Cell Division

• Cell Division is the process by which a parent cell divides into two or more daughter cells. Cell division usually occurs as part of a larger cell cycle.
• Cell Division is a crucial biological process for the creation of daughter cells from parent cells aimed at growth, development, and reproduction.
• Why Do Cells Divide? Cells divide basically for three reasons. They are:

  1. For the growth & development of our body
  2. To repair the dead and damaged tissues.
  3. For reproduction
• The process of Mitosis that cells use to make exact replicas of themselves, is observed in almost all the body's cells, including eyes, skin, hair, and muscle cells.
• In meiosis, sperm or egg cells are produced with haploid set of chromosomes instead of identical daughter cells as in mitosis.
• During the Cell Cycle, the (1) cell grows. (2) Deoxyribonucleic Acid or DNA is replicated. (3) Mitotic cell division produces cell identical to the parent.
• There are two primary phases in the cell cycle:

  Interphase - This phase was thought to represent the resting stage between subsequent cell divisions, but new research has shown that it is a very active phase.

  M Phase (Mitosis Phase) - This is where the actual cell division occurs. There are two key steps in this phase, namely cytokinesis and karyokinesis.
• The interphase comprises three phases:

  G0 Phase (Resting Phase) - The cell neither divides nor prepares itself for the division.

  G1 Phase (Gap 1) - The cell is metabolically active and grows continuously during this phase.

  S Phase (Synthesis) - The DNA replication or synthesis occurs during this stage.

  G2 Phase (Gap 2) - Protein synthesis happens in this phase.
• The G0 stage is also known as the Quiescent Stage, the cells that do not undergo further division exits the G1 phase and enters an inactive phase.
• The interphase is a Non-dividing State. In this state, the cell activities happen when (1) the cell grows in size, (2) the organelles replicated, (3) the DNA replicated, (4) synthesis of proteins associated with DNA, and (5) synthesis of proteins associated with mitosis.
• The Division of somatic cells in eukaryotic organisms is called Mitosis. In this process, a single cell divides into two identical daughter cells. Daughter cells have same set of chromosomes as does the parent cell.
• In Prophase, 3 major events take place:

  1. Chromosomes condense.
  2. Spindle fibers form.
  3. Chromosomes are captured by the spindle fibers.
• In Metaphase, chromosomes align along the equator of the cell, with one kinetochore facing each pole.
• In the Anaphase stage, the sister chromatids separate from each other. Spindle fibers attached to kinetochores shorten and pull the chromatids towards the opposite poles.
• In this Telophase stage, the spindle fibers between the poles disintegrate. The nuclear envelopes start reforming around both the groups of chromosomes at the poles.
• ADDITIONAL: Chromosomes revert to their extended state by absorbing water from the cytoplasm. There appears a constriction in the cytoplasm between the two groups of dividing chromosomes. Cytokinesis completes the enclosing of each daughter nucleus into a separate cell.
• Cytokinesis in an animal cell is the last phase of the normal cell cycle. In this phase, the cell physically divides into two identical daughter cells. In animal cells, the cell membrane breaks apart where it was pinched and now it is two daughter cells. In both cells, the DNA is identical. In plant cells, a cell plate forms down the middle of the cell and the cell breaks apart where the cell plate was formed. The two daughter cells will often stay attached to each other side-by-side.
• Meiosis (Reduction) is a process where a single cell divides twice to produce four daughter cells containing half the original amount of genetic information These cells are called as gametes sperm in males, eggs in females.
• Genetics Terminology: Ploidy refers to the number of sets of chromosomes in cells.
• Haploid - It only contains one copy of a chromosome (designated as "n").

  Diploid - It contains two sets of chromosomes (designated as "2n")
• Diploid organisms receive one of each type of chromosome from female parent and one of each type of chromosome from male.
• Genetics Terminology: Homologues

  Chromosomes exist in homologous pairs in all diploid (2n) cells except in Sex Chromosomes in males (XY). Apart from the Sex Chromosomes, the other chromosomes are known as autosomes and all they have homologues.
• Meiosis - there are two nuclear divisions. They are Meiosis I and Meiosis II.
• MEIOSIS I: Meiosis is a reduction division. The salient features of meiotic division that make it different from mitosis are as follows:

  1. It occurs in two stages of the nuclear and cellular division; Meiosis I and Meiosis II, DNA replication occurs, however, only once.
  2. It involves the pairing of homologous chromosomes and recombination between them.
  3. Four haploid daughter cells are produced at the end, unlike two diploid daughter cells in mitosis.
• METAPHASE I - the maternal and paternal chromosomes (homologous chromosomes) align along the equator of the cell. A process called independent assortment occurs where the maternal and paternal chromosomes line up randomly and align themselves on either side of the equator. At this stage, 50% of the chromosomes start migrating to the opposite pole of the dividing cell.
• ANAPHASE I - In this stage, Kinetochore microtubules shorten and pull the homologous chromosomes to opposite poles. The sister chromatids remain tightly bound together at the centromere.
• ANAPHASE-I

  The chiasmata are broken in as the microtubules attached to the fused kinetochores pull the homologous chromosomes apart.

  Non-kinetochore microtubules lengthen, pushing the centrosomes farther apart and the cell starts elongating.
• TELOPHASE-I

  This is the end of the First meiotic division. Each daughter cell now has half the number of chromosomes but each chromosome consists of a pair

  of chromatids.

  The microtubules that make up the spindle network disappear, and a new nuclear membrane surrounds each haploid set.
• TELOPHASE-I

  The chromosomes uncoil back into chromatin. Cytokinesis occurs, completing the creation of two daughter cells. However, cytokinesis does

  not fully complete resulting in "cytoplasmic bridges" which enable the cytoplasm to be shared between daughter cells until the end of Meiosis II.
• MEIOSIS-II

  Meiosis II initiates immediately after cytokinesis, usually before the chromosomes have fully decondensed. In contrast to meiosis I, meiosis Il resembles a normal mitosis. In some species, cells enter a brief interphase, or interkinesis, before entering meiosis.
• MEIOSIS-II

  However, this interphase lacks an S phase, so chromosomes are not duplicated. The two cells produced in meiosis I go through the events of meiosis Il together. During meiosis II, the sister chromatids within the two

  daughter cells separate, forming four new haploid gametes. The mechanics of meiosis II are similar to mitosis, except that each dividing cell has only one set of homologous chromosomes.
• Meiosis - chromosomes contain genetic information. Genetic information is passed from parents to offspring through the processes of meiosis and fertilization, ensuring the continuity of genetic traits across generations.
• Meiosis introduces genetic diversity through two key mechanisms:

  Crossing Over - exchange of genetic material between homologous chromosomes.

  Independent Assortment - random distribution of chromosomes to the gametes.
• No Crossing Over - daughter cells are identical to parent cells.

  Crossing Over occurs, causing genetic variation or the daughter cells are not identical to parent cells.