Phases of Cell

Cell division:

A method in which new cells are originated from pre-existing cells.

Amitosis:

When the nucleus divides several times without each division being accompanied by the division of the cytoplasm, e.g., prokaryotes.

Mitosis:

Mitosis is a type of cell division that results in two daughter cells, each having a nucleus containing the same number of chromosomes of the mother cell. It is also called equational division. It occurs in all vegetative or somatic cells.

PHASES OF THE CELL

Interphase:

The period when the cell grows in size and performs the physiological functions. Chromatin is in a decondensed state in the nucleus of an interphase cell. Before cell division begins, the non-dividing or resting cells are said to be in interphase.

Prophase:

Chromatin condenses to form chromosomes which have two chromatids joined by a centromere. Nuclear envelope disappears during prophase. Centrioles move to the opposite poles (sides) of the cells and spindle fibres are formed from the centrioles.

Metaphase:

Chromosomes move to the centre of the cell and form an equatorial plate. Spindle fibres grow in length and attach themselves to the centromere of each chromosome.

Anaphase:

The chromosomes are pulled apart and each half moves to the opposite side of the cell.

Telophase:

Nuclear membrane appears around the chromosomes and chromosomes decondense into chromatin.

Karyokinesis:

The dividing nucleus involves a number of sequential changes. It is divided into four phases called prophase, metaphase, anaphase and telophase.

Cytokinesis:

A membrane or a cell wall appears in the middle of the cells and divides the cytoplasm of the cell into two. With this the cell division is completed and two daughter cells are formed.

Meiosis:

Meiosis or reduction division is so named as it results in the formation of four daughter cells, each with half the number of chromosomes. All sexually reproducing organisms undergo meiosis, either for the purpose of gamete formation (gametic meiosis) or to reduce the diploid zygote to a haploid condition (zygotic meiosis). During meiosis, the nucleus divides twice.

Division I:

This division is meiotic in which the chromosome number is reduced to half (n).

Prophase I:

For the convenience of study, it is divided into five substages called leptotene, zygotene, pachytene, diplotene and diakinesis.

Leptotene (Leptonema):

The chromatin reticulum which was rolled up and tightly packed around the nucleolus undergoes condensation and shortening to form definite thread-like structures called chromosomes. The chromosomes are replicated during interphase but separate sister chromatids cannot be distinguished due to the presence of nucleoprotein core between them. The chromosome has a characteristic beaded appearance.

The nucleus increases in volume. Centrioles duplicate and start moving towards the poles.

Zygotene (Zygonema):

Identical (homologous) chromosomes, one paternal and one maternal, come together and lie side by side. There is strong attraction between them and pairing takes place along their entire length. This is called synapsis. The chromosomes are said to be in bivalent condition. Soon after, the paired chromosomes thicken and shorten.

Synapsis is not a fusion of chromosomes but simply a lateral association between identical pairs.

Pachytene (Pachynema):

The chromosomes now split longitudinally, and four chromatids (two from each homologue) are produced. This is termed as tetrad. The homologous chromosomes remain connected at some points called chiasmata (singular-chiasma).

The non-sister adjacent chromatids of a tetrad twist round each other.

Diplotene (Diplonema):

A repulsive force develops between the homologous pairs of chromosomes and they begin to separate from each other.

The separation does not take place at the chiasmata where the chromatids exchange corresponding portions by crossing over. This cytogenetic activity brings about recombination of genetic material.

Diakinesis:

The chromosomes further condense. The nuclear membrane and nucleolus disappear. The centrioles move to the poles and initiate spindle formation.

Metaphase I:

The chromosomes collect at the equator of the spindle. The four chromatids (in two pairs) are attached to the spindle fibres by two centromeres, and face opposite poles.

Anaphase I:

The two pairs now separate and move towards the opposite poles of the spindle.

Telophase I:

The chromatid pairs (chromosomes) form a compact group at each pole. The two daughter nuclei thus formed, contain haploid or 'n' chromosomes.

Division II:

This division is mitotic. Each of the two cells formed in division I further divide into two, resulting in the formation of four daughter cells, each with haploid (n) chromosomes.

Prophase II

• The nucleolus reappears in each nucleus.
• The two chromatids of each chromosomes remain separate but connected at the chiasma.

Metaphase II:

• The chromosomes arranged themselves along the equator of the newly formed spindle.
• The paired chromatids are separated.

Anaphase II:

Chromatids move apart towards opposite poles of the spindle. 

Telophase II:

Cytokinesis occurs and four nuclei are formed, each having one set of chromatids (now called chromosomes) reorganized in it.