CELL-DIVISIONCELL LIFE CYCLE

a)Division

b)Elongation

c)Differentiation

zygote

embryo

plantlet

baby plant

LIFE CYCLE1.INTERPHASE(resting)

2.DIVISION PHASE

INTERPHASE

*Gets ready for division.

* Cell is metabolically active.

* Hence, it is called non resting stage.

It is divided into three periods.they are G1,S,G2

Gap 1 (G1): Cells increase in size in Gap 1, produce RNA and synthesize protein. An important cell cycle control

mechanism activated during this period (G1 Checkpoint) ensures that everything is ready for DNA synthesis. (Click on the Checkpoints animation, above.)

S Phase: To produce two similar daughter cells, the complete DNA instructions in the cell must be duplicated. DNA replication occurs during this S (synthesis) phase.

Gap 2 (G2): During the gap between DNA synthesis and mitosis, the cell will continue to grow and produce new proteins. At the end of this gap is another control checkpoint (G2 Checkpoint) to determine if the cell can now proceed to enter M (mitosis) and divide.

Mitosis or M Phase: Cell growth and protein production stop at this stage in the cell cycle. All of the cell's energy is focused on the complex and orderly division into two similar daughter cells. Mitosis is much shorter than interphase, lasting perhaps only one to two hours. As in both G1 and G2, there is a Checkpoint in the middle of mitosis (Metaphase Checkpoint) that ensures the cell is ready to complete cell division. Actual stages of mitosis can be viewed at Animal Cell Mitosis.

 

DIVISION PHASEIt is of two types 1)MITOSIS,2)MEIOSIS

MITOSISIt is again divided into two stages 1.KARYOKINESIS.

2.CYTOKINESIS.

KARYOKINESIS

1.It is “division of nucleus”.

Karyo=nucleus;kinesis=division.

2.It occurs in four different phases.

a)PROPHASE

b)METAPHASE

c)ANAPHASE

d)TELOPHASE

PROPHASEIt is divided into there sub stages i)early prophase

ii)mid prophase

iii)late prophase

EARLY PROPHASE

1.”CHROMATIN” which is present in nucleus condenses and form rod like structures called “CHROMOSOMES”.

MID PROPHASE

1.’CHROMOSOMES’ splits longitudinally and forms “SISTER CHROMATIDS” and they are attached to the “CENTROMERE”.

LATE PROPHASE

1.NUCLEAR MEMBRANE and NUCLEOLUS dissappeares.

2.Chromosomes will scatter in the cytoplasm.

3.EVENTS are opposite in the TELOPHASE.

METEPHASE

1.Formation of by polar spindle appratus takes place.

2.Spindle fibers are spindle shaped.

3.Spindle fibers are made up of “MICROTUBULES”.

4.It has a protein called “TUBULIN”.

5.Spindle fibers forms the METAPLATE at centre portion of the cell.

6.CHROMOSOMES which are scattering in the CYTOPLASM are brought togethere by the “SPINDLE FIBERS”.

7.These CHROMOSOMES are along with INTERZONAL spindle fibers forms hard portion called METAPLATE.

8.SPINDLE FIBERS are of three types

a)CONTINUOUS Spindle fibers are the fibers which extends from one pole to the other pole passing through mid line and are not attached to the CHROMOSOMES.

b)CHOMOSOMAL Spindle fibers are the fibers which are attached to the CHROMOSOMES at CENTROMERE.

c)INTERZONAL Spindle fibesr are the fibers which are present at either side of the equotorial plate but in small hair like strutures.

ANAPHASE1.During ANAPHASE pressure is exerted on CENTROMERE.

2.As a result,the Chromatids break forming daughter CHROMOSOMES.

3.These reach either ends of the cell.

4.CONTINUOUS Spindle fibers breaks into MICROTUBULES .

TELOPHASE1.NUCLEOLUS and NUCLEAR MEMBRANE reappears during this phase.

2.This phase is opposite to LATE PROPHASE.

3.Each of the Chromosomes are surrounded by NUCLEAR MEMBRANE.

CYTOKINESIS

1.The MICROTUBULES gather to the centre and forms “PHRAGMOPLAST”.(Barrel shaped)

2.Due to disapperance of vacoules,it condenses into flat structure.

3.Ths develops into CELL PLATE.

4.This finally forms MIDDLE

LAMELLUM.