Chapter 10: Cell Cycle and Cell Division Revision Notes Part 2
Cell division cycle is a process whereby a single parent cell divides into two or more identical daughter cells. Cell division is a process that occurs as a part of the cell cycle.
What is meant by the cell cycle or cell division?
Cell division cycle is a process by which a single parent cell divides itself into 2 or more daughter cells. This generally occurs as a part of the cell cycle, and hence the cells increase in number through this process of division. It is one of the prime aspects of the growth and development of multicellular organisms, as new cells are needed to replace dead and damaged cells.
Types of cell division:
There are 2 main types of cell division:
It is a process where a parent cell gives rise to two identical daughter cells.
There are 4 phases in mitosis: Prophase, Metaphase, Anaphase, and Telophase.
The main purpose of this type of division is to grow and replace worn-out cells.
They occur only in the somatic cells.
The chromosomal numbers remain the same in this type of divisional process.
They also do not allow for genetic recombination.
Meiosis is divided into 2 phases: Phase I and Phase II.
The division of cells happens twice in this process.
There are 4 phases in Meiosis I; they are :
i) Prophase I– further divided into:
- Leptotene: Here, the chromosomes undergo compaction.
- Zygotene: Chromosomes start pairing together in a process called synapsis.
- Pachytene: Nuclear membrane is intact, nucleolus completely disappears, there is also crossing over of non-sister chromatids of homologous chromosomes.
- Diplotene: Nuclear membrane starts disappearing, centrioles move towards the poles and chiasmata is seen.
- Diakinesis: Nuclear membrane completely disappears, and crossed over chromosomes are separated through a disjunction process.
Source: Stages of prophase
ii) Metaphase I – The bivalent chromosomes are arranged on the equatorial plate. The microtubules from the opposite poles attach to the kinetochore of homologous chromosomes.
iii) Anaphase I– Homologous chromosomes separate and sister chromatids remain attached to the centromeres.
iv) Telophase I – The nuclear membrane and nucleolus reappears. Cytokinesis follows after this phase.
- The meiosis II phase is initiated after cytokinesis and is the same as the Mitosis phase with 4 sages: Prophase, Metaphase, Anaphase, and Telophase, and, lastly, cytokinesis.
Significance of Meiosis:
- Specific chromosome number of each species is achieved across generations in sexually reproducing organisms.
- Increases the genetic variability in the population of organisms from one generation to the next.