
The genes allow the cell to grow to a particular size, assume particular shape and come to perform certain specific functions.
Various phases of cell cycle are controlled by proteins cyclins and kinases that take part in phosphorylation and dephosphorylation.
Cell division or M-phase is of three types - amitosis, mitosis and meiosis.
It was discovered by Remak (1841, 1855) and studied by Flemming (1882).
In amitosis, nucleus elongates, constricts in the middle and divides directly into two daughter nuclei. Chromatin does not condense to form chromosomes.
Spindle is not produced. Nuclear division is followed by cytokinesis or division of cytoplasm. It occurs through cleavage or constriction, e.g., cartilage cells, degenerate cells, meganucleus of Paramecium, cells of foetal membranes and endosperm.
Some workers include procaryotic cell division under amitosis. As amitosis does not distribute chromatin equitably, it leads to structural and functional irregularities.
Mitosis is studied in plants in the regions of meristems, e.g., stem tip, root tip (Onion 2n = 16).
In animals it is studied in bone marrow, skin, base of nails, etc.
Mitosis is equational division in which a parent cell divides into two identical daughter cells, each of which contains the same number and kind of chromosomes as are present in the parent cell.
It is centric in animal cells and acentric (without participation of centrosome) in plants.
Mitosis occurs in two steps, karyokinesis and cytokinesis.
prophase,
metaphase,
anaphase and
telophase.
It is the longest phase of karyokinesis. In early prophase or spireme stage,
the chromatin fibres condense through spiralisation to form elongated chromosomes.
There is increased viscosity and refractivity of cytoplasm.
Animal cells become nearly rounded. The ends of chromosomes are not distinguishable.
The nucleus appears as a ball of wool.
Centrosome has already divided. The daughter centrosomes begin to move away from each other.
In mid prophase, chromosomes shorten and become distinct with each having two chromatids attached to narrow point called centromere.
The centrosomes develop astral rays and migrate farther. In late prophase, the centrosomes reach the poles, form asters and begin to develop spindle fibres.
Nucleolus degenerates and nuclear envelope starts breaking.
In plant cells, centrosome are absent. Spindle fibres develop without them. Polar ends are negatively charged.
They are organised in both plants and animals with the help of Ca2+ containing protein calmodulin.
During this stage the nuclear envelope disappears so that nucleoplasm comes in contact with cytoplasm.
A differentiation between the two disappears. A fluid area occurs in the centre of the cell. Chromosomes can move freely in this area.
A spindle fibre consists of 4 - 20 microtubules formed of protein tubulin.
Spindle fibres converge at the two ends or poles.
Spindle has the maximum diameter in the middle. It is called equator. In dividing animal cells, the spindle bears asters at the two poles. Such a spindle is called amphiaster or centric.
In plant cells spindle is acentric or anastral. Fibres of the spindle are of two main types,
continuous (from pole to pole) and discontinuous (radiating from one pole but not reaching the other).
The term eumitosis or extranuclear mitosis is used for organisation of spindle in which nuclear envelope degenerates.
In many protists, fungi and algae, the nuclear envelope does not degenerate during mitosis. Instead, spindle is formed inside the nucleus. It is called intranuclear mitosis or premitosis.
Chromosomes are the shortest and the thickest in metaphase.
Each chromosome gets attached to two discontinuous spindle fibres or chromosome fibres, one from each pole,
in the region of its kinetochores (a complex of proteins associated with the centromere of a chromosome during cell division, to which the microtubules of the spindle attach.).
The latter have plus ends for this purpose.
Chromosome fibres contract and bring the chromosomes over the equator. The phenomenon is called congression. Centromeres of all the chromosomes are present over the equator. Therefore, chromosomes form a sort of apparent plate called equatorial or metaphasic plate.
Limbs of chromosomes project in different directions.
Metaphase is the best time to see chromosomes.
APC (anaphase promoting complex) develops. It degenerates proteins binding the two chromatids in the region of centromere. As a result, the centromere of each chromosome divides.
This converts the two chromatids into daughter chromosomes each being attached to the spindle pole of its side by independent chromosomal fibre.
The chromosomes move towards the spindle poles with the centromeres projecting towards the poles and the limbs trailing behind.
There is corresponding' shortening of chromosome fibres. Anaphasic chromosomes appear differently depending upon the position of their centromeres - V -shaped (metacentric), L-shaped (submetacentric), J-shaped (acrocentric) and I-shaped (telocentric). The two pole-ward 'moving chromosomes of each type remain attached to each other by interzonal fibres. Ultimately, two groups of chromosomes come to lie at the spindle poles.
It is the stage of reconstitution of nuclei.
Spindle fibres converge at the two ends or poles.
Spindle has the maximum diameter in the middle. It is called equator. In dividing animal cells, the spindle bears asters at the two poles. Such a spindle is called amphiaster or centric.
In plant cells spindle is acentric or anastral. Fibres of the spindle are of two main types,
continuous (from pole to pole) and discontinuous (radiating from one pole but not reaching the other).
The term eumitosis or extranuclear mitosis is used for organisation of spindle in which nuclear envelope degenerates.
In many protists, fungi and algae, the nuclear envelope does not degenerate during mitosis. Instead, spindle is formed inside the nucleus. It is called intranuclear mitosis or premitosis.
Chromosomes uncoil, elongate, lose their stainability and form chromatin fibres.
Nucleolar organisers form nucleoli in one or more pairs of chromosomes.
It is type of nuclear division found in dinoflgellates in which the nuclear envelope persists. Microtubular spindle is not formed.
Chromosomes move while attached to inner membrane of nuclear envelope.
Karyochorisis. Intranuclear spindle is formed with spindle pole bodies (SPBs) developing at the two ends.
Cytokinesis is the division of cytoplasm of a cell undergoing karyokinesis to form two daughter cells.
I t begins towards the middle of anaphase and is completed simultaneously with the completion of telophase.
Non occurrence of cytokinesis produces multinucleate coenocyte or syncytium. Cytokinesis also brings about nearly equitable distribution of various cell organelles like mitochondria, plastids, lysosomes, Golgi apparatus, endoplasmic reticulum, cytoplasmic matrix, etc. Mitochondria and plastids are known to multiply through fission.
Details of multiplication of other organelles are not -yet known.
Cytokinesis is of two types, cleavage and cell plate.
A mid body of dense vesicular and fibrous material appears in the equatorial region.
Microfilaments appear in the peripheral region.
Their activity constricts the cell membraY1e. As a result a centripetal furrow or cleavage develops in the middle.
The furrow deepens and divides the parent protoplast into two uninucleate protoplasts or cells. Cleavage is the usual method of cytokinesis in animals.
It also occurs in some lower plants where wall material is deposited in the furrow between the two daughter protoplasts.
Cell Plate Cytokinesis.
It occurs in plants. Middle part of spindle persists.
It gets interdigited with microtubules to form a complex structure called phragmoplast. Vesicles having pectic compounds and other wall materials appear in the middle of phragmoplast. They fuse and form a film or cell plate with membrane on either side.
This divides the parent binucleate cell into two daughter uninucleate cells.
Cell plate grows centrifugally and functions as middle lamella. Primary wall is deposited on its either side by the two daughter protoplasts.
(ii) Repair. Old and worn out cells are regularly replaced. The new cells are formed through mitosis. It is estimated that daily 5 X 109 cells are being lost and replaced in the upper layer of epidermis, lining of gut, RBCs and WBCs.
(iii) Maintenance of Surface Volume Ratio.
(iv) Maintenace of Nucleocytoplasmic Ratio.
(v) Reproduction. Mitosis is a method of multiplication in unicellular organisms
(vi) Opportunity for Differentiation. Multicellular condition provides opportunity for differentiation and division of labour
(vii) Maintenance of Chromosome Number. It maintains similar number and type. of chromosomes in all the cells. The latter is important in healing, asexual reproduction, replacement of worn out cells and regeneration.
(viii) Somatic Mutations. It maintans somatic mutations by vegetative reproduction, e.g., Navel Orange.
Meiosis
1. It occurs only once in the life cycle. The cells undergoing meiosis are called meiocytes (e.g., oocytes, spermatocytes, microsporocytes, megasporocytes).
2. Meiosis is a double division in which a diploid cell divides twice to form four haploid cells. Interphase is single so that DNA or chromosome replication occurs once. It changes single stranded or monad chromosome into double stranded chromosome.
3. Meiosis can be
(a) gametic (occurring at the time of gamete formation, e.g., animals),
(b) zygotic (occurring at the time of zygote or zygospore germination, e.g., Ulothrix, Spirogyra, Chlamydomonas) and
(c) sporic (at the time of microspore and megaspore formation, e.g., most plants comprising bryophytes, pteridophytes, gymnosperms and angiosperms).
Meiosis is studied in anthers of unopencd flowersl buds and testis of Grasshopper.
Interphase
It is a stage prior to karyokinesis.
Interphase has three phases -G1, S and G2. Chromosomes replicate during S-phase except for small parts (0.3% of total which occurs during zygotene).
Size of nucleus increases to about 3 times. Centrosome replicates in G2 phase which is short.
M-phase has two steps or divisions,
meiosis I and
meiosis II.
The essential processes that occur in meiosis are:
(i) Two successive divisions without replication of DNA in the period of interkinesis.
(ii) Crossing over and chiasma formation.
(iii) Segregation of sister chromatids.
In meiosis I, the two chromatids of a chromosome often become different due to crossing over.
leptotene,
zygotene,
pachytene,
diplotene and
diakinesis.
Chromatin fibres condense and form chromosomes.
The chromosomes often show chromomeres.
They are attached to nuclear envelope by both of their ends through a specialised structure called attachment plate.
They may also develop basket-like arrangement called bouquet stage (diverging from a common point lying near centrosome).
Chromosome number is diploid where there are two chromosomes of each type called homologous chromosomes.
Their chromatids are not clear because of the development of nucleoprotein core between them.
Homologous chromosomes join laterally in the process of synapsis (Montgomery, 1901) syndesis to form bivalents.
Number of bivalents is half the number of individual chromosomes.
Bivalents are actually tetrads but the individual chromatids of the two chromosomes are not clear due to the presence of nucleoprotein core between them.
Depending upon the area of initiation, synapsis can be procentric (starting from centromeres),
proterminal (pairing beginning from telomeric regions and proceeding inwards) and intermediate (= random pairing starts at several points).
Pairing proceeds from the starting regions towards other parts in a zipper like manner.
It brings alleles of the two homologous chromosomes exactly opposite each other.
The two chromosomes of a bivalent are held together by nucleoprotein core.
The whole structure is called synaptinemal or synaptonemal complex (Moses, 1956).
Bivalents may remain in pachytene stage for days.
Dense areas or recombination nodules (Zickler, 1977), having multienzymes, appear here and there over the bivalents.
There is breakage and re-union of chromatid segments.
At places it results in exchange of segments between nonsister chromatids of a bivalent.
The phenomenon is called crossing over. It brings about chromosome recombinations.
However, individual chromatids are not clear except towards the end of pachytene when synaptinemal complex begins to dissolve.
Chromatids become clear and the bivalents are now called tetrads.
At places homologous chromosomes remain attached to each other. The points of attachment between nonsister chromatids of two homologous chromosomes are called chiasmata.
They are sites of previous crossing over where synaptinemal cdmplex persists. Chiasmata were first seen by Johannsen (1909).
In animal oocytes, diplotene stage is often prolonged. In oocytes of many fishes, amphibians, reptiles and birds,
the diplotene chromosomes decondense, elongate and form lampbrush chromosomes. They take part in rapid synthesis of various RNAs.
Nuclear envelope breaks at places. A spindle begins to develop, with (in animals) or without centrioles.
The chromosome pairs get attached to spindle poles by discontinuous fibres. In a pair, each chromosome is attached to only one spindle pole of its side. Congression brings the bivalents/tetrads over the equator of the spindle.
The chromosome limbs lie over the equator while the centromeres are projected outwardly towards the poles.
On the equator, the chromosome bivalents form an apparent double whorl or double metaphasic plate.
Chiasmata disappear completely and the homologous chromosomes separate. The process is called disjunction.
The separated chromosomes (univalents) show divergent chromatids and are called dyads.
They move towards the spindle poles and ultimately form two groups of haploid chromosomes.
Chromosomes elongate. Nucleoplasm and nuclear envelope appear over each
chromosome group forming nuclei. Nucleolus is rarely formed.
Interkinesis.
It is brief intrameiotic interphase, which is found in some cases in order to synthesise deficient biochemicals. DNA synthesis does not occur.
Significance.
Meiosis I gives stimulus for formation of gametes or spores.
It reduces the chromosome number to half, performs random separation of paternal and maternal chromosomes, shows gene recombinations due to crossing over and produces occasional aberration due to non¬disjunction.
Meiosis II
DNA replication is absent.
Prophase II. The chromatin fibres shorten to form chromosomes.
Nucleolus and nuclear envelope break down. Spindle is formed in the area of each nucleus.
Both telophase I and prophase II are omitted in some organisms where anaphase I directly leads to metaphase II, e.g., Trillium.
Metaphase II.
Chromosomes come to lie at the equator of the spindle forming a single metaphasic plate.
The centromere of each chromosome gets attached by both its surfaces to the spindle poles of their sides by distinct chromosome fibres.
Anaphase II.
Centromere of each chromosome divides into two.
This separates the two chromatids of a chromoosome into two independent daughter chromosomes.
Each daughter chromosome is attached to spindle pole of its side by a chromosome fibre.
Chromosomes move towards the spindle poles forming two groups. Since there were two spindles, a total of four groups are formed.
Telophase II.
The four groups of chromosomes organise themselves into four haploid nuclei. Chromosomes de condense and elongated to return to interphase condition.
Nuclear envelope is formed from remains of old nuclear envelope and endoplasmic reticulum.
Nucleolus develops from NOR of certain chromosomes. It is caused by formation of rRNA and its association of ribosomal proteins.
Cytokinesis.
Cytokinesis may occur after each division (successive type) or simultaneously at the end of meiosis. It is generally through cleavage.
In case of plants, wall material is deposited in the furrows. Cytokinesis gives rise to four haploid cells.
Importance.
(i) Variations. Meiosis produces a lot of variations due to
(a) independent assortment of chromosomes
(b) crossing over (c) irregular disjunction
(d) gene mutations during replication and nicking for crossing over
(ii) Polyploidy. Failure of chromosomes to separate during anaphase I leads to polyploidy.
(iii) Maintenance of chromosome number generation after generation.
(iv) Sexual Reproduction. It produces gamete forming structures and is, therefore, essential for sexual reproduction.
Spindle fibres converge at the two ends or poles.
Spindle has the maximum diameter in the middle. It is called equator. In dividing animal cells, the spindle bears asters at the two poles. Such a spindle is called amphiaster or centric.
In plant cells spindle is acentric or anastral. Fibres of the spindle are of two main types,
continuous (from pole to pole) and discontinuous (radiating from one pole but not reaching the other).
The term eumitosis or extranuclear mitosis is used for organisation of spindle in which nuclear envelope degenerates.
In many protists, fungi and algae, the nuclear envelope does not degenerate during mitosis. Instead, spindle is formed inside the nucleus. It is called intranuclear mitosis or premitosis.
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