CELL CYCLE

The ability to grow and reproduce is a fundamental property of living organisms. Cell growth is accomplished through the synthesis of new molecules of proteins, nucleic acids, carbohydrates and lipids.  As the accumulation of these molecules causes the volume of a cell to increase, the plasma membrane grows to prevent the cell from bursting. But, cells cannot continue to enlarge indefinitely; as a cell grows larger, there is an accompanying decrease in its surface are / volume ratio and hence in its capacity for effective exchange with the environment. Therefore, cell growth is generally accompanied by cell division. In unicellular organisms, cell division increases the total number of individuals in a population. In multi cellular organisms, cell division either increases the number of cells, leading to growth of the organism or replaces dead cells.

When cells grow and divide, the newly formed daughter cells are usually genetic duplicates of the parent cell, containing the same DNA sequences. Therefore, all the genetic information in the nucleus of the parent cell must be carefully duplicated and distributed among the daughter cells. To accomplish this, the cells passes through a series of stages, collectively known as the cell cycle.

The cell cycle can be divided into four stages – G1, S, G2 and M phase. The first three phases are together referred to as the interphase. Most cellular contents are synthesized continuously during interphase, so cell mass gradually increases as the cell approaches division. During interphase the amount of nuclear DNA doubles, this occurs during the S phase. A time gap called G1 phase separates the S phase from the preceding M phase, and a second gap, The G2 phase, separates the end of S phase from the beginning of the next M phase.

In mammals the total cell cycle takes about 18 – 24 hours. The S Phase lasts about 6- 8 hours and the M phase usually lasts less than an hour  ( 30- 45 minutes). The length of the G1 phase is quite variable among mammalian cells. A typical G1 phase lasts 8 – 10 hours but some cells enter the G0 (quiescent phase) phase in a state of transient arrest and re-enter the cell cycle when required. Other cells undergo terminal differentiation, i.e. they are destined to never divide again. The G2 phase is considerably shorter than the G1 phase and lasts for 4- 6 hours.

MITOSIS

Mitosis is the form of cell division occurring in somatic cells. It gives rise to two daughter cells that are clones of the parent cell. Mitosis is also called as the equational cell division. It can be divided into four phases – prophase, metaphase, anaphase and telophase.

1.       Prophase :

·         Cell enters G2 phase after the end of S phase.

·         Chromosome condensation begins at the end of G2 phase. A cell is said to be in prophase when individual chromosomes have condensed to the point of being visible objects in the light microscope.

·         Each prophase chromosome is composed of two sister chromatids.

·         The centromere is duplicated in the S phase and the two centromeres separate from each other and begin to move towards opposite ends of the nucleus.

·         Each centromere acts as a nucleation site for microtubule assembly and the region between the two centromeres begin to fill with growing microtubules that wll eventually form the mitotic spindle.

·         Cytoskeletal microtubules disassemble and their tubulin subunits are added to the growing mitiotic spindle.

·         During late prophase ( or prometaphase) the nuclear membrane starts disappearing.

·         The spindle microtubules attach to protein containing structures associated with the centromere called kinetochore.

2.       Metaphase:

·         The cell is said to be in metaphase when the condensed chromosomes align at the metaphase plate.

·         The cell pauses at the metaphase that takes place in about 20 minutes.

·         The chromosomes start moving towards the poles though the movement is barely noticeable.

3.       Anaphase:

·         Usually the shortest  phase of mitosis, anaphase typically lasts a few minutes

·         At the beginning of anaphase the two sister chromatids separate and begin moving towards opposite spindle poles at a rate of about 1µm / min.

·         In anaphase A, the chromosomes are pulled, centromere first, towards the spindle poles as the kinetochore microtubules get shorter.

·         In anaphase B, the poles themselves move away from each other as the polar microtubules lengthen.

·         Depending on the cell type, anaphase A and B might take place at the same time or anaphase B might follow anaphase A.

4.       Telophase:

·         By the beginning of the telophase, the daughter chromosomes have arrived at the poles of the spindle fibres.

·         The chromosomes uncoil and revert to chromatin form.

·         Nucleoli develop, the spindle disassembles and nuclear envelops form around the two daughter chromosomes.

·         The cell undergoes cytokinesis which divides the cell into two daughter cells.

MEIOSIS

Since gametes are haploid, they cannot be produced from diploid cells by mitosis because mitosis creates daughter cells that are genetically identical to the parent cell. The hypothetical zygote created by the fusion of such diploid gametes would be tetraploid. Moreover, the chromosome number would continue to double for each succeeding generation. Thus, for the chromosome number to remain constant from generation to generation, a different type of cell division must occur during the formation of gametes. That special type of reductional division is called Meiosis.

Meiosis involves one round of chromosomal DNA replication followed by two successful nuclear divisions. This results in the formation of four daughter nuclei containing one haploid set of chromosomes per nucleus.

Meiosis can be divided into two phases – Meiosis I and Meiosis II.

Meiosis I

1.      Prophase 1 :

·         Leptotene – condensation of chromatin fibres

·         Zygotene – homologous chromosomes pair with each other using synaptonemal complex

·         Pachytene -  crossing over i.e. DNA exchange between homologous chromosomes

·         Diplotene – homologous chromosomes of each bivalent separate from each other except at the point of chiasmata

·         Diakinesis  –   chromosomes recondense ,

-         centromeres of homologous chromosomes separate further and      are only attached at the chiasmata,

-          nucleoli disappear,

-         spindle forms,

-         nuclear envelope breaks down.

2.      Metaphase 1: The bivalents attach via their kinetochores to spindle microtubules and migrate to the spindle equator.

3.      Anaphase 1: the members of each homologous chromosome separate from each other and start moving towards opposite spindle poles, pulled by their respective kinetochore microtubules.

4.      Telophase 1: haploid setoff chromosomes arrive at each spindle pole. Nuclear envelope develop around the cells before cytokinesis. Two daughter cells are formed.

Meiosis II:

1.       Prophase :

·         Cell enters G2 phase after the end of S phase.

·         Chromosome condensation begins at the end of G2 phase. A cell is said to be in prophase when individual chromosomes have condensed to the point of being visible objects in the light microscope.

·         Each prophase chromosome is composed of two sister chromatids.

·         The centromere is duplicated in the S phase and the two centromeres separate from each other and begin to move towards opposite ends of the nucleus.

·         Each centromere acts as a nucleation site for microtubule assembly and the region between the two centromeres begin to fill with growing microtubules that wll eventually form the mitotic spindle.

·         Cytoskeletal microtubules disassemble and their tubulin subunits are added to the growing mitiotic spindle.

·         During late prophase ( or prometaphase) the nuclear membrane starts disappearing.

·         The spindle microtubules attach to protein containing structures associated with the centromere called kinetochore.

2.       Metaphase:

·         The cell is said to be in metaphase when the condensed chromosomes align at the metaphase plate.

·         The cell pauses at the metaphase that takes place in about 20 minutes.

·         The chromosomes start moving towards the poles though the movement is barely noticeable.

3.       Anaphase:

·         Usually the shortest  phase of mitosis, anaphase typically lasts a few minutes

·         At the beginning of anaphase the two sister chromatids separate and begin moving towards opposite spindle poles at a rate of about 1µm / min.

·         In anaphase A, the chromosomes are pulled, centromere first, towards the spindle poles as the kinetochore microtubules get shorter.

·         In anaphase B, the poles themselves move away from each other as the polar microtubules lengthen.

·         Depending on the cell type, anaphase A and B might take place at the same time or anaphase B might follow anaphase A.

4.       Telophase:

·         By the beginning of the telophase, the daughter chromosomes have arrived at the poles of the spindle fibres.

·         The chromosomes uncoil and revert to chromatin form.

·         Nucleoli develop, the spindle disassembles and nuclear envelops form around the four daughter chromosomes.

·         The cell undergoes cytokinesis which divides the cell into four daughter cells.

Synaptonemal Complex:

The synaptonemal’s complex form on the chromosomes during leptotene and its central or axial element which appears during zygotene and zips the homologous chromosomes together. At pachytene, the homologues are held tightly together align their lengths.

GAMETE FORMATION

Spermatogenesis :

Spermatogonium ( 2n)

   (Matures and Grows)

Primary Spermatocyte (4n)

(Meiosis I)

Secondary Spermatocyte (2n)

(Meiosis II)

Spermatids (n)

(Differentiation)

Spermatozoa

Oogenesis :

Oogonium ( 2n)

   

(Matures and Grows)

Primary oocyte (4n)

(Meiosis I)

                                             

            Secondary Oocyte (2n)          

                                                                                                      +Primary Polar Body

(Meiosis II)

                                                           

           Ootids (n)                  

                                                                                                       + Secondary Polar Body

(Differentiation)

Ovum

( Notes made from Beckers World of Cell, Pearson Publications)