Life - as it continues

LIFE- as it continues to grow 

Cell growth and cell division 

Cell division involves the distribution of identical genetic material , that is the DNA (Deoxyribonucleic acid) to two daughter cells .

Cell division is of two types :

• Mitosis 
• Meiosis 

Mitosis is the type of cell division which takes place in general body cells or somatic cells .

Eg- Liver, Bone marrow ,etc.

Meiosis is the type of cell division which takes place in germ cells or reproductive cells 

Eg- Sperm , Ovum 

Before moving ahead to the mechanism of cell division , I would like you all to get Familiar to some basic terminologies:

• gene - basic unit of heredity, codes for a specific trait 
• somatic cells - all body cells except reproductive cells
• gamete - reproductive cells , eg - sperm and ovum 
• chromosome - elongate cellular structure composed of DNA and                               protein, they are vehicles which carry DNA in cells.
• diploid - cellular condition where each chromosome type is                              represented in a pair.
• haploid- cellular condition where chromosome is represented only                  by single chromosome.
• homologous chromosome - chromosome of the same size and shape which carry the same type of genes.
• chromatid - one of the two duplicated chromosomes connected at                        the centromere.
• centromere - region of chromosome where microtubules attach during mitosis and meiosis. 

 chromosome diagram 

Example - an organism is 2n = 4. 

• Chromosomes 1 & 2 are homologous chromosomes
• Chromosomes 3 & 4 are homologous chromosomes
• Chromosomes 1 & 3 came from the mother
• Chromosomes 2 & 4 came from the father

Now lets move on to the mechanism of cell division, 

Cell division undergoes a series of cyclic phases namely , G1 - S- G2 - M phase thereby calling it  the cell cycle . 

G1 - first gap

S - DNA synthesis (replication)

G2 - second gap
M - mitosis

Diagrammatic representation of the phases of cell cycle .


The stages of cell cycle are divided into 5 stages :

1. Interphase - It is the resting or non - mitotic portion of the cell                               cycle . It is comprised of G1, S, G2 stages of the cell                           cycle. 
2. Prophase-  The first stage of mitosis , the chromosomes condense                       and become visible. The centrioles form and move                              towards opposite ends of the cell. The nuclear                                    membrane dissolves . The mitotic spindle forms from                       the centrioles in case of animal cell and microtubules in                     case of plant cell. Spindle fibers from each centriole                           attach to each sister chromatid at the kinetochore.

                                          Prophase

   

      3. Metaphase - The centrioles complete their migration to the poles. The chromosomes line                                   up in the middle of the cell. 

Metaphase 

4. Anaphase - Spindles attached to kinetochores begin to shorten . This exerts a force on sister                           chromatids that pulls them apart. Spindle fibers continue to shorten, pulling                                 chromatids to opposite poles. This ensures that each daughter cell gets identical                           sets of shromosomes.

 Anaphase  

5. Telophase - The chromosomes decondense. The nuclear envelope forms.                                           Cytokinesis reaches completion creating two daughter cells.

 Telophase 

Note : To understand the stages of mitosis in a better way ..... you can watch this video 

MEIOSIS 

Meiosis   is divided into meiosis 1 and meiosis 2 

Meiosis 1 is divided into prophase 1, metaphase 1, anaphase 1 and telophase 1.

Meiosis 2 is divided into prophase 2, metaphase 2, anaphase 2 and telophase 2. 

MEIOSIS-I

Prophase I - The chromosomes condense and become visible. The centrioles form and move towards poles . The nuclear membrane begins to dissolve. The nuclear membrane begins to dissolve. The homologous pair up forming a tetrad. Each tetrad is comprised of four chromatids, the two homologue each with their sister chromatid.

Homologous chromosomes will swap genetic material in a process known as crossing over. Crossing over serves to increase genetic diversity by creating four unique chromatids.

prophase - 1

Before moving on with other stages let us know about the concept of crossing over .

CROSSING OVER

• Genetic material from the homologous chromosomes is randomly swapped
• This creates four unique chromatids
• Since each chromatid is unique, the overall genetic diversity of the gametes is greatly increased

Metaphase-I -  Microtubules grow from the centrioles and attach to the centromeres . The                                     tetrad line up along the cell equator.  

 metaphase - I 

Anaphase - I- The centromeres break and homologous chromosomes separate . Cytokinesis begins. 

 Anaphase - I

Telophase-I  - The chromosomes may decondense. Cytokinesis reaches completion, creating two haploid cells. 

 Telophase - I

MEIOSIS-II

Prophase - II -  Centrioles form and move towards the poles . The nuclear membrane dissolves.

 Prophase - I 

Metaphase-II  - Microtubules grow from the centrioles and attach. The sister chromatids line up along the cell equator. 

 metaphase - II 

Anaphase- II -  The centromeres break and sister chromatids separate. cytokinesis begins.

Telophase-II   - The chromosomes may decondense . Cytokinesis reaches completion, creating four haploid daughter cells. 

Telophase-II 

 

NOTE-  TO UNDERSTAND MEIOSIS IN A BETTER WAY YOU CAN WATCH THIS VIDEO 

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Thankyou ....