lecture3

Differences between Prokaryotes and Eukaryotes. Cell division – mitosis

• Cell is the basic unit of organization or structure of all living matter.

• It was first coined by Robert Hook.

• The cell has also been defined as “a unit of life that is the smallest unit which can carry on the activities indispensable to life to grow, to synthesize new living material and to produce new cells”.

• The cell theory was formulated by M.J. Schleiden and Theodor Schwann in 1838-39.

• The term protoplasm was coined by Purkinje.

• Protoplasm is a Greek word which means first organization

• It is a juicy living substance.

• The protoplasm occurs everywhere in the cell i.e. the plasma membrane, the nucleus and the portion in between the plasma membrane and the nucleus.

• The portion between plasma membrane and nucleus is cytoplasm.

• The portion of the protoplasm occurring in the nucleus is named as nucleoplasm.

CLASSIFICATION OF CELL:

• The body of all living organisms (blue green algae, bacteria, plants and animals) except viruses has cellular organization and may contain one or many cells.

• The organisms with only one cell in their body are called unicellular organisms (E.g.: Blue green algae, bacteria, protozoa etc.).

• The organisms having many cells in their body are called multicellular organisms (E.g.: Most plants and animals). Cellular organisms may have only one kind of cell.

o   Prokaryotic cell

o   Eukaryotic cell.

CELL DIVISION:

     All cells originate through division of pre-existing cells. Bodies of all multicellular organisms are derived from unicellular zygote through repeated divisions of zygote and the cells derived through its division. The division of chromosomes and cytoplasm of a cell into daughter cells is known as 'Cell division'. The cell that undergoes division is termed as 'parent cell', while the cells derived from the division of a parent cell are known as ‘daughter cells’.

Functions of cell division:

• To produce two daughter cells, which are involved in the following;

• Growth and development of somatic tissue of organism

• Regeneration of damaged tissues

• Production of new tissues

• Reproduction

• Keeping the size of cells within a limited range.

The various events occurring in division may be grouped into 

• Karyokinesis - Division of chromosomes

• Cytokinesis - Division of cytoplasm.

Cell cycle: Cell cycle can be defined as the entire sequence of events happening from the end of one nuclear division to the beginning of the next.  A cell cycle consists of two phases, viz.,

• Interphase

• The cell division proper.

The time required for the completion of cell cycle differs from species to species.

Interphase: Interphase is generally known as DNA synthesis phase. Interphase consists of G1, S and G2 sub phases. G1 is the resting phase, S is the period of DNA replication and G2 again is a resting stage after DNA replication. 

G1 Phase: It is a pre-DNA replication phase. Thus, this is a phase between telophase and S phase. This is the longest phase which takes 12 hours in Vicia faba.  It is the most variable period of cell cycle. Synthesis of proteins and RNA take place during this phase.

S (Synthetic) Phase: This phase comes after G1and takes lesser time than G1phase. In Vicia faba, it takes six hours. The chromosome and DNA replications take place during this phase. 

G2 Phase: This is the post-DNA replication phase and last sub stage of interphase. This phase also takes 12 hours in Vicia faba. Synthesis of protein and RNA occur during this stage. 

Two types of cell division

• Mitosis 

• Meiosis

Mitosis: The term mitosis was coined by Flemming in 1882. Mitosis occurs in somatic organs like root tip, stem tip and leaf base etc. Hence it is also known as somatic cell division.

o   The daughter cells are similar to the mother cell in shape, size and chromosome complement. Since the chromosome number is same in the daughter cells as compared to that of mother cell, this is also known as homotypic or equational division.

o   Mitotic cell cycle includes the following stages:

Interphase: This is the period between two successive divisions. Cells in interphase are characterized by deeply stained nucleus that shows a definite number of nucleoli. The chromosomes are not individually distinguishable but appear as extremely thin coiled threads forming a faintly staining network. The cell is quite active metabolically during interphase.

   Mitosis consists of four stages, viz.,

• Prophase

• Metaphase

• Anaphase

• Telophase

Prophase: The nucleus takes a dark colour with nuclear specific stains and also with acetocarmine / orcein.

o   The size of the nucleus is comparatively big and the chromosomes that are thin in the initial stages slowly thicken and shorten by a specific process of coiling.

o   The two chromatids of a chromosome are distinct with matrix coating and relational coiling. The disintegration of nuclear membrane denotes the end of prophase.

Metaphase: After the disintegration of nuclear membrane, the shorter and thicker chromosomes will spread all over the cytoplasm. Later, the size of the chromosomes is further reduced and thickened.

o   The distinct centromere of each chromosome is connected to the poles through spindle fibres.

o   The chromosomes move towards equator and the centromere of each chromosome is arranged on the equator. This type of orientation of centromeres on the equator is known as auto orientation.

o   The chromosomes at this stage are shortest and thickest. The chromatids of a chromosome are held together at the point of centromeres and the relational coils are at its minimum.

Anaphase: The centromere of each chromosome separates first and moves to towards the poles.

o   Depending on the position of the centromeres (metacentric, acrocentric and telocentric), the chromosomes show ` V `, ` L ` and ` I ` shapes respectively as the anaphase progresses.

o   The sister chromatids move to the poles. The chromosome number is constant but the quantity of each chromosome is reduced to half.

Telophase: Chromosomes lose their identity and become a mass of chromatin. The nucleus will be re-organized from the chromatin. At late telophase stage, the cell plate will divide the cell into two daughter cells.

Cytokinesis: The division of cytoplasm usually occurs between late anaphase and end of telophase.

o   In plants, cytokinesis takes place through the formation of cell plate, which begins in the centre of the cell and moves towards the periphery in both sides dividing the cytoplasm into two daughter cells.

o   In animal cells, cytokinesis occurs by a process known as cleavage, forming a cleavage furrow.

Significance of Mitosis:

Mitosis plays an important role in the life of living organisms in various ways as given below:

• Mitosis is responsible for development of a zygote into adult organism after the fusion of male and female gametes.

• Mitosis is essential for normal growth and development of living organisms. It gives a definite shape to a specific organism.

• In plants, mitosis leads to formation of new organs like roots, leaves, stems and branches. It also helps in repairing of damaged parts.

• It acts as a repair mechanism by replacing the old, decayed and dead cells and thus it helps to overcome ageing of the cells.

• It helps in asexual propagation of vegetatively propagated crops like sugarcane, banana, sweet potato, potato, etc. mitosis leads to production of identical progeny in such crops.

• Mitosis is useful in maintaining the purity of types because it leads to production of identical daughter cells and does not allow segregation and recombination to occur.

• In animals, it helps in continuous replacement of old tissue with new ones, such as gut epithelium and blood cells.