CELL CYCLE AND CELL DIVISION

 Objectives  

This  blog post provides readers with the following objectives. The reader will be able to:

o   Explain some terms under cell cycle

o   Outline the process of the cell cycle

o   Describe the process mitosis and meiosis.

CELL CYCLE

The cell cycle is the sequence of events that takes place in cells. It leads to cell division and replication. In eukaryotes, the cell cycle can be divided into four distinct phases: G₁ phase, S phase (synthesis), G₂ phase and M phase. The G₁, S phase and G₂ phase together are known as interphase. The M phase or the mitotic phase (cell division) occurs after interphase. Cell division consists of two phases: nuclear division followed by cytokinesis. Nuclear division divides the genetic material in the nucleus, while cytokinesis divides the cytoplasm.

There are two kinds of nuclear division: mitosis and meiosis.

INTERPHASE

It is the resting stage or the non dividing phase of the cell cycle. It takes about 90% of the cell's life span.

The cell prepares itself, grows and accumulates nutrients for DNA replication and division.

The nucleus is visible and the chromosomes are uncoiled and invisible. The cells grow by producing proteins and cytoplasmic organelles.

Interphase is divided into three phases  G₁, S and G₂

Ø    G₁ Phase (Gap or Growth phase): During G₁ phase the cell becomes metabolically active and continuously grows in size. It is the period which the cell synthesizes its structural proteins and enzymes. The synthesis of cytoplasmic organelles also occurs here.

Ø  S phase (synthesis phase):  There is replication of genetic material in this phase. DNA replicates in the nucleus and the centriole duplicates in the cytoplasm. Chromosomes duplicate, each of the 46 chromosomes has two chromatids.

Ø  G₂ phase: During this phase the cell continues to grow. The synthesis of enzymes and other proteins in preparation for mitosis occurs during this period. Proteins organize themselves to form a series of fibers called the spindle. The cell also stores much energy needed for its division. 

 

MITOSIS

Mitosis is a type of cell division in which a single cell produces two genetically identical daughter cells. The daughter cells contain the same genetic material as the parent cell. It is the way in which new body cells are produced for growth and repair.

There are four stages of mitosis: Prophase, Metaphase, Anaphase, Telophase.

Prophase

1.  The chromosomes condense and become visible. 

2. Each chromosome is seen as two chromatids joined together at a point called centromere.   

3. The nucleolus disappears and nuclear membrane breaks down. 

4.  Centrioles move to opposite ends of the cell.    

5.   Spindle fibers extend from the centrioles.

Metaphase

1. Chromosomes arranged at the equator of the cell.

2 Spindle fibers attach to kinetochores of chromosomes.

Small disc-shaped structure at the surface of the centromere is called kinetochore

Anaphase

1. Spindles begin to shorten and separate the sister chromatids at the centromere. 

2. Spindle fibers continue to shorten, pulling the chromatids to opposite poles. This ensures that each daughter cell gets identical sets of chromosomes

Telophase 

1.  Chromatids reach the poles. 

2. The nuclear membrane and nucleolus reappears. 

3.   The chromosomes uncoil. 

4.   The spindle apparatus breaks down. 

5.  Cytokinesis may also begin during this stage 

Cytokinesis

Cytokinesis is not a phase of mitosis. It is a separate process necessary for completing cell division. 

In animal cells a cleavage furrow containing a contractile ring develops at the equatorial plane separating the nuclei. 

In plant cells, a cell plate is formed across the middle of the cell to divide the cytoplasm and cell wall is between the two daughter cells.

Significance of Mitosis

1.       It brings about growth.

2.      The daughter cells have the same genetic material as that of the parent cell. 

3.       It is responsible for development of a single-celled zygote into a multicellular organism. 

4.       The daughter cells have the same characters as those of the parent cell.

5.        It helps in repairing and replacing dead or damaged cells.

6.     It’s a means of reproduction in unicellular organisms. 

7.    It may result in uncontrolled growth of cells leading to cancer or tumor.

Difference between Mitosis in Plant and Animal Cells

Plant cell	Animal cell
Centriole is absent	Centriole present
Does not form aster	Aster is formed
Cell plate is formed to divide daughter cell	Does not form cell plate
Occurs in meristem	Occurs in all cells

MEIOSIS

Meiosis is a reduction division of a diploid cell to form haploid daughter cells. Haploid cells contain half the number of chromosomes as the parent cell. It takes place in all sexually reproducing organisms. Meiosis is restricted to the gonads or sex cells. Meiosis is divided into meiosis I and meiosis II.

Meiosis I 

It involves the splitting of parent cell and separation of the homologous chromosomes from each other into different cells.

The resulting daughter cells contain one entire haploid set of chromosomes.

It produces two haploid cells (N chromosomes, 23 in humans).

Prophase I

1.    Prophase I is the longest phase of meiosis I.

2.   The chromosomes condense and become visible within the nucleus 

3. The nuclear membrane begins to disintegrate.

4.      The centrioles form and move toward the poles.

5.    In this phase, there is exchange of DNA between homologous chromosomes, this process is known as homologous recombination.

6.      The homologous chromosomes pair up forming bivalents or tetrads. Each chromosome comes from each parent.

7.      Pairing of homologous chromosomes is called synapsis. At the stage of synapsis, the non-sister chromatids may cross-over at points called chiasmata.  Crossing over result in exchange of fragments.

8.     Crossing over serves to increase genetic diversity by creating four unique chromatids

 Metaphase I

1.  Microtubules or spindle fibers grow from the centrioles.    

2.   The bivalents line up along the cell equator and attach to spindle fibers at the centromeres.

Anaphase I

The homologous chromosomes separate and move towards the opposite pole of the cell while sister chromatids remain associated at their centromeres.

Telophase I

1. The nuclear membrane and nucleolus reappear.     

2.  The spindle fibers disappear.   

3.    The chromosomes uncoil and return back to the chromatin stage.

4.  The process of cytokinesis occurs, creating two haploid daughter cells. 

5.    The daughter cells have half the number of chromosomes.

Meiosis II

Meiosis II is an equational division similar to mitosis, where the sister chromatids split forming 4 haploid cells, two from each daughter cells of meiosis I. 

Prophase II 

1.  The nucleoli and the nuclear envelope disappear.

2.     Chromosomes condense.

3.     The centrioles move toward the poles.

Metaphase II

1.      The spindle fibres grow from the centrioles and attach to the centromeres. 

2. The sister chromatids line up along the cell equator. 

3.   The equatorial plane formed here is rotated by 90 degrees, compared to meiosis I and is perpendicular to the previous equatorial plane.

Anaphase II

1. The centromeres break and sister chromatids separate.

2.   The sister chromosomes move towards the opposing poles.

Telophase II

1.   Chromatids reach the opposite poles of the cells.

2. The nuclear membrane and nucleolus reappears. 

3.    The chromosomes uncoil.  

4.    The spindle apparatus breaks down.  

5. Cleavage or cell wall forms which eventually produces a total of four daughter cells, each cell having haploid set of chromosomes.

Significance of meiosis

1. It ensures continuity of species and brings about variation.

2. It enables sexual recombination to occur by reducing the chromosome number by half.

 

Stages in Mitotic Cell Division

Stages in Meiotic Cell Division

Difference between Mitosis and Meiosis

Mitosis	Meiosis
Nucleus divides only once	Nucleus divides twice
Chromosome number is maintained	Chromosome number is halved
No pairing of chromosomes	Pairing occurs
Chiasmata are never formed	Chiasmata are formed
Crossing - over never occurs/ no exchange of genetic materials	Crossing - over occurs /exchange of genetic materials occurs
Daughter cells identical to parent cell	Daughter cells generally different from parent cell
Two daughter cells are formed	Four daughter cells are formed
Chromosomes form single row at the equator	Chromosomes form double row at the equator
Occurs in somatic cell	Occurs in reproductive cell
Occurs in asexual reproduction	Occurs in sexual reproduction

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