Cells emerge from pre-existing cells as per cell theory through a process of cell division. For any entity that reproduces sexually, its life cycle originates through a single-celled zygote. The cell does not stop dividing with the formation of mature entities. It is a continuous process throughout one’s lifetime. A cell cycle is a stage through which a cell undergoes division from one to the next. It has two phases, namely – the interphase and the M phase or Mitosis. Cycles of division and growth permit a single cell to form a structure possessing lakhs of cells. Stay tuned to learn more about cell cycle and cell division.
Very Short Answer Type Questions
Q.1. Which cell between a eukaryote and a prokaryote has a shorter cell division time?
A.1. A prokaryotic cell has a shorter cell cycle compared to a eukaryotic cell.
Q.2. Name the cell cycle phase that has the longest duration.
A.2. The interphase
Q.3. Which stain is usually used to colour chromosomes?
A.3. Giemsa and Acetocarmine
Q.4. Name the plant and animal tissue that undergoes meiosis.
A.4. It occurs in the sex cells or the germ cells of female and male reproductive organs in animals and plants, which produces female and male gametes that participate in sexual reproduction.
Q.5. How much time will two E.coli cells take to become 32 cells if the average duplication time of E.coli is 20 minutes?
A.5. It takes 1 hour and 20 minutes. There are four succeeding cell divisions which generate 16 cells where each division takes a total of 20 minutes. Hence the total time = 20 x 4 = 80 minutes or 1 hour and 20 minutes. Thus, 1 cell produces 16 cells in 80 minutes and 2 cells produce 32 cells in 1 hour and 20 minutes.
Q.6. Which human body part can be utilized to illustrate mitosis phases?
A.6. Except for the germinal cells, all the cells in the human body are somatic cells. The somatic cells divide by mitosis for regeneration and growth which can be used to demonstrate mitosis.
Q.7. For a chromatid to be classified as a chromosome, what attributes does it need to possess?
A.7. The attribute of crossing over.
Q.8. During which phase of the cell cycle does the DNA get synthesized?
A.8. The S-phase or Synthetic phase of the interphase.
Short Answer Type Questions
Q.1. What is the role of centrioles apart from spindle formation?
A.1. In the centrosome, the two centrioles align perpendicular to each other, organized in a cart-wheel pattern. Besides spindle fibre formation seen in animal cell division, they form the basal body of cilia and flagella of animal/plant cells. They also assist in the formation of the sperm tail and microtubules.
Q.2. What happens to the DNA of the plastids and mitochondria during nuclear divisions such as mitosis?
A.2. Chloroplasts and mitochondria possess DNA in the form of extrachromosomal DNA and have no role in nuclear division. Only nuclear DNA takes part in mitosis.
Q.3. A cell having 32 chromosomes undergoes mitotic division. During metaphase, what will the chromosome number(N) of the cell? During anaphase, what will the DNA content of the cell be?
A.3. Mitosis occurs in somatic cells of entities. The number of chromosomes is the same in both parent and daughter cell and remains unchanged even at anaphase or metaphase. The content of DNA, however, is doubled at the interphase or synthetic phase. The division takes place at the anaphase but the number of chromosomes remains unchanged.
Q.4. 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
A.4. a) Prophase
Q.5. Under uncontrolled cell division, what is the pathological condition that occurs?
A.5. It is cancer. In this condition, the cells lose control of cell division and result in malformation of the organs where cell division takes place.
Q.6. Which is the cell that is captured in the diplotene phase for months and years? How does it complete its cell cycle?
A.6. Diplotene can last for months or years in the oocytes of a few vertebrates.
- The diplotene chromosome present in diplotene phase of a few animal oocytes of amphibians or frogs.
- In meiotic prophase, the lampbrush chromosomes are noticed. These chromosomes tend to turn normal after growth thereby complete the cell cycle.
Long Answer Type Questions
Q.1. Telophase is the reverse of prophase. Elucidate the statement.
A.1. The condensation of chromosomal material initiates prophase. During the process of chromatin condensation, the chromosomal material untangles. During the start of the final stage of mitosis, i.e., the telophase, upon arrival at the respective poles, the chromosomes decondense and lose their individuality. When observed under a microscope, the cells at the end of prophase do not show Golgi complexes, nucleolus, endoplasmic reticulum, and the nuclear envelope. The nuclear envelope at the telophase stage gathers around the cluster of the chromosomes. The Golgi complex, ER, nucleolus reform.
Q.2. Describe the different phases of meiotic prophase – I. Mention the chromosomal events during each stage.
A.2. During the prophase – I, genetic recombination and variation in sexually reproducing entities takes place due to events of this stage.
- Chromosomes are long, thin and slender
- Chromatin network exposes and threads appear clear
- The diploid number of chromosomes
- Similar chromosomes turn intimately associated
- Synapse is exact hence pairing is not just between chromosomes but corresponding individual units.
- Chromosomes appear thicker and shorter
- Synaptic chromosomes become intimately related
- Thick and short pair of chromosomes
- Cross over occurs, Chiasmata visible clearly
- Homologous chromosomes start detaching from each other.
- Chiasmata tends to shift away from chromosomes, termed as terminalisation of chiasmata
- Chromosomes detach out but it is an incomplete separation
- Nucleolus and nuclear membrane start to fade.
- The bivalents are randomly distributed after further condensation
- Paired chromosomes separate completely
- Terminalisation of chiasmata is almost concluded
- The disappearance of the nucleolus and nuclear membrane
Q.3. State differences between the events of meiosis and mitosis.
A.3. Following are the differences
|Place of occurrence||Somatic cells||Germ cells|
|Nature of Organisms||Asexually and sexually reproducing organisms||Sexually reproducing organisms|
|Nuclear and cell division||One cycle||Two sequential cycles – Meiosis I & II|
|DNA replication||Once for each cell division||Once for two cell divisions|
|Duration of prophase||Short||Long|
|Nature of prophase||Simple||The first meiotic division is in comparison to prophase of mitosis|
|Cell division and chromosome division||Both divide once each||Two cell divisions, but one chromosome division|
|End product||Two cells||Four haploid cells|
a) Synaptonemal complex
b) Metaphase plate
A.4. a) These are zipper-structures that are assembled during the prophase of the meiosis – I between homologous chromosomes. This disassembly and assembly are interlinked with the continuous rearrangements of the chromatin during the meiotic prophase such as the poring, recombination, condensation, and dysfunction of homologous chromosomes. These regulate the number and distribution of reciprocal exchanges between the homologous chromosomes. They convert cross over to functional chiasmata.
b) The centromeres of the chromosomes in the metaphase gather on the metaphase plate which is an imaginary line at an equal distance from the two centrosome poles. The alignment is even due to the opposite kinetochore microtubules. The chromosomes at this plate, the sister chromatids, in particular, are connected to the package of four to eight spindle fibres.
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