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Indian Institute of Technology Bombay

Cell division:

It is a process by which one single cell gives rise to two or more cells.

Cell division is of two types, namely:

  • Mitosis
  • Meiosis.

During the process of cell division, cell growth takes place, followed by DNA replication, nuclear division and finally cytoplasmic division. The process of cell division must take place in a coordinated manner so that each cell divides correctly and the genomes of the progeny cells remain intact.

Mitosis:

In this, cell division leading to growth and development and seen in vegetative cells of the body.

During mitosis, the parent cell divides into two daughter cells, which are not only identical to each other but also to the parent cell. Mitosis is also known as equational division, as both the parent and the progeny cells have the same number of chromosomes.

Mitosis is divided into four stages of nuclear division, namely

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

Meiosis:

In this cell division leading to the production of gametes and seen only in the reproductive cells of the body.

Meiosis consists of two cell divisions, namely meiosis one and two.

Meiosis one is divided into four stages – prophase one, metaphase one, anaphase one and telophase one.

Similarly, meiosis two is divided into four phases – prophase two, metaphase two, anaphase two and telophase two.

Diploid condition:

It is the normal condition in all cells where two sets of chromosomes are present. It is called 2n condition

Ploidy is the number of sets of chromosomes in a biological cell.
Haploid number is the number of chromosomes in a gamete ( n).
Diploid number is made up of two complete haploid sets of homologous chromosomes (2n).

Cytokinesis::

Mitosis results in the segregation of duplicated chromosomes into two daughter nuclei and is followed by the division of cytoplasm, that is, cytokinesis, to produce two daughter cells.

The process by which a furrow appears in the cell membrane in the middle, which deepens and finally splits the cytoplasm into two, thus producing two daughter cells is called cytokinesis.

Binary fission is a simplified version of mitosis, but they are not identical processes.

Homologous chromosomes are identical in all respects. Only the sex chromosomes are heterozygous.

The zygote is a single cell, which multiplies to form a multicellular organism. Each cell of the organism will have the same number of chromosomes, but only the reproductive organs can show meiosis.

No animal can live if it has an extra set of chromosomes, but plants can have polyploid character (several sets of chromosomes).

The prophase I of meiosis alone is different from prophase of mitosis, the other stages are the same in the procedure.

Division of nuclear material is called karyokinesis.

The importance of Mitosis:

  • Ensures that chromosome number and genetic stability in organisms is maintained. It is an equational division and results in daughter chromosomes that are exactly like the parent chromosome.
  • It provides new cells required for repair of wounds and regeneration and aids in replacement of old cells.
  • It helps in asexual reproduction in lower organisms.
  • It checks overgrowth of cells by an inbuilt mechanism called checkpoint.

Basics of Meiosis:

Meiosis is called reduction division. It is seen only in reproductive cells and is important for gamete formation.

Meiosis consists two stages: meiosis I and meiosis II.

Meiosis one is divided into four stages – prophase one, metaphase one, anaphase one and telophase one.

Similarly, meiosis two is divided into four phases – prophase two, metaphase two, anaphase two and telophase two.

Before dividing, a cell enters meiosis, it goes through the interphase where the cell increases in mass, synthesises DNA and proteins and duplicates its chromosomes in preparation for cell division. However, there is no interphase between the first and second meiotic division. The cell immediately enters into the second meiotic division

Significance of Meiosis

  • It results in haploidy of chromosomes. Thus, when zygote formation occurs, the normal condition called diploid condition is restored
  • It results in variation in the offspring as it causes mixing of maternal and paternal genes during crossing over.
  • It maintains the stability of the species by ensuring that the gametes get only half the number of chromosomes found in the adult of the species.
  • All the four chromatids of the homologous chromosomes go to four different daughter cells. Thus no two daughter cells are the same genetically. This ensures genetic variation.

The structure of Chromosomes:

In the nucleus of each cell, the DNA molecule is packaged into thread-like structures called chromosomes. Each chromosome is made up of DNA tightly coiled many times around proteins called histones that support its structure.

Chromosomes are made of 50% proteins, and 50% DNA (deoxyribonucleic acid).Chromosomes are made of many chromatin threads, each containing DNA and proteins.

Somatic cells contain 46 chromosomes, gametic cells contain 23 chromosomes.44 of our chromosomes are AUTOSOMES, while 2 are called sex chromosomes. Each chromosome contains many GENES that act as a blueprint or set of instructions for the cell.

Chromatin: The substance distributed in the nucleus of a cell that condenses to form chromosomes during cell division. It consists mainly of DNA and proteins called histones.

Chromatid:  A chromatid is one-half of two identical copies of a replicated chromosome.

Centromere: The centromere is the constricted point at which the two chromatids forming the chromosome are joined together.

Gene: A portion of a DNA molecule that serves as the basic unit of heredity. Genes control the characteristics that an offspring will have by transmitting information in the sequence of nucleotides on short sections of DNA. A gene is a part of the DNA made up of a specific number of nucleotides. It is not a separate entity

Depending on the position of the centromere, different shapes of chromosomes have been identified.

A chromosome is made up of two chromatids which are joined by the centromere. The chromatids separate from each other during mitosis to form two new chromosomes. The DNA making up a chromosome is dispersed as chromatin.

The complete set of chromosomes in the cells of an organism is its karyotype.

The karyotype of the human female contains 23 pairs of homologous chromosomes: 22 pairs of autosomes and 1 pair of X chromosomes

The karyotype of the human male contains: 22 pairs of autosomes, one X chromosome and one Y chromosome

The DNA in chromosomes forms the basis for the genetic code

DNA is made of long chains of nucleotides. 2 strands of nucleotides join and twist around to form a spiral staircase. This is called double helical structure. Each nucleotide has 3 parts:

  • a phosphate group which is the backbone of the DNA molecule
  • a pentose sugar (deoxyribose)
  • nitrogenous bases – These are of 4 types (guanine, cytosine, adenine, thymine).Hydrogen bonds are found between the bases on the helix, forming the steps of the DNA ladder

The sequence of the bases makes up the genetic code.Each molecule of DNA can be subdivided into smaller segments consisting of a few thousand of nucleotides. Each of these subunits is a gene.

Each gene in a DNA molecule carries the instructions for making a single kind of protein. Proteins are very important molecules that perform many vital functions in living organisms.

 

Section 1Cell Cycle
Section 2Mitosis
Section 3Cytokinesis and the Significance
Section 4Meiosis
Lecture 4Meiosis - 1
Lecture 5Meiosis - 2
Section 5Nucleus