Thinking Process

Sexual reproduction involves the union of male and female gametes. Fusion of gametes is accomplished by the plasmogamy (union of protoplasm) followed by karyogamy (union of nucleus). This results in doubling of chromosome numbers in the resultant cell.

Answer

(d) Meiosis is a reduction division which reduces the chromosomes number to half in gametes. Since, it is a special kind of cell division in which exchange of genetic material takes place that brings about variation in next generations. So all options are characteristic features of meiotic cell division.

• Option (a) is not correct because meiosis does result in the production of gametes.
• Option (b) is not correct because meiosis does result in a reduction in the number of chromosomes.
• Option (c) is not correct because meiosis does introduce variation through the exchange of genetic material.

Answer

(d) The genetic constitute of gametes is finally decided at the anaphase-I after which each cell receives half the chromosome numbe,r i.e., from ’ $2 n$ ’ in parent cell it changes to ’ $n$ ’ in daughter cells. During this phase, the two homologou of each chromosome pair separate and move toward opposite poles drawn by microtubule of the spindle apparatus.

These are separated further by anaphase-II, wherein sister chromatids of each chromosome separate and move toward opposite poles. It connot be metaphase-I or metaphase-Il because during these stages. The chromosomes or chromatids, merely arrange themselves at the metaphasic plate.

• (a) Metaphase-I: During metaphase-I, homologous chromosomes align at the metaphase plate, but no separation occurs at this stage. The genetic constitution is not finalized because the chromosomes are still paired and have not yet been distributed to daughter cells.

• (b) Anaphase-II: In anaphase-II, sister chromatids separate and move toward opposite poles. However, the genetic constitution of gametes is already determined by this stage, as the reduction in chromosome number occurs during anaphase-I.

• (c) Metaphase-II: During metaphase-II, chromosomes align at the metaphase plate again, but similar to metaphase-I, no separation occurs at this stage. The genetic constitution is not finalized because the chromatids are still together and have not yet been distributed to daughter cells.

Answer

(a) Meiosis occurs in sexually reproducing organisms to reduce the chromosome number to half before their gametes unit, so, as to maintain the constant chromosome number $(2 n)$ in the progery.

Vegetative reproduction is a kind of asexual reproduction occurring in plants and does not involve the formation and fusion of gametes.

• (b) Vegetative reproduction: This is a type of asexual reproduction that occurs in plants and does not involve the formation and fusion of gametes. Therefore, meiosis does not occur during vegetative reproduction.

• (c) Both (a) and (b): This option is incorrect because meiosis only occurs in sexually reproducing organisms and not in vegetative reproduction, which is a form of asexual reproduction.

• (d) None of these: This option is incorrect because meiosis does occur in sexually reproducing organisms, making option (a) the correct answer.

Answer

(a) During Anaphase-I homologous chromosomes separate, while sister chromatids remain associated at their centromeres.

• (b) non-homologous autosomes separate: This is incorrect because non-homologous autosomes do not pair up or separate during Anaphase-I. Instead, it is the homologous chromosomes that align and then separate.

• (c) sister chromatids separate: This is incorrect because sister chromatids do not separate during Anaphase-I of meiosis. They remain attached at their centromeres and only separate during Anaphase-II.

• (d) non-sister chromatids separate: This is incorrect because non-sister chromatids do not separate during Anaphase-I. Non-sister chromatids are part of homologous chromosomes, and it is the homologous chromosomes themselves that separate during this phase.

Thinking Process

Growth and multiplication is the characteristic feature of all living organisms. It is accomplished by mitotic cell division by which there is on increase in the number of cells. This is also involved in asexual reproduction in lower organisms and plants.

Answer

(b) Mitosis is characterised by equal division because the chromosome numbers in the daughter cells remain same as that of parent cell. While reduction division is the characteristic of meiosis.

• (a) Reduction division is incorrect because it refers to meiosis, where the chromosome number is halved in the daughter cells.
• (c) Both (a) and (b) is incorrect because mitosis does not involve reduction division; it only involves equal division.
• (d) Pairing of homologous chromosomes is incorrect because this process occurs during meiosis, not mitosis.

Answer

(c) The complex formed by a pair of synapsed homologous chromosome is called bivalent or a tetrad. It has two centromeres and four chromatids.

Rest of the options are incorrect.

• (a) two chromatids and one centromere: This option is incorrect because a bivalent consists of two homologous chromosomes, each with two chromatids, making a total of four chromatids. Additionally, each chromosome has its own centromere, so there should be two centromeres, not one.

• (b) two chromatids and two centromeres: This option is incorrect because a bivalent consists of two homologous chromosomes, each with two chromatids, making a total of four chromatids. This option only accounts for two chromatids, which is insufficient.

• (d) four chromatids and four centromeres: This option is incorrect because a bivalent consists of two homologous chromosomes, each with one centromere. Therefore, there should be a total of two centromeres, not four.

Thinking Process

Cell division is a very important process in all living organisms. The cell prepares itself, and DNA duplication and cell growth takes place prior to cell division.

Answer

(c) $G_{0}, G$ signifies growth. and zero means no growth takes place at this stage. It is also called quiescent stage $\left(G_{0}\right)$. Some cells of the body like heart cells, neuron which do not divide, exit at $G_{1}$ stage and enter $G_{0}$ of the cell cycle which is an inactive stage.

The cells in this stage remain metabolically active but no longer proliferate unless called on to do so depending on the requirement of the organism.

• (a) G₁: The G₁ phase is the first growth phase after mitosis and before the S-phase. During this phase, the cell grows and prepares for DNA replication. Cells in G₁ are actively preparing for division, not in a non-dividing state.

• (b) G₂: The G₂ phase is the second growth phase after DNA replication and before mitosis. During this phase, the cell continues to grow and prepares for mitosis. Cells in G₂ are actively preparing for cell division, not in a non-dividing state.

• (d) S-phase: The S-phase is the phase of the cell cycle in which DNA replication occurs. During this phase, the cell synthesizes a complete copy of the DNA in its nucleus. Cells in the S-phase are actively replicating their DNA, not in a non-dividing state.

Answer

(d) Crossing Over is the phenomenon of genetic exchange between homologous pair of chromosomes and is a characteristic feature of meiotic cell division. It does not occur in mitosis. Rest of the options represent stages in mitosis.

• Chromatin condensation: This is a key event in mitosis where the chromatin fibers condense into visible chromosomes, making them easier to separate.

• Movement of centrioles to opposite poles: During mitosis, centrioles move to opposite poles of the cell to help in the formation of the mitotic spindle, which is essential for chromosome separation.

• Appearance of chromosomes with two chromatids joined together at the centromere: This occurs during mitosis when each chromosome is replicated into two sister chromatids that are joined at the centromere, ready to be pulled apart into daughter cells.

Answer

(d) Two cycles of DNA replication does not occur in meiosis.

Other options (a), (b) and (c) define the meiotic cell division.

• (a) Pairing of homologous chromosomes: This statement is correct because during meiosis, homologous chromosomes do pair up in a process called synapsis.

• (b) Four haploid cells are formed: This statement is correct because meiosis results in the formation of four haploid cells from one diploid cell.

• (c) At the end of meiosis the number of chromosomes are reduced to half: This statement is correct because meiosis reduces the chromosome number by half, resulting in haploid cells from an original diploid cell.

Answer

(b) $\mathrm{G}_{1}$ phase means gap 1 phase. It is the interval between mitosis and initiation of DNA replication. The cell is metabolically active and continuously grows but does not replicate the DNA content. The cell also synthesizes proteins that are required for DNA replication.

Rest of the options are not characteristics of $G_{1}$ phase.

• (a) The cell is metabolically active during the G1 phase, as it is involved in various cellular processes and prepares for DNA replication.
• (c) The G1 phase is indeed a phase of synthesis of macromolecules, including proteins and RNA, which are essential for cell growth and preparation for DNA replication.
• (d) The cell does not stop growing during the G1 phase; instead, it continues to grow and increase in size as it prepares for the subsequent phases of the cell cycle.

Thinking Process

Prokaryotic cell has simple cell structure and cellular organisation. It’s nucleus does not contain nuclear membrane.

Answer

Prokaryotic cell has shorter cell cycle than the eukaryotic cell.

Answer

Interphase is of the longest duration. It has following events and is divided in the following phases

(a) $G_{1}$ phase (Gap 1) Corresponds to the interval between mitosis and initiation of DNA replication. Cell is metabolically active and continuously grows but does not replicate its DNA.

(b) S-phase (Synthesis phase) Marks the period during which DNA synthesis or replication takes place.

(i) Amount of DNA per cell doubles.

(ii) No increase in chromosome number.

(c) $G_{2}$ Phase (Gap 2) Proteins are synthesised for the preparation of cell division, while cell growth continues.

Thinking Process

The chromosomes are the thickest and the shortest at metaphase. They are stained for karyotyping for further study of chromosomes.

Answer

Acetocarmine and Giemsa stain can be used to stain the chromosomes.

Answer

Meiosis is a special kind of cell division, also called as reduction division, which occurs in germ cells or sex cells of male and female reproductive organs of plants and animals. They produce male $(♂)$ and female $(♀)$ gametes that take part in sexual reproduction.

Answer

1 hour -20 minutes

These are the 4 subsequent cell divisions to produce 16 cells and each division takes $=20 \mathrm{~min}$ Total time $=20 \times 4=80$ minutes or $1 \mathrm{hr} 20$ minutes

So, 1 cell produces 16 cells in 1 hour 20 minutes and 2 cells produces 32 cells in 1 hour 20 minutes.

Answer

All the cells in the human body are somatic cells except germinal cells in the male and female reproductive organs. The somatic cells divide by mitotic cell division for growth and regeneration. These can be used to demonstrate mitosis.

Answer

Chromatid is one copy of a duplicated chromosome, which is joined to the other copy centromere. The duplication of chromosome material takes place at synthetic phase of cell cycle.

During mitosis, late metaphase and early anaphase, there is longitudinal splitting of chromosomes and thus two chromosome get separated, to get divided into the daughter cells.

Answer

The homologous chromosomes lie parallel to each other in leptotene stage. Each chromosome has four chromatids and are bivalent. The non-sister chromatids of homologous chromosomes cross over in pachytene stage of prophase- I.

Answer

The parental cell undergoes 10 divisions of mitotic cycle to give 1024 cells.

Thinking Process

The pollen mother cell (2n) undergoes meiotic cell divisions, each such cell produces four daughter cells with haploid ( $n$ ) number of chromosomes.

Answer

Three hundred pollen mother cells have to be there to produce 1200 pollen grains. because one pollen mother cell will produce four pollen grains.

Answer

Synthetic phase or S-phase of interphase, is the stage of cell cycle where DNA synthesis or replication takes place.

Thinking Process

Cell cycle is a sequential event and is under genetic control. Every cell prepares itself before it starts dividing. This preparation takes place in interphase stage of the cell cycle.

Answer

If a cell takes 24 hours to divide, it will have 18-20 hours time to spend on interphase stage to prepare itself to undergo cell division.

Answer

It is observed that heart cells do not exhibit cell division. Such cells do not divide further and exit $G_{1}$ phase to enter an inactive stage called quiescent stage $\left(G_{0}\right)$ of cell cycle.

Muscle cells when reach a level of maturity, no longer divide and just perform their function all through its life. Similar is the case with nerve cells too which are highly specific and lack the ability to divide once they get specialised. They remain in that state until they die.

Answer

(a) Synaptonemal complex zygotene.

(b) Recombination nodules pachytene.

(c) Appearance/activation of enzyme recombinase telophase-I/after, meiosis-I

(d) Termination of chiasmata diakinesis.

(e) Interkinesis after telophase-I/before prophase of meiosis-II.

(f) Formation of dyad of cells pachytene.

Thinking Process

The animal cell has few membrane less cell organelles. Centrosome is one of them. Two cylindrical structures are the part of centrosome, these are called centrioles.

Answer

The two centrioles in the centrosome lie perpendicular to each other. Each has organisation like a cart wheel. These form the basal body of cilia and flagella of plant/animal cells besides forming spindle fibre in animal cell division. It also helps in the formation of microtubuless and sperm tail.

Answer

The DNA present in mitochondria and chloroplast is extrachromosomal DNA. It has nothing to do with the nuclear divisions. In mitosis, only nuclear DNA takes part.

Thinking Process

Cell division is an important event in the cell cycle. The cell passes through various phases in which number of changes are seen in its nuclear content.

Answer

The diagram shows the transition stage between prophase and metaphase stage of mitotic cell division.

Thinking Process

Mitotic cell division helps in the growth of organism and its development. It also plays a vital role in asexually reproducing organisms.

Answer

The mitotic cell division occurs in somatic cells of an organism. The chromosome number in the daughter cells remain same as that of the parent (dividing) cell, so even at metaphase or anaphase, the chromosome number does not change.

The DNA content gets doubled at the synthetic phase of interphase and gets divided at anaphase but the chromosome number remains same.

Answer

Such a condition may arise in case of a mosaic or mosaicism, which denotes preserve of two or more populations of cells with different genotypes in one individual.

It can result from various mechanisms including non-disjunction, anaphase lagging and endoreplication. It may also result from a mutation during development which is propagated to only a subset of the adult cells.

In this case, cells with 16 chromosomes could have arisen from cells with 32 chromosomes.

Answer

(a) Prophase

(b) Telophase

(c) Anaphase

(d) S-phase

Thinking Process

Cell division is under genetic control and is highly precised phenomenon. However, some mistake occur sometimes at genetic and molecular level in cell cycle which may lead to certain abnormalities.

Answer

(a) If nuclear membrane fails to disintegrate, the spindle fibres would not be able to reach chromosomes and they would not move towards opposite poles of the cell. In certain protozoans, such as Amoeba, the spindle is formed within the nucleus and this is called intranuclear mitosis or pre-mitosis.

(b) If DNA duplication does not occur than cell might not be able to surposs S-phase of cell-cycle as no chromosome formation will take place, and will not be able to enter $\mathrm{M}$-(mitotic phase) or if in case it enters mitosis the cycle will cease,

(c) If the centromeres do not divide, one of the daughter cell will receive a complete pair of chromosomes and other cell would not get any of them. This may result in trisomy.

(d) If cytokinesis does not occur, then multinucleate condition called coenocyte, syncytium is produced, as in Rhizopus and Vaucheria, etc.

Answer

In unicellular organisms, the type of cell division is known as amitosis in which somatic cell is divided into two parts directly. In multicellular organisms occurs which is an indirect process.

Both cell division in unicellular and multicellular organisms undergo mitosis have differences as below

Cell Division in Unicellular Organism (Amitosis)	Cell Division in Multicellular Organism (Mitosis)
It is direct division of cellular and nuclear content. Without the formation of chromosomes.	Nucleus and cellular content do not divide directly and involves formation of chromosomes.
Different phases of cell divisions are not seen	It involves different phases of cell division.

Answer

Cancer is the pathological condition in which the cells loose control over cell division and this results into malformation of the organ in which such cell division occurs.

Answer

DNA replication takes place in nucleus and duplication of centriole occurs in the cytoplasm. Centriole forms spindle fibres during cell division in animal cells which direct the movement of chromosomes within the cell. The position of centriole determines the position of the nucleus and plays a crucial role in the spatial arrangement of the cell.

Answer

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 results in reduction of chromosome number by half, it is gradually conserved by union of male gamete $(n)$ and female gamete $(n)$ in next generation.

Meiosis also increases the genetic variability in the population of organisms from one generation to the next.

Answer

Meiotic arrest at diplotene stage commonly occurs in mammalian occytes. In females, meiosis starts in the embryo and proceeds as for as diplotene, when the chromosomes become diffused and the cells are referred to as being in the dictuate stage. This arrest is under hormonal control.

In many amphibian oocytes binds and insects with a long period of immaturity, the oocyte may be arrested in the dictyate stage for many years and spend a prolonged period in diplotene.

This stage is characterised by formation of lampbrush chromosomes where intense RNA synthesis occurs and most of the genes in the DNA loops are actively transcribed and expressed.

Answer

Difference between cytokinesis in plant cell and animal cell is as follows

Cytokinesis in Plant Cell	Cytokinesis in Animal Cell
The division of cytoplasm takes place by cell plate formation.	The division of cytoplasm takes place by cleavage.
The cell plate formation starts at the centre of the cell and grows outward, toward the lateral walls.	Cleavage starts at the periphery and then moves inward, dividing the cell into two parts.

Long Answer Type Questions

Answer

The following contrasting differences reveals that telophase is reverse of prophase, in cell division

Prophase	Telophase
I’st stage of (karyokinesis ) in cell division, Viscosity of cytoplasm increases.	Last stage of karyokinesis in cell division. Viscosity of cytoplasm decreases.
The indistinct and interind DNA condense to form elongated chromosomes.	Chromosome groups reorganize themselves into nuclei.
The chromatin disappears and chromosome fibers get shortened and thickened.	Chromosomes elongate and overlap each other to form chromatin.
Spindle fibers appear towards the poles from the centriole connected in animals with astral rays and in plants without asters.	Spindal fibres disappear around the poles Astral rays also disappear in plants.
Nucleolus degenerates completely.	Nuclear envelope appears and two daughter nuclei are formed at the poles.
Cell organelles such as ER, Golgi complex disorganize, and the difference between cytoplasm and nucleoplasm disappears.	Cell organelles, i.e., ER and Golgi complex, appear in the chromatin area, making it distinct from the rest of the cytoplasmic area.

Answer

Prophase-I occurs over a long duration and involves several complicated changes in meiotic cell division. It is important because genetic recombination and variation in sexually reproducing organism occurs due to the events of this phase.

Leptotene

(i) The chromatin network opens out and threads become clear.

(ii) The chromosomes are thin, slender and long.

(iii) Chromosome number is diploid.

Zygotene

(i) Corresponding chromosomes become intimately associated.

(ii) The process of pairing is known as synapse. It is so exact that pairing is not merely between corresponding chromosomes but between corresponding individual units.

(iii) The chromosomes become shorter and thicker.

Pachytene or Pachynema

(i) The synaptic chromosomes become very intimately associated.

(ii) The pair of chromosomes becomes short and thick.

(iii) Crossing over occurs at this stage. Chiasmata are clearly seen.

Diplotene

(i) Homologous chromosomes start separating from one another.

(ii) Chiasmata tend to slip out of the chromosomes. This is known as terminalisation of chiasmata.

(iii) Chromosomes start separating out but the separation is not complete.

(iv) Nuclear membrane and nucleolus start disappearing.

Diakinesis

(i) The bivalents condense further and get randomly distributed.

(ii) The separation of paired chromosomes is almost complete.

(iii) Terminalisation of chiasmata is almost complete.

(iv) Nuclear membrane and nucleolus disappear.

Answer

Mitotic cell division results into the increase in the number of cells that have same genetic composition whereas meiosis has its importance in the life cycle of sexually reproducing organisms.

Answer

(a) Synaptonemal complexes are zipper-structures which are assembled between homologous chromosomes during the prophase of the first meiosis. Their asssembly and disassembly correlate with the successive chromatin rearrangements of meiotic prophase namely the condensation, pouring, recombination and dysfunction of homologous chromosomes.

They are considered to be the structures that control the number and distribution of reciprocal exchanges between homolgous chromosomes. They also are known to convert cross over into functional chiasmata .

(b) In metaphase, the centromeres of the chromosomes assemble themselves on the metaphase plate (equatorial plate), an imaginary line that is equidistant from two centrosome poles. This even alignment is due to the opposing kinetochore microtubules. At this plate, chromosomers, especially sister chromatids are attached to the bundle of four to eight spindle fibres.

Answer

Multicellular organisms grow and develop with the involvement of mitotic cell division. Meiosis occurs in them to form gametes in their reproductive phase of life cycle.

Significance of Mitosis

(i) Multicellular plants and animals start life as single cells. The process of mitosis gives rise to many cells which differentiate to form tissues, organs and organ-systems of the organism.

(ii) It results in increase in size and growth of an organ.

(iii) Cell reproduction is used to form new cells to renew certain tissues and to replace worn out cells.

(iv) Mitosis is also involved in asexual reproduction in some organisms like in unicellular. Amoeba and multicellular Hydra as well as in vegetative reproduction in plants.

Significance of Meiosis

(i) Meiosis is the mechanism by which conservation of specific chromosome number of each species is achieved across generations in sexually reproducing organisms.

(ii) Meiosis 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.

Thinking Process

Meiosis is a reduction in which division, chromosome number reduces to half in daughter cells. The number reduces as half set of chromosomes move to 2 daughter cells of the meiosis-I. Thus two cells with half set of chromosomes again re-enter meiosis-ll which is a similas to mitotic cell division.

Answer