Explain cell division as per class 9th
Answers
cell division is a process of dividing a cell into 2 and more cells continue........
Explanation:
According to the theory old cells split into new cells and the formation of new cells is known as cell division or cell production. This was firstly observed by Flemming in 1882 but an extensive in details was given by Belar in 1920.
Usually cell division is of three types:
(i) Amitosis (ii) Mitosis (iii) Meiosis
Amitosis: This cell division takes place in less developed cell of unicellular organism and firstly nucleus of the cell is divided and then later cytoplasm, ultimately two new cells are formed. In bacteria, blue-green algae, yeast, amoeba, protozoa etc this type of cell division takes place.
Mitosis: This cell division is also known as Somatic cell division in which two identical cells are produced. Although cell is divided but number of chromosomes remains the same and mitosis is a continuous process. Various phases like prophase, metaphase, anaphase, telophase, cytokinesis occur and cell divides itself stage by stage.
Prophase: First stage of mitosis.
- In all animal cells and in some plants like fungi and some algae.
- The centriole duplicates itself and divides two new centrioles (centrosomes) move to opposite ends of the cell (poles).
- The spindle fibres or series of fibres radiates from vicinity of each centriole towards the nucleus.
- Except fungi and some algae the spindle fibres develop without the presence of centrioles.
- The chromosomes which are already duplicated become shorten and thicken.
- Chromatids are the duplicated halves of each chromosome which are held together by the centromere.
- The nucleus and the nuclear membrane begin to disintegrate in the late prophase.
Metaphase:
- The pairs of chromosomes align themselves in such a way that the centre of the cell and each centromere becomes attached to one spindle fibre from each pole.
- The centromere divides and the separated chromatids become independent daughter chromosomes.