CELL CYCLE 

             Or 

Write a note on the cell cycle.

              Or

Explain cell cycle with a labelled diagram.

Introduction:

When we talk of the Life cycle it is the cycle showing the changes occurring in the organism through its life e.g. life cycle of a butterfly. Where it passes through different changes from egg_ larvae_ pupa_ butterflies. Similarly, the cell cycle is the different stages to which a cell passes until a new cell is formed.

 Definition: Cell Cycle 

The cell cycle is a series of cyclic changes that take place in a Cell from the time it divides until another division of the newly formed cell.

Cell Cycle: Diagram 

So, the cell cycle begins when a new cell is formed. It ends when the cell completes a division. The time interval between two successive divisions is called "generation time".

Phases of Cell Cycle:

In Eukaryotic cells (the cell with a nucleus), the cell cycle has two phases;

Long non-dividing I- phase ( interphase) and short dividing M- phase( dividing phase).

Interphase ( I- Phase):

It is a complex of changes that occur in the newly formed cell before it is again able to divide. It is although called the resting phase, it is the time when the cell is preparing for division by undergoing both cell growth and DNA replication in a sequence.

Interphase is further divided into three stages,

1. G1 phase ( Gap 1)

2. S-phase    ( synthesis phase)

3. G2 phase ( Gap 2)

G1 Phase ( Gap 1):

It is post mitotic gap phase or growth phase. The cell is metabolically active and continuously growing but does not replicate its DNA. RNA and Protein synthesis occur ( including histone).

S-Phase ( synthesis phase):

The S phase or synthesis phase is marked as the period during which DNA synthesis or replication takes place. So the amount of DNA per cell doubles.

* If the initial amount of DNA is denoted as 2C, then it increases to 4C, but there is no increase in the number of chromosomes. If the cell is diploid or 2n at G1, even after the S phase, the number of chromosomes remains the same i.e. 2n.

G2 Phase ( Gap 2):

It is the second growth or premitotic gap phase when proteins are synthesized in preparation for mitosis while cell growth continues. In this phase, macromolecules including protein tubulins are synthesized for cell division. A new centriole pair is formed.

G0 stage:

Some cells in adult animals do not exhibit cell division e.g. heart cells and many other cells divide occasionally as needed to replace the cells lost due to injury or cell death. These cells exit the G1 phase to enter an inactive stage called the Quiescent stage ( G0 stage) of the cell cycle. These cells remain metabolically active but no longer proliferate as long as not needed by the organism.

M- Phase ( mitotic phase):

It is the phase of cell division. It is the most dramatic phase of the cell cycle where all components of the cell are reorganised. In M- phase or mitotic phase the number of chromosomes in the parent and the progeny cells is the same, it is also called equational division.

Cell division or mitotic division consists of nuclear division called karyokinesis and protoplasmic division or cytokinesis.

Cell division or M-phase is of three types

1. Amitosis

2. Mitosis

3. Meiosis

Amitosis: 

It is a type of direct cell division ( by Robert Remarks in 1841) in which parent cells nucleus and cytoplasm divide between two daughter cells by a simple constriction. Here the nucleus elongates, constricts in the middle and divides directly into two daughter nuclei. This is followed by the division of cytoplasm through constriction. e.g. cartilage cells, and degenerate cells.

Mitosis:

Also called somatic division because it occurs during the formation of somatic or body cells. Mitosis is equational division in which a parent cell divides into two identical daughter cells, each of which contains the same number and kind of chromosomes as in the parent cell. Mitosis occurs in stages: 

Karyokinesis: Division of nucleus.

Cytokinesis: Division of cytoplasm,

giving rise to two daughter cells. Each of these newly formed cells again undergoes a cell cycle to form two new cells. This way the cell cycle goes on forming a full-grown organism.

Meiosis:

 Meiosis is a type of cell division that takes place in the germ cell (sex cells or gametes), at some point in the life cycle of sexually reproducing eukaryotic species. 

Meiosis consists of a single duplication division. In animals and lower plants, meiosis is terminal or gametic ( i.e. it occurs before the formation of gametes). In the male, four haploid sperms are produced, in the female, one ovum and three polar bodies are produced. In most plants, meiosis is intermediary or sporadic (i.e. It occurs sometime between fertilization and the formation of gametes). cells in meiosis are called meiocytes.

In animals, it typically occurs during the maturation of sex cells ( gametes). Sexual reproduction in animals involves the union of two gametes ( egg and sperm), each bearing one set of chromosomes, resulting in a cell, usually a fertilized egg, or zygote, which is diploid i.e. containing two sets of chromosomes.

If no means existed for reducing the gametic chromosome to half, in each generation the chromosome number would double, which would ultimately spell a quick demise for sex! So the reduction of gametes to the haploid state( i.e. to half the normal chromosome number) must be accomplished with precision so that every gamete will still possess one chromosome of each homologous psir. In all eukaryotes, meiosis brings this about. 

In humans, the normal chromosome number of 46 ( 23 homologous pairs) per cell is reduced meiotically to just 23 in the sperm and egg. The diploid state in the zygote is restored by fertilization bringing together the two homologous gametes.

Meiosis is divided into division I and II.

Meiosis I

In division I, there is a long prophase of which the stages are leptotene, zygotene, pachytene, diplotene and diakinesis.

During prophase I, the two members of each chromosome pair behave remarkably. They come together and "zip up" so that each allele lies in intimate contact with its allele in the homologous chromosome. This is called synapsis and is essential for the orderly separation of two members of each chromosome pair ensuing anaphase. Since each chromosome already consists of two daughter chromatids held together at the centromere, the result is a tetrad, a bundle of four strands.

These intimately entwined strands can break and rejoin in such a way that part of one strand exchanges position with an equivalent portion of another strand. This crossing over has the effect of moving genes into new linkage groups.

During metaphase I, the tetrads are located in the middle of the spindle, and spindle fibres attach to the centromeres, but the centromeres do not divide.

As a result during anaphase I, when the spindle fibres shorten, the two members of each homologous chromosome pair are separated.

During telophase I chromosomes elongate, nucleolus, nucleoplasm and nuclear envelope appear over each chromosome group forming nuclei.

Meiosis I is often called the "reduction division"  where at its end each daughter cell contains only one member of each chromosome pair, although each chromosome still consists of two DNA molecules, or chromatids, held together by the undivided centromere.

Meiosis II:

Depending on the species, meiosis II may begin at once or be delayed, but in either case, DNA does not replicate between meiosis I and II.

1. When meiosis II commences, the chromosome, which remains in its shortened condition between the two meiotic divisions, promptly moves to the middle of the new spindle.

2. The centromeres finally divide, and one of the two chromatids of each chromosome passes to each daughter cell so that meiosis II  is often called the "equational division".

3. The result is four haploid cells, with each chromosome now consisting of only one DNA molecule.

 

Cytokinesis: Cytokinesis may occur after each division ( successive type) or simultaneously at the end of meiosis. It is generally through cleavage. In the case of plants, wall material is deposited in the furrows. Cytokinesis gives rise to four haploid cells.

These newly formed cells again undergo a cell cycle to form new cells. This way cell cycle goes on forming a full-grown organism.

Some Cell-Cycle related Short QnA

Question: what is cell cycle in short?

Answer: 

The sequence of events denoting the various physiological activities in a cell is known as a cell-cycle.  There are four major stages in a Cell-Cycle. The M-phase (divisional phase), the G1 (gap I phase), S-phase (synthetic phase ) and G2 (gap II phase). The G2, S and G2 phases are together called interphase.

Question: state the importance of divisional stages.

Answer:

The stages of cell cycle when the parent cell produces its own replicate is known as divisional stages. It has very small duration in comparison to the interphase. The different divisional stages are prophase, metaphase, Anaphase, and telophase. 

The importance are: 

1. One single cell produces many replicates.
2. An organism shifts from unicellular to multicellular condition.
3. It assists in growth of an organism and development of organs.
4. The germ cell produces the gametes.
5. It results in healing of wounds.

Question: what are S.P.A., M.P.F and M.D.F in relation to Cell-Cycle?

Answer:

S P.A. -  S-phase activator: It is a protein that helps in the DNA replication at S-.

M P.F.-  M-phase promoting factor: It is the cytoplasmic protein of metaphase stage that helps in the aggregation of chromosomes.

M.D.F. -  M-phase delaying factor: It is a cytoplasmic protein formed in the S-phase that inhibits the activity of M.P.F and thereby delays the process of metaphase.

Question: what are contractile ring and stem body? where are they present?

Answer: 

A contractile ring is formed of microscopic fibres that are distributed in the cytoplasm of certain animal cell like muscle cell and helps in process of cytokinesis.

Stem body is the contracted spindle fibres that contract gradually at the end of karyokinesis and occupy the equatorial plane during cytokinesis. It helps in the process of cytokinesis by invagination.

They are both found in dividing animal cell.

Question: Name the different phases of cell cycle.

Answer:

The different phases of cell-cycle are, G1, S, G2 and M.

Question: what is G0 stage? where is it located in Cell-Cycle?

Answer:

G0 stage is the arrested G1 stage in cells where division is suspended e.g., Nerve cell.

It is present at the onset of G1 stage.

Question: in which phase of cell cycle, the cell becomes double in size?

Answer:

G1 phase.

Question: In which phase of cell cycle, DNA replication takes place?

Answer: S-phase or synthetic phase.