State true or false:
(i) Chromonema represents a chromatid in the early stages of condensation.
(ii) The disc-shaped structure found on chromosome is called chromomere.
(iii) Chromatin fibres condense to form chromosomes.
(iv) These are 23 pairs of autosomes (chromosomes) in human male.
Answer
(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 23rd pair is of sex chromosomes.
The genetic material in the form of very long, thin and dark staining is called
- Centriole
- Chromatin
- Chromatid
- Centromere
Answer
Chromatin
Reason — The genetic material is seen as a network of very long, extremely thin, darkly stained fibres called chromatin (chromatin fibres).
Each chromosome consists of two symmetrical structures called
- Chromatids
- Centromere
- Chromatin fibres
- Arms
Answer
Chromatids
Reason — A chromosome consists of two symmetrical structures called chromatids, which are attached to each other at the centromere.
Rod-shaped chromosome, having centromere at the proximal end is called
- Acrocentric
- Metacentric
- Submetacentric
- Telocentric
Answer
Telocentric
Reason — A telocentric chromosome is rod-shaped and has its centromere at the proximal end.
A pair of corresponding chromosomes of the same shape, size and one from each parent is called:
- Autosomes
- Allosomes
- Analogous chromosomes
- Homologous chromosomes
Answer
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.
A sequence of DNA has 200 nitrogenous base pairs, of which 100 are Thymine-Adenine pairs. What is the number of Cytosine-Guanine pairs in this sequence:
- 50
- 200
- 100
- 25
Answer
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.
Pentose sugar, phosphate and nitrogenous base together form
- chromatids
- nucleotide
- nucleoside
- centromere
Answer
nucleotide
Reason — Each nucleotide consists of three subunits — a pentose sugar, a phosphate and a nitrogenous base.
Three hydrogen bonds are present between
- Guanine and cytosine
- Adenine and thymine
- Adenine and Guanine
- Cytosine and thymine
Answer
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).
The sex chromosome in human ovum is
- X-chromosome
- Y-chromosome
- Both X and Y chromosome
- Either X or Y chromosome
Answer
X-chromosome
Reason — In humans the female is XX, so every ovum (female gamete) always carries an X-chromosome.
The physical carriers of heredity are
- Nuclei
- Chromosomes
- Cells
- Mitochondria
Answer
Chromosomes
Reason — Chromosomes carry the genes, which are the units of heredity, and thus transfer characters from one generation to the next.
Assertion (A): A nucleosome is composed of histones and DNA.
Reason (R): Nucleosome is the basic unit of chromatin organisation.
- Both (A) and (R) are true
- Both (A) and (R) are false
- Both (A) and (R) are true and (R) is the correct explanation of (A)
- Both (A) and (R) are true but (R) is not the correct explanation of (A)
Answer
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).
The nitrogenous base Adenine always pairs with ..................... .
- Thymine
- Guanine
- Cytosine
- Thiamine
Answer
Thymine
Reason — Adenine always pairs with Thymine (A = T) through two hydrogen bonds.
Centromere :
- It is the point of attachment of two sister chromatids
- It is the point of attachment of two centrioles
- It is the point of attachment of two centrosomes
- It is the point of attachment between two daughter nuclei
Answer
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.
Pooja was studying the structure of chromosomes in her biology class. She observed a diagram showing a chromosome with two identical arms joined at a specific point. She was asked to identify the structures. What did Pooja correctly identify?
- The arms are chromatids, and the joining point is the centromere.
- The arms are centromeres, and the joining point is the chromatid.
- The arms are nucleosomes, and the joining point is the centromere.
- The arms are chromatin fibres, and the joining point is the chromonema.
Answer
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.
A biology teacher asked her students to identify the key components of a chromosome.
Raj said: Chromosomes are made up of chromatin, which is a complex of DNA and histone proteins.
Sonu said: Each chromosome consists of two chromatids joined at the centromere.
Lata said: Chromosomes are only visible during the cell's interphase.
Abhay said: The centromere is where the spindle fibres attach during telophase.
Who were correct?
- Raj and Sonu
- Sonu and Abhay
- Raj and Abhay
- Lata and Abhay
Answer
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.
State whether the following statements are true or false. If false, write the correct statement by choosing the incorrect word/words only :
(i) Genes are some specific sequence of nitrogenous bases on a chromosome.
(ii) Cytosine, Adenine, Thyroid and Guanine are the nitrogenous bases in DNA.
(iii) A cell containing 46 chromosomes will have 23 chromatin fibres inside the nucleus during interphase.
(iv) A nitrogenous base alongwith a sugar and sulphate form a nucleotide.
(v) Nitrogen bonds are present between the complementary nitrogenous bases of DNA.
Answer
(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.
Name the following :
(i) The nitrogenous base that pairs with adenine.
(ii) The components of nucleotides.
(iii) Type of bond between the complementary bases.
(iv) A pair of corresponding chromosomes of the same size and shape, one from each parent.
(v) Four nitrogenous bases in a DNA molecule.
(vi) The number of chromosomes present in a nerve cell of a human being.
(vii) The complex consisting of a DNA strand and a core of histones.
Answer
(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
Mention if the following statements are True or False :
(i) Chromatids are attached to each other by centromeres.
(ii) The dark stained fibres found in the nucleus are called chromatin fibres.
(iii) The term 'chromosomes' literally means coloured bodies.
(iv) Histones are a particular type of nucleic acid.
(v) Each DNA strand is composed of repeating nucleotides.
(vi) Genes are the specific parts of nucleolus.
(vii) Telocentric chromosomes look like 'L' shape.
Answer
(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.
What is the structure of DNA as proposed by Watson and Crick?
Answer
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.
Mention the two pairs of nitrogenous bases that are held by together by hydrogen bonds.
Answer
The two pairs are:
- Adenine and Thymine (A = T) — held by two hydrogen bonds.
- Guanine and Cytosine (G ≡ C) — held by three hydrogen bonds.
Define the term allele.
Answer
An allele is one of the alternative forms of a gene that occupies the same locus on homologous chromosomes and controls the same character.
How are sex-chromosomes in man different from those in woman?
Answer
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).
Give difference between chromatin fibres and chromosome.
Answer
Differences between chromatin fibres and chromosome are:
| Chromatin fibres | Chromosome |
|---|---|
| These are long, thin, thread-like, darkly stained fibres present in the nucleus. | These are short, thick, rod-like structures formed by the condensation of chromatin fibres. |
| They are seen during interphase (non-dividing stage). | They are seen clearly during cell division. |
| They are uncoiled or loosely coiled. | They are highly coiled and condensed. |
(i) Expand the abbreviation – DNA.
(ii) What are the strands of DNA made up of?
Answer
(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.
What is crossing over? Give its significance also.
Answer
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.
What is chromatin? When is it observed clearly?
Answer
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.
State the exact location of centromere.
Answer
The centromere lies within a thinner segment of the chromosome called the primary constriction, where the two sister chromatids are joined together.
Mention the two pairs of nitrogenous bases that are present in a DNA strand and pair with each other by hydrogen bonds.
Answer
The two pairs are:
- Adenine and Thymine (A = T) — held by two hydrogen bonds.
- Guanine and Cytosine (G ≡ C) — held by three hydrogen bonds.
Draw a neat labelled diagram of a nucleotide.
Answer
Below is a neat labelled diagram of a nucleotide:
What are nucleosomes? Draw the structure of one nucleosome alongwith DNA strand.
Answer
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:
Explain the structure of a chromosome.
Answer
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.
Mention the changes you observe in the structure of chromosomes in different stages of cell cycle.
Answer
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.
Look at the given below figure and write the name of each part (a) to (d) :
Answer
The parts are as follows:
- (a) → Chromatid
- (b) → Centromere
- (c) → Spindle microtubule
- (d) → Kinetochore
Give the correct sequence of the process of DNA replication :
Answer
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.
Look at the given below diagram and answer the questions followed :
(i) Name the parts labelled as (a), (b), (c), (d) and (e).
(ii) Name the possible combination of nitrogenous bases.
Answer
(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)
Given below is a diagram of a double helical structure of DNA :
(i) Name the four nitrogenous bases that form a DNA molecule.
(ii) Give the full form of DNA.
(iii) Name the unit of heredity.
(iv) Mention two points of differences between Mitosis and Meiosis.
Answer
(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:
| Mitosis | Meiosis |
|---|---|
| It produces two daughter cells. | It produces four daughter cells. |
| The chromosome number in each daughter cell remains the same as that of the parent cell (2n → 2n in a diploid cell). | The chromosome number in each daughter cell is reduced to half that of the parent cell (2n → n). |
Depending upon the position of centromere, a chromosome may have the following shapes. On the basis of this answer the questions given.
(i) The rod shaped chromosome having centromere at the proximal end is called
- Acrocentric
- Metacentric
- Submetacentric
- Telocentric
(ii) Acrocentric chromosomes can be identified with their feature i.e.
- L – shaped structure
- Centromere at sightly below the one end
- T – shaped structure
- V – shaped structure
(iii) V – shaped chromosomes are called
- Metacentric
- Submetacentric
- Telocentric
- Acrocentric
(iv) During cell-division the microtubules stick to the chromosomes at
- Chromosomal arms
- Centromere
- Centrosome
- Kinetochore
Answer
(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.