CELL CYCLE
FILL IN THE BLANKS
Q.01: Fill in the blanks:
(i) Mongolism is also known as _______. (Down’s syndrome)
(ii) During _______, homologous chromosomes get close to each other. (leptotene)
(iii) _______ phase precedes G2 phase. (S)
(i) Polar microtubules ______ during anaphase. (elongate)
(iv) Mitotic apparatus is formed during _______ of cell division. (prophase)
(v) The chromosome number (44+1) denotes ______ Syndrome. (Turner’s syndrome)
(vi) Intracellular contents are released during the type of cell death called _______. (necrosis)
TRUE / FALSE
Q.02: Indicate True or False:
(i) Meiosis occurs in haploid cells only. (FALSE)
CORRECT: Meiosis occurs in diploid cells.
(ii) Cell cycle is comprised of two phases i.e. karyokinesis and cytokinesis. (TRUE)
(iii) A point where non-sister chromatids cross each other is called kinetochore. (FALSE)
CORRECT: kinetochore is an area on centromere with specific base arrangement and special proteins where kinetochore fibers of mitotic apparatus attach.
(iv) G0 stands for no gap. (TRUE)
(v) Full life cycle of yeast cells requires 90 seconds to be completed. (FALSE)
CORRECT: Full life cycle of yeast cells requires 90 minutes to be completed.
(vi) Crossing over takes place during metaphase I. (FALSE)
CORRECT: Crossing over takes place during prophase I.
(vii) Autosomal non disjunction may occur in chromosomes other than 21st chromosome. (TRUE)
(viii) Benign tumors are always non localized. (FALSE)
CORRECT: Benign tumors are always localized.
(ix) Cancer is caused mainly by mutations in germ cells. ()
(x) Genetic information remains unchanged during mitosis. (TRUE)
(xi) Homologous chromosomes are necessarily identical. (FALSE)
CORRECT: Homologous chromosomes are similar but not necessarily identical.
(xii) The cells are kept alive due to trophic factors. (TRUE)
(xiii) Cytokinesis involves the division of cytochromes. (FALSE)
CORRECT: Cytokinesis involves the division of the whole cell.
(xiv) Phragmoplast is a type of fragmentation. (FALSE)
CORRECT: Phragmoplast is a membrane structure formed from vesicles originated from Golgi complex.
MCQs
Q.03: Each question has four possible options. Encircle the correct answer.
(i) In Klinefelter’s syndrome.
(a) One X chromosome missing
(b) Additional sex chromosome present
(c) Sex chromosome fails to segregate
(d) None of these
ANSWER: (b) Additional sex chromosome present
EXPLANATION: These individuals have additional sex chromosome e.g., 47 chromosomes (44 autosome + XXY).
(ii) Mitosis is divided into.
(a) Karyokinesis
(b) Cytokinesis
(c) Interphase
(d) Both a and b
ANSWER: (d) Both a and b
EXPLANATION: Mitosis is a continuous process, but conventionally it may be divided into two phases, i.e., karyokinesis, which involves the division of nucleus and cytokinesis that refers to the division of the whole cell
(iii) Separation of homologous chromosomes occur during.
(a) Prophase
(b) Metaphase
(c) Telophase
(d) Anaphase
ANSWER: (d) Anaphase
EXPLANATION: The separation of homologous chromosomes occurs during the first division of meiosis, specifically in anaphase I. During anaphase I, homologous chromosomes, each composed of two sister chromatids, are pulled apart and move to opposite poles of the cell.
Q.04: SHORT QUESTIONS
(i) Differentiate between necrosis and apoptosis.
ANSWER:
Necrosis:
(1) The cell death due to tissue damage is called necrosis.
(2) During necrosis, typical cell swells and bursts, releasing the intracellular contents, which can damage neighbouring cells and cause inflammation.
(3) Intracellular constituents are released freely in extracellular atmosphere which might have deleterious effects.
Apoptosis:
(1) Internal programme of events and sequence of morphological changes by which cell commits suicide is collectively called as apoptosis (Greek word that means dropping off or falling off).
(2) During this process the dying cells shrink and condense ultimately split up, thus releasing small membrane bounded apoptotic bodies, which are generally phagocytosed by other cells.
(3) Intracellular constituents are not released freely in extracellular atmosphere which otherwise might have deleterious effects.
(ii) What are the functions of mitotic apparatus?
ANSWER:
Functions of Mitotic Apparatus:
“The specialized microtubule structure, formed during prophase of mitosis, including aster and spindle is called mitotic apparatus.” This is larger than the nucleus, and is designed to attach and capture chromosomes, aligning them and finally separating them so that equal distribution of chromosomes is ensured.
(iii) How can you identify the cancer cells?
ANSWER:
Cancer cells can be distinguished from normal cells because:
(1) They are less differentiated than normal cells.
(2) They exhibit the characteristics of rapidly growing cells, i.e., high nucleus to cytoplasm ratio, prominent nucleoli and many mitoses.
(iv) Give importance and significance of meiosis.
ANSWER:
Importance & Significance of Meiosis:
(1) In mitosis, the hereditary material is equally distributed in the daughter cell.
(2) As there is no crossing over or recombination, the genetic information remains unchanged generation after generation, thus continuity of similar information is ensured from parent to daughter cell.
(3) Asexual reproduction in plants & animals, regeneration, healing of wounds and replacement of older cells all are the gifts of mitosis.
(4) Development and growth of multicellular organisms depends upon orderly, controlled mitosis.
(5) Tissue culture and cloning seek help through mitosis. An organism requires managed, controlled and properly organized process of mitosis, which otherwise may result malfunction, unwanted tumors and lethal diseases like cancer.
(v) Define chromosomal non disjunction.
ANSWER:
CHROMOSOMAL NON-DISJUNCTION:
“Chromosomal on-disjunction is a meiotic abnormality in which chromosomes fail to segregate during anaphase and telophase and do not finish with equal distribution of chromosome among all the daughter nuclei.” This results either increase or decrease in the number of chromosomes, causing serious physical, social and mental disorders. This non-disjunction may be in autosome or in sex chromosome.
Examples: Down’s Syndrome 47 (2n+1), Klinefelter’s syndrome 47 (44+XXY), Turner’s syndrome 45 (2n (autosomes) + X) etc.
(vi) What are symptoms of turner’s syndrome?
ANSWER:
Symptoms of Turner’s Syndrome:
Individuals affected with Turner’s Syndrome have female appearance with short stature, webbed neck, without ovaries and complete absence of germ cells.
(vii) Define cell cycle. Highlight its importance and significance.
ANSWER:
Cell Cycle: “The cell undergoes a sequence of changes, which involves period of growth, replication of DNA, followed by cell division. This sequence of changes is called cell cycle.”
It comprises two phases:
(1) Interphase ___ the period of non-apparent division.
(2) Mitotic Phase ___ the period of division.
Importance & Significance:
The significance of the cell cycle lies in its role in:
(1) Maintaining organismal growth and development by producing new cells.
(2) Repairing tissues by replacing damaged cells with new ones, thus promoting healing and regeneration.
(3) Forming gametes for sexual reproduction.
Disturbance in cell cycle can lead to uncontrolled cell division (cancer) and developmental problems.
(viii) Is interphase a resting phase? Why?
ANSWER:
The interphase is misleadingly called resting phase. It is, actually, a period of great biochemical activity involving active cellular processes such as cell growth, DNA replication (S phase), and preparation for the cell division.
(ix) In what respect does mitosis in plant cells differ from that in animal cells?
ANSWER:
Difference Between Mitosis in Plant & Animal Cells:
Mitotic events in plant cells are generally similar to the events observed in animal cells but there are some major differences:
(1) Most higher plants lack visible centrioles, instead they have its analogous region from which the spindle microtubules radiate.
(2) Moreover, shape of the plant cell does not change greatly compared with an animal cell because it is surrounded by a rigid cell wall.
(3) At cytokinesis, in place of contractile ring a membrane structure, phragmoplast is formed from vesicle which originate from Golgi complex. These vesicles originate actually during metaphase, line up in the center of the dividing cell, where they fuse to form phragmoplast at the end of telophase.
Q.05: EXTENSIVE QUESTIONS
(i) How does cytokinesis occur in animal cells? In which way does it differ from that in plant cell?
ANSWER:
Cytokinesis in Animal Cells:
During late telophase, the astral microtubules send signals to the equatorial region of the cell, where actin and myosin are activated which form contractile ring, followed by cleavage furrow, which deepens towards the center of the cell, dividing the parent cell into two daughter cells.
Cytokinesis in Plant Cells:
Mitotic events in plant cells are generally similar to the events observed in animal cells but there are some major differences.
- Most higher plants lack visible centrioles, instead they have its analogous region from which the spindle microtubules radiate.
- Moreover, shape of the plant cell does not change greatly compared with an animal cell- because it is surrounded by a rigid cell wall.
- At cytokinesis, in place of contractile ring a membrane structure, phragmoplast is formed from vesicle which originate from Golgi complex. These vesicles originate actually during metaphase, line up in the center of the dividing cell, where they fuse to form phragmoplast at the end of telophase.
- The membrane of vesicles becomes the plasma membrane of daughter cells. These vesicles also contain materials for future cell wall such as precursors of cellulose and pectin.
(ii) Why and how do the chromosomes get separated during anaphase of mitosis?
ANSWER:
- During anaphase of mitosis, the distribution of chromatids in the daughter cells takes place.
- The kinetochore fibers of spindle contract towards their respective poles.
- The polar microtubules exert force and elongate.
- The sister chromatids separate from centromere, half travelling towards each pole.
(iii) What is the role of centriole in an animal cell? How is this function carried out in plant cell?
ANSWER:
In an animal cell, the partition of the centriole takes place in the beginning of karyokinesis. Two pair of centrioles separate and migrate to opposite sides of the nucleus, establishing the bipolarity of the dividing cells.
In most higher plants, the cells lack visible centrioles. Instead, they have its analogous region from which the spindle microtubules radiate.
(iv) In what respect can cell death be regarded beneficial?
ANSWER:
Benefits of Cell Death:
Programmed cell death helps in proper control of multicellular development, which may lead to deletion of entire structure (e.g., the tail of developing human embryos) or part of structure (e.g., tissue between developing digits).
Cell death even controls the number of neurons, because most of the neurons in the human body die during development.
(v) Compare mitosis with meiosis and describe their importance.
ANSWER:
| MITOSIS | MEIOSIS | |
| Purpose | Somatic cell division for growth, repair, and maintenance. | Gamete formation for sexual reproduction. |
| No. of Divisions | One division. | Two divisions. |
| Daughter Cells | Two genetically identical diploid cells. | Four genetically diverse haploid cells. |
| Genetic Diversity | No exchange of genetic material. | Involves crossing over and random assortment, leading to genetic variation. |
| Occurrence | Occurs in somatic cells. | Occurs in germ cells (e.g., egg and sperm cells). |
(vi) Define disjunction and discuss its effect.
ANSWER:
Consult textbook at page 158 — 159.
(vii) Describe meiosis and explain its significance.
ANSWER:
Consult textbook at page 155 — 158.
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