A chromosome that lies in the middle of the centromere is called a 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.

1. Double Helix: There are two strands in the DNA molecule. It is called double strand or double helix. The arrangement of the two sutras is like a stairwell. Each helix is a polynucleotide.
2. Reverse Strands: The two strands of a DNA molecule lie side by side in opposite directions. One is 5-3 carbon oriented and the other is 3-5 carbon oriented. The positions of the two sutras are called antiparallel. These two formulas are positioned around an imaginary central axis.
3. Railings: Deoxyribose sugars and phosphates form the railings on both sides of the ladder. Deoxyribose sugars and phosphates are sequentially linked to form two railings.
4. Sopan: There are many steps in the middle of the stairs. These steps are called steps. Different types of nitrogenous alkalis are added to form this terrace.
5. Hydrogen Bonding: Nitrogenous bases are bonded to each other by hydrogen bonding. According to the bonding principle, Adenine and Thymine are bonded by two hydrogen bonds (A=T) and Cytosine and Guanine are bonded by three hydrogen bonds (G≡C). There are 25 hydrogen bonds per patch of DNA molecule.
6. Carbon attachment: Nitrogenous bases are always attached to the 1st carbon of the pentose sugar. A phosphate is attached to carbon 5 of one nucleotide while a phosphate is attached to carbon 3 of another nucleotide.
7. Alkalinity: DNA molecules contain equal amounts of adenine and thymine and guanine and cytosine.
8. Diameter: The diameter of the double helix of each DNA molecule is 20 Å. But this diameter may vary according to the length.
9. Patch Length: Each patch of double helix consists of 10 pairs of mononucleotides. A pair of mononucleotides is 3.4 Å in length. Thus the length of each patch is 10×3.4= 34 Å or 3.4 nm.
10. Grooves: There are two grooves in each turn of the double helix. One is the major (deep) groove 22 nm and the other is the minor (shallow) groove 12 nm.
11. Molecular Weight: Molecular weight of DNA is 106-109 daltons.

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.

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.

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.

Pseudopolar neuron has two pronged parts first. Later merge into one. It arises from bipolar neurons. Spinal ganglia and carotic ganglia contain pseudopolar neurons.

Multipolar neurons have many elongated parts. The mammalian brain and spinal cord contain multipolar neurons.

Bipolar neurons have two elongated parts. One is the axon and the other is the dendrite. These later transform into unipolar or multipolar neurons. In the human embryo, there are bipolar neurons in the retina, nose and cochlea.

A unipolar neuron has an elongated segment. It is a sensor. The peripheral nervous system of vertebrates consists of unipolar neurons.

A non-polar neuron has no protracted segment. Apolar neurons are located in the outer layer of the cerebral hemisphere and in the intermediate nuclear layer of the retina.