Discuss the phases of cell cycle.
Cell cycle or cell division refers to the series of events that take place in a cell leading to its maturity and subsequent division. These events include duplication of its genome and synthesis of the cell organelles followed by division of the cytoplasm. Human cells exhibit a typical eukaryotic cell cycle and take around 24 hours to complete one cycle of growth and division. The duration of the cycle, however, varies from organism to organism and cell to cell. A typical eukaryotic cell cycle is divided into two main phases:-
Interphase
Also known as the resting phase of the cell cycle; interphase is the time during which the cell prepares for division by undergoing both cell growth and DNA replication. It occupies around 95% time of the overall cycle. The interphase is divided into three phases:-
G1 phase (Gap 1) – G1 phase is the phase of the cell between mitosis and initiation of replication of the genetic material of the cell. During this phase, the cell is metabolically active and continues to grow without replicating its DNA.
S phase (Synthesis) – DNA replication takes place during this phase. If the initial quantity of DNA in the cell is denoted as 2N, then after replication it becomes 4N. However the number of chromosomes does not vary, i.e. if the number of chromosomes during G1 phase was 2n, it will remain 2n at the end of the S phase. The centriole also divides into two centriole pairs in the cells that contain centriole.
G2 phase (Gap 2) –During this phase, RNA, proteins, and other macromolecules required for multiplication of cell organelles, spindle formation, and cell growth are produced as the cell prepares to go into the mitotic phase.
Some cells like cardiac cells in the adult animals do not exhibit division and some others only divide to replace those cells which have been either damaged or lost due to cell death. Such cells which do not divide further attain an inactive G0 phase also known as the quiescent phase after they exit the G1 phase. These cells remain metabolically active but do not divide unless called upon to do so.
M phase
This is the mitotic phase or the phase of the equational division as the cell undergoes a complete reorganization to give birth to a progeny that has the same number of chromosomes as the parent cell. The other organelles are also divided equally by the process of cytokinesis which is preceded by mitotic nuclear division. The mitotic phase is divided into four overlapping stages:-
Prophase, Metaphase, Anaphase, and Telophase
I. Mitosis
It is a process of cell division where chromosomes replicate and get equally distributed into two daughter cells. Hence, it is also called equational division.
The process of mitosis keeps the chromosome number equal in the daughter as well as the parental cell.
Mitosis Phases:
Prophase
It involves the initiation and condensation of chromosomes.
Nucleolus and nuclear membrane disappear.
Metaphase
Chromosomal material condenses to form compact chromosomes that get aligned in the middle of the nucleus at the equatorial plate.
Anaphase
Centromere splits and chromosomes move apart towards two opposite poles due to the shortening of spindle fibers.
Telophase
Chromosomes finally reach their respective poles.
The nuclear envelope assembles around each chromosome cluster.
Nucleolus and other organelles reform.
Karyokinesis and Cytokinesis
Karyokinesis is the division of the nucleus during mitosis or meiosis which is followed by cytokinesis. Cytokinesis involves the division of the cytoplasm of a cell.
Figure : Mitosis
II . Meiosis
It is the process that involves the reduction in the amount of genetic material.
It mainly occurs in gametes.
At the end of meiosis II, four haploid cells are formed.
It is comprised of two successive nuclear and cell divisions with a single cycle of DNA replication.
Phases-
Meiosis I
1. Prophase I – It comprises of 5 stages:
i. Leptotene
Chromosomes start condensing.
ii. Zygotene
The pairing of chromosomes called synapsis occurs.
A pair of synapsed homologous chromosomes is called bivalent or tetrad.
iii. Pachytene
Exchange of genetic material (crossing over) between non-sister chromatids occurs. Chiasmata formation.
iv. Diplotene
Bivalents formed during pachytene separate from each other (except at chiasmata) due to the dissolution of the synaptonemal complex.
v. Diakinesis
Terminalization of chiasmata can be observed.
By the end of this stage, the nucleolus disappears and the nuclear envelope breaks.
2. Metaphase I
Bivalents (tetrad) get aligned along metaphase plate through spindle fibres.
3. Anaphase I
Homologous chromosomes separate while chromatids remain attached at their centromere.
4. Telophase
Nucleolus and nuclear membrane reappear around chromosome clusters at each pole. Inter-kinesis – It is the stage between two meiotic divisions.
Meiosis II
1. Prophase II
Chromosomes become compact.
The nuclear membrane disappears.
2. Metaphase II
Chromosomes align at the equator.
Kinetochores of sister chromatids attach to spindle fibers at each pole.
3. Anaphase II
Chromatids separate by splitting of centromere.
As a result, chromatids move towards their respective poles in the cell.
4. Telophase II
Nuclear envelope and nucleolus reform around the chromosome clusters.
Cytokinesis: After meiosis II, the process of cytokinesis results in the formation of four haploid cells.
Figure : Meiosis
Cell cycle or cell division refers to the series of events that take place in a cell leading to its maturity and subsequent division. These events include duplication of its genome and synthesis of the cell organelles followed by division of the cytoplasm. Human cells exhibit a typical eukaryotic cell cycle and take around 24 hours to complete one cycle of growth and division. The duration of the cycle, however, varies from organism to organism and cell to cell. A typical eukaryotic cell cycle is divided into two main phases:-
Interphase
Also known as the resting phase of the cell cycle; interphase is the time during which the cell prepares for division by undergoing both cell growth and DNA replication. It occupies around 95% time of the overall cycle. The interphase is divided into three phases:-
G1 phase (Gap 1) – G1 phase is the phase of the cell between mitosis and initiation of replication of the genetic material of the cell. During this phase, the cell is metabolically active and continues to grow without replicating its DNA.
S phase (Synthesis) – DNA replication takes place during this phase. If the initial quantity of DNA in the cell is denoted as 2N, then after replication it becomes 4N. However the number of chromosomes does not vary, i.e. if the number of chromosomes during G1 phase was 2n, it will remain 2n at the end of the S phase. The centriole also divides into two centriole pairs in the cells that contain centriole.
G2 phase (Gap 2) –During this phase, RNA, proteins, and other macromolecules required for multiplication of cell organelles, spindle formation, and cell growth are produced as the cell prepares to go into the mitotic phase.
Some cells like cardiac cells in the adult animals do not exhibit division and some others only divide to replace those cells which have been either damaged or lost due to cell death. Such cells which do not divide further attain an inactive G0 phase also known as the quiescent phase after they exit the G1 phase. These cells remain metabolically active but do not divide unless called upon to do so.
M phase
This is the mitotic phase or the phase of the equational division as the cell undergoes a complete reorganization to give birth to a progeny that has the same number of chromosomes as the parent cell. The other organelles are also divided equally by the process of cytokinesis which is preceded by mitotic nuclear division. The mitotic phase is divided into four overlapping stages:-
Prophase, Metaphase, Anaphase, and Telophase
I. Mitosis
It is a process of cell division where chromosomes replicate and get equally distributed into two daughter cells. Hence, it is also called equational division.
The process of mitosis keeps the chromosome number equal in the daughter as well as the parental cell.
Mitosis Phases:
Prophase
It involves the initiation and condensation of chromosomes.
Nucleolus and nuclear membrane disappear.
Metaphase
Chromosomal material condenses to form compact chromosomes that get aligned in the middle of the nucleus at the equatorial plate.
Anaphase
Centromere splits and chromosomes move apart towards two opposite poles due to the shortening of spindle fibers.
Telophase
Chromosomes finally reach their respective poles.
The nuclear envelope assembles around each chromosome cluster.
Nucleolus and other organelles reform.
Karyokinesis and Cytokinesis
Karyokinesis is the division of the nucleus during mitosis or meiosis which is followed by cytokinesis. Cytokinesis involves the division of the cytoplasm of a cell.
Figure : Mitosis
II . Meiosis
It is the process that involves the reduction in the amount of genetic material.
It mainly occurs in gametes.
At the end of meiosis II, four haploid cells are formed.
It is comprised of two successive nuclear and cell divisions with a single cycle of DNA replication.
Phases-
Meiosis I
1. Prophase I – It comprises of 5 stages:
i. Leptotene
Chromosomes start condensing.
ii. Zygotene
The pairing of chromosomes called synapsis occurs.
A pair of synapsed homologous chromosomes is called bivalent or tetrad.
iii. Pachytene
Exchange of genetic material (crossing over) between non-sister chromatids occurs. Chiasmata formation.
iv. Diplotene
Bivalents formed during pachytene separate from each other (except at chiasmata) due to the dissolution of the synaptonemal complex.
v. Diakinesis
Terminalization of chiasmata can be observed.
By the end of this stage, the nucleolus disappears and the nuclear envelope breaks.
2. Metaphase I
Bivalents (tetrad) get aligned along metaphase plate through spindle fibres.
3. Anaphase I
Homologous chromosomes separate while chromatids remain attached at their centromere.
4. Telophase
Nucleolus and nuclear membrane reappear around chromosome clusters at each pole. Inter-kinesis – It is the stage between two meiotic divisions.
Meiosis II
1. Prophase II
Chromosomes become compact.
The nuclear membrane disappears.
2. Metaphase II
Chromosomes align at the equator.
Kinetochores of sister chromatids attach to spindle fibers at each pole.
3. Anaphase II
Chromatids separate by splitting of centromere.
As a result, chromatids move towards their respective poles in the cell.
4. Telophase II
Nuclear envelope and nucleolus reform around the chromosome clusters.
Cytokinesis: After meiosis II, the process of cytokinesis results in the formation of four haploid cells.
Figure : Meiosis
