Cell Cycle and Cell Division
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An excerpt from the Biology Term 2 → Slip Test 2 question paper of DON BOSCO SCHOOL OF EXCELLENCE SENIOR SECONDARY SCHOOL; conducted on 17.12.2021
I. CHOOSE THE CORRECT ANSWER ( 6X1=6)
When there is an increase in the condensation of chromatin during the process of cell division –
A) Heterochromatin increases
B) Euchromatin increases
C) Differentiation of euchromatin & heterochromatin decreases
D) Differentiation of euchromatin & heterochromatin increases
The condensation of chromosomes is observed in ______
A) Prophase 1
B) Anaphase 1
C) Metaphase 1
D) None of the above
Nuclear DNA replicates in the ________ phase.
A) G2 phase
B) M phase
C) S phase
D) None of the above
_________ is a form of cell division which results in the creation of gametes or sex cells.
A) Mitosis
B) Meiosis
C) Miosis
D) None of the above
The longest stage in the cell cycle is
A) Interphase
B) Anaphase
C) Metaphase
D) None of the above
Choose the correct sequence. A. Chromatin condensation B. Protein synthesis C. Duplication of centrioles D. Centrioles moves towards opposite poles
(a) C, A, B, D
(b) C, B, A, D
(c) C, D, B, A
(d) A, C, D, B
II. ANSWER THE FOLLOWING: (2X2=4)
Why is mitosis an equational division?
Write the phases of the cell cycle against each of the events
a) The disintegration of the nuclear membrane
b) The appearance of the nucleolus
c) Division of centromere
d) Replication of DNA
Notice
The upcoming section includes copyrighted material from:-
The cell cycle consists of distinct phases, including interphase and the M phase. The M phase is also known as mitosis, where cells reproduce asexually to create new cells.
During interphase, the cell goes through G1, S, and G2 stages. In G1, all the cell's contents except for the chromosomes are duplicated. In the S phase, the chromosomes are replicated. In G2, the cell checks for errors and makes needed fixes before entering the M phase.
Some cells, like those in the liver, may enter a phase called G0, where they spend a long time without dividing after completing mitosis.
Mitosis is the process in eukaryotic cells that produces two genetically identical daughter cells from one parent cell. It is asexual and involves the splitting of chromosomes. Mitosis consists of prophase, prometaphase, metaphase, anaphase, and telophase. Each phase has its own specific events, such as chromosome condensation, spindle attachment, chromosome alignment, chromatid separation, and membrane reformation.
Telophase marks the end of mitosis, where the chromosomes de-condense, the nuclear membrane is reassembled, and the cell divides into two identical daughter cells.
Overall, the M phase of the cell cycle involves the replication and division of chromosomes to create new cells.
Meiosis is a complex cell division process that plays a crucial role in sexual reproduction in animals, humans, and plants. Unlike mitosis, which produces two identical daughter cells, meiosis results in the formation of four haploid daughter cells, each containing half the number of chromosomes as the parent cell.
Meiosis is divided into two parts: meiosis I and meiosis II. Each part consists of several phases or stages of cell division. Meiosis I includes prophase I, metaphase I, anaphase I, and telophase I. Meiosis II consists of prophase II, metaphase II, anaphase II, and telophase II.
In meiosis I, the parent cell undergoes interphase before division. Interphase consists of three subphases: G1 phase, S phase, and G2 phase. During interphase, the parent cell prepares for division by increasing in size, synthesizing DNA, and synthesizing proteins. Following interphase, prophase I begins. In prophase I, the chromosomes condense and pair up to form tetrads through a process called synapsis. This allows for genetic recombination or "crossing over" of genes. The chromosomes then thicken, detach from the nuclear envelope, and migrate to the metaphase plate. In metaphase I, the tetrads align themselves at the metaphase plate, with the centromeres of the chromosome pairs facing opposite poles. Anaphase I is characterized by the movement of chromosomes to opposite sides of the cell, pulled by kinetochore fibers. Telophase I concludes meiosis I, as the spindle fibers continue to pull the chromosome pairs to opposite poles, resulting in the formation of two daughter haploid cells through cytokinesis.
Meiosis II follows a similar pattern to meiosis I, but with some differences. In metaphase II, the chromosomes line up at the metaphase II plate, with the kinetochore fibers of sister chromatids pointing towards opposite poles. Anaphase II involves the separation of sister chromatids, which become individual chromosomes and migrate to opposite poles. Telophase II marks the end of meiosis II, as nuclei form at each opposite pole and cytokinesis occurs to form two additional daughter haploid cells.
The key difference between meiosis and mitosis lies in the number of daughter cells, genetic composition, length of prophase, formation of tetrads, chromosome alignment in metaphase, and method of chromosome separation.
1. What is the average cell cycle span for a mammalian cell?
Solution:
The average cell cycle span for a mammalian cell is 24 hours.
2. Distinguish cytokinesis from karyokinesis.
Solution:
Cytokinesis
Karyokinesis
It is the cell division of cytoplasm that occurs during the M phase of the cell cycle.
It is the separation of daughter chromosomes corresponding to the M phase of the cell cycle
3. Describe the events taking place during interphase.
Solution:
Events taking place during interphase are as follows:
G1 phase (Gap 1) – During this stage, the cell is metabolically active. It grows and prepares the DNA to replicate.
S phase (Synthesis) – During this stage, the synthesis of DNA takes place. The DNA quantity doubles, whereas the number of chromosomes remains unchanged
G2 phase (Gap 2) – During this phase, the cell advances to grow and prepares itself for division. It is during this stage that the RNA and proteins that are required for mitosis are generated.
4. What is Go (quiescent phase) of cell cycle?
Solution:
In adult animals, some cells will not exhibit cell division, and many other cells occasionally divide when there is a need to replace cells that have been lost because of injury or cell death. These cells exit the G1 phase to enter an inactive stage of the cell cycle called the G0 phase. Cells in the G0 phase do not proliferate unless called on to do so. Hence, the cells in this phase tend to become inactive, stop dividing and become specialized through the differentiation process.
5. Why is mitosis called equational division?
Solution:
Mitosis is called equational division because the number of chromosomes in the parent and progeny cells is the same.
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.
Solution:
i) Chromosomes are moved to the spindle equator in the Metaphase. ii) Centrosomes split, and chromatids separate in the Anaphase iii) Pairing between homologous chromosomes takes place in the Zygotene stage of prophase 1 in meiosis iv) Crossing over between homologous chromosomes takes place during the Pachytene stage of prophase 1 in meiosis
7. Describe the following: (a) synapsis (b) bivalent (c) chiasmata Draw a diagram to illustrate your answer.
Solution:
a) Synapsis – Homologous chromosomes pair together during Zygotene of prophase-I of meiosis. This pairing is called synapsis.
b) Bivalent or tetrad is the pair of complexes formed by a pair of synapsed homologous chromosomes during the zygotene of prophase I of meiosis.
c) Chiasmata
During diplotene, the paired chromosomes form an X-shaped structure known as chiasmata. At chiasmata, the crossing over between two non-sister chromatids takes place.
8. How does cytokinesis in plant cells differ from that in animal cells?
Solution:
Plant cytokinesis
Animals cytokinesis
Occurs by cell plate formation
Takes place by cleavage
Cell plate moves to the centre and extends towards the exterior
Cleavage begins at the periphery and advances inwards
The fusion of vesicles originates in cell plate formation
Cleavage starts with the contraction of a peripheral ring of microfilaments
Midbody is not formed
Midbody is formed with dense material in the middle of the cell.
9. Find examples where the four daughter cells from meiosis are equal in size and where they are found unequal in size.
Solution:
During the formation of male gametes in human beings (sperms), the four daughter cells formed during meiosis are equal in size. The formation of the female gamete (ovum) during meiosis results in the formation of four daughter cells, unequal in size. The unequal daughter cells are – one big mature ovum and 3 small polar bodies.
10. Distinguish anaphase of mitosis from anaphase I of meiosis.
Solution:
The differences are as follows:
Anaphase of mitosis
Anaphase I of meiosis
Centromere splits and chromatids separate
centromere does not split, and sister chromatids remain associated at their centromere
11. List the main differences between mitosis and meiosis.
Solution:
The differences are as follows:
Mitosis
Meiosis
Occurs in somatic cells
Occurs in germ cells
The number of chromosomes stays the same as the parent cell
The number of chromosomes gets halved in comparison to parent cells.
Two daughter cells are formed
Four daughter cells are formed
Chromosomes replicate before each mitotic division
Chromosomes do not replicate before each meiotic division
12. What is the significance of meiosis?
Solution:
Significances of Meiosis:
It conserves the specific chromosome numbers of each species achieved across generations.
Enhances the genetic variability in the population of organisms from generation to generation. These variations are significant for the evolution process.
It produces gametes for sexual reproduction
Promotes crossing over. It introduces a new combination of variations or traits
Chromosomal mutations can occur due to abnormalities during meiosis. A few of these can be beneficial to organisms.
13. Discuss with your teacher about (i) haploid insects and lower plants where cell division occurs. (ii) some haploid cells in higher plants where cell division does not occur.
Solution:
i) Haploid insects where cell division occurs are drones of the honey bee, and lower plants are Spirogyra, Chlamydomonous, and Pteridophytes. These haploid gametes are produced by them through mitosis and not meiosis. ii) Spermatozoa and ova of higher animals and microspores of higher plants will not undergo cell division.
14. Can there be mitosis without DNA replication in the ‘S’ phase?
Solution:
During the S phase, DNA synthesis or replication of DNA takes place. DNA replication is essential for cell division. Without DNA replication, cell division will not take place.
15. Can there be DNA replication without cell division?
Solution:
Yes, DNA replication can take place without cell division. In order to prepare for cell division, DNA replication is necessary. Cell division is the succeeding logical step that occurs post cell division.
16. Analyse the events during every stage of the cell cycle and notice how the following two parameters change (i) the number of chromosomes (N) per cell (ii) the amount of DNA content (C) per cell
Solution:
i) Yes, DNA replication can take place in the G1 phase of the cell cycle. The number of chromosomes remains the same, and each chromosome is formed from one chromatid. In the S phase, a chromosome is formed by two sister chromatids joined at the centromere. Similar conditions continue in the G2 phase, while in the M phase, sister chromatids separate and move to different cells. The number of chromosomes stays the same in mitosis.
ii) Amount of DNA content in the cell remains the same in the G1 phase, but in the S phase, it doubles as the DNA replication takes place. It remains double in the G2 phase but halved in the M phase of the cell cycle.
1. Meiosis in diploid organisms results in
a. Production of gametes
b. Reduction in the number of chromosomes
c. Introduction of variation
d. all of the above
Solution:
Option (d) is the answer.
2. At which stage of meiosis does the genetic constitution of gametes is
finally decided
a. Metaphase I
b. Anaphase II
c. Metaphase II
d. Anaphase I
Solution:
Option (d) is the answer.
3. Meiosis occurs in organisms during
a. Sexual reproduction
b. Vegetative reproduction
c. Both sexual and vegetative reproduction
d. None of the above
Solution:
Option (a) is the answer.
4. Mitosis is characterised by
a. Reduction division
b. Equal division
c. Both reduction and equal division
d. Pairing of homologous chromosomes
Solution:
Option (b) is the answer.
6. A bivalent of meiosis-I consists of
a. Two chromatids and one centromere
b. Two chromatids and two centromere
c. Four chromatids and two centromere
d. Four chromatids and four centromereSolution:
Option (c) is the answer.
7. Cells which are not dividing are likely to be at
a. G1
b. G2
c. Go
d. S phase
Solution:
Option (c) is the answer.
8. Which of the events listed below is not observed during mitosis?
a. Chromatin condensation
b. Movement of centrioles to opposite poles
c. The appearance of chromosomes with two chromatids joined together
at the centromere.
d. Crossing over
Solution:
Option (d) is the answer.
9. Identify the wrong statement about meiosis
a. The pairing of homologous chromosomes
b. Four haploid cells are formed
c. At the end of meiosis, the number of chromosomes is reduced to
half
d. Two-cycle of DNA replication occursSolution:
Option (d) is the answer.
10. Select the correct statement about G1 phase
a. The cell is metabolically inactive
b. DNA in the cell does not replicate
c. It is not a phase of synthesis of macromolecules
d. Cell stops growing
Solution:
Option (b) is the answer.
1. Between a prokaryote and a eukaryote, which cell has a shorter cell
division time?
Solution:
Prokaryotes take a shorter time for cell division.
2. Which of the phases of the cell cycle is of the longest duration?
Solution:
Interphase. In a 24-hour cell cycle, 23 hours are for I-phase and 1 hour for M phase.
3. Name a stain commonly used to colour chromosomes.
Solution:
To make the chromosomes visible, the cell is stained with Acetocarmine and mounted in acetocarmine only.
4. Which tissue of animals and plants exhibits meiosis?
Solution:
Germ cells present in the reproductive tract undergo meiosis to produce gametes.
5. Given that the average duplication time of E.coli is 20 minutes, how much time will two E.coli cells take to become 32 cells?
Solution:
2 →4 →8 →16 →32
It takes 80 mins.
6. Which part of the human body should one use to demonstrate stages in mitosis?
Solution:
Tissues near to the cheeks can be taken to demonstrate stages in mitosis.
7. What attributes does a chromatid require to be classified as a chromosome?
Solution:
When chromatids are attached to the centromere, it is termed as a chromosome.
8. The diagram shows a bivalent at prophase-I of meiosis. Which of the four chromatids can cross over?
Solution:
Sister chromatids of homologous chromosome
9. If the tissue has at a given time 1024 cells, how many cycles of mitosis had the original parental single-cell undergone?
Solution:
2x=1024
X=10
Therefore, the parent will undergo 10 rounds of mitosis.
10. An anther has 1200 pollen grains. How many pollen mother cells must have been there to produce them?
Solution:
1 pollen mother cell →4 pollen grain
Therefore, no. of PMC = 1200/4 = 300 pollen grains.
11. At what stage of the cell cycle does DNA synthesis take place?
Solution: At the S-phase of the cell cycle, DNA synthesis takes place.
12. It is said that the one cycle of cell division in human cells (eukaryotic cells) takes 24 hours. Which phase of the cycle, do you think occupies the maximum part of the cell cycle?
Solution: Interphase is the phase of the cycle that takes 23 hours out of 24 hours.
13. It is observed that heart cells do not exhibit cell division. Such cells do not divide further and exit ___________ phase to enter an inactive stage called ___________ of cell cycle. Fill in the blanks.
Solution: G1 phase, G0 phase.
14. In which phase of meiosis is the following formed? Choose the answers from hint points given below.
a. Synaptonemal complex
b. Recombination nodules
c. Appearance/activation of enzyme recombinase
d. Termination of chiasmata
e. Interkinesis
f. Formation of dyad of cells
Hints: 1) Zygotene, 2) Pachytene, 3) Pachytene, 4) Diakinesis, 5) After Telophase-I /before Meosis-II, 6) Telophase-I /After Meiosis-I.
Solution:
a. Zygotene
b. Zygotene
c. Pachytene
d. Diakinesis
e. After telophase I/before meiosis
f. After Telophase I/after meiosis I
1. State the role of centrioles other than spindle formation.
Solution:
Centriole forms the microtubule and forms the basal body of flagella.
2. Mitochondria and plastids have their DNA (genetic material). What is known about their fate during nuclear division, like mitosis?
Solution:
DNA present in mitochondria in plastids is called the extrachromosomal DNA.
They are independent of the nuclear division of nuclear DNA. Hence, no effect of mitosis is seen on extrachromosomal DNA.
3. Label the diagram and also determine the stage at which this structure is visible.
Solution:
4. A cell has 32 chromosomes. It undergoes mitotic division. What will be the chromosome number (N) during metaphase? What would be the DNA content (C) during anaphase?
Solution:
Both N and C will be 32. This is because DNA is replicated during the S phase, after that it would remain the same during equational division.
5. While examining the mitotic stage in a tissue, one finds some cells with 16 chromosomes and some with 32 chromosomes. What possible reasons could you assign to this difference in chromosome number? Do you think cells with 16 chromosomes could have arisen from cells with 32 chromosomes or vice versa?
Solution:
Mosaicism refers to the presence of two different types of chromosome numbers in different cells of the body. This can occur as a result of chromosome non-disjunction or a delay in the separation of chromosomes during cell division.
6. The following events occur during the various phases of the cell cycle, Name the phase against each of the events.
a. Disintegration of nuclear membrane _________________________
b. Appearance of nucleolus _________________________
c. Division of centromere _________________________
d. Replication of DNA _________________________
Solution:
a. Early Prophase
b. Late Telophase
c. Anaphase
d. S phase
7. Mitosis results in producing two cells which are similar to each other. What would be the consequence if each of the following irregularities occur during mitosis?
a. Nuclear membrane fails to disintegrate
b. Duplication of DNA does not occur
c. Centromeres do not divide
d. Cytokinesis does not occur
Solution:
a. Nuclear membrane fails to disintegrate, then the spindle fibre would not reach the chromosomes.
b. Duplication of DNA does not occur, then the whole cell cycle would get arrested and will enter an inactive stage called the G0 stage.
c. Centromeres do not divide- Sister chromatids would not split to opposite poles, and one daughter cell would get two sister chromatids, whereas the other will get none.
d. Cytokinesis does not occur- It will result in polyploidy.
8. Both unicellular and multicellular organisms undergo mitosis. What are the differences, if any, observed in the process between the two?
Solution:
In prokaryotes, amitosis takes place, the cell directly divides into two daughter cells. Whereas in multicellular organisms, proper steps of cell division occur; first the karyokinesis takes place then cytokinesis occurs.
9. Name the pathological condition when uncontrolled cell division occurs.
Solution:
This is called malignancy caused due to cancer.
10. Two key events take place, during S phase in animal cells, DNA replication and duplication of the centriole. In which parts of the cell do events occur?
Solution:
DNA replication occurs in the nucleus and centriole duplicates in the cytoplasm.
11. Comment on the statement – Meiosis enables the conservation of specific chromosome number of each species even though the process per se, results in a reduction of chromosome number.
Solution:
A diploid zygote is formed when the two haploid cells called gametes fused during fertilization. If meiosis is absent than two diploid gametes would be produced which will result in the formation of tetraploid zygote. To conserve the chromosome, number meiosis is very important.
12. Name a cell that is found arrested in diplotene stage for months and years. Comment in 2-3 lines how it completes the cell cycle?
Solution:
The oocyte of mammalian females gets arrested in the diplotene stage. The oocyte grows and forms secondary follicle and then it gets arrested at the diplotene stage. As the woman attains sexual maturity first, meiosis gets completed. Meiosis occurs one by one in the egg, which is released from the ovary.
13. How does cytokinesis in plant cells differ from that in animal cells?
Solution:
Cytokinesis in plants occurs by cell plate formation. Cell plate formation starts from the centre and grows to the periphery.
LONG ANSWER TYPE QUESTIONS
1. Comment on the statement – Telophase is reverse of prophase.
Solution:
In Prophase- Chromosome condenses. The nuclear membrane disintegrates. Organelles like mitochondria, endoplasmic reticulum, and Golgi apparatus disappears. Cells at the end of prophase do not show Golgi complexes, endoplasmic reticulum and nucleolus. The chromosome clusters will be surrounded by the nuclear envelope at the telophase stage.
2. What are the various stages of meiotic prophase-I? Enumerate the chromosomal events during each stage.
Solution:
Prophase I-
a. Leptotene stage:
• Nuclear membrane starts to disappear.
• Chromatin material starts condensing.
• Centriole reaches opposite poles.
• In some members of Liliaceae, chromosomes remain attached to the nuclear membrane–bouquet stage.
b. Zygotene:
• Pairing of homologous chromosomes start.
• To hold these homologous chromosomes a protein called synaptonemal complex develops.
c. Pachytene
• Biavalent [homologous chromosomes] or tetrad becomes visible.
• Crossing over takes place resulting in genetic recombination.
• The enzyme recombinase, for crossing over comes from the recombination nodule of the synaptonemal complex.
d. Diplotene
• Synoptenemal complex dissolves
• But homologous chromosomes are still held by an X-like structure called the chiasma.
• Chiasmata develops at the point of crossing over.
e. Diakinesis
• Terminalisation of chiasmata: Chiasmata starts opening in a zipper-like fashion towards the periphery
• Nuclear membrane and nucleolus nearly disappear.
3. Differentiate between the events of mitosis and meiosis.
Solution:
MITOSIS
Prophase will be short duration
Only a single metaphasis plate is formed during metaphase.
Sister chromatids tend to move opposite pole during anaphase
Cytokinesis occurs after mitosis
MEIOSIS
Prophase I is a long duration and prophase II is a short duration
Metaphase I – two metaphasic plates and Metaphase II is a single metaphasic.
Two homologous chromatids separate during anaphase I.
It doesn’t occur after meiosis I, occurs after meiosis II
4. Write a brief note on the following a. Synaptonemal complex b. Metaphase plate Solution: a. Synaptonemal complex It is made up of protein formed between two homologous chromosomes. The complex formed during zygotene which is the substage of Prophase I. It contains a recombination nodule which consists of enzyme recombinase involved in crossing over. b. Metaphase plate The metaphase plate is the imaginary line which is formed in the centre of the cell during metaphase. The chromosomes will get close to the spindle fibres and align themselves on the plate. The movement of chromosomes towards the metaphase plate is called congression. 5. Write briefly the significance of mitosis and meiosis in a multicellular organism. Solution: Mitosis creates the cells necessary to add more mass to the body, as well as more cells to cope with the growth, such as new blood cells. In case of injury, mitosis is involved in the healing of the tissues. Meiosis also produces genetic variation by way of the process of recombination. It helps to conserve the no. of chromosomes in a species. 6. An organism has two pair of chromosomes (i.e., chromosome number = 4). Diagrammatically represent the chromosomal arrangement during different phases of meiosis-II. Solution:
Cell cycle and cell division are a few different processes that form a part of your body. The tendency of two daughter cells to multiply from a single parent cell is termed Cell division. Prokaryotic cells rely on the process of binary fission, while Eukaryotic cells rely on the process of mitosis. The cell cycle can be defined as the events which take place during the process of cell division.
Prophase – Prophase is the first stage of mitosis.
Metaphase – The second stage of mitosis is metaphase, where sister chromatids are formed.
Anaphase – The third stage of mitosis is anaphase, where the splitting of centromeres takes place, and the sister chromatids move to opposite poles.
Telophase – The fourth stage in mitosis is telophase, where the chromosome cluster is covered with the nuclear envelope.
Cycle 1 is called meiosis 1, and the second cycle is called meiosis 2.
Meiosis I produces two new cells, each haploid in their DNA, but with 2 copies. These cells take a short rest before entering the second division of meiosis, meiosis II. In meiosis II, the duplicated chromosomes align on a second spindle, and the sister chromatids separate to produce cells with a haploid DNA content. Telophase 2 is basically splitting the 2 sisters apart, but, as each contains the same genetic info, it is still haploid. In meiosis 1, homologous chromosomes separate from each other, whereas, in meiosis 2, sister chromatids separate.
