1
(CELL CYCLE)
Phases of cell cycle
Every individual of a given species has a characteristic number of chromosomes in its body cells. The normal
chromosomal constitutions are called karyotypes.
Usually when cells reach a certain size, they must either stop growing or divide, Some cells such as nerve, skeletal
muscle, and red blood cells do not normally divide once they as nerve, skeletal muscle, land red blood cells do not normally
divide once they are mature. The actities of growing and dividing cells, can be described in terms of the life cycle of the cell
or the cell cycle. Such cycles start each time when new cells are formed, and they end when those cells compete their own
division. The cycle starts again for each new daughter cell. The process that occurs during the division of eukaryotic cells is
conveniently diagramed as a cells cycle:
G1 S G M C
Interphase
Most of the cell cycle is spent in interphase. Chromosomes duplicate during this period, many cell parts are made,
and the cell does most of its growing. Typically, intyerphases lasts for at least 90% of the total time required for the cell
cycle. DNA synthesis, the main event in chromosome duplication, occurs in the middle of interphase and serves as the basis
for dividing interphase into three sub-phases.
G1 phase
The firs subphase, designated G1 is the period befor DNA synthesis begins. G stands for gap and G 1 refers to the
firs gap between cell division and DNA synthesis. This phase is a gap between cell activity. In G 1 phase, the cell increases
its supply of proteins, increases the number of many of its organelles, such as mitochondria and ribosomes, and grows in
size. Towards the end of G1 phase there is an increased activity of enzymes required for DNA synthesis.
S Phase: Following G1 phase is the sub-phase S, in which DNA synthesis (replication) actually occurs. At the beginning of
the S phase, each chromosome is single. At the end of this phase, after DNA replication, the chromosomes are double, each
consisting of two sister chromatids.
G2 Phase: The third sub-phase, called G2 phase, spans the time from the complition of DNA synthesis to the onset of cell
division. In this phase, increased protein synthesis occurs as the final step in the cells preparation for cell division. The
completion of G2 phase in marked by the beginning of mitosis. The sequence of the sub-stages of interphase is therefore:
G1 phase S Phase G2 Phase
M phae: In this the micro-tubular apparatus is assembled, binds to the chromosomes, and moves the siter chromosomes
apart. This phase is called mitosis.
C phase: In this phase the cell itself divides, creating two dauthter cells. This phase is called cytokinesis.
Length of each phase is variable. In this case of human cells, average cell cycle is of about 24 hours, mitosis takes
30 minutes, G1 phase 9 hours, S phase 10 hours, and G1 phase 4 hours and 30 minutes. In yeast the full cycle is of only 90
minutes.
Amitotic Cell Division in Prokaryotes
Prokaryotes (bacteria) reproduce by amitosis or binary fission (Fig No. 21.2). There is a single chromosomes
having a circular DNA molecule. When the DNA in bacterial cell is replicating, it is attached to the plasma membrane at two
separate points. Continued growth of the cell gradually separates the chromosomes, which are still attached lto the
membrane. Eventually, the plasma membrane and the bacterial cell wall grow inward, dividing the cell into two.
Cell Division
There are two types of cell division: Mitosis and Meiosis.
Mitosis in animal Cell
The word mitosis comes from the Greek word ‘mitos’ which means thread, and refers to the threadlike appearance
of chromosomes during this period. Mitosis was first studied by Walther Flemming in animals and by Strassburger in plants.
Definition: Mitosis can be define as “ The division of cell in such a manner that the chromosomes are duplicated and
distributed equally to the daughter cells”. The process of mitosis reproduces cells and distributes equal DNA to each
daughter cell. Thus, mitosis is a qualitative division of the cell. It takes place in the somatic (body) cells. The process of cell
division can be divided into two main phases, karyokinesis and cytokinesis.
Karyokinesis: Division of Nucleus.
Traditionally karyokinesis is studied in four stages, but actually it is a continuous process.
(1) Phophase (Gk: Pro; before)
(2) Metaphase (meta ; after)
(3) Anaphase (ana; again)
(4) Telophase (telo; end)
Intraphase: (Preparing the Scene)
Interphase (inter; between) is the phase between cell division. By G 2 stage the cell has doubled much of its cell
contents. The cytoplasm contains two microtubule organizing centre (MTOCs) each with a pair of centriole.
The chromatin resembles interoven fine threads. One or more nucleoli are present in the nucleus. Duplication of
chromosomes takes place DNA replicates. Synthesis of RNA and proteins occur. The chromosomes are mot visible under a
light microscope, as they are still in the from of lossely packed chromatin.
Prophase: Formatio of Mitotic Apparatus)
Activities in the nucleus: The chromosomes being to shorten and thicken, coiling upon themselves a process called
condensation. The condensation process continues throughout the prophase.
Nucleolus diappears: The synthesis of r-RNA ceases when that portion of the chromosome bearing rRNA genes, is
condensed, as a result the nucleolus diappears.
2
Activities outside the nucleus: Early in the prophase, the two centriole pairs start to move a part, and continue to move until
they reach the opposite poles of the cell establisheing the bipolarity of the cell. Three sets of fibers i.e. microtubules
originate from each pair of centriole. The microtubules are composed of protein tubulin and RNA. Two sets of microtubules
(half spindle and spindle) compose the spindle. The polar microtubules do not interact with the chromosomes. They
interdigitate with polar molecules from opposite poles.
Nuclear membrane disappears: During the formation of the spindle the nuclear membrane breaks down, and its
components are reabsorbed into the endoplasmic recticulum.
Aster: When centrioles reach the poles of the cell, they radiate the third set of mictotubules outward, thus bracing the
centrioles against the cell membrane, this arrangement of microtubules is called aster. The function of the aster is brobably
mechanical (acting to stiffen the point of micro-tubular attachment during the later contraction of the spindle).
Mitotic apparatus: The aster, spindle and centrioles are collectively called the mitotic apparatus.
Chromatids: At the beginning of prophase the chromatin material is condensed and folded, as a result chromosomes appear
as thin threads which range in length from 0.25µm to 50 µm in length. By late prophase, each chromosome appears double.
The two halves are called chromatids. Each pair of chromatids have a centromere. Specialized protein complexes called
kinetochore having specific base arrangement develop on either side of each centromere. The second set of microtubules of
spindle fibres are called each centromere. The second set of microtubules of spindle fibres are called kinetochore spindle
fibres. The kinetochores of sister chromatids capture these fibers coming fron opposite poles. Forces associated with spindle
microtubules move the chromosomes toward centre of the cell.
Metaphase: (Division of Centromers)
The chromosomes move towards the equator of spindle, (half way between the poles, the equatorial or metaphase
plate). It is not a physical structure but an imaginary one. The kinetochore extends as spindle form fibers, the two poles of
the spindle are attached to the kinetochore of centromere.
The centromeres of all chromosomes are lined up on the metaphase plate. For each chromosome, the kinetochores
of the two sister chromatids face opposite pole of the spindle. The microtubules attached to a particular chromatid come
from one pole of the spindle, and those attached to its sister chromatid come from the opposite pole. At the end of the
metaphase, the centromeres divide, freeing the two sister chromatids from their attachment to one another. Centomere
replication is simultaneous for all the chromosomes.
Anaphase: (Separation of the Sister Chromatids)
The beginning of the Anaphsase is marked by the separation of the sister chromatids. Each sister chromatid now
rapidly moves towards the poles, to which its microtubule is attached.
The poles move apart: The spindle fiber consists of microtubules which occur in the form of a pair. Each member of the
pair is attached to opposite poles. The pair of microtubules of spindle fibre slide over one another, as a result the poles move
apart and increase in length.
The centromeres mover toward the poles: The kinetochore of the chromosomes are attached to the poles by half spindle
fibers, so with the movement of the pole apart, the chromosomes also move towards each pole. The spindle fibers do not get
shorten by condensation. At the end of the poles the spindle fiber is broken down into its subunits by the action of enzymes.
The tubulin from the spindle fibre is removed, as a result the spindle fiber becomes shorter and shorter, pulling the
chromosomes closer to the pole of the cell. Anaphase is over when equivalent and complete collection of chromosomes have
reached the two opposite of the cell.
Telophase: (Reformation of Nuclei)
At the beginning of the telophase the two sets of chromosomes reach the opposite poles of the cell. The condensed
and coiled chromosomes begin to uncoil. They increase in their length, and become thinner i.e. the chromosomes become
like the chromosomes of interphase stge.
Disappearance of spindle: The spindle fibers break into tubulin subunits reach the opposite poles of the cell. The
condensed and coiled chromosomes begin to uncoil. They increase in their length, and become thinner i.e. the chromosomes
become like the chromosomes of interphase stage.
Formation of the nuclear membrane: Each set of sister chromatids becomes suttounded by endoplasmic reticulum. This
ER form the nuclear membrane around each set of chromosomes.
Appearance of nucleolus: The chromosome in each set continue to uncoil. One of the early genes to regain expression are
the genes for rRNA. The rRNA are synthesized. These newly formed rRNA form the nucleoulus. At the end of the
telophase the nuclear division is over.
Cytokinesis: The Division of Cytoplasm
Nuclear division is over, but not the cell division. The new nuclei are still in the same cytoplasmic unit. Commonly
nuclear division is followed by separation of the cytoplasm into two parts. This separation accomplished by pinching of the
cell membrane when the astral microtubukles send signals to equatorial region of the cell, where actin and myosin are
activated which form the contractile ring. The pinching near the middle of an animal cell forms a cleavage furrow. The
process of cytoplasmic division is called cytokinesis. (Gk: Cyto; cell, Kinesis; movement). Cell organelles are distributed to
the towo daughter cells.
Significance of Mitosis
Diploid No: The diploid (2n) number of chromosomes is maintained in the cell.
Asexual reproduction: Asexual reproduction of single called eukaryotes takes place by mitosis.
Development: It is the basis of growth in the multicellular eukaryoptes.
Replacement of cells: Healing of wounds and replacement of damaged cells is accomplished by mitosis.
New cells: New cells are produced by mitosis.
Genetic continuity: The genetic informationi.e. DNA is regularly and equally distributed to the daughter cells.
Cancer: It is caused by uncontrolled mitosis.
TABL;E: 20.1 COMPARISON
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Mitosis in Animal Cell Mitosis in Plant Cell
Mitotic apparatus is Mitotic apparatus is
formed by aster, formed by spindle fibers
centriole and spindle only, as the aster and
fibers centrioles are absent in the
cells of higher plants.
In cytokinesis the It is achieved by the
cytoplasm is pinched in formatuion of a partition
the centre and forms two wall. The interzonal region
cells. spindle fibers at the
anaphase get transformed
into a structure called cell
plate or phragmoplast. It
extends round the equatorial
plate separating the two
cells.
Cancer
Cancer, is a disease of the cell cycle. Unlike normal cells of the body, cancer cells do not have properly functioning
cell-cycle control system and therefore divide excessively. This excessive growth can result in an abnormal mass of cell
called a tumor. Not all tumors are cancerous, however, a benign tumor is and abnormal mass of essential normal cells.
Benign tumors can cause problems if they grow in certain organs, Such as the brain but usually they can be completely
removed by surgery. They always remain at their original site in the body.
In contrast to a bengn tumor, a malignant tumor is cancerous. It is a mass of cancer cells, which are capable of
spreading into neighbouring tissues and often to other parts of the body. A malignant tumor arises from a single cancer cell
and displaces normal tissue as it grows. If the tumor is not killed or removed, some of the cancer cells spread into
surrounding tissue, enlarging the tumor. Cells may also split off from the tumor, invade the circulatory system (lymph
vessels and blood vessels), and travel to new locations, where they can form new tumors. The spread of cancer cells beyond
their original site is called metastasis.
Cancers are named according to the organ or tissue in which they arise, and over 200 different tupes are recognized
in humans. Form simplicity they are grouped into four categories. Carcinomas are cancers thet originate in the external or
internal coverings of the body; such as the skin or the lininig of intestine. Sarcoma arise in tissues that support the body,
such as bone and muscle. Cancers of blood-forming tissues, such as bone marrow, spleen are called leukemias and that of
lymph nodes are called lymphomas. Cancer cells can go on dividing indefinitely, as long as they have a supply of nutrients.
If cancer cells stop dividing, the seem to do so at random points in the cell ctycle, rather than just at the normal cell-cycle
checkpoints.
Treatment
Two types of cancer treatment, chemotherapy and radiation therapy, attempt to halt the spread of cancer cells by
stopping them from dividing. In radiation therapy, parts of the body that have cancerous tumors are exposed at high-energy
radiation, which disrupts cell division. Chemotherapy uses the same strategy as radiation; in theis case, drugs that disrupt
cell division are administered to the patient.
Cancer cells can be distinguished from normal cells because they have high nucleus to cytoplasm ratio, prominent
mnucleoli and many mitosis. The presence of invading cells in otherwise normal tissue is an indication of malignancy.
Cancer is caused mainly mutations in somatic cells. Secondary, the cancer results from the accumulation of a few as three to
as many as twenty mutations, in genes that regulate cell division.
Characteristic of Cancer Cells
At the least four features characterize all cancer cells.
(1) Their plasma membrane and cytoplasm change profoundly.
(2) Cancer cells grow and divide abnormally.
(3) Cancer cells have a wakened capacityu for adhesion.
(4) Cancer cells are lethal.
Meiosis in an animal cells
Sexual reproduction takes place in animal and plants by forminggametes. The gametes fuse to form zygote. They
zygote forms the organism. The number of chromosomes remain constant in the successive generations. How the number of
chromolsomes remoan constant from generations to generations? August Weismann proposed the hypothesis that “ there
must be a kink of cell division in which the chromosome member is halved”. There are two types of cell. (i) Somatic cells
which make up the body. (ii) Germline cells which produce the gametes. Both germline cells and somatic cells are diploid
(2n). When a germline cell undergoues division it produces cells with half of the diploid number of chromosomes i.e.
haploid (n) and the process of division is called reduction division or meiosis. Sperms and eggs are produced by meiosis in
anuimals. Spores are produced by meiosis in plants.
Definition: Meiosis is a process of nuclear division in which the number of chromosomes in a cell is halved during cell
division.
The Stages of Meiosis
Meiosis is a continuous process. It can be described most easily by dividing it into two arbitrary stages. The two
stages of meiosis are called meiosis I and meiosis II. These are further subdivided into prophase, metaphase, anaphase and
telophase.
The First Meiotic Division
Interphase I: The DNA duplicates by its replicating process. The fine chromosomes. It is further divided into five stages:
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(1) Leptoten (2) Zygotene (3) Pachytene (4) Dipotene (5) Diakinesis.
Leptoten: (Thin Thread Stage)
Chromosomes look like interweaving threads. Due to condensation of chromatin material; chromosomes become
more apparent and distinct, nucleus increases in size. Initially thin chromosomes become shorter and thicker. The
chromosomes in size. Initialy thin chromosomes become shorter and thicker. The chromosome number of the cells is seen.
Homologous chromosomes start getting closer to each other. In the cell there are two of each type of chromosome. The
identical chromosomes are called homologous chromosomes (Gk: Homologue; agreement). The morphology and position
of the centromere of the homologous chromosome is same. For example in man, the number of chromosome is 46 and the
homologous pair of chromosome is 23.
Zygotene: (Pairing Stage)
The homologous chromosomes begin to pair length wise with its homologue. The process of pairing is called
synapsis. (si-nap-sis) (Gk: Union). The synapsis may start from any point with the corresponding complementary DNA stand
of the other.
Pachytene: (Thickening Stage)
After the completion of synapsis the chromosomes shorten and thicken due to condensation of the chromatin
material land their double nature is evident. Each pair of synapsed chromosome consists of four chromatids, two
centromeres and is called a tetrad or bivalent.
Crossing over: The chromatids of the homologues my cross each other and the point of crossing is X shaped figure under
the light microscope. It is called chiasma (Gk: chiasma; cross, plural: chiasmata). Chromosomes segment is exchanged
between the two homologous chromosomes at the chiasma and is called crossing over.
Diplotene: (Duplication Stage)
The paired homologous chromosomes begin to separate by repelling. (The bivalent is still attached at chiasmata or
chiasma). Diplotene can last for months or years. The chromatids are clearly visible due to the progress in condensation
cycle.
Diakinesis: (Moving Apart Stage)
The tetrad are more evenly distributed in the nucleus against the nucleoli envelope. The number of chiasmata
begins ro reduce as the diakinesis reaches to its end, but the terminal chiasmata are still present. The nucleoli disappear. The
nuclear membrane is still present. The diad (the two chromatids attached to a single centromere is called diad) behaves as
a single unit because they are held together by a common centromere, throughout meiosis.
Metaphase I
The nuclear membrane disintegrate. The microtubules form the spindle. The chromosome lineup in double row
i.e. pair on the equator. Microtubules bind only to one kinetochore of each centromere. The centromere of one homologue
becomes attached to microtubules extending to one pole, whereas the centromere of the other homologue becomes attached
to microtubules extending to other pole.
Anaphase I
The attachmend of chromosomes to the spindle is complete. The microtubules that are attached to the
homologous chromosomes begin to slide over one another, as a result the spindle poles move apart. The spindle fibers
become shorter as the spindle fibers are dragged to the poles. They are broken by the action of enzymes. With the
shortening of spindle fibers, the chromosomes that are attached to the spindles are also pulled towards each pole. When the
shortening of spindle fiber is completed, each pole has single set of chromosomes, consisting of one member of each
homologous pairs.
Telophase I
In telophase I, each set of chromosomes is present at their respective poles. By cytokinesis two cells are formed,
which are haploid.
The second Metotic Division: Meiosis II is simply a mitotic division. It occurs to separate the sister chromatids as in
mitosis.
InterphaseII: DNA does not duplicate in interphase II interphase II is very brief. Each of the two cells resulting from
meiosis I progress into meiosis II very quickly.
ProphaseII: The complicated nuclear events of prophase do not take place.
Metaphse II and Anaphase II: The chromosomes lineup in the same fasion as they did in metaphase I. Here again a randon
distribution of chromosomes take place. The centromere divides, as a result the two chromatids are seoarated. As each
chromatid is now a sparated structure they are called chromosomes. Chromosomes are distributed in equal number, to each
pole. DNA does not replicate but the centomere divides.
Telophase II: The nuclei are reconstructed in the typical manner. Each nucleus now constains haploid set of chromosomes,
because DNA has duplicated only once during the cell division.
Fate of haploid cells: Each of the four haploid products of meiosis contains a basic set of chromosomes. These haploid cells
may function directly as gametes, as they do in animals or may continue to divide by mitosis, as they do in plants, fungi and
many protests.
Table: 20.2 Comparison
Mitosis Meiosis
1. It takes place in somatic 1. It takes place in the
cells. germline cells
2. It consists of one division 2. It consists of two divisions
3. The centromere divides in 3. The centromere does not
mitosis at metaphase divide at metaphase of
meiosis I.
4. The chromosomes are 4. The chromosomes are
monovalent in the bivalent in the daughter
dauthter cells cells of meiosis I.
5
5. The number of 5. The number of
chromosomes remains chromosomes is halved in
constant meiosis I and this half
number remains same in
meiosis II.
6. Chiasmata, tetread, 6. Chiasmata, tetrad, crossing
crossing over do not take over take place
place
7. Two diploid cells are 7. Four haploid cells are
formed formed
8. The daughter cells remain 8. The daughter cells are
as cells transformed into sperms or
eggs cells.
9. Genetic content of mitotic 9. Genetic content of meiotic
products is identical products is different
10. Mitotic products are 10. Meiotic products cannot
usually capable of underfo further meiotic
undergoing additional divisions.
mitotic divisions
Significance of Meiosis
Reduction of chromosome number:
The number of chromosomes become half (haploid) in the gametes due to which the number of chromosomes
remain constant from generation to generation.
Distribution of genetic material: It provides a mechanism for the transmisision of genetic material form one generation to
the next.
Mixing of genetic material: There is a mixing of genetic material in the zygote formed by the fusion of gametes from
different parents. As a result there is also genetic variability.
Role in adaptation: It is the meiosis which offers a chance for the production of the best adapted individual and better
chance for survival and better chance for survival in the changed environment.
Role in evolution: Meiosis provides a raw material for evolution. Evolution is impossible without a change. Meiosis serves
as a source for the change.
Non-Disjunction
Meiosis occurs repeatedly, as testes or ovaries produce gametes. Almost always, the meiotic spindle distributes
chromosomes to daughter cells without an error. But occasionally there is an accident, called a nondisjuction, in which the
members of a chromosome pair fail to separate. Figure No. 21.9. A and B illustrate two ways that nondisjuction can occur.
For simplicity, we use a hypothetical organism whose diploid chromosome number is 4. In both figures, the cell at the top is
diploid (2n), with two pairs of homologous chromosomes undergoing meiosis I.
Sometimes, as in Figure 21.9, a pair of homologous chromosomes does not separate during meiosis I. In this case,
even though the rest of meiosis occurs normally, all the resulting gametes end up with abnormal numbers of chromosomes.
Two of the gametes have three chromosomes, two of which are the same; the others two gametes have only one
lchromosome each. In figure B, meiosis I is normal, but a pair of sister chromatids fail to move apart in one of the pair of the
cells during meiosis II. In this case, two gametes have the normal complement of two chromosomes each, but the other two
gametes have the abnormal complement.
Figure 21.9 shows what happens when an abnormal gamete produced by nondisjuction unites with a normal gamete
in fertilization. Here, an egg cell with two copies of one of its chromosomes (a total of n + 1 chromosomes) is fertilized by a
normal sperm cell (n). The resulting zygote has an extra chromosome (a total of 2n + 1 chromosomes). Mitosis will then
transmit the abnormality to all wmbryonic cells. If this were a real organism and if survived, it would have an abnormal
karyotype and probably a syndrome of disorders caused by the abnormal number of genes. Nondisjunction can lead to an
abnormal chromosome number in either sex of any sexually reproducing, diploid organism, including humans. Three
chromosome disorders of interest are Down syndrome, Klinefelter syndrome and Turner syndrome, Klinefelter syndrome
and Turner syndrome.
Down Syndrome (Trisomy 21)
Down syndrome (Down’s is not used these days) is also known as Mongolism (Fig: 20.10) is easily recognized by
these characteristics: short stature; an eyelid fold; stubby fingers; a wide gap between the first and second toes; a large,
fissured tongue; a round head; a palm crease, the so-called simian line; and, unfortunately, mental retardation, along with an
enlarge tongue. Down syndrome is also called trisomy 21 because the individual usually has three copies of chromosome 21.
In most instances, the egg had two copies instead of one of this chromosome. The chances of a woman having a Down
syndrome child increase rapidly with age, starting at about age 35. The frequency of Down syndrome is 1 in 800 births for
mothers under 40 years of age and 1 in 80 for mothers over 40 years of age.
Turner Syndrome (XO)
Turner Syndrome occurs in one in 6,000 births. The individual is XO with one sex chromosome, in XO the O
signifies the absence of a second sex chromosome. These females are short, have a broad chest, and webbed neck. The
ovaries, oviducts, and uterus are very small and nonfunctional. Turner females do not undergo puberty or menstruation, and
there is a lack of breast development. They are usually of normal intelligence and can lead fairly normal lives, but they are
infertile even if they receive hormone supplements.
Klinefelter Syndrome (XXY)
Klinefelter syndrome occurs in one in 1,500 briths. These males with two or more X chromosomes in addition to a
Y chromosome are sterile. The testes and prostate gland are underdeveloped, and there is no facial hair. Also, they may be
some breast development. Afffected individuals have large hands and feet and very long arms and legs. They are usually
slow to learn but mentally retarded unless they inherit more than tow X chromosomes.
6
Apoptosis and Necrosis
The first cell of a new multicelled individual contains marching orders that will guid its descndants along a program
of growth, development, and reproduction, then on to death. As part of the program, many cells proceed to self-destruction
when they compete a prescribed function. If they become altered in ways tat might pose a threat to the body as a whole, as
by infection or cancer, they can execute themselves. Apoptosis (Gk; dropping off or falling off) is the name for this form of
cell death. It starts with molecular signals that activate and unleash lethal weapon of self extraction, which were stockild
earlier within the cell. Protein cleavage enzymes are such weapons. Think of them as folded pocketknives. When popped
open, they chop apart structural proteins, including the building blocks of cytoskeleton elements and nucleosomes that
organize the DNA.
The dying cells shrink, condense and ultimately split up, thus release small membrane bounded apoptic bodies,
such are generally phagocytosed by other cells. Intracellular constituents are not released freely in extracellular atmosphere
which otherwise might have deleterious effects. In contrast to suicide the cell death due to tissue damage is called necrosis
during which the typical cells swells and bursts, releasing the intracellular contents, which can damage neighbouring cells
and cause inflammation.
EXERCISE – SECTION – I – OBJECTIVE QUESTIONS
1. Fill in the blanks
i. During cell division. ____ sets of microtubules originate from each pair of centrioles.
ii. In Down syndrome, chromosme no. ____ fails segregate.
iii. A person with Klinefelters syndrome has an extra _____ chromosome.
iv. The cell death due to tissue damage is called ______.
v. Division of the nucleus is called ______.
vi. Mitotic apparatus is formed during ______.
vii. Spindles are composed of mainly a protein called ____.
viii. In man there are ____ pair of homologous chromosomes.
ix. Exchange of chromosomal segment is called ___.
x. In plants meiosis takes place during the formation of ____.
2. Mark the statements as “True” or “False”.
i. Interphase is the period of great biochemical activity.
ii. Meiosis occurs in haploid cells.
iii. During S-phase, DNA is synthesized and chromosome number is doubled.
iv. Division of the cytoplasm is called cytokinesis.
v. Equatorial plate is formed in plants.
3. Select the correct answer and encircle it.
i. It is period of extensive metabolic activity: (a) G0 (b) G1 (c) G2 (d) S
ii. The period of cell cycle between two consecutive divisions is termed as: (a) prophase (b) metaphase (c) telophase (d)
interphase
iii. Cancer is caused mainly by mutations in: (a) somatic cells (b) sex cell (c) G 2 Stage (d) S-phase
iv. The sex chromosme complement in individuals with klinefelter syndrome is: (a) XXY (b) XY (c) XXYY (d) XYYY
v. Mitosis is divided into: (a) karyokinesis (b) cytokinesis (c) interphase (d) both a and b
vi. Small localized tumors are called: (a) benign (b) malignin (c) cancer (d) interdigitate
vii. The significane of mitosis is: (a) to bring about cell division (b) to produce daughter cells with the same genetic make
up as the parent cell. (c) to control the size of the cell (d) to control cell contents.
viii. Which of the following is not for crossing over? (a) it occurs during prophase I (b) it occurs during prophase II (c) it
occurs between homologues (d) it is seen as X-spahped structure called chiasmata.
4. Match the following columns.
Column ‘A’ Column ‘B’
a. Phragmoplats i. Somatic cells
b. Cancer ii. Panchytene
c. Tetrad iii. Anaphase I
d. Crossing over iv. Plant cell
v. Zygotene
5. Draw a label:
(i) stages of mitosis. (ii) stages of meiosis
SECTION – II – SHORT QUESTIONS
(1) Distinguish between:
mitosis and meiosis - chromosme and chromatin
chromosome and chrmatids – centromere and centriole
cytokinesis and karyokinesis – centromerere and kinetochore
chiasmata and chiasma – diad and bivalent
apoptosis and necrosis – haploid and diploid number of chromosomes.
2. Write notes on:
karyotypes chromatine kinetochore
cytokinesis crossing over leptotene
zygotene pachytene diplotene
diakinesis apoptosis Down syndrome
klinefelter syndrome turner syndrome cancer
nondisjuctionof chromosme
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3. Write the differences between:
(a) mitosis in animal cell and plant cell
(b) Mitosis and meiosis
4. Write the significance of (a) mitosis (b) meiosis
5. What is mitotic apparatus?
6. How does meiosis contribute to genetic recombination?
7. What is synapis and crossing over?
8. Define cell cycle and outline main features of each stage.
SECTION – III – EXTENSIVE QUESTIONS
1. Give a detail account of mitosis in an animal cell.
2. Give a detail account of meiosis in an animal cell.
3. Write a comprehensive note on cancer.
4. Give a detailed account of nondisjunction, and the syndromes caused.
ANSWERS
1.
(i) 3 (ii) 21 (iii) X
(iv) necrosis (v) karypkinesis (vi) prophase
(vii) tubulin (viii) 23 (ix) crossing over
(x) spores
2.
(i)T (ii) F (iii) T (iv) T (v) F
3.
(i) b (ii) d (iii) a (iv) a (v) d (vi) a
(vii) b (viii) b
4.
(a) iv (b) I (c) v (d) ii