Structure of Chromosomes

(i) True.

(ii) False.
Corrected Statement — The disc-shaped structure found at the centromere is called the kinetochore.

(iii) True.

(iv) False.
Corrected Statement — There are 22 pairs of autosomes (chromosomes) in a human male; the 23^rd pair is of sex chromosomes.

Chromatin

Reason — The genetic material is seen as a network of very long, extremely thin, darkly stained fibres called chromatin (chromatin fibres).

Chromatids

Reason — A chromosome consists of two symmetrical structures called chromatids, which are attached to each other at the centromere.

Telocentric

Reason — A telocentric chromosome is rod-shaped and has its centromere at the proximal end.

Homologous chromosomes

Reason — Homologous chromosomes are a pair of corresponding chromosomes of the same shape and size, one member of which is inherited from each parent.

100

Reason — The total number of base pairs is 200. Since 100 of these are Thymine–Adenine pairs, the remaining 200 - 100 = 100 are Cytosine–Guanine pairs.

nucleotide

Reason — Each nucleotide consists of three subunits — a pentose sugar, a phosphate and a nitrogenous base.

Guanine and cytosine

Reason — There are three hydrogen bonds between Guanine and Cytosine (G ≡ C), whereas Adenine and Thymine are joined by two hydrogen bonds (A = T).

X-chromosome

Reason — In humans the female is XX, so every ovum (female gamete) always carries an X-chromosome.

Chromosomes

Reason — Chromosomes carry the genes, which are the units of heredity, and thus transfer characters from one generation to the next.

Both (A) and (R) are true but (R) is not the correct explanation of (A)

Explanation

A nucleosome is formed when a DNA strand wraps around a core of eight histone proteins, so (A) is true. The nucleosome is also the basic repeating unit of chromatin, so (R) is true. However, (R) only states the role of the nucleosome and does not explain its composition, so (R) is not the correct explanation of (A).

Thymine

Reason — Adenine always pairs with Thymine (A = T) through two hydrogen bonds.

It is the point of attachment of two sister chromatids

Reason — The centromere is the region of the chromosome at which the two sister chromatids are joined together.

The arms are chromatids, and the joining point is the centromere.

Reason — A chromosome consists of two symmetrical arms called chromatids, which are joined together at the centromere.

Raj and Sonu

Reason — Chromosomes are made of chromatin (DNA + histone proteins) and each chromosome has two chromatids joined at the centromere, so Raj and Sonu are correct. Lata is wrong because chromosomes are seen clearly during cell division, not interphase. Abhay is wrong because spindle fibres attach to the kinetochore at the centromere during metaphase, not telophase.

(i) True.

(ii) False.
Corrected Statement — The correct word is Thymine in place of Thyroid.

(iii) False.
Corrected Statement — The correct word is 46 in place of 23.

(iv) False.
Corrected Statement — The correct word is phosphate in place of sulphate.

(v) False.
Corrected Statement — The correct word is Hydrogen bonds in place of Nitrogen bonds.

(i) Thymine

(ii) Pentose sugar, phosphate and nitrogenous base

(iii) Hydrogen bond

(iv) Homologous chromosomes

(v) Adenine, Guanine, Cytosine and Thymine

(vi) 46

(vii) Nucleosome

(i) True

(ii) True

(iii) True

(iv) False
Corrected statement — Histones are proteins around which DNA is wrapped to form nucleosomes.

(v) True

(vi) False
Corrected statement — Genes are specific segments of DNA present on chromosomes.

(vii) False
Corrected statement — Telocentric chromosomes are rod-shaped because the centromere is located at the terminal (proximal) end.

The structure of DNA was proposed by Watson and Crick in 1953. Its main features are:

• It is made of two strands helically coiled with each other, giving the appearance of a spiral staircase called a double helix.
• Each strand is made up of small units called nucleotides. Each nucleotide consists of a pentose sugar, a phosphate and a nitrogenous base.
• It contains four nitrogenous bases — Adenine, Guanine, Cytosine and Thymine.
• Adenine pairs with Thymine by two hydrogen bonds (A = T) and Guanine pairs with Cytosine by three hydrogen bonds (G ≡ C).
• DNA has the property of replication.

The two pairs are:

• Adenine and Thymine (A = T) — held by two hydrogen bonds.
• Guanine and Cytosine (G ≡ C) — held by three hydrogen bonds.

An allele is one of the alternative forms of a gene that occupies the same locus on homologous chromosomes and controls the same character.

In a man (male), the sex chromosomes are one X and one Y chromosome (XY). In a woman (female), the sex chromosomes are two X chromosomes (XX).

Differences between chromatin fibres and chromosome are:

(i) DNA — Deoxyribonucleic acid.

(ii) Each strand of DNA is made up of small units called nucleotides. Each nucleotide consists of a pentose sugar, a phosphate and a nitrogenous base.

Crossing over is the exchange of chromatid material (segments) between the non-sister chromatids of a pair of homologous chromosomes during cell division.

Significance — It brings about recombination of genes, which produces variations and diversity among the offspring.

Chromatin is the network of very long, extremely thin, darkly stained fibres, made up of DNA and histone proteins, present in the nucleus of a cell. It is observed clearly during the interphase (non-dividing stage) of the cell.

The centromere lies within a thinner segment of the chromosome called the primary constriction, where the two sister chromatids are joined together.

The two pairs are:

• Adenine and Thymine (A = T) — held by two hydrogen bonds.
• Guanine and Cytosine (G ≡ C) — held by three hydrogen bonds.

Below is a neat labelled diagram of a nucleotide:

A nucleosome is formed when a DNA strand is wrapped around a core (histone octamer) of eight histone proteins. Thus, each unit consisting of the histone octamer together with the associated DNA is called a nucleosome. Nucleosomes are the basic repeating units of a chromatin fibre.

Below is the structure of one nucleosome along with the DNA strand:

A chromosome has the following structures:

• Chromatid — A chromosome consists of two symmetrical structures called chromatids, which are attached to each other at the centromere.
• Centromere — It is the region at which the two sister chromatids are joined. The kinetochore is present on the centromere, and spindle microtubules attach to it during cell division.
• Chromonema — It represents a chromatid in the early stages of condensation and forms the gene-bearing portion of the chromosome.
• Chromomere — These are bead-like thickenings present along the chromonema.
• Euchromatin — It is the lightly stained, partially condensed portion of the chromosome.
• Heterochromatin — It is the dark stained, condensed portion of the chromosome.

The changes observed in the structure of chromosomes are:

• Interphase — Chromosomes exist as very long, thin, uncoiled chromatin fibres.
• Early prophase — The chromatin fibres begin to condense and coil, becoming shorter and thicker.
• Late prophase — The chromosomes become distinct and appear as two chromatids joined at the centromere.
• Metaphase — The chromosomes become fully condensed, short and thick, and spindle microtubules attach to the kinetochores at the centromeres.

The parts are as follows:

• (a) → Chromatid
• (b) → Centromere
• (c) → Spindle microtubule
• (d) → Kinetochore

The correct sequence of DNA replication is:

• (a) The DNA double helix (parental molecule).
• (b) The molecule unwinds and unzips at one end, so that the two strands separate.
• (c) Free nucleotides pair with the complementary bases on each old (template) strand.
• (d) Two identical DNA molecules are formed, each having one old (parental) strand and one new strand.

(i) The parts are as follows:

• (a) → Nitrogenous base
• (b) → Hydrogen bonds
• (c) → Nitrogenous base
• (d) → Pentose sugar
• (e) → Phosphate

(ii) The possible combinations of nitrogenous bases are:

• Adenine – Thymine (A – T)
• Guanine – Cytosine (G – C)

(i) The four nitrogenous bases are Adenine, Guanine, Cytosine and Thymine.

(ii) DNA — Deoxyribonucleic acid.

(iii) The unit of heredity is the Gene.

(iv) Difference between Mitosis and Meiosis:

(i) Telocentric

Reason — A telocentric chromosome is rod-shaped and has its centromere at the proximal (terminal) end. According to the classification used in the chapter, an acrocentric chromosome is rod-shaped with the centromere near one end, a submetacentric chromosome is T- or L-shaped with the centromere almost at the middle, and a metacentric chromosome is V-shaped with the centromere at the centre.

(ii) Centromere at slightly below the one end

Reason — An acrocentric chromosome is rod-shaped and has its centromere near one end, slightly below the terminal position. This distinguishes it from a telocentric chromosome, whose centromere is at the proximal end.

(iii) Metacentric

Reason — A metacentric chromosome has the centromere at the centre, forming two equal arms and giving it a V-shaped appearance. In the chapter, submetacentric chromosomes are described as T- or L-shaped, while telocentric and acrocentric chromosomes are rod-shaped.

(iv) Kinetochore

Reason — The kinetochore is a protein structure present on the centromere where spindle microtubules attach during cell division.
Spindle fibres do not attach to the chromosome arms. They attach to the kinetochore. The kinetochore, which is located on the centromere, is the actual site of spindle fibre attachment. The centrosome organizes spindle fibres but does not serve as the attachment site on chromosomes.