What is Cell Cycle? – Definition
It is the entire sequence of events happening right from the end of one nuclear division to the start of the next, simply, it is the orderly program of events which happen in the lifetime of a cell.
Cell cycle is divided in four phases or stages (By Howard and Pele). G1, S, G2 and M
L.G. Lajtha (1963) proposed that additional stage G0 occurs in the cell cycle of eukaryotic cells, positioned somewhere in G1.
G0 phase, G1 phase, S phase and G2 phase are collectively called as interphase.
Interphase
G0 phase
This is a time when a cell will leave the cycle and quit dividing. This may be a temporary resting period or more permanent.
G1 phase
1st gap phase, as no DNA synthesis. 1st growth phase as synthesis of RNA, proteins, membranes occur in this phase. This phase generally takes about 30 to 50% part of time of entire cell division. Chromatin is fully extended, no more distinguishable chromosomes. Transcription of rRNA, tRNA, mRNA and protein synthesis (enzymes, regulatory proteins, tubulin and other mitotic apparatus proteins) occur in this phase. rRNA synthesis is indicated by appearance of nucleolus.
Sometimes, fast dividing cells lack this phase e.g. striated muscle cells, neurons.
S phase
Replication of DNA and synthesis of histone proteins are the major events of this phase. At the end of S phase each chromosome has two duplicate set of genes. This phase generally takes about 35 to 45% part of time of entire cell division.
G2 phase
It is second gap or second growth phase. Synthesis of RNA and proteins continues.
This phase generally takes about 10 to 20% part of time of entire cell division. This phase can be absent in division giving rise to pollen grains in higher plants. Nuclear envelop remains intact during interphase. Nucleolus greatly increased in size due to accumulation of rRNA and ribosomal proteins.
Mitosis or M phase
What is Mitosis? – Definition
W. Flemming coined the term ‘Mitosis’. Mitosis is the process by which a single cell divides and produces two daughter cells, each containing the same number of chromosomes and genetic content as that of the parent cell.
Cell growth and protein production stop at this stage in the cell cycle. Prime focus is on the complex and orderly division of parent cell into two similar daughter cells. Mitosis takes much less time period than interphase.
It consists of nuclear division (karyokinesis) and cytoplasmic division (cytokinesis). Technically, mitosis is specifically the process of chromosome division; while cytokinesis is officially the process cytoplasm division to form two cells (technically not a phase of mitosis, but is a separate process). In most cells, cytokinesis follows or occurs along with the last part of mitosis.
Preprophase
In plant cells only, prophase is preceded by a pre-prophase stage. In highly vacuolated plant cells, the nucleus has to migrate into the center of the cell before mitosis can begin. This is achieved through the formation of a phragmosome, a transverse sheet of cytoplasm that bisects the cell along the future plane of cell division. In addition to phragmosome formation, preprophase is characterized by the formation of a ring of microtubules and actin filaments (called preprophase band) underneath the plasma membrane around the equatorial plane of the future mitotic spindle and predicting the position of cell plate fusion during telophase.
The cells of higher plants (such as the flowering plants) lack centrioles. Instead, spindle microtubules aggregate on the surface of the nuclear envelope during prophase. The preprophase band disappears during nuclear envelope disassembly and spindle formation in prometaphase.
Prophase
Appearance of thin threadlike condensing chromosomes marks prophase. As a result of DNA replication in S phase, each chromosome contains two coiled filaments, called chromatids. At the onset of prophase, chromosomes become shorter and thicker. Two sister chromatids are bound together at the centromere by the cohesion complex. In early prophase, chromosomes are evenly distributed in the nucleus. Further chromosomes approach towards nuclear membrane, because of which central space of the nucleus becomes empty.
Spindle formation takes place. Mitotic spindle consists of three types of fibers namely polar fibers, kinetochore fibers and astral fibers.
- Polar fibers – Extend from two poles of the spindle, towards the equator.
- Kinetochore fibers – Attach to kinetochore and extend towards pole.
- Astral fibers – Radiating outwards from the poles towards periphery or cortex.
Nucleolus gradually disintegrates. Disappearance and degeneration of nuclear envelop marks the end of prophase.
Prometaphase
Breakdown of nuclear envelop marks the start of Prometaphase. Spindle tries to align chromosomes at metaphase plate. Here, chromosomes are violently rotated and oscillated back and forth between the spindle poles. This is because, kinetochore (one for each chromatid). Kinetochore contains some form of molecular motor. When a microtubule connects with the kinetochore, the motor activates, using energy from ATP to “crawl” up the tube toward the originating centrosome. This motor activity, along with polymerisation and depolymerisation of microtubules, provides the pulling force necessary to later separate the chromosome’s two chromatids.
When the spindle grows to sufficient length, kinetochore microtubules start searching for kinetochores to attach to. A number of nonkinetochore microtubules find and interact with corresponding nonkinetochore microtubules from the opposite centrosome to establish the mitotic spindle. Chromosomes are violently rotated and oscillated back and forth between the spindle poles as their kinetochores have captured ‘+’ ends of microtubules and being pulled by captured microtubules.
Metaphase
Chromosomes are shortest and thickest. Centromere of chromosomes occupy plane of the equator of the mitotic apparatus i.e. metaphase plate. Chromosomal arms extend in any direction. Kinetochores of sister chromatids face opposite poles. Tubulin dimers (subunits) are added to the plus end of microtubule at the kinetochore and are removed from minus end. Thus poleward flux of tubulin subunits occur, microtubules remain stationary, under tension. As for proper chromosome separation every kinetochore should be attached to a bundle of microtubules, it is thought that unattached kinetochores generate a signal to prevent premature progression to anaphase without all chromosomes being aligned. The signal creates the mitotic spindle checkpoint.
Anaphase
Starts with synchronous splitting of each chromosome into sister chromatids called daughter chromosomes. This synchronous splitting is due to the cytosolic Ca++. Ca++ containing membrane vesicles appear at spindle poles and release calcium ions to initiate anaphase.
Two steps in anaphase are
- Anaphase A (early anaphase) – Separation of the sister chromatids and their poleward movement due to shortening of microtubules. During migration centromere remains ahead.
- Anaphase B (late anaphase) – Separation of poles themselves by the elongation of polar microtubules and the microtubules being pulled farther apart.
Telophase
Telophase is a reversal of prophase and prometaphase events. End of polar migration of daughter chromosomes marks beginning of telophase. Nuclear envelop reassembles around each set of separated sister chromosomes, using fragments of the parent cell’s nuclear membrane. Mitotic apparatus except centriole disappears. High viscosity of cytoplasm decreases. Chromosomes resume their long, slender appearance. RNA synthesis restarts. Nucleolus reappears at the region called secondary constriction where rRNA genes are located.
Cytokinesis
Usually begins in anaphase and continues through telophase and into interphase. In plant cell, more rigid cell plate is usually initiated at centre and is completed toward periphery. After the cell plate is laid down, primary walls are deposited on either side. The thick secondary cell walls of cellulose may be laid down later on.
Visit Mitosis – Significance and Consequences to know both of these. This page also summarizes chromosomes and chromatids during mitosis.