Phases of the Cell Cycle

Phases of Cell Cycle

Cell division cycle or the cell cycle is a 4-stage process in a somatic cell during which two significant molecular processes occur – parent chromosome duplication (occurring in S phase) and equal detachment of the chromosome to the daughter cells (occurring during M phase)

In eukaryotic cells, the cell cycle phases are split into two significant phases – interphase and the mitotic phase. While in interphase, the cell significantly grows and replicates a DNA copy, in the mitotic phase or the M phase, the cell splits its DNA into two sets and hence the division of the cytoplasm to form two daughter cells.

Interphase

Interphase is the time lapse between two successive M phases of cell division. The cell prepares for division, grows and DNA replication takes place. Interphase is further divided into three phases: G₁, S, G₂

G1 (Gap 1) Phase

This is the primary stage of the interphase, known as the G1 or first gap phase as diminutive changes are observed due to the hyperactivity of the cell at the biochemical degree

This phase is characterized by changes in the chromosomes from the condensed to the extended state in addition to a range of metabolic activities leading to the initiation of replication of DNA.

Characteristics of the chromatin fibres in this phase are – less coiled and slender, extended fully and ready for transcription. The process of transcription results in the production of RNAs and also a sequence of protein molecules vital for DNA replication to be initiated

G1 phase is lengthier than the other three phases and varies from cell to cell

This is a significant phase as cell grows and assembles building blocks of chromosomal DNA and the linked proteins. In addition, it also reserves adequate energy to accomplish chromosome replication

DNA synthesis in this phase is initiated at a distinct checkpoint. The cell progresses towards division once all the biochemical events at this particular point have concluded.

Synthesis (S) phase

It is an active DNA synthesis and histone synthesis phase of the interphase

Here the chromosomes replicate, enabled by linked proteins and DNA replication. Most of the histone protein synthesis occurs in this phase, though some of it occurs in G1 phase

Identical pair of DNA molecules are formed as the process of DNA replication is discontinuous and semi-conservative

Even after the chromosomes have doubled, the sister chromatids are securely attached to the centromeric region. The chromosome count of the cell remains the same

Centrosomes of animal cells at the centre of each animal cell is linked with centrioles positioned perpendicular to each other. The centrioles are functional in organizing the cell division process

During this phase, the centrosome is duplicated, producing the mitotic spindle, the apparatus which liaises chromosomal movement while mitosis is taking place

G2 (Gap 2) Phase

This phase is succeeded by the S phase. Here the chromosomes comprise two chromatids thus cell has double the quantity of DNA

Here, the cell restores its energy, producing proteins essential for chromosomes to manipulate

Few of the cell organelles are replicated. Cytoskeleton dismantles to render resources for mitosis

Additional growth of cell may be observed. Before the cell enters the first phase of mitosis, the concluding preparations of the mitotic phase must be done.

M (Mitotic) Phase

This phase is succeeded by the G2 phase. Here the cell divides into two daughter cells along with equal distribution of chromosomes between the daughter cells. Once the M phase steps into the G1 phase, the next cell cycle is initiated to be repeated. Some cells, however, do not enter into the G1 phase. These are referred to as G0 cells

It comprisesq  the following sub-phases –

Prophase – in this stage, the nucleus disappears, spindle fibres are formed, DNA condenses into sister chromatids

Metaphase – the sister chromatids orient alongside the cell-equator by linking their centromeres to the spindle fibres

Anaphase – separation of sister chromatids at the centromere, being pulled towards the opposite poles of the cell by mitotic spindle

Telophase – At the opposite poles, the chromosomes arrive to unwind into fine DNA strands. Spindle fibres vanish. Nuclear membrane resurfaces

Cytokinesis – cell membrane splits, animal cells drift away. Plant cells form a cell plate which turns into a new cell wall

Cells arriving at the G0 phase, which is the inactive phase once they exit the cell cycle when they are not preparing actively to divide. Few of these cells tend to remain in this stage permanently.