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Comprehensive Overview of Cell Division: Mitosis and Meiosis Processes

Detailed explanation of cell division, including mitosis, meiosis, cell cycle phases, and their biological significance in growth, repair, and reproduction.

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Cell Division
Defination: 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 In cell division, the cell that is dividing is called the "parent" cell. The parent cell divides into two "daughter" cells and the process then repeats in what is called the cell cycle.
Why Do Cells Divide? Cells divide basically for three reasons. They are: a) For the growth & development of our body b) To repair the dead and damaged tissues c) For reproduction
The cells of higher organisms divide by two processes. They are : 1) Mitosis: The process that cells use to make exact replicas of themselves is mitosis. Mitosis is observed in almost all the body’s cells, including eyes, skin, hair and muscle cells. 2) Meiosis: In this type of cell division, sperm or egg cells are produced with haploid set of chromosomes instead of identical daughter cells as in mitosis.
CELL DIVISION Mitosis & Meiosis
Cell Cycle During the cell cycle, – Cell grows. – DNA is replicated. – Mitotic cell division produces daughter 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. Quiescent Stage (G0): The cells that do not undergo further division exits the G1 phase and enters an inactive stage. This stage is known as the quiescent stage (G0) of the cell cycle. Interphase
Interphase It is actually a Non-dividing state. In this state…. – cell grows in size – organelles replicated – replication of DNA – synthesis of proteins associated with DNA – synthesis of proteins associated with mitosis
Division of somatic cells in eukaryotic organisms is called as 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. Mitosis Cell Division
Mitosis It has 4 sub-phases: 1st – Prophase 2nd – Metaphase 3rd – Anaphase 4th – Telophase followed by Cytokinesis
1. Prophase In Prophase 3 Major Events take place. 1) Chromosomes condense 2) Spindle fibers form (spindle fibers are specialized microtubules radiating out from centrioles) 3) Chromosomes are captured by spindle
chromatin nucleolus nucleus centrioles condensing chromosomes Fig; Main events of Prophase
In this phase chromosomes align along equator of the cell, with one kinetochore facing each pole centrioles spindle fibers chromosomes 2. Metaphase Kinetocores not pictured in this illustration.
3. Anaphase In anaphase stage the sister chromatids separate from each other. Spindle fibers attached to kinetochores shorten and pull the chromatids towards the opposite poles. The cell appears almost oval in shape as it starts becoming longer.
4. Telophase • In the telophase stage, the spindle fibers between the poles disintegrate. • The nuclear envelopes start reforming around both the groups of chromosomes at the poles • 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 ANIMAL CELL . Cytokinesis 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 pinches together and the membrane breaks apart where it was pinched and now it is two daughter cells. In both new 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
REDUCTION CELL DIVISION Meiosis 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 gamates sperm in males, eggs in females.
Diploid organisms receive one of each type of chromosome from female parent and one of each type of chromosome from male Ploidy refers to the number of sets of chromosomes in cells. ● Haploid : * It contains only one copy of chromosome * It is designated as “n” ● Diploid : * It contains two sets of chromosomes * It is designated as “2n” Genetics Terminology: Ploidy
GENETICS TERMINOLOGY: HOMOLOGUES Chromosomes exist in homologous pairs in all diploid (2n) cells except in Sex Chromosomes in male (XY) Apart from the Sex chromosomes, the other chromosomes are known as autosomes and all they have homologues.
MEIOSIS – THE GAMETE FORMATION In meiosis, there are 2 nuclear divisions. They are Meiosis I & 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 as 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.
Meiosis-I has following four sub stages : Prophase - I Metaphase - I Anaphase - I Telophase - I Prophase - I Prophase - I is longer than the mitotic prophase and is further subdivided into 5 sub stages. They are : Leptotene Zygotene Pachytene Diplotene Diakinesis
LEPTOTENE Leptotene is the first of five stages of Prophase - I and it consists of the condensation of the already replicated chromosomes The chromosomes become visible and now they can be distinguished easily. The chromosomes at this stage are likened to a string with beads, called chromomeres. Each sister chromatid is attached to the nuclear envelope and they are so close together that they can be mistaken as one chromosome.
Fig : Leptotene Stage
ZYGOTENE In this stage the homologous chromosomes begin to pair. This process is also known as zygonema (Synapsis). These synapsis can form along the entire length of the chromosomes allowing numerous points of contact called 'synaptonemal complex‘. The synaptonemal complex facilitates synapsis by holding the chromosomes together. along the entire length. After the homologous pairs synapse they are either called bivalents.
Fig: Zygotene stage
PACHYTENE Once the synapse is formed, now the cell is ready for crossing over. In this stage further thickening and shortening of chromosomes take place. During this stage, exchanges of chromosome material between maternal and paternal homologous chromosomes occur by crossing over. At the points of crossing over, X–shaped chiasmata are seen in variable numbers.
Fig : Pachytene stage
DIPLOTENE In the "Diplotene" stage, the paired chromosomes begin to get separated from each other. While getting separated, the homologous chromosomes remain united at the points of interchange of "Chismata". Chiasma are formed as the result of crossing over in the Diplotene stage. With the progression of diplotene, the nuclear membrane gets disorganized and the nucleolus disappears.
Fig : Diplotene Stage under Electron Microscope
In this stage, bivalents get distributed in the nucleus. The nuclear membrane breaks down and the nucleolus disappears. Chiasma moves towards the end, and this process is called as terminalization. Chromatids remain attached only at the terminal chiasmata and enter the metaphase stage. Recondensation of chromosomes takes place. Tetrad move towards equatorial metaphase plate and spindle formation is initiated. DIAKINESIS
Fig : Diakinesis
METAPHASE - I In Metaphase-I of meosis 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.
Fig : Metaphase-I
ANAPHSE-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. 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 elongatting.
Fig : Anaphase-I
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. 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
Fig : Telophase-I
MEIOSIS-II Meiosis-II initiates immediately after cytokinesis, usually before the chromosomes have fully decondensed. In contrast to meiosis I, meiosis II resembles a normal mitosis. In some species, cells enter a brief interphase, or interkinesis, before entering 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 II together. During meiosis II, the sister chromatids within the two daughter cells separate, forming four new haploid gametes. The mechanics of meiosis II is similar to mitosis, except that each dividing cell has only one set of homologous chromosomes.
Comprehensive Overview of Cell Division: Mitosis and Meiosis Processes