Structure of Chromosomes, Cell Cycle and Cell Division

1. Chromatin fibre is made up of DNA and Histones.
2. The two sister chromatids of a chromosome are attached to each other at centromere.
3. The structure of DNA was first discovered by Watson and Crick.
4. The three components of a nucleotide are phosphate, sugar and nitrogenous base.
5. DNA strand wound around a histone octamer forms a complex called a nucleosome.
6. A specific sequence of nucleotides on a chromosome constitutes a gene.

DNA and Histones

Reason — DNA surrounds a core of 8 Histones to form a unit called nucleosome. These nucleosomes coil to form chromatin fibres.

Coloured bodies

Reason — The chromosomes readily pick up certain dyes and get coloured, hence the name chromosomes.

both ovary and testis

Reason — In Meiosis, the number chromosomes is halved and it occurs in the sex cells or gametes. It takes place in the reproductive organs — testis and ovary.

DNA

Reason — In synthesis phase, more DNA is synthesised and the chromosomes are duplicated.

Movement

Reason — New cells need to be produced for Growth, Repair, Replacement and Reproduction.

S phase

Reason — In the S phase (i.e., Synthesis phase) more DNA is synthesised and the chromosomes are duplicated.

Thymine = Cytosine

Reason — Purines pair with pyrimidines. Since, thymine and cytosine both are pyrimidines, they can't pair with each other. Thymine pairs with Adenine.

22 pairs

Reason — Out of total 23 pairs of chromosomes, 22 pairs are autosomes and 1 pair is sex chromosome.

Crossing over

Reason — Crossing over results in change of gene sequence which is responsible for variation.

44 + XX chromosomes

Reason — After mitotic cell division, the resulting daughter cells will have the same number and type of chromosomes as the parent cell. A human female cell has 46 chromosomes. Out of these 46, two are sex chromosomes (XX).

Telophase

Reason — In telophase, chromatids thin out in the form of chromatin fibres. In prophase, chromosomes become distinct.

Pentose sugar

Reason — The lengthwise arrangement of DNA mainly consists of a phosphate group attached with five-carbon pentose (ribose) sugar.

8

Reason — A nucleosome has 8 histone proteins.

22 + X chromosomes

Reason — Chromosome number is halved in gametes (sex cells) so the female gamete/egg cell of a human cell will have 22 + X chromosomes.

Anaphase, 4 chromosomes

Reason — During mitosis the chromosome number remains the same, and in anaphase the sister chromatids move towards opposite ends. During this phase, in animal cell, the furrow formation starts in the cell membrane at the middle by which the cytoplasm will divide after mitosis.

(II), (III), (IV), (I)

Reason — During prophase, chromatin fibres change to chromosomes. During metaphase, chromosomes split into sister chromatids. During anaphase, chromatids move to opposite poles. During telophase, chromatids convert into chromatin fibres.

A is False and R is True.

Explanation
Centromere is the place where spindle fibres attach and centrosome is the organelle which helps in forming spindle fibres. Centromere and centrosome are not parts of chromosome.

A is True and R is False.

Explanation
DNA is composed of repeating nucleosome which are made up of nucleotide and histone proteins. Nucleotide are composed of pentose sugar, phosphate group and nitrogenous bases.

A is False and R is True.

Explanation
Centromere is the point of attachment of two chromatids of a chromosome. Centrosomes or centrioles are structures present in cell that initiate and regulate cell division.

A is True and R is False.

Explanation
During meiosis, chromosome number is halved. During fertilisation, two haploid gametes/cells fuse to restore normal chromosome count.

(a) Nucleotides

(b) Nucleosome

(c) Hydrogen Bond

(d) Phosphate, Sugar and Nitrogenous base.

Cell A produces 2 cells
Cell B produces 4 cells

(a) DNA replicates in the synthesis phase of the cell cycle.

(b) Mitosis occurs in our somatic (body) cells.

(c) Meiosis occurs only in reproductive cells.

(d) Modern humans have 46 chromosomes. Their sperms and eggs will have 23 chromosomes each.

(e) During the pairing of chromosomes in meiosis, the homologous chromosomes come to lie side by side.

(f) The two non-sister chromatids of a paired chromosome are attached to each other at chiasma during the process of crossing over.

(a) DNA and Histones

(b) RNA and DNA

(c) Phosphate, Sugar and Nitrogenous bases

(d) Adenine, Guanine, Thymine and Cytosine

(e) Purine and Pyrimidine

(f) DNA and Histone

(g) Mitosis and Meiosis

(h) Prophase, Metaphase, Anaphase, Telophase

(i) Karyokinesis and Cytokinesis

(j) Male gamete (22 + Y) and Female gamete (22 + X)

The rungs of the DNA ladder are made of four types of nitrogenous bases. These are:

1. Adenine (A)
2. Guanine (G)
3. Cytosine (C)
4. Thymine (T)

Corrected statements are as follows:

(a) The four nitrogenous bases in the DNA are Guanine, Thymine, Adenine and Cytosine.

(b) Genes are specific sequences of nucleotides on a chromosome.

(c) A nucleotide is composed of a phosphate, a sugar (pentose) and a nitrogenous base.

(d) Nucleosomes are groups of histone molecules surrounded by DNA strands.

(e) If there are 46 chromosomes in a cell there will be 46 chromatin fibres inside the nucleus during interphase.

(a) Chromosome — Chromosomes are formed of very long, highly coiled and condensed chromatin fibres which are made of DNA (about 40%) and histones (about 60%). They are present in the nucleus of the cell. They carry the chemical instructions for the reproduction of the cell.

(b) Gene — Genes are specific sequences of nucleotides on a chromosome that encode particular proteins which express in the form of some particular feature of the body. They are the units of heredity which are transferred from parents to offsprings and are responsible for some specific characteristics of the offspring.

(c) Cell Division — Cell division is the method in which the cell divides and the duplicated chromosomes get evenly distributed into the daughter cells.

(d) Chromatid — Duplicated chromosomes consist of two identical strands, each of these is called a chromatid. Before replication, one chromosome is composed of one DNA molecule. In replication, the DNA molecule is copied, and the two molecules are known as chromatids. During the later stages of cell division these chromatids separate longitudinally to become individual chromosomes.

(e) Aster — During mitosis in an animal cell, after the centrosome splits into two along with simultaneous duplication of the centrioles contained in it, each centriole is surrounded by radiating rays and is termed aster (meaning star).

(a) Gametes must be produced by meiosis for sexual reproduction because the numbers of chromosomes are reduced to half during meiosis and then the normal diploid numbers of chromosomes are regained during the process of fertilization.

(b) Meiosis is referred to as 'reductional division' because the number of chromosomes are reduced to half i.e. out of the 23 pairs of chromosomes in humans, only single set of chromosomes are passed on to the sex cells. This is essential because when the male and female gametes fuse during fertilization, the normal double (diploid) number of chromosomes is reacquired. The diploid number, as a rule, is expressed as “2n” and the haploid number as "n".

(c) The mixing up or recombination of genes during meiotic division provides for the innumerable variations and diversity in the progeny. That is how, the children of the same parents, howsoever similar, are different from each other in certain aspects.

(a) Difference between Cytokinesis and Karyokinesis

(b) Difference between DNA and RNA

(c) Difference between Nucleosome and Nucleotide

(d) Difference between Centrosome and Centromere

(e) Difference between Haploid and Diploid

(a) DNA — Deoxyribonucleic acid

(b) RNA — Ribonucleic acid

(a) Odd one out — Adrenaline
Adenine, Guanine, Thymine — Nitrogenous bases

(b) Odd one out — Histones
Pentose sugar, Phosphate group, Nitrogenous bases — form nucleotide

(c) Odd one out — Interphase
Metaphase, Anaphase, Telophase — Phases of Mitosis

(d) Odd one out — M phase
G1 phase, G2 phase, S phase — Phases of Interphase

(e) Odd one out — Chromosome
Chromoplast, Chloroplast, Leucoplast — Plastids

(a) 2 strands are shown in the diagram.

(b) 2 on each strand.

(c) The parts are as follows:

• 1 → Phosphate
• 2 → Sugar
• 3 → Bases
• 4 → Hydrogen Bond
• 5 → Base

(d) The DNA unit constituted by the parts 1, 2 and 3 collectively is called Nucleotide.

(a) The parts are as follows:

• 1 → Centrosome
• 2 → Spindle fibres
• 3 → Chromatids

(b) The stage described in the diagram is the late anaphase of mitosis in an animal cell. The stage can be identified by the presence of separated chromatids which are found at the two poles of the cell. The appearance of the furrow in the cell membrane classifies the stage as the late anaphase.

(c) The division is mitotic and this kind of cell division occurs in all the cells of the body except for the reproductive cells.

(d) The stage before anaphase is metaphase. Below diagram shows metaphase:

Below diagram shows the metaphase stage of mitosis in an animal cell having 6 chromosomes:

The phenomenon shown in the diagram is termed Crossing-Over. It is the exchange of chromatid material between the two members of a homologous pair of chromosomes while the maternal and paternal chromosomes are separating.

(a) By observing the given figure we can say that it is the late prophase stage because the nuclear membrane and nucleolus have disappeared.

(b) Centrioles is the cell organelle that form the aster.

(c) The parts are as follows:

• 1 → Centromere
• 2 → Chromatids
• 3 → Spindle fibre

(d) The stage that follows the one shown here is Metaphase. In Metaphase, the centromeres of chromosomes are drawn to the equator by equal pull of two chromosomal spindle fibres that connects each centromere to the opposite poles, forming a metaphasic plate.

(e) Difference between mitosis and meiosis :

(a) It is Metaphase.

(b) The diploid number of chromosomes shown in them is 4.

(c) Categorisation of cell A, B and C are as given below:

• Cell A - It is an animal cell as the centrosome has split into two centrioles and the centrioles have formed asters. Also, no cell wall is present.
• Cell B - It is an animal cell as the centrosome has split into two centrioles and the centrioles have formed asters. Also, no cell wall is present.
• Cell C - It is a plant cell as no centrioles are seen and cell wall is present.

(a) This figure is of the animal cell because:

1. Centrosomes on centrioles are present.
2. Cell wall is absent

(b) It is undergoing mitosis.

(c) The correct sequence is B, C, D and A

(d) The stage that should precede the earliest of these stages is interphase.

(e) Below diagram shows interphase stage of mitosis:

(i) Chromatin fibres become distinct and develop into chromosomes during prophase.

Reason — During prophase, chromatin fibres condense and organize into chromosomes.

(ii) Interphase also occurs between equational and reductional division phases of meiosis.

Reason — There is no interphase between the reductional and equational phases of meiosis, but there is a short stage called interkinesis.

(iii) Four daughter cells are produced with haploid chromosome number.

Reason — In meiosis, two successive divisions occur. In the first division, called reductional division, the chromosome number is reduced to half. In the second division, called equational division, the chromosome number remains the same. Therefore, four haploid daughter cells are produced.