CELL CYCLE AND CELL DIVISION ..............................( PART 1)

 What is cell .........? 

Cell is the fundamental, structural and functional unit of living organisms.

 image :- cell 

Robert Hooke studied and discovered the cell from the thin slice of cork but that was dead cell . Anton von leeuwenhoek  was the first person who observed few living cell. 

What is cell cycle ......? 

The cell cycle is an ordered series of events involving cell growth and cell division that produces two new daughter cells. 

 Fig :- cell cycle. 

- Cells on the path to cell division proceed through a series of precisely timed and carefully regulated stages of growth, DNA replication, and division that produces two identical (clone) cells.

What is cell division .......? 

The process by which a cell divides into two or more cells. 

 image :- cell division. 

- Cell division is used as a means of reproduction in organisms that reproduce asexually, as by fission or spore formation, and sexually reproducing organisms form gametes through cell division.

Cell cycle basically consist of two phage namely  -

1) Interphase 

2) M phase :- a) mitosis 

                           b) meiosis 

INTERPHASE :- 

Interphase is the longest part of the cell cycle. This is when the cell grows and copies its DNA before moving into mitosis. 

- During mitosis, chromosomes will align, separate, and move into new daughter cells.

- The prefix inter- means between, so interphase takes place between one mitotic (M) phase and the next.

By studying molecular events in cells, scientists have determined that interphase can be divided into 3 steps:

1) Gap 1 (G1),

2) S (synthesis) phase,

3) Gap 2 (G2).

 Fig :- Interphase and M phase. 

1) Gap 1 ( G1) :- 

- During this stage, the cell performs its normal functions, and grows in size.

- The G1 phase takes place right after the division of cells. A lot of protein synthesis occurs during G1. 

- This facilitates the increase in the amount of cytosol in the cell. Cytosol refers to the liquid inside the cell. Furthermore, cell proteins are outside the organelles.

- Proteins refer to the molecular machines that sustain the various activities of the cell.

 - The increase in cell size occurs not merely for the production of new proteins, but also because of the more water which the cell takes in. Also, the protein concentration in a mammalian cell is roughly 100 milligrams per millilitre.

G0 phase :- 

The G0 phase (referred to the G zero phase) or resting phase is a period in the cell cycle in which cells exist in a quiescent state. 

-G0 phase is viewed as either an extended G1 phase, where the cell is neither dividing nor preparing to divide, or a distinct quiescent stage that occurs outside of the cell cycle.

2) S phase ( Synthesis phase ) :- 

- The synthesis stage is a very important stage because at this stage a cell copies its DNA.

- DNA replication is certainly a huge effort that requires a lot of proteins. Furthermore, the DNA does not exist by itself in a cell. Moreover, the DNA is packaged by proteins.

- The production of more packaging proteins takes place during the S phase. Histones are proteins exist around the DNA wraps. 

-The production of new histone proteins and the DNA synthesis starts at the same time.

- Blocking DNA synthesis with a chemical drug certainly means the blocking of histone synthesis. Therefore, the linking of the two processes takes place during the S phase.

3) Gap 2 ( G2) :-

- During the G2 phase, the cell undertakes preparation to enter mitosis. The duplication of the DNA has already taken place during the S phase.

- Therefore the G2 phase is when the cell’s organelles need to duplicate.

- The duplicated DNA would be divided evenly during cell division . Similarly, an even division of the organelles would also take place.

- There are some organelles like mitochondria and chloroplasts which are discrete units and they do not bud off from larger organelles. 

- Discrete organelles increase in number when their division takes place during G2.