Mitosis

Mitosis

Mitosis is a type of cell division involves a series of complex changes in both, the nucleus and cytoplasm. During mitosis the diploid (2n) number of chromosomes kept constant in daughter cells.

Interphase

It is an active phase of cell division which occurs before the onset of mitosis. The basic components of cell duplicated during this phase particularly the DNA and histones. Moreover various proteins are also synthesized during interphase, which are essential for proper completion of mitosis.

Main Phases of Mitosis

The process of mitosis has been studied in both plants and animals. Walther studies it in animals wile Strassburger studied it in plants. According to these studies, mitosis takes place in somatic cells and comprises two phases.

1- Cytokinesis (division of cytoplasm)
2- Karyokinesis (division of nucleus)

Cytokinesis

Cytokinesis refers to the division of cytoplasm. In animals, it is carried out by the formation of cell membrane and contraction of cytoplasm. Cell membrane moves inward deeply and divides the cell into two halves. In plants, some special structures are formed by the transformation of spindle fibers at equatorial; these structures are called phragmoplast which leads to the division of one cell into two.

Karyokinesis

Karyokinesis refers to the division of nucleus. It comprises four stages:

1- Prophase
2- Metaphase
3- Anaphase
4- Telophase

1- Prophase

It involves the appearance of chromosomes as thin threads by the condensation of chromatin fiber in the nucleus. Each chromosome comprises two chromatids with centromere present in between. As mitosis proceeds, nucleolus and nuclear membrane also disappears gradually

2- Metaphase

During this phase spindle fibers starts to appear. Chromosomes get attached with spindle fibers and migrates from equator to opposites poles to take the final shape.

3- Anaphase

During anaphase, Spindle fibers attached to centromere pull them and two chromatids get separated from each other and move towards opposite poles.

4- Telophase

This phase is an opposite or reversal phase of prophase. All changes took place during prophase reverse to their original states. On reaching the poles, chromosome start uncoiling and nucleoli and nuclear membrane begins to appear, which results in the formation of two daughter cells.

Significance of Mitosis

Mitosis is a significant cell division which ensures the provision of correct and complete genetic information to the daughter cells. As chromosome have heredity material and number of chromosomes must be kept constant in daughter cells in order to transfer complete genetic information and mitosis keeps this number of chromosome constant. In case of abnormal number of chromosome, several physiological and psychological disorders can cause in an organism. So we can say that the product of mitosis is not only quantitative but qualitative as well. Healing of wounds and formation of new cells takes place due to the process of mitosis.

Nucleic Acid

Nucleic Acid

A nucleic acid is a macromolecule composed of polynucleotide chains. These polynucleotide chains are made up of several monomers which are called nucleotides. These nucleotides carry genetic information or form structures within cells. Ncleic acids are found in every living organism including viruses as well. Nucleic acids were first discovered in 1871 by Friedrich Miescher.

Composition of Nucleic Acid

Each nucleotide comprises three components; Pentose Sugar, Phosphase Group and nitrogenous bases. Nitrogenous bases are of two types; purine and pyrimidine. Purines are based on Adenine (A) and Guanine (G) while Pyrimidine are based on Cytosine (C), Thymine (T) and Uracil (U). Nitrogenous bases found in the two nucleic acid types are different: adenini, cytosine and guanine found in both RNA and DNA, while thymine only occurs in DNA and uracil only occurs in RNA. Some other types of nucleic acid also occur, for example inosine found in strands of mature tRNA.

Pentose Sugar in ribonucleotide is ribose and in deoxyribonucloetides is deoxyribose. Phosphate linkage with pentose sugar is called Phosphodiester Linkage. Phosphoric acid has ability to develop ester linkage with hydro oxyle(OH) groups of pentose sugar.

Types of Nucleic Acid

There are two types of nucleic acids.

1- RNA
2- DNA

1- RNA

RNA stands for Ribonucleic acid, is a single polynucleotide chain comprising nucleotide monomers. These monomers plays significant role during transcription. It transcribed genetic information from deoxyribonucleic acid (DNA) into proteins during protein synthesis. In this way RNA acts as a messenger between DNA and ribosomes which are protein synthesis complexes present in cell. RNA forms vital portions of ribosomes and serves as an essential carrier of amino acids during protein synthesis. RNA exists in three types which include:

1- tRNA (transfer RNA)
2- mRNA (messenger RNA)
3- rRNA (ribosomal RNA)

2- DNA

DNA stands for Deoxyribonucleic acid, comprises two polynucleotide chains linked together by weak hydrogen bonds. There are two hydrogen bonds between A and T and three hydrogen bonds between C and G. DNA is heredity material contain genetic information passed from generation to generation. Evidence from the study of bacterium (Pneumococcus) that DNA is heredity material. This genetic information plays essential role in the development and functioning of all known living organisms.

The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes or are involved in regulating the use of this genetic information. The amount of DNA depends upon the number of chromosomes present.

DNA comprises four types of nitrogenous bases i-e cytosine, thymine, guanine and adenine. These bases are linked together to form a polynucleotide chain. Two polynucleotide chains coil around each other to form the double helical sucture of DNA molecule.

What are Chromosomes?

Chromosomes

Chromosomes are important part of a cell discovered in 1876 by Waldeyer. In 1908 W.S. Sutton observed that genes are located on chromosomes and genetic phenomenon can be explained in terms of chromosomal behavior during cell division particularly during metaphase and anaphase.

Structure of Chromosome

Chromosome comprises two chromatids attached to centromere. Centromere is the point where spindle fibers get attached with them. The position is very important to distinguish the type of chromosome. You can say that on the basis of its position the type and shape of a chromosome can be defined.

Types of Chromosomes

On the basis of centromere’s position, chromosomes can be divided into four types.

1- Telocentric (Centromere present at one end of chromosome)
2- Acrocentric (Centromere present close to one end, so acrocentric chromosomes have very small arms)
3- Metacentric (Centromere present in centre with equal arms)
4- Submetacentric (Centromere present near the centre but not exactly in the centre therefore have an equal arms)

Karytype

It is the term used to describe the total chromosome complement of cell. Study of karytype helps in determining the total number of chromosome present in human beings and in other species.

Chemical Composition of Chromosomes

Chromosomes are chemically composed of DNA and basic protein. These basic proteins are called Histones. DNA and Histones combine together to form a structure which is called Nucleosome. Nucleosome comprises eight molecules of various types of histones with two turns of DNA molecules. Under electron microscope nucleosome appears as a beaded structure of 10nm diameter.