Category: Biology Second Paper

Chromosomes are divided into four parts based on the position of centromere.
(i) Metacentric Chromosome: Chromosome which lies in the center of centromere is called metacentric chromosome. During the anaphase phase of cell division, the metacentric chromosomes look like the English letter V. Amphibians have metacentric chromosomes. All chromosomes of Solanum nigrum are metacentric. Human chromosomes 1, 3, 6, 7, 8, 11, 16, 19, 20 and X are metacentric types.
(ii) Sub-metacentric chromosomes: Chromosome which is slightly away from the center of centromere is called sub metacentric chromosome. During the anaphase phase of cell division, the sub-metacentric chromosomes look like the English letter L. Human chromosomes 2, 4, 5, 9, 10, 12, 17 and 18 are of sub metacentric type.
(iii) Acrocentric Chromosome: Chromosome which is near the edge of centromere is called acrocentric chromosome. In the anaphase phase of cell division, acrocentric chromosomes look like the English letter O. All acrocentric chromosomes are sat-chromosomes. Human chromosomes 13, 14, 15, 21, 22 and Y are acrocentric types.
(iv) Telocentric Chromosome: Chromosome which is located at the extreme end of centromere is called telocentric chromosome. During the anaphase phase of cell division, the telocentric chromosomes look like the English letter I. Telocentric chromosomes are rare in organisms. Humans do not have telocentric chromosomes. Telocentric chromosomes are found in gymnosperms (Welwitschia) and onion (Allium).
[The dark purple variant of Typhonium trilobatum has 11 metacentric, 4 sub-metacentric and 2 acrocentric chromosomes]

Uncontrolled mitosis causes cell division due to various factors external and internal to the cell. Uncontrolled mitosis causes tumors and cancer in the body. The process of tumor formation is called oncogenesis. Causes of uncontrolled mitosis or cancer are mentioned.

1. Cell cycle control system: Abnormalities in the cell cycle control system lead to uncontrolled growth of cells and tumor or cancer.

(i) Dysregulation of cyclin-CDK complex in cells leads to uncontrolled mitosis and cancer.

(ii) p53 protein arrests the cell cycle when the cell’s DNA is damaged. If the p53 protein is defective for any reason, cell cycle control is lost. As a result, cancer occurs. This is probably one of the reasons for the high rate of cancer in humans.

(iii) Some growth factors act for cell division. Cancer cells themselves produce growth factors. Due to this, cell division accelerates and cancer occurs.

(iv) Two types of proteins that regulate the cell cycle are kinase proteins and cyclin proteins. Cancer occurs when protein kinases and cyclin proteins fail to control the cell cycle.

2. Proto-Oncogene: Genes that create positive cell cycle regulators in the body are called proto-oncogenes. Proto-oncogenes become oncogenes due to mutations. Oncogenes cause cell cycle disruption and cancer. CDK is a proto-oncogene.
3. Papilloma virus: Different types of papilloma virus help to cause cancer. Papillomaviruses contain B₆ and B₇ genes. This gene displaces two proteins that regulate cell division. It destroys the normal cell division ability. The result is the formation of tumors that can later turn into cancer.
4. Tumor suppressor genes: Tumor suppressor genes are negative cell cycle regulatory genes. It prevents the uncontrolled growth of cells. Stops the formation of tumors and cancer in the body.
5. Carcinogenic agents: All substances or agents that cause cancer in the body are called carcinogenic agents. Carcinogenic agents like UV rays, cigarette tar, X-rays etc. These substances cause mutations or changes in genes. Mutagenic genes cause cancer.
6. Metastasis: The spread of cancer cells to different parts of the body is called metastasis. Malignant tumor cells that cause cancer spread through the blood and lymph to different parts of the body and form new tumors. Cancer formation is accelerated due to metastasis.
7. Cell Death: Cell death occurs in two ways. Necrosis and apoptosis. Cell death due to lack of nutrients or toxic substances is called necrosis. Apoptosis is the process of death of unnecessary cells or organs in the body. Apoptosis is the destruction or death of tissue between human fingers. Apoptosis causes red blood cells, white blood cells and platelets to die after a certain period of time.
8. Cancer: The meaning of the word Cancer is crab. Cancer cells look like crab legs when they spread around. Dysregulation of cyclin-cdk in cancer cells. Cancer cells make their own growth factors or do not need growth factors for cell division. Hence the possibility of cancer increases.
9. Mitotic index: The ratio of the total number of cells present in a tissue to the number of cells undergoing mitosis is called the mitotic index (MI). Cancer cells have a higher mitotic index than normal cells.

Chromosomes are divided into two groups based on characteristics.
(i) Autosomes: All the chromosomes present in the body cells of organisms are called autosomes. The number of autosomes is 22 pairs or 44. It does not carry hereditary characteristics.
(ii) Sex Chromosomes: Chromosomes which determine the sex of an organism are called sex chromosomes. Sex chromosomes are called allosomes/heterochromosomes. The number of sex chromosomes is one pair or 2. X and Y.
In 1891, the scientist Henking observed that certain elements of the nucleus were involved in the production of spermatozoa in insects. He named it X-body. It is recognized as the X chromosome in sex determination. Later another sex chromosome was discovered in males. It is called Y chromosome. Sex chromosomes can be carriers of color blindness or hemophilia. This problem is called Sex Linked Inheritance.

The number, size and shape arrangement of chromosomes is called Karyotype. Chromosomes are arranged according to shape and the diagram is called Karyogram. The scientific study of the chromosome set of organisms is called Karyology. The arrangement of chromosome sets is called Chromosome banding.

In 1921, scientist Theophilus Painter first published the number of human chromosomes. Less than 10% of living organisms have chromosome numbers counted. Chromosome 2n number can be 2-1600 depending on species characteristics. Each cell in higher organisms has 2-80 chromosomes. The number of chromosomes in the human body is 23 pairs or 46. Among them are 2 sex chromosomes and 44 autosomes.
The least number of chromosomes
1. Plants: Moss-Mucor heimalis 2n=2 and Haplopappus gracilis 2n=4
2. Animals: Ants-Myrmecia pilosula 2n=1 and roundworms- Ascaris megalocephalus sub. sp. univalens 2n=2
Maximum number of chromosomes
1. Plants: Poa littarosa 2n=506-530 and Fern-Ophioglossum reticulatum 2n=1260 |
2. Animals: Protozoa-Aulacantha (Olacantha sp.) 2n=1600 |
[A ciliated protozoa (Oxytrichatrifallax) has 2n = 16000+ chromosomes]

Chromosomes are usually 3.5-30 µm in length and 0.2-2.0 µm in diameter. Average human chromosome length is 4-6 µm, Drosophila chromosome length is 3 µm and Bhutra chromosome length is 8-12 µm. Lamp brush chromosomes are 1500-2000 micrometers in length. Polytene chromosomes are 2000 micrometers in length. Chromosome length of Trillium is 32 µm. At the metaphase stage of cell division, the chromosomes are thick, well-organized and clearly visible.

In 1842, Karl Nageli first observed chromosomes in the nucleus of plant cells. In 1875, scientist Strasburger first observed thread-like structures during cell division. He is called the inventor of chromosomes. In 1888, scientist Walter Fleming named the thread-like structure of the nucleus as chromatin. In 1888, scientist Wilhelm Waldeyer named it chromosome for its color-carrying capacity. In 1902, scientists Sutton and Boveri described chromosomes as the containers and carriers of heredity. In 1921, scientist Theophilus Painter first published the number of human chromosomes.

The word chromosome is formed from the Greek word chroma meaning color and soma meaning body. Chromosome means colored body or colored body. Chromosome is a small, replicable thread-like organelle composed of nucleoproteins located in the nucleus of a true cell that carries hereditary characteristics and is capable of causing variation and mutation. In primitive cells, the main component of chromosomes is DNA or adichromosomes.

The ratio of the total number of cells present in a tissue to the number of cells undergoing mitosis is called the mitotic index (MI). Cancer cells have a higher mitotic index than normal cells.

The meaning of the word Cancer is crab. Cancer cells look like crab legs when they spread around. Dysregulation of cyclin-cdk in cancer cells. Cancer cells make their own growth factors or do not need growth factors for cell division. Hence the possibility of cancer increases.