Cell Cycle

August Weismann in 1887 predicted that the number of chromosomes must be reduced by one half during gamete formation. The two divisions of meiosis are called the first and the second mieiotic divisions. In meiosis I, the number of chromosomes are reduced from diploid to haploid condition whereas in meiosis II, the two chromatids of each chromosome separate from each other and go to separate daughter cells, as a result the number of chromosomes remains the same as produced by meiosis – I.

During the S phase or synthetic phase the replication of DNA takes place. For replication of DNA histone proteins are required, so they are also synthesised during this phase. It takes about 30%-50% of the total cell cycle.

Metaphase is the best time to count and study the number and morphology of chromosomes. The distinctly visible chromosomes arrange themselves at the equatorial or metaphasic plate. The centromeres lie at the equational plate while the limit are placed variously according to their size and spiral arrangement.

Chromatin in the nucleus begins to condense and becomes visible in the light microscope as chromosomes. The nucleolus disappears. Centrioles begin moving to opposite ends of the cell and fibres extend from the centromeres. Some fibres cross the cell to form the mitotic spindle.

A spindle of fine fibres begins to develop during prophase. It consists of microtubules, which are made of protein called tubulin and certain other associated proteins. These delicate fibres radiate from the centriole and constitute aster.

The zygotene stage is called the bouquet stage because of the way the telomeres cluster at one end of the nucleus. At this stage, the synapsis (pairing/coming together) of homologous chromosomes takes place, facilitated by assembly of central element of the synaptonemal complex.

In the relatively short M (mitosis) phase the two copies of the replicated chromosomes are separated and the cell divides. In this phase the chromosomes are highly condensed, with the DNA tightly wrapped around histones. In this state they are inactive and neither DNA replication or transcription takes place.

Mitosis occurs both in somatic cells as well as in germ cells of the gonads. In plants mitosis occurs in the meristematic cells of root tip or shoot tip. These cells divide at a faster rate. So, the root tip shows active cell division and is used in the laboratory to study mitosis.

The major event in S phase is DNA replication. The goal of this process is to create exactly two identical semi-conserved chromosomes. The cell prevents more than one replication from occurring by loading pre-replication complexes onto the DNA at replication origins during G1 phase, which are dismantled in S phase as replication begins.

The spindle pole body (SPB) is the microtubule organising centre in yeast cells, functionally equivalent to the centrosome. Unlike the centrosome the SPB does not contain centrioles. The SPB organises the microtubule cytoskeleton, which plays many roles in the cell. It is important for organising the spindle and thus in cell division.

Mitosis occurs only in eukaryotic cells and the process varies in different groups. For example, animals undergo an "open" mitosis, where the nuclear envelope breaks down before the chromosomes separate while fungi, such as Aspergillus nidulans and Saccharomyces cerevisiae (yeast) undergo a "closed" mitosis, where chromosomes divide within an intact cell nucleus.

In 1835, German botanist Hugo von Mohl discovered the process of mitosis.

Abundant evidence indicates that facultative sexual protists tend to undergo sexual reproduction under stressful conditions. For instance, the budding yeast Saccharomyces cerevisiae switches to meiosis (sexual reproduction) under starvation conditions.This proves that in protists and simple multicellular eukaryotes, meiosis is an adaptation to deal with stress.

In prophase II  one can observe the disappearance of the nucleoli and the nuclear envelope again as well as the shortening and thickening of the chromatids. Centrioles move to the polar regions and arrange spindle fibres for the second meiotic division.

A contractile ring, made of non-muscle myosin II and actin filaments, assembles equatorially (in the middle of the cell) at the cell cortex (adjacent to the cell membrane). Myosin II uses the free energy released when ATP is hydrolysed to move along these actin filaments, constricting the cell membrane to form a cleavage furrow.