Category: Biology Second Paper

Five-carbon sugar (sugar) is called pentose sugar. Nucleic acids contain two types of pentose sugars. Ribose sugar and deoxyribose sugar.
(i) Ribose sugar: RNA molecule contains ribose sugar. It is a type of monosaccharide. Its 2nd carbon contains oxygen.
(ii) Deoxyribose sugar: DNA molecule contains deoxyribose sugar. It is a type of monosaccharide. Its 2nd carbon does not contain oxygen (deoxy = less of one oxygen) so named. That is why it is called 2-β-D deoxyribose sugar.

Adenine and Thymine are named after Thymus. They were isolated from the thymus gland. Adeno means gond. Cytosine is named after cyto. Cyto means cell. Guanine comes from the Spanish word guano. Guano means the droppings of bats. Alkalies are usually denoted by the alphabet AGTCU. DNA molecules have the letters AGTC and RNA molecules have the letters AGCU.

This alkali is formed by carbon, hydrogen, oxygen and nitrogen. It contains 15% nitrogen. Alkaline compounds form rings. Based on the number of rings, nitrogenous bases can be divided into two groups.
(i) Purines: Dicyclic nitrogenous bases are called purines. Its common symbol is C5H4N4. It is composed of Adenine and Guanine.
(ii) Pyrimidine: A cyclic nitrogenous base is called pyrimidine. Its common symbol is C4H4N2. It is composed of Thymine, Cytosine and Uracil.

Nucleic acids are made up of three main components. These are-
1. Nitrogen Alkali: This alkali is formed by carbon, hydrogen, oxygen and nitrogen. It contains 15% nitrogen. Alkaline compounds form rings. Based on the number of rings, nitrogenous bases can be divided into two groups.
(i) Purines: Dicyclic nitrogenous bases are called purines. Its common symbol is C5H4N4. It is composed of Adenine and Guanine.
(ii) Pyrimidine: A cyclic nitrogenous base is called pyrimidine. Its common symbol is C4H4N2. It is composed of Thymine, Cytosine and Uracil.

Nomenclature of Nitrogen Alkalies
Adenine and Thymine are named after Thymus. They were isolated from the thymus gland. Adeno means gond. Cytosine is named after cyto. Cyto means cell. Guanine comes from the Spanish word guano. Guano means the droppings of bats. Alkalies are usually denoted by the alphabet AGTCU. DNA molecules have the letters AGTC and RNA molecules have the letters AGCU.
2. Pentose Sugar: Five-carbon sugar (sugar) is called pentose sugar. Nucleic acids contain two types of pentose sugars. Ribose sugar and deoxyribose sugar.
(i) Ribose sugar: RNA molecule contains ribose sugar. It is a type of monosaccharide. Its 2nd carbon contains oxygen.
(ii) Deoxyribose sugar: DNA molecule contains deoxyribose sugar. It is a type of monosaccharide. Its 2nd carbon does not contain oxygen (deoxy = less of one oxygen) so named. That is why it is called 2-β-D deoxyribose sugar.
3. Phosphoric acid: One of the chemical components of chromosomes is phosphoric acid. It contains 10% phosphorus element. Its molecular symbol is H3PO4. It contains one divalent oxygen atom and three monovalent hydroxyl groups. Oxygen and hydroxyl groups combine with a pentavalent phosphorus atom to form phosphoric acid.
Nucleic acids are composed of carbon, hydrogen, oxygen, nitrogen and phosphorus. It contains 15% nitrogen and 10% phosphorus.

In 1869, the Swiss physician and chemist Friedrich Miescher discovered a new chemical substance from the nucleus of the white blood cell of the stem cell and named it nucleon. In 1889, Richard Altmann discovered the religion of acid in nucleic and named it nucleic acid. In 1894, Albrecht Kossel identified purines, pyrimidines, sugars and phosphoric acid in nucleic acids. For this he was awarded the Nobel Prize in Chemistry in 1910. In 1921 Phoebus Lavine discovered two types of nucleic acids DNA and RNA. In 1950, Yerwin Chargaff demonstrated that the nucleotide composition of DNA varies among species. In 1952, Hershey and Chase proved that DNA is the genetic material of organisms.

Nucleotides form polymers called nucleic acids. Nucleic acids are composed of nitrogenous bases, pentose sugars and phosphoric acid. It is the largest chemical molecule in the cell. They are called master molecules as they carry all the characteristics of heredity.

The process by which parts of bivalent homologous chromosomes are exchanged between two non-sister chromatids is called crossing over. This is called genetic recombination. In 1909 scientist Thomas Hunt Morgan first observed the crossing over at Bhutra. He won the Nobel Prize in 1933.

White House Model of Crossover
According to the White House model, crossing over is done in four steps. These are-
1. Synapsis: The process by which homologous chromosomes join together is called synapsis. At this stage attraction is observed between homologous chromosomes. Homologous chromosomes come close to each other and pair along their length. The process of pairing of homologous chromosomes is called synapsis. Each pair of chromosomes is called bivalent.
2. Duplication of Chromosome: Each chromosome of bivalent divides longitudinally without centromere to form two chromatids. As a result, four chromatids are formed in each bivalent. This condition is called tetrad. Two chromatids of the same chromosome are called sister chromatids and two chromatids of different chromosomes are called non-sister chromatids. Siddique Publications
3. Interchange of body parts: Two non-sister chromatids of bivalents come close to each other and form X-shaped zygoma. Chromatids are broken by the action of endonuclease enzyme at the chiasma segment. A broken chromatid joins another non-sister chromatid with the help of ligase enzyme. In this way parts are exchanged between two non-sister chromatids. It is called crossing over. Dr. Siddique Publications
4. Marginalization: Repulsion begins between the bivalent chromosomes after crossing over is completed. Chromosomes move away from each other. The kaiazmas gradually move towards the edge. This is called terminalization.

Importance of crossing over
1. Rearrangement of genes: During crossing over, segments are exchanged between two non-sister chromatids. As a result, new arrangements of genes occur in the chromosomes.
2. Genetic Variation: Crossing over results in genetic variation in organisms. New varieties are created in crop plants through crossing over.
3. Genetic modification: Genetic modification can be done by artificial crossing over. This is why crossing over plays an important role in breeding.
4. Creation of Biodiversity: Biodiversity is the characteristic and characteristic difference between one organism and another organism. Differences between two plants or animals of the same species belong to biodiversity. Biodiversity is created due to changes in the position and arrangement of genes. Siddique Publications
5. Survival in new environments: Characteristic changes occur in organisms due to crossing over. In this the organism acquires the ability to survive in the new environment and survive.
6. Preparation of genetic map: By determining the percentage of crossing over, the position of the gene on the chromosome is determined and the chromosome map is prepared.
7. Genetic research: Genetic research has created a great stir worldwide. Crossing over is an interesting topic in gene theory research.
8. Aid in evolution: Crossing over results in trait changes, biodiversity and increased ability to survive in new environments. As a result, evolution is possible.
9. Breeding of crop plants: Crossover can be used to produce desirable traits in crop plants. Improved varieties of crops can be created.
10. In breeding: Alteration in heredity by artificial crossing over. Hence it has a wide role in reproductive science.
11. Linear arrangement of chromosomes: Linear arrangement of genes in chromosomes occurs due to crossing over.
12. Preparation of Chromosome Map: First the percentage of crossing over is determined. Then the location of the gene on the chromosome is determined and a map of the chromosome is made.
Crossing over is a very important process in the living world. Rearrangement of hereditary traits in chromosomes occurs through crossing over. The result is variation. The early beginnings of evolution in species. Dr. Siddique Publications

Catalyst of Crossing Over
1. High temperature: High temperature and radiation increase the tendency of crossover occurrence. Siddique Publications
2. Aging: As the organism ages, the incidence of crossing over decreases.
3. Mutation: The level of crossing over is reduced due to mutation.
4. Inversion: Inversion stops the crossing over process.
5. Interference: Interference with the formation of another chiasm near the site of chromosomal formation. This phenomenon is called interference.
6. Radiomimetic substances: Chemical substances that increase the rate of somatic crossing over are called radiomimetic substances. Eg- ethylmethane sulphonate. Dr. Siddiq Publications
7. Chemicals: Various chemicals reduce crossover. For example, colchicine, selenium, etc. Dr. Siddiq Publications

Chromosomes in meiosis-1 and chromatids in meiosis-2 do not separate normally, it is called non-disjunction. Non-disjunction results in either too many or too few chromosomes in the gamete. Cohesin protein increases the probability of non-disjunction in the egg. Genetic abnormalities in human babies due to non-disjunction are:
1. Patao syndrome: Patao syndrome is caused by trisomy on human chromosome 13.
2. Edward’s syndrome: Edward’s syndrome occurs when human trisomy occurs on chromosome 18. Dr. Siddique Publications
3. Down syndrome: Down syndrome occurs when trisomy occurs on human chromosome 21.
4. Klinefelter syndrome: Klinefelter syndrome occurs when the male zygote has an excess of X chromosomes. For example- XXY.
5. XYY syndrome: XYY syndrome occurs when a male zygote has an extra Y chromosome.
6. Turner syndrome: If there is less X chromosome in the female zygote, Turner syndrome occurs. Siddique Publications
7. Triple X Syndrome: Triple X syndrome occurs when there is an extra X chromosome in the female zygote. For example, XXX.

In animal cells, chromosomes are clustered near the nuclear membrane to form a bouquet-like structure. This is called polarized mode.

Substances that stimulate cell division are called mitogens. Some of the mitogens are – cytokinins, steroids, lymphokines etc.