CELL CYCLE AND CELL DIVISIONS

The cell cycle is the series of events that takes place inside a cell thus leading to cell
division and cell duplication. The cell cycle is divideds into two brief stages : (A) Interphase
– during which the cell grows and accumulates nutrients needed for mitosis and DNA material
duplicates in this stage It is further divided into G1
, S and G2
(B) Mitosis (M) phase – during
which the cell divides itself into two distinct cells, called “daughter cells”. Mitosis is also
divided into four stages viz. prophase, metaphase, anaphase and telophase. During prophase
condensation of chromosomes takes place. Metaphase can be indicatd by arrangement of
chromosomes at the equatorial plate. During anaphase centromeres divide and chromatids
start moving towards the opposite poles. Each chromatid behaves like an individual
chrosomosome during telophase. Nuclear membrane appeared and two nuclei are formed.
Nuclear division (karyokinesis) is followed by cytoplasmic division and is called cytokinesis.
There are two stages of meiosis, namely, meiosis I and meiosis-II. Meiosis – I is
called reduction division or heterotypic division while meiosis – II is called homotypic
division.
The parent cell or the dividing cell undergoes a preparatory phase, known as interphase,
before entering the two stages of meiosis. Meiosis – I and II consist four common phases
viz. prophase, metaphase, anaphase and telophase. The prophase of meiosis – I is a long
phase which is further divide into five phases. These are leptotene, zygotene, pachytene,
diplotene, and dikenesis. Due to formation of bivalent spindle the chromosomes which are
arranged at the equatorial plate during metaphase are pulled towards the opposite poles during
anaphase. Each pole receives half the chromosome number of the parental cell during
telophase. At the completion of telophase, nuclear membrane and nucleolus reappear. Meiosis
– II is similar to mitosis. Both the daughter cells formed by meiosis – I undergo meiosis –
II and produce four haploid daughter cells.
The stage between two meiotic stages in called interkinesis and is generally short lived.

Read More  EXCRETORY SYSTEM -BIOLOGY- TNPSC,TET,RRB,SSC SCIENCE (GK) STUDY MATERIAL

Be the first to comment

Leave a Reply

Your email address will not be published.


*