Cell Reproduction

All the living organisms are composed of cells. They grow and reproduce. The growth and development of every living organism depends upon the growth and multiplication of the constituent cells. Unicellular bacteria which are minute and microscopic, alligators and the weeds which are giant sized, produce offspring like themselves and pass on to them the hereditary information that makes them what they are. In unicellular organisms, cell division is the

of reproduction. Multicellular organisms develop from a single cell, the zygote, which is formed by the fusion of preceding cells, the gametes. The ways by which new cells originate have been thoroughly investigated, only after 1846 when Nageli pointed out that new cells are always formed through the division of pre-existing cells. Rudolf Virchow (1855) supported the above idea and investigated the process of cell division. R. Virchow gave the first modifcation of cell- theory. He gave the statement 'Omnis cellulae cellula', which means that the new cells are formed from pre-existing cells. Strassburger proposed that nuclei are formed from the pre-existing

ones. Boveri and Flemming (1873) revealed the details of somatic cell division. The term ‘mitosis' was coined for this by

y Flemming (1882). August Weismann (1887) predicted on theoretical grounds that the number of chromosomes must be reduced by one half during gamete formation. Benedin (1887) demonstrated the reduction division. The division was termed 'meiosis' by Farmer and Moore (1905).

                                CELL CYCLE

The sequence of events involving growth and division a cell undergoes from the time of its formation by division of the parent cell to its own division into daughter cells is called the cell cycle (Fig. 24.1).

The cell cycle consists of the following five phases :

G1 → S → G2 → M →C Duration of Cell Cycle

Duration of cell cycle is constant for a particular type of cells. Duration of interphase depends on the type of cell and external factors such as temperature, food and oxygen supply. Interphase normally comprises of 90% or more of the total cell cycle. In rapidly dividing cells of higher eukaryotes the interphases are interrupted by the successive cell divisions (Mphases). Each interphase is of 16—24 hours duration, while each M-phase is of 1–2 hours duration only. Interphase is divisible into three stages.

(a) G1 phase (First growth phase). The growth phase of the newly formed cell. The cell carries on active metabolic activity and prepares itself for DNA replication. Amino acids, ATP and other nucleotides are formed. Mitochondria and chloroplasts multiply, Golgi apparatus, endoplasmic reticulum and other cell organelles are produced ; various types of RNAS and ribosomes

formed. Carbohydrates, lipids and proteins (including some non-histones) are also synthesized. The duration of G1 phase is variable. For many organisms, this occupies the major portion of the life span of a cell.

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In G1 phase, the chromosomes begin the long process of condensation, coiling into more and more tightly compacted bodies.

(6) S-phase

(Synthetic Replication of DNA occurs. DNA content doubles and duplicate set of genes are formed. New chromatin fibres are formed, which, however, remain attached in pairs. Each chromosome comes to have two chromatin threads or sister chromatids which remain attached at common point, called centromere. This stage is also called invisible phase of

because chromosomes prepare themselves equitabledistribution later

on, in this Centrosome also divides to form two centrosomes or centriole pairs.

(c) G2-phase (Second growth phase) has double DNA content as compared to the original cell. The macromolecules like RNAs and proteins, required for the multiplication of cell organelles, spindle formation and cell growth, are synthesized. The cell gets ready to undergo cell division,