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.

Phases of Cell Cycle

Human cells in culture provide an example of a normal eukaryotic cell cycle. Every 24 hours or so, these cells divide once. The length of the cell cycle can, however, differ from organism to organism and from cell type to cell type. For instance, yeast may complete the cell cycle in about 90 minutes.

Interphase and M Phase(Mitosis phase) are the two fundamental phases of the cell cycle.

Interphase

A cell spends the majority of its time in what is known as interphase, where it develops, duplicates its chromosomes, and gets ready to divide. The cell then exits interphase, goes through mitosis, and finishes dividing. The phases of interphase are G1 phase (cell growth), S phase (DNA synthesis), and G2 phase (cell growth). The mitotic phase, which consists of mitosis and cytokinesis and produces two daughter cells, begins after interphase. In the cell cycle, the interphase is a protracted resting phase during which , DNA is replicated, RNA is synthesised, and proteins are produced.

The transitional period between mitosis and the start of DNA replication is known as the G1 phase. The cell is metabolically active and continues to develop during the G1 phase but does not duplicate its DNA. The time when DNA synthesis or replication occurs is known as the "S" phase. The amount of DNA in each cell doubles throughout this period. DNA grows from 2C (the initial amount) to 4C (the final amount). However, the number of chromosomes does not grow; if the cell had diploid or 2n chromosomes at G1, the number of chromosomes continues to be 2n even after S phase.In animal cells, the centriole doubles in the cytoplasm and DNA replication starts in the nucleus during the S phase. Proteins are created during the G2 phase as cells continue to expand in preparation for mitosis.Thus interphase is vital in the cell cycle as it permits the cell to grow and develop into a mature cell before it is able to reproduce.

Heart cells are one type of cell that does not appear to divide in adult animals, and many other cells only sporadically divide when it is necessary to replenish cells lost to damage or cell death. These cells leave the G1 phase and enter the quiescent stage (G0), which is an inactive phase of the cell cycle. While still metabolically active, cells in this stage no longer divide unless specifically instructed to do so by the organism. Only the diploid somatic cells in animals undergo mitotic cell division. In contrast, both haploid and diploid cells in the plants can exhibit mitotic divisions.

M Phase

It is the most dramatic phase of the cell cycle and involves a significant reorganisation of almost all cell components. It is also known as equational division since both the parent and progeny cells have the same number of chromosomes. Although mitosis has been conveniently split into four stages of nuclear division, it is crucial to realise that cell division is a progressive process, and extremely obvious distinctions between different stages cannot be made. Two processes make up the M phase: cytokinesis (or cell division), in which a cell's cytoplasm splits in half to create two different daughter cells, and mitosis, in which the cell's chromosomes are divided equally between the two daughter cells.