When a single cell divides into two or more cells, we say that cell division has occurred. The single cell that divides itself is known as a parent cell or parental cell. The resulting cells from the parent cell are known as daughter cells. A cell cycle can be defined as the sequence of events in which cell synthesizes the other constituents of cells by duplicating themselves and eventually dividing themselves into daughter cells.
Phases in Cell Cycle
Interphase
- This phase is between the two successive phases of the M-phase.
- In this phase, the cell prepares themselves for cell division.
- Cells replicate their genetic material.
- Cells duplicate their organelles.
It includes 4 phases:
- G1 phase: Phase in which most organelles duplicate themselves. This phase is the interval between the mitosis and initiation of DNA.
- G0 phase: Phase in which cells remain metabolically active but the cell doesn’t replicate themselves until they are asked to do so.
- S phase: Phase in which replication or synthesis of DNA occurs.
- G2 phase: Phase in which chloroplast, Golgi bodies, and mitochondria duplicate. This phase is the interval between the S phase and mitosis.
M-phase or Mitosis
- Phase in which actual cell division occurs.
- In this phase, identical daughters are formed by cell division of their same constituents.
It includes 2 processes:
- Karyokinesis – In this nucleus division occurs.
- Cytokinesis – In this cytoplasm division occurs.
Meiosis
This diploid cell forms 4 haploid daughter cells, each having only half the number of chromosomes.
It takes place in 2 divisions:
- Meiosis – I: In this chromosome number is reduced to half that is why this division is known as reductional division.
- Meiosis – II: In this chromosome numbers remain the same till the end of the division that is why this division is known as equational division.
Meiosis I
Prophase I: It is further divided into five stages that are –
- Leptotene: This word is made up of two words that are “leptos” and “tene” and their meanings are “thin” and “thread”. In this stage, you will observe that a thin thread of chromatin fibers will appear and they will get condensed by dehydration. Here the nuclear membrane remains undamaged and chromosomes appear thread-like and are made up of two sister chromatids.
- Zygotene: During this chromosomes will get paired then this whole process of association is known as synapsis. Now there will be the formation of bivalent (pair of synapsed chromosomes form a complex called bivalent).
- Pachytene: Crossing of two non-sister chromatids of homologous chromosomes takes place in this stage. While this crossing there will be the exchange of genetic material, this is an enzyme-mediated process and the enzyme involved in this process is known as recombinase. By the end of this stage, the recombination of chromosomes will be completed.
- Diplotene: In this stage nuclei and the nuclear membrane will start disappearing. Homologous chromosomes will separate in this stage. Chiasmata(X-shaped structure) will be formed.
- Diakinesis : chiasmata will terminate.
Metaphase- I
- Pairs of chromosomes align themselves on the equator of the cell.
- The spindle fibers at each pole of the cell are attached to each chromosome of each pair.
- Centrioles are now at the opposite poles of the cell.
Anaphase- I
- Pair of chromosomes will be apparated by the spindle fibers.
- Now one chromosome will be at one pole of the cell and another chromosome will look at the opposite pole.
- In this stage, sister chromatids will stay together.
Telophase- I
- Chromosomes have completed their whole moment and reach the poles of the cell.
- Around each set of chromosomes, there will be a formation of a membrane to create two nuclei.
- The nuclear membrane will reappear.
- Nucleolus will reappear.
Cytokinesis: Each parent cell will now divide itself to form haploid daughter cells.
Interkinesis
- The stage between the telophase-I and prophase-II.
- No DNA replication will take place.
- Chromosomes will elongate in this stage
Meiosis II
Prophase- II
- Two daughter cells are formed will have condensed chromosomes that are visible in an X-shaped structure which can be seen under a microscope.
- Duplication of centrioles takes place.
- Spindle fibers will be formed again
Metaphase- II
- Pairs of chromosomes align themselves on the equator of the cell.
- Centrioles are now at the opposite poles of the cell.
- The spindle fibers at each pole of the cell are attached to each chromosome of each pair.
Anaphase- II
- The pair of chromatids are now pulled to opposite poles of the cell due to the action of the spindle.
- Now, these chromatids will become individual chromosomes.
Telophase- II
- Chromatids will reach to pole and decondense themselves.
- The nuclear membrane will reappear.
- Nucleolus will reappear.
- Nuclei of four haploid cells are formed.
Cytokinesis
Each daughter cell at meiosis-I will divide itself to form new daughter cells. Basically, two haploid daughter cells will convert themselves into four haploid cells.
Frequently Asked Questions
Question 1: During which phase of interphase does the synthesis of DNA occur?
Answer:
In the Synthetic phase or S-phase of the interphase synthesis of DNA occurs.
Question 2: Name the phases of the cell cycle?
Answer:
Phases of the cell cycle :
- Interphase and
- M-phase or Mitosis.
Question 3: What are reductional and equational divisions?
Answer:
In cell division If the chromosome number is reduced to half then this division is known as reductional division. In cell division If the chromosome number remains the same till the end of the division then this division is known as equational division.
Question 4: Name and define the stages of Prophase-I in Meiosis?
Answer:
Stages of Prophase-I in Meiosis:
- Leptotene: This word is made up of two words that are “leptos” and “tene” and their meanings are “thin” and “thread”. In this stage, you will observe that a thin thread of chromatin fibers will appear and they will get condensed by dehydration. Here the nuclear membrane remains undamaged and chromosomes will appear thread-like and made up of two sister chromatids.
- Zygotene: During this chromosomes will get paired then this whole process of association is known as synapsis. Now there will be the formation of bivalent (pair of synapsed chromosomes form a complex called bivalent).
- Pachytene: Crossing of two non-sister chromatids of homologous chromosomes takes place in this stage. While this crossing there will be the exchange of genetic material, this is an enzyme-mediated process and the enzyme involved in this process is known as recombinase. By the end of this stage, the recombination of chromosomes will be completed.
- Diplotene: In this stage nuclei and the nuclear membrane will start disappearing. Homologous chromosomes will separate in this stage. Chiasmata (X-shaped structure) will be formed.
- Diakinesis : chiasmata will terminate.
Question 5: What is the Interkinesis stage?
Answer:
Interkinesis stage –
- This is the stage between the telophase-I and prophase-II.
- No DNA replication will take place in this stage.
- Chromosomes will elongate in this stage.