Question 1. Meiosis
Answer:
- Meiosis is cell division which reduces the number of chromosomes to half.
- It occurs in all sexually reproducing single celled and multicellular eukaryotes.
- In meiosis, DNA replication is followed by two rounds of cell division to produce four potential daughter cells
- Before meiosis begins, during S phase of the cell cycle, DNA of each chromosome is replicated so that it consists of two identical sister chromatids
Read And Learn More: BDS Previous Examination Question And Answers
Meiosis Phases:
- Meiosis is divided into meiosis I and meiosis II
- Meiosis I
- Meiosis I seggregates homologous chromosomes which are joined by tetrads
- It produces two haploid cells
- Prophase I
- It is longest phase of meiosis
- In it homologous chromosomes pair and exchange DNA called homologous recombination
- It is divided into series of substages like
- Leptotene
- Zygotene
- Pachytene
- Diplotene
- Diakinesis
- Prophase I
- Metaphase I
- In this stage, the paired homologous chromosomes align along the equatorial plane
- Anaphase I
- Pulling of homologous chromosomes to opposite poles occurs
- Telophase I
- It is last stage of meiosis I
- It ends when the chromosomes arrive at the poles
- Each daughter cell now has half the number of chromosomes and each chromosomes consists of a pair of chromatids
- Meiosis II
- It involves separation of sister chromatids
- It results in production of four haploid cells from two haploid cells.
- Meiosis I
Question 1. Chromosome.
Answer:
- A typical chromosome is made up of two rod-shaped chromatids which are placed more or less parallel to each other.
- Both chromatids are united to each other at a light staining area called centomere.
- Each chromatid has two arms – one on either side.
Chromosome Signficance:
- Each chromosome bears very large number of structures called genes which are made up of nucleic acid called deoxyribonucleic acid (DNA).
- These genes are involved in synthesis of proteins.
- Chromosome control the development and functioning of cells, by determining what types of proteins will be synthesized in them.
Question 2. DNA (Deoxyribonucleic acid).
Answer:
- It consists of two strands of fibers that are twisted to form double helix.
- The strands are linked to each other at regular intervals.
- Each strand consists of a chain of nucleotides.
- Each nucleotide molecule is made up of
- One molecule of sugar – deoxyribose.
- One molecule of phosphate.
- It is attached to fifth carbon atom of sugar and third carbon atom of sugar of next nucleotide.
- A nitrogenous base [adenine, guanine, cytosine/thymine].
- It is attached to the first carbon atom.
- The DNA molecule is made up of two such polynucleotide chains running in opposite direction.
- Adenine
(A) always pairs with thymine
(T) and cytosine
(C) always pairs with guanine (G).
Question 3. Gene.
Answer:
- It is a segment of DNA which contains the information (code) for the synthesis of one complete polypeptide chain (or an enzyme).
- It is a set of instructions for making proteins.
- Genes control the development of functioning of cells.
- They play an important role in the development of tissues and organ of an individual.
Question 4. Barr body.
Answer:
- The inactivation of an X-chromosome forms sex – chromatin also called barr body.
- It can be seen in the nuclei of somatic cells in the female only.
Barr body Appearance:
- Chromatin mass lying against the inner surface of the nuclear membrane
- Or as a small drumstick’ of chromatin projecting from then nuclei of the polymorphonuclear leucocytes.
Question 5. Karyotyping.
Answer:
- Individual chromosomes differ from one another in the following aspects.
- Total length of chromosome.
- Relative length of the two arms.
- Various other characteristics.
- These differences enables us to identity the individual chromosome.
- Classification of chromosome in this way is called karyotyping.
Question 6. Sex chromosome
Answer:
- Sex chromosome may be of two kinds
- X and Y
- In males there are 44 autosomes and one X and one Y chromosome
- Similarly females has 44 autosomes and two X chromosomes
- Thus in any individual, 22 pair of autosomes and one pair of sex chromosome is present
- One chromosome of each pair is derived from mother and other from father
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