All living things, including human beings, are made up of cells. Some organisms have only one cell during their whole lifespan by which they carry out all the physiological processes they require to survive. These are called unicellular organisms (e.g. bacteria, yeast etc.). But there are also many other organisms whose bodies are made up of two or more cells. These organisms are called multicellular organisms (e.g. higher plants, animals, human beings etc.). Multicellular organisms grow up and reproduce by dividing their cells.
What is cell division?
Cell division is the basic process by which a parent cell divides into two or more daughter cells. It occurs as a part of a larger cell cycle. Both prokaryotic & eukaryotic organisms grow up and reproduce by this process.
There are three stages of cell division:
- DNA replication
- Nuclear division (karyokinesis)
- Cytoplasmic division (cytokynesis)
3 types of cell divisions occur in living organisms:
- Mitosis and
Discovery of cell division
- In 1879, Dr. Schleicher first gave the word ‘Karyokinesis’ . Here, karyo = nucleus & kinesis= division.
- In 1882, Walther Flemming, a German biologist and a founder of cytogenetics, observed , along with nucleus, some thread-like structures were also divided equally and gave the name ‘Mitosis’ ( mito =thread, sis= division ).
- In 1888, Waldeyer observed that the thread-like structure took certain stain. He gave the name ‘chromosome’. Here, chroma = color, soma = absorb.
- T. O. Caspersson said that DNA replication occurs in Prophase.
- In 1925, Polyster et. al , through ‘Quantitative Feulgen Micro-spectrophotometry DNA Stain method’, they observed that if light is being passed through interphase, the cot values were:
Interphase – 4C
Prophase – 4C
Metaphase – 4C
Anaphase – 2C
Telophase – 2C
Cell – 2C
- In the same species, the amount of DNA in diploid somatic cell is 2C.
- In the same species, the amount of DNA in gametic cell is C.
- In the same species, the amount of DNA in zygote is 2C.
- In the same species, the amount of DNA in interphase is 4C.
- In the same species, the amount of DNA in mitotic anaphase, it becomes 2C.
Therefore, the DNA replication or chromosome duplication occurs in interphase stage, not in prophase.
Cot Value: A technique for measuring the complexity (size) of DNA or genome. Co= concentration of DNA and t= time taken for renaturation. Low cot value indicates more no. of repetitive sequences. High cot value shows more no. of unique sequences or less no. of repetitive sequences.
Cell cycle is the series of events that take place in a cell when it grows and divides. Howard & Pele gave the cell cycle in 1953. They used H3 Thymidin isotoph to discover cell cycle. They saw 4 steps in cell cycle:
- Replication of DNA
- Non- function
In short, there are 2 stages in a cell cycle:
- A long, undividing stage or, interphase or, I-phase
- A short, dividing stage or, mitotic phase or, M-phase
Mitotic phase is further divided into 5 sub-stages:
- Prophase ( pro = initial )
- Prometaphase ( pro= initial, meta = middle )
- Metaphase ( meta = middle )
- Anaphase ( ana = move )
- Telophase ( telo = end )
- Size of cell
- Growth regulator
- DNA damage
- DNA replication completeness
- DNA damage
This checkpoint is also known as spindle checkpoint.
- Chromosome attachment to spindle
Interphase or I-phase
The time between the end of the telophase and the beginning of the next M-phase is called interphase or I-phase.
- It lasts about 10-30 hours depending on different conditions.
- During this phase, the cell grows by synthesizing biological molecules such as carbohydrates, lipids, proteins, nucleic acids etc.
Interphase has 3 sub-phases:
The gap between the end of telophase and the beginning of DNA synthesis is called G1-phase. Things that happens during this phase are as follows:
- Initial growth of newly formed cells.
- Various biological molecules are synthesized.
- Take preparation for DNA replication.
The part of interphase where DNA is synthesized is called S-phase.
- Each chromosome is duplicated by DNA replication.
- Formation of new nucleosome.
- Lasts for about 6 to 8 hours.
- Cytoplasmic organelles ( centrioles, mitochondria, golgi body, chloroplast etc.) are doubled.
- Formation of proteins for spindle aster synthesis.
- Lasts for about 2 to 5 hours.
Mitotic phase or M-phase
- During this phase, all the chromosomes are divided into two daughter cells.
- Many structural and physiological changes take place.
- Chromatins are packed into chromosomes.
- Disappearance of nucleolus and nuclear membrane occurs.
- Endoplasmic reticulum and golgi bodies get fragmented into small vesicles and prevent movement.
- Microtubules are assembled into spindle fibre.
- Actin filament ( one kind of protein ) form a contractile ring for the cytoplasmic division.