Biology
Answer
The cell cycle is a series of events that take place in a cell leading to the duplication of its DNA and the subsequent division of the cell to produce two daughter cells.
A cell cycle consists of two phases:
- A non-dividing phase called the interphase, and
- A dividing phase called the M-phase or simply mitosis.
Interphase
The two daughter cells produced from a mother cell are relatively small, with a full-sized nucleus but relatively little cytoplasm. These cells are said to be in interphase. The interphase itself has three phases —
- First growth phase (G1) — RNA and proteins are synthesised, the volume of cytoplasm increases.
- Synthesis phase (S) — More DNA is synthesised, the chromosomes are duplicated.
- Second growth phase (G2) — This is a shorter growth phase in which RNA and proteins necessary for cell division continue to be synthesised.
Mitosis
Mitosis is the cell division in which one parent cell divides into identical daughter cells. Mitosis has two phases —
- Karyokinesis — It is the division of the nucleus during cell division. It occurs in four phases:
- Prophase — Chromosomes have become short and thick and clearly visible inside the nucleus.
- Metaphase — The duplicate chromosomes arrange on the equatorial plane. Each chromosome gets attached to a spindle by its centromere.
- Anaphase — The centromere attaching the two chromatids divides and the two sister chromatids of each chromosome separate and are drawn apart towards opposite poles.
- Telophase — Spindle apparatus disappears. Chromosome become thinner and turns into a network of chromatin threads.
- Cytokinesis — At the end of telophase, a furrow appears in the cell membrane in the middle, which deepens and finally splits the cytoplasm into two, thus producing two new cells.
Related Questions
Given below is a diagram representing a stage during mitotic cell division in an animal cell. Examine it carefully and answer the questions which follow.
(a) Identify the stage. Give one reason in support of your answer.
(b) Name the cell organelle that forms the 'aster'.
(c) Name the parts labelled 1, 2 and 3.
(d) Name the stage that follows the one shown here. How is that stage identified?
(e) Mention two points of difference between mitosis and meiosis with regard to:
- The number of daughter cells produced.
- The chromosome number in the daughter cells.
Given below are three diagrammatic sketches (A, B and C) of one and the same particular phase during mitotic type of cell division.
(a) Identify the phase.
(b) What is the diploid number of chromosomes shown in them?
(c) Identify whether A, B, C are animal or plant cells? Give reasons.
Shown below are four stages (A, B, C, D) (not in sequence) of a certain kind of cell division.
(a) Is it a plant cell or an animal cell? Give two reasons.
(b) Is it undergoing mitosis or meiosis?
(c) What should be the correct sequence of these four stages among themselves?
(d) Name the stage that should precede the earliest of these stages.
(e) Draw the stage named above inside the blank space provided.
Cell division is a part of a long series of events called the cell cycle. The cell cycle consists of two main phases — interphase and the division phase. There are two types of cell divisions which occur in our body for growth, repair replacement and gamete formation. Mitosis and meiosis mainly differ from each other with respect to the number of daughter cells produced, the number of chromosomes in the daughter cells and the purpose of division, etc.
(i) Both kinds of cell divisions start after the completion of interphase which includes G-1 phase, S-phase and G-2 phase. Interphase is the preparatory phase. Choose the correct statement from the options given below :
- Mitosis occurs in each and every cell of the human body.
- Chromatids split into two homologous chromosomes during prophase.
- Chromatin fibres become distinct and develop into chromosomes during prophase.
- RNA and histamines are the components of chromatin.
(ii) Interphase is the longest part of the cell cycle which lies between two successive cell divisions. Choose the incorrect statement from the options given below:
- Interphase is followed by karyokinesis and then cytokinesis at last.
- Synthesis of RNAs and proteins occurs actively in the first growth phase (G-1 phase).
- Interphase also occurs between equational and reductional division phases of meiosis.
- Replication or duplication of DNA occurs in the synthesis phase (S-phase).
(iii) Daughter cells formed from one parent cell after the two phases of meiosis (equational and reductional) are:
- Identical and diploid
- Four daughter cells are produced with haploid chromosome number.
- Two daughter cells are produced with diploid chromosome number.
- Both (a) and (c)