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The formation of the new cell wall begins with the formation of a simple precursor, called the ‘cell-plate’ that represents the middle lamella between the walls of two adjacent cells.

Necrosis is a form of cell injury which leads to the premature death of cells. For example: due to scrape or a harmful chemical.

The answer is (b) Zygotene 

Howard & Pelc (1953).

The answer is Dumbble – shaped nucleus.

The answer is (b) Mitosis

The answer is (b) Mitosis

The point of exhcnage of genetic material between homologous chromosomes.

This is a simplest cell division. It takes place in prokaryotes eg Bacteria and some lower plants eg algae, fungi etc. In this division, the nucleus elongates to become dumble shaped and finally divides into two. Simultaneously, the cytoplasm also divides into 2 and there is formation of 2 daughter cell. The genetic material is approximately divided in this division

1. It keeps the number of chromosomes constant in a species.
2. The exchange of genetic material causes new gentic variations which help in speciation.
3. It is essential to complete sexual cycle.
4. Lack of meiosis will cause polyploidy. It may be useful in plants but it will be deleterious for animals.

1. True

2. True

3. True

The correct answer is (B) centromere

Two haploid cells formed during first meiotic division divide further into four haploid cells this division is called homotypic division. It consists of five phases: prophase – II, metaphase – II, anaphase – II, telophase – II, and Cytokinesis – II. 

1. Prophase-II: 

a. The chromosomes are distinct with two chromatids.

b. Each centriole divides into two resulting in formation of two centrioles which migrate to opposite poles and form asters. 

c. Spindle fibres are formed between the centrioles. 

d. The nuclear membrane and nucleolus disappears in this phase. 

2. Metaphase -II: 

a. Chromosomes are arranged at the equator. 

b. The two chromatids of each chromosome are separated by division of the centromere. 

c. Some of the spindle fibres are attached to the centromeres and some are arranged end to end between two opposite centrioles. 

3. Anaphase – II: 

In this phase, the separated chromatids become daughter chromosomes and move to opposite poles due to the contraction of the spindle fibres attached to centromeres. 

4. Telophase – II: 

a. In this stage daughter chromosomes starts to uncoil. 

b. The nuclear membrane surrounds each group of chromosomes. 

c. Nucleolus reappears in this phase. 

5. Cytokinesis – II 

a. Cytokinesis takes place after the nuclear division. 

b. Two haploid cells are formed from each haploid cell. 

c. Thus, four haploid daughter cells are formed. 

d. These cells then undergo changes to form gametes.

Telophase is nuclear membrane and nucleotide reappear.

Anaphase of Mitosis: During this phase of mitosis the centromeres divide, the spindle fibres contract and move towards opposite poles, pulling the daughter chromosomes apart.

Anaphase of meiosis-I: With the contraction of microtubules of the spindle apparatus each homologous chromosome with its two chromatids and unbroken centromeres (unlike anaphase of mitosis) start moving towards the opposite poles of the cell.

Gametic meiosis: When the reproductive cells of a diploid organism undergoes meiosis to produce haploid gametes, it is called gametic meiosis.

Zygotic meiosis: Some algae and fungi are haploid adults. They produce haploid gametes which upon fertilization form a diploid zygote. This zygote undergoes meiosis to form haploid spores, which on repeated mitotic division form the adult body.

(i) It is plant cell because centrosome is absent and spindle apparatus not connected to it

(ii) Prophase.

(iii) Metaphase: In this stage the chromosome lie in one plane at equator and gets attached to a spindle fibre by its centromere.

(iv) Mitosis: Same diploid number of chromosomes are present in the daughter cell.

Meiosis: Haploid number of chromosomes are present in the daughter cells.

1. During interphase, the cell is metabolically very active. 

2. In this phase, a cell grows to its maximum size, chromosomal material (DNA and histone proteins) duplicates and the cell prepares itself for next mitotic division. Hence, inteiphase is known as preparatory phase.

Centrioles: Two tiny structures located in the cytoplasm near the nuclear envelope (membrane that surrounds the nucleus).

Centriole plays an important role in cell division. Centrioles help organize microtubule assembly and forms spindle apparatus that separate the chromosomes during cell division.

S – phase (Synthesis phase): 

In this phase DNA is synthesized (replicated), so that amount of DNA per cell doubles. Synthesis of histone proteins takes place in this phase. Karyokinesis is the nuclear division which is divided into prophase, metaphase, anaphase and telophase. 

1. Prophase: 

a. In this phase, condensation of chromatin material, migration of centrioles, appearance of mitotic apparatus and disappearance of nuclear membrane takes place. 

b. Due to condensation, each chromosome with its sister chromatids connected by centromere is clearly visible under light microscope. 

c. The nucleolus starts to disappear. 

d. Centrosome start moving towards the opposite poles of the cell. 

e. Mitotic apparatus is almost completely formed.

2. Metaphase: 

a. Chromosomes are completely condensed and appear short. 

b. Centromere and sister chromatids become very prominent. 

c. All the chromosomes are arranged at equatorial plane of cell. This is called metaphase plate. 

d. Mitotic spindle is fully formed in this phase. 

e. Centromere of each chromosome divides horizontally into two, each being associated with a chromatid. 

[Note: The centromeres divide at the beginning of anaphase so that the two chromatids of each chromosome become separated from each other. Source: Cell Division, Donald B. McMillan, Richard J. Harris, in An Atlas of Comparative Vertebrate Histology, 2018.] 

3. Anaphase: 

a. In this phase, chromatids of each chromosome separate and form two chromosomes called daughter chromosomes. 

b. The chromosomes which are formed are pulled away in opposite direction by spindle apparatus. 

c. Anaphase ends when each set of chromosomes reach at opposite poles of the cell. 

4. Telophase: 

a. This is the final stage of karyokinesis. 

b. The chromosomes with their centromeres begin to uncoil at the poles. 

c. The chromosomes lengthen and lose their individuality. 

d. The nucleolus reappears and the nuclear membrane appear around the chromosomes. 

e. Spindle fibres breakdown and get absorbed in the cytoplasm. Thus, two daughter nuclei are formed. 

These are small disc-shaped structures at the surface of the centromeres which serve as the sites of attachment of spindle fibres to the chromosomes.

(c) interkinesis – III

(d) homologous chromosomes

(b) Metaphase

1. The type of cell division shown in diagram is meiosis II.

 2. Meiotic division produces gametes or spores. 

1. If it is absent, the number of chromosomes would double or quadruple resulting in the formation of monstrosities (abnormal gametes). 

2. The constant number of chromosomes in a given species across generations is maintained by meiosis and fertilization.

3. Because of crossing over, exchange of genetic material takes place leading to genetic variations, which are the raw materials for evolution.

1. In diakinesis, the chiasmata begin to move along the length of chromosomes from the centromere towards the ends of chromosomes.

2. The displacement of chiasmata is termed as terminalization. The terminal chiasmata exist till the metaphase. 

3. The nucleolus disappears and the nuclear membrane also begins to disappear. 

4. Spindle fibres starts to appear in the cytoplasm.

The correct answer is (B) Synapsis

a. Recombination is exchange of genetic material between paternal and maternal chromosomes during gamete formation. 

b. The points where crossing over takes place is known as chiasmata. 

c. Chromatids acquire new combinations of alleles by physically exchanging segments in crossing-over. 

d. The exchange of genetic material between homologous chromosomes involves accurate breakage and joining of DNA molecules through a complex mechanism. 

e. It is catalyzed by enzymes.

The correct answer is (C) Leptotene, Zygotene, Pachytene, Diplotene, Diakinesis

Zygotene:

Pairing of non-sister chromatids of homologous chromosomes takes place by formation of synaptonemal complex. This pairing is called synapsis. Each pair consists of a maternal chromosome and a paternal chromosome. Chromosomal pairs are called bivalents or tetrads.

1. Prophase -I: 

It is the most complicated and longest phas0e of meiotic division. It is further divided into five sub-phases viz. leptotene, zygotene, pachytene, diplotene and diakinesis. 

a. Leptotene: 

1. The volume of the nucleus increases. 

2. The chromosomes become long distinct and coiled.

3. They orient themselves in a specific fonn known as bouquet stage. This is characterized with the ends of chromosomes converged towards the side of nucleus where the centrosome lies. 

4. The centriole duplicates into two and migrates to opposite poles.

b. Zygotene: 

1. Pairing of non-sister chromatids of homologous chromosomes takes place by formation of synaptonemal complex. This pairing is called synapsis. 

2. Each pair consists of a maternal chromosome and a paternal chromosome. Chromosomal pairs are called bivalents or tetrads. 

c. Pachytene:

1. Each individual chromosome begins to split longitudinally into two similar chromatids. Therefore, each bivalent now appears as a tetrad consisting of four chromatids. 

2. The homologous chromosomes begin to separate but they do not separate completely and remain attached to one or more points. 

3. These points are called chiasmata (Appear like a cross-X). 

4. Chromatids break at these points and broken segments are exchanged between non-sister chromatids of homologous chromosomes resulting in recombination.

d. Diplotene: 

The chiasma becomes clearly visible in diplotene due to beginning of repulsion between synapsed homologous chromosomes. This is known as desynapsis. Synaptonemal complex also starts to disappear. 

e. Diakinesis: 

1. The chiasmata begin to move along the length of chromosomes from the centromere towards the ends of chromosomes. The displacement of chiasmata is termed as terminalization. 

2. The terminal chiasmata exist till the metaphase. 

3. The nucleolus and nuclear membrane completely disappear and spindle fibres begin to appear.

(C) exit the cell cycle

Karyokinesis is the division of Nucleus.

Cell division divides into two equal daughter cells.

Cell division occurring in somatic cells is Mitosis.

The mitotic cell division occurs in somatic cells of the body.

The stage at Late prophase or early Metaphase spindle fibres begin to be formed.

The crossing-over takes place in pachytene. 

Metaphase the stage when chromosomes arrange at the equator.

Amitosis type of cell division present in unicellular organisms.

1. True

2. False

3. True

4. False

Mitosis, Meiosis

1. The spindle fibres are made of Microtubules.

2. The pairing of homologous chromosomes is called Synapsis.

3. Chromosomes are Hereditary material.

4. Polytene chromosomes are found in Salivary- glands of fly larvae.

Mitosis takes place in Somatic cells.

Anaphase chromosomes occurs reach the opposite poles.