Category: Biology Second Paper

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

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

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.
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. Terminalization: Repulsion begins between the bivalent chromosomes after crossing over is completed. Chromosomes move away from each other. The chiasma gradually move towards the edge. This is called terminalization.

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.

All parts found in an ideal chromosome are as follows-
1. Pellicle: Chromosomes are covered by a membrane called pellicle. Modern studies have found no attachment to the pellicle. Scientists Mc Clinton and Swanson mentioned the pellicle. But scientists Darlington, Novikoff and Rees deny Pellicol’s existence.
2. Matrika: The fluid inside the pellicle is called Matrika. But the remains of the mother have not been found till date.
3. Centromere: The word Centromere is formed from the Greek word Kentron meaning center and meros meaning part. The round, colorless and constricted area in the chromosome is called centromere. Each chromosome has only one centromere. However, some chromosomes may have two or more centromeres. The position of the centromere creates a groove in the chromosome. This is called Mukhyakunchan or Mukhyakhan. Scientist Darlington named it centromere.
4. Kinetochore: The disc or disk like structure at the centromere of chromosome is called kinetochore. The kinetochore contains DNA, tubulin protein and microtubules. Each kinetochore is composed of two plates. The outer plate is associated with microtubules and the inner plate with centromeric heterochromatin. It regulates chromosomal movement.
5. Arms: The chromosomal parts on either side of the centromere are called arms. Chromosome arms can be equal or unequal.
6. Secondary Contractions: Chromosomes contain one or more other sites of contraction other than the centromere. This is called secondary shrinkage. A secondary contraction called SAT (Sine Acid Thymonucleic) forms the nucleolus. Each diploid chromosome has a pair of secondary chromosomes. It is called Nucleolar Organizer Region-NOR. Human chromosomes 13, 14, 15, 21 and 22 have a total of five pairs of NORs.
7. Chromomeres: The small beads like beads found in the chromosomes are called chromomeres. Its other name is Idiomere. Chromomeres are formed by DNA coiling. The region between two chromomeres is called interchromomeres.
8. Satellite: The small segment adjacent to the secondary end of the chromosome is called satellite. Such chromosomes are called SAT chromosomes. Chromosome 1 of chickpea has satellite. Human chromosomes 13, 14, 15, 21 and 22 have been identified as SAT chromosomes. Cotton and jute have satellite chromosomes.
9. Telomere: The word Telomere is formed from the Greek words telos meaning end and meros meaning part. Scientist H. J. According to Müller, the characteristic regions at both ends of chromosomes are called telomeres. The repeated sequence of DNA at the head of the chromosome is the telomere. Because of telomeres, chromosome arms can never come together. Here some nucleotides are rearranged. It contains the telomerase enzyme that works to prevent human aging. Telomere length decreases with each cell division. Therefore, the age of an organism can be determined by measuring the length of the telomere. Human telomere length is 8000 base pairs at birth, 3000 base pairs at age 35, and 1500 base pairs at age 65. Telomeres are always in the TTAGGG state in higher plants and TTTAGGG in vertebrates. The function of telomeres is to protect the coding region of DNA from destruction during cell division.
10. Chromatin: Each chromosome divides longitudinally to form two thread-like structures. These are called chromatin. It is initially 11 nm thick and gradually becomes 30 nm, 300 nm and 700 nm thick. Heitz (1928) divided chromatin fibers into two parts. Heterochromatin and Euchromatin.
11. Chromonema: Each chromatid divides longitudinally to form two linear structures. It is called chromonema. Scientist Vejdovsky (1921) called them Chromonema. Chromonema has two types of patches. Paranemic panch and plactonemic panch. Each chromoneme is made up of 26 or 32 molecules. However, in 1965, scientist Drup proved that chromonema does not have any formula. During the anaphase phase the chromatin becomes organized into chromosomes.

Chemical structure of Chromosome
The chemical components of chromosomes are nucleic acids, proteins and other components.
1. Nucleic acid: Chromosome contains 45% nucleic acid. There are two types of nucleic acids. DNA and RNA.
(i) DNA: Deoxyribonucleic acid is abbreviated as DNA. The smallest part of the chromosome that is self-reproducing, regulates biological functions and transmits heritable characteristics and is capable of causing variation, mutation and evolution of organisms is called DNA. It is composed of three chemical substances. Nitrogen bases, pentose sugars and phosphoric acid. Nitrogenous alkalis are of two types. Purines and pyrimidines. Purines are adenine and guanine and pyrimidines are thymine and cytosine. According to scientists Swift (1964) and Bonner (1968), the ratio of DNA and histone proteins in chromosomes is 50:50 or 1:1. About 90% of an organism’s DNA is contained in chromosomes.
(ii) RNA : Ribonucleic acid is abbreviated as RNA. The monomeric units of nucleotides of nucleic acid are made up of ribose sugar, adenine, guanine, cytosine, uracil and phosphoric acid is called RNA. It is composed of three chemical substances. Nitrogenous bases, pentose sugars and phosphoric acid. Nitrogenous alkalis are of two types. Purines and pyrimidines. Purines are adenine and guanine and pyrimidines are cytosine and uracil.
2. Proteins: Proteins form the basic structure of chromosomes. Chromosomes contain 55% protein. Chromosomes contain two types of proteins. Histone proteins and non-histone proteins.
(i) Histone proteins: The major proteins of chromosomes are histone proteins. Histone proteins are alkaline because they contain more lysine and arginine. It forms nucleosome. There are five types of histone proteins. These are-
* Histone-1 (H1): Its molecular weight is 23,000 daltons. It is very high in lysine.
* Histone-2A (H2A): Its molecular weight is 14,000 daltons. It contains slightly more lysine than arginine.
* Histone-2B (H2B): Its molecular weight is 13,800 daltons. It contains a little more lysine.
* Histone-3 (H3): Its molecular weight is 15,300 daltons. It is high in arginine.
* Histone-4 (H4): Its molecular weight is 11,300 daltons. It is high in arginine. Lysine content is slightly higher.
(ii) Non-histone proteins: All proteins other than the histone proteins of chromosomes are called non-histone proteins. The protein structure that remains on the chromosome after the removal of histone proteins is called the scaffold. Non-histone proteins are acidic in nature when tyrosine and tryptophan are added.
3. Other components: Chromosomes contain small amounts of lipids, DNA polymerase enzyme, RNA polymerase enzyme, nucleoside trifatase, calcium ion, magnesium ion, iron ion etc.

Giant chromosomes
Chromosomes that are much larger than normal chromosomes are called giant chromosomes. These special types of chromosomes are:
1. Lamp Brush Chromosome: The chromosome which looks like lamp brush or lamp brush is called lamp brush chromosome. These chromosomes have several pairs of loops centered on their axis and look like chimney brushes or lamp brushes. The axis of the chromosome is composed of chromomeres and inter-chromomeres and loop transcribing genes. Its length is 1500-2000 micrometers. It is required for the formation of sperm and egg yolk. In 1882, the embryologist Rishabh Sarham was the first to discover this chromosome from the egg of an asymmetric amphibian. It is found in oocytes or immature eggs of fish, amphibians, reptiles, birds and insects. However, it is not found in the ovum of mammals. Embryologist Flemming first discovered this chromosome from the egg of the amphibian Amblystoma maxicanum. It is found in oocytes or immature eggs of fish, amphibians, reptiles, birds and insects. However, it is not found in the ovum of mammals.

2. Polytene Chromosomes: Those chromosomes which are large, multi-armed and consist of thousands of DNA are called polytene chromosomes. It is also called salivary gland chromosome. Its length is 2000 micrometers. It increases the nucleus and cell shape. Their multicopy genes result in high levels of gene expression in the organism.
Polytene chromosomes have 5 long and 1 short arm. Each arm has a black and white band. Black arms are euchromatin regions. Some parts of it swell and form large formations. It is called Balbiani ring. The chromosomal puff/ballbiani ring synthesizes mRNA. This chromosome is found in salivary gland of insects like Drosophila, Chironomus, Rhynchosciara etc. In 1881, French entomologist E.G. Balbiani discovered polytene chromosomes.

3. B Chromosome : One or more extra chromosomes other than the normal chromosome in the nucleus are called B chromosomes or supplementary chromosomes. B chromosomes are small, non-vital and have heterochromatin. This is sefish genetic material and can be easily distinguished. It contains few genes and does not follow Mendelian pattern. In 1905, Wilson first discovered the B chromosome from the beetle Metapodius. B chromosome is found in 475 species of plants belonging to 163 genera of 42 families. B chromosome is found in organisms such as Ulat Chandal, Bhuta, Brachycome iberidifolia, Lilium callosum, Secale cereale etc.

4. Sat Chromosome: The chromosomes that have satellites are called sat chromosomes. The satellite is separated from the parent chromosome by a thin chromatin. Sat chromosome plays a major role in the formation of cell nucleolus. In 1930, scientist Heitz first used the term SAT.

Function of Chromosome
1. Container and carrier of heredity: Chromosomes are the container and carrier of heredity. It contains, carries and transmits the hereditary characteristics of the organism.
2. Cell Division: Chromosomes play a direct role in cell division. A cell that does not contain chromosomes does not divide. Primary cells do not contain chromosomes.
3. Containing DNA: The smallest part of chromosome is DNA. The entire segment of chromosome contains DNA.
4. Protein synthesis: Chromosomes contain DNA. mRNA is made from DNA. The translation process of mRNA leads to protein synthesis.
5. Sex determination: Sex chromosomes determine the sex of the organism. If the sex chromosome is XY, it is male and if it is XX, it is female.
6. Blue print of life: Genes are carriers of heredity and act as the blue print of life. The blue print is located on the chromosome.
7. Evolution: Changes in the number and structure of chromosomes play a role in evolution.
8. Creation of Variation: Structural and numerical changes in chromosomes create variation in organisms.
9. Shape of Nucleus: Chromosomes shape the nucleus.
10. Production of nuclear material: Chromosomes produce nuclear material.
11. Carrying genetic messages: Chromosomes contain mRNA. mRNA carries the genetic message in the cytoplasm.
12. Self-reorganization: Chromosomes are self-reorganizing.

Role of Chromosomes
Genes or DNA located in chromosomes carry all the signals of the cell. It initiates cell division by signaling. DNA replication completes preparations for cell division. Cell division does not begin unless DNA is replicated. Hence, chromosomes play an important role in cell division. Replication, duplication, division and polarization of chromosomes are prerequisites for cell division. Cells without chromosomes never divide. Abnormal distribution of chromosomes during cell division has adverse effects on cell characteristics and survival. So it can be said that chromosomes play a direct role in cell division. Cell division will be mitosis or meiosis depending on how many times the chromosomes divide.

Genes or DNA located in chromosomes carry all the signals of the cell. It initiates cell division by signaling. DNA replication completes preparations for cell division. Cell division does not begin unless DNA is replicated. Hence, chromosomes play an important role in cell division. Replication, duplication, division and polarization of chromosomes are prerequisites for cell division. Cells without chromosomes never divide. Abnormal distribution of chromosomes during cell division has adverse effects on cell characteristics and survival. So it can be said that chromosomes play a direct role in cell division. Cell division will be mitosis or meiosis depending on how many times the chromosomes divide.

1. Container and carrier of heredity: Chromosomes are the container and carrier of heredity. It contains, carries and transmits the hereditary characteristics of the organism.
2. Cell Division: Chromosomes play a direct role in cell division. A cell that does not contain chromosomes does not divide. Primary cells do not contain chromosomes.
3. Containing DNA: The smallest part of chromosome is DNA. The entire segment of chromosome contains DNA.
4. Protein synthesis: Chromosomes contain DNA. mRNA is made from DNA. The translation process of mRNA leads to protein synthesis.
5. Sex determination: Sex chromosomes determine the sex of the organism. If the sex chromosome is XY, it is male and if it is XX, it is female.
6. Blue print of life: Genes are carriers of heredity and act as the blue print of life. The blue print is located on the chromosome.
7. Evolution: Changes in the number and structure of chromosomes play a role in evolution.
8. Creation of Variation: Structural and numerical changes in chromosomes create variation in organisms.
9. Shape of Nucleus: Chromosomes shape the nucleus.
10. Production of nuclear material: Chromosomes produce nuclear material.
11. Carrying genetic messages: Chromosomes contain mRNA. mRNA carries the genetic message in the cytoplasm.
12. Self-reorganization: Chromosomes are self-reorganizing.

Chromosomes that are much larger than normal chromosomes are called giant chromosomes. These special types of chromosomes are:
1. Lamp Brush Chromosome: The chromosome which looks like lamp brush or lamp brush is called lamp brush chromosome. These chromosomes have several pairs of loops centered on their axis and look like chimney brushes or lamp brushes. The axis of the chromosome is composed of chromomeres and inter-chromomeres and loop transcribing genes. Its length is 1500-2000 micrometers. It is required for the formation of sperm and egg yolk. In 1882, the embryologist Rishabh Sarham was the first to discover this chromosome from the egg of an asymmetric amphibian. It is found in oocytes or immature eggs of fish, amphibians, reptiles, birds and insects. However, it is not found in the ovum of mammals. Embryologist Flemming first discovered this chromosome from the egg of the amphibian Amblystoma maxicanum. It is found in oocytes or immature eggs of fish, amphibians, reptiles, birds and insects. However, it is not found in the ovum of mammals.

2. Polytene Chromosomes: Those chromosomes which are large, multi-armed and consist of thousands of DNA are called polytene chromosomes. It is also called salivary gland chromosome. Its length is 2000 micrometers. It increases the nucleus and cell shape. Their multicopy genes result in high levels of gene expression in the organism.
Polytene chromosomes have 5 long and 1 short arm. Each arm has a black and white band. Black arms are euchromatin regions. Some parts of it swell and form large formations. It is called Balbiani ring. The chromosomal puff/ballbiani ring synthesizes mRNA. This chromosome is found in salivary gland of insects like Drosophila, Chironomus, Rhynchosciara etc. In 1881, French entomologist E.G. Balbiani discovered polytene chromosomes.

3. B Chromosome : One or more extra chromosomes other than the normal chromosome in the nucleus are called B chromosomes or supplementary chromosomes. B chromosomes are small, non-vital and have heterochromatin. This is sefish genetic material and can be easily distinguished. It contains few genes and does not follow Mendelian pattern. In 1905, Wilson first discovered the B chromosome from the beetle Metapodius. B chromosome is found in 475 species of plants belonging to 163 genera of 42 families. B chromosome is found in organisms such as Ulat Chandal, Bhuta, Brachycome iberidifolia, Lilium callosum, Secale cereale etc.

4. Sat Chromosome: The chromosomes that have satellites are called sat chromosomes. The satellite is separated from the parent chromosome by a thin chromatin. Sat chromosome plays a major role in the formation of cell nucleolus. In 1930, scientist Heitz first used the term SAT.

The chromosomes that have satellites are called sat chromosomes. The satellite is separated from the parent chromosome by a thin chromatin. Sat chromosome plays a major role in the formation of cell nucleolus. In 1930, scientist Heitz first used the term SAT.

One or more extra chromosomes other than the normal chromosome in the nucleus are called B chromosomes or supplementary chromosomes. B chromosomes are small, non-vital and have heterochromatin. This is sefish genetic material and can be easily distinguished. It contains few genes and does not follow Mendelian pattern. In 1905, Wilson first discovered the B chromosome from the beetle Metapodius. B chromosome is found in 475 species of plants belonging to 163 genera of 42 families. B chromosome is found in organisms such as Ulat Chandal, Bhuta, Brachycome iberidifolia, Lilium callosum, Secale cereale etc.