Cell cycle 

Introduction 

• Cell division is a really important process in all living organisms.
• During the division of the cell, DNA replication and cell growth also take place.
• The sequence of events by which a cell duplicates its genome synthesizes the other constituents of the cell and eventually divided into two daughter cells is termed cell cycle.

Phases of cell cycle

The cells divide once in approximately every 24 hours.

In the 24 hours average duration of the cell cycle of a human cell, cell division proper lasts for only about an hour.

The interface lasts more than 95% of the duration of the cell cycle.

The cell cycle is divided into two basic phases:

• Some cells in the adult animals do not appear to exhibit division (e.g. heart cells) and many other cells divide only occasionally, as needed to replace cells that have been lost because of injury or cell death.
• These cells that do not divide further exit the G1 phase to enter an inactive stage called the quiescent stage (G0).
• Cells in this stage remain metabolically active but no longer proliferate unless called on to do so depending on the requirement of the organism.

M phase

• This is the most dramatic period of the cell cycle, involving a major reorganization of virtually all components of the cell.
• Since the number of chromosomes in the parent and progeny cells is the same, it is called equatorial division.
• Mitosis is divided into the following four stages:

Prophase

• Prophase which is the first stage of mitosis follows the S and G2 phases of interphase.
• Prophase is marked by the initiation of condensation of chromosomal material.
• The chromosome material becomes untangled during the process of chromatin condensation.
• The centrioles which had undergone duplication during the S phase of interphase, now begin to move towards opposite poles of the cell.

The completion of prophase can be marked by the following characteristics events-

• Chromosomal material condenses to form compact mitotic chromosomes. Chromosomes are seen to be composed of two chromatids attached together at the centromere.
• Initiation of the assembly of the mitotic spindle, the microtubules, the proteinaceous components of the cell cytoplasm help in the process.

Metaphase

• The complete disintegration of the nuclear envelope marks the start of the second phase of mitosis, hence the chromosomes are spread through the cytoplasm of the cell.
• By this stage, the condensation of chromosomes is completed.
• At this stage, the metaphase chromosome is made up of two sister chromatids, which are held together by the centromere.
• Small disc-shaped structures at the surface of the centromere are called kinetochores.
• These structures serve as the sites of attachment of spindle fibers to the chromosomes that are moved into position at the center of the cell.
• Hence, the metaphase is characterized by all the chromosomes coming to lie at the equator with one chromatid of each chromosome connected by its kinetochore to spindle fibers from one pole and its sister chromatid connected by its kinetochore to spindle fibers from the opposite pole.
• The plane of alignment of the chromosomes at metaphase is referred to as the metaphase plate.
• The key features of metaphase are:
• Spindle fiber attaches to the kinetochore of chromosomes.
• Chromosomes are moved to the spindle equator and get aligned along with the metaphase plate through spindle fibers to both plates.

Anaphase

• At the onset of Anaphase, each chromosome arranged at the metaphase plate is split simultaneously and the two daughter chromatids, now referred to as chromosomes of the future daughter nuclei, begin their migration towards the opposite poles.
• As each chromosome moves away from the equatorial plane, the centromere of each chromosome is towards the pole and hence at the leading edge, with the arms of the chromosomes trailing behind.
• Thus, the anaphase stage is characterized by the following key events:
• Centromere split and chromatids separated
• Chromatids move to opposite poles.

Telophase

• At the beginning of the final stage of mitosis, i.e., telophase, the chromosomes that have reached their respective poles decondensed and lose their individuality.
• The individual chromosomes can no longer be seen and chromatin material tends to collect in a mass in the two poles.
• This is the stage which shows the following events.
• Chromosomes cluster at opposite spindle poles and their identity is lost ad discrete elements.
• The nuclear envelope assembles around the chromosome cluster.
• Nucleolus, golgi complex and ER reform.