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Cell Division Part 4

Cell Division Part – 4

Meiosis - II:
It is similar to the mitotic division and four cells are formed at the end of meiosis-II. All four new cells contain same number of chromosomes as were in the two cells. However, in the period between two meiotic divisions, no replication occurs during interphase. Meiosis is divided into four sub phases.

Prophase II
Metaphase II
Anaphase II
Telophase II

Prophase – II:
In this phase, bipolar spindle is reformed and nucleolus disappears gradually by decreasing in their size and nuclear membrane is disintegrated.

Metaphase – II:
In this phase, centromere of each chromosomes moves towards the equatorial plane of bipolar spindle and arranged in one plane.

Anaphase – II:
Centromere divides and each chromatid receives its own separate centromere and moves towards opposite poles. The number of chromatids collected at each pole is the same as the number of chromosomes in parent cell.

Telophase – II:
Chromosomes uncoil and nucleolus and nuclear membrane is formed. The number of chromosomes is maintained in each nucleus.

Cytokinesis:
It follows nuclear division and cytoplasmic spindle disappears.

Significance of meiosis:
· Essential for maintaining number of chromosomes generation after generation.
· Exchange of genes results into variation in characters in the offspring. Such variations are important in the evolution of the species.

Amitosis:
It occurs in amitotic type of cell like Protista and ciliate organisms. Degenerating plant cells are also divides in this way.

· Nucleus constricted from the middle and becomes dumbbell shaped.
· Nuclear membrane is present and individual chromosomes cannot be identified.
· Bipolar spindle is not formed.
· When constriction of nucleus completed, two nuclei are formed.
· When cytoplasm constricts, two cells are formed.

Comparison between mitosis and meiosis:

Mitosis	Meiosis
Division occurs in somatic cells	Division occurs in reproductive cells
Two cells are formed with same number of chromosomes as in the parent cell.	Four cells are formed with half number of chromosomes as in the parent cell.
Cell division occurs once after replication of genetic material	Cell division occurs twice after replication of genetic material
Prophase is simple and chromosomes behave as independent unit.	Prophase is complex and last longer. Chromosomes become synapsed.
Exchange of genes does not occurs	Exchange of genes occurs at various places of chromosomes
Newly formed cells have identical genetic material	Variation occurs in genetic material and is important in the evolution of the species.

Self study:
Example-1: How many cells are formed at the end of meiosis-II?

a) Two cells
b) Four cells
c) Five cells
d) One cell

Answer: Four cells are formed at the end of meiosis-II.

Example-2: Meiosis occurs in …………………….
a) somatic cells
b) Amniotic cells
c) Reproductive cells
d) None

Answer: Meiosis occurs in reproductive cells.

Example-3: Which of the following cell division is useful in degeneration of plant cells?
a) Meiosis
b) Mitosis
c) Amitosis
d) Cytokinesis

Answer: Amitosis is useful in degeneration of plant cell.