A single cell divides many times and forms a multicelled organism. Injured tissues are replaced by new cells through cell division. Thus cell division is one of the most important activities in all organisms.
Majority of cells in a multicellular organism grow and then can divide. But cells like the nerve and muscle cells of animals do not divide.
The process of cell division is almost same in all organisms. A cell passes through phases of growth after which are able to duplicate their chromosomes before they divide. These phases in the life of a cell constitute the cell cycle.
3.1 The cell cycle
You can use the term mother or parent cell for the cell that undergoes division and the daughter cells for the ones that are the result of this division. Before each daughter cell undergoes division, it must grow to the same size as its mother cell. We can distinguish two main phases in the life of a cell.
(i) Interphase – Non-dividing period (Growth phase)
(ii) M-phase – Dividing phase (M for Mitosis or Meiosis)
i. Interphase – (Inter = in between)
The interval between two successive cell divisions is termed interphase (phase at which the cell is not dividing). It is the longest period in the cell cycle. The interphase is subdivided into three main periods – G1, S and G2.
- G1 (Growth Phase 1 ) Phase i.e. First phase of growth – This is the longest phase. The first phase of interphase and the cell cycle is called G1. During G1, the cell is preparing to replicate DNA by synthesising the mRNAs and proteins required to execute the future steps. The cell usually grows larger, and some organelles are copied.
- S or synthetic Phase – It comes next. Lot of DNA is (synthesised). During the S phase, all the genetic information in the cell is copied by the process of DNA replication. This process of replication generates sister chromatids, which are identical pairs of chromosomes. These sister chromatids are attached to each other by a centromere. A centromere is a specialised sequence of DNA that links the sister chromatids and is important throughout mitosis.
- G2 (Growth Phase 2) phase – More protein is synthesised in this phase. Cytoplasmic organelles such as mitochondria, golgi bodies get duplicated. Centriole also divides into two centrioles contained in a single centrosome.
Figure 12- Cell Cycle
ii. M-phase or Dividing phase – Represented by the symbol M (Mitosis or meiosis). Mitosis occurs so that during this period the chromatids separate and form daughter chromosomes. The daughter chromosomes go to daughter nuclei and cytoplasm divides forming two identical daughter cells.
Types of cell division
There are two kinds of cell division- mitosis and meiosis.
- Mitosis : Cell division for growth and replacement wherein the two daugher cells are identical and similar to mother cell in all respects.
- Meiosis : It occurs in the gonads for sexual reproduction to produce gametes. The resultant cells, egg (in female) and sperms (in male), possess half the chromosome number of the parent cell.
- Mitosis (mitos = thread)
Mitosis is divided into 4 phases or stages termed as
(These phase refer to the changes taking place in the nucleus.)
The nucleus divides first and then the whole cell divides. Division of one nucleus to give two daughter nuclei (karyokinesis). Division of cytoplasm to give two daughter cells (cytokinesis).
a) Chromosomes are visible
b) Nucleolus and Nuclear envelop disappears.
c) Each centromere mpves to an opposite pole of the cell.
- chromosomes move, towards the equator of the cell.
- Each chromosome becomes attached to the spindle fibre by centromere.
- The sister chromatids are not yet separated.
- Centromeres divide
- Two daughter chromatids separate
- Each chromatid now contains a centromere and is now termed a chromosome.
- Half the number of now chromosomes (daugher chromatids) move toward one pole and the other half to the other pole.
- Cytokinesis begins as the cleavage furrow starts in animal cells.
- Chromosomes begin to form a chromatin network as in a nucleus.
- New nuclear membrane is formed around each daughter nucleus.
- Nucleolus becomes visible again.
It is the process of the division of cytoplasm into two. It is initiated in the beginning of telophase and is completed by the end of telophase. In an animal cell, invagination of plasma membrane proceeds from the periphery of the cell towards the interior.
Siginificance of Mitosis
- It is an equational division, and the two daughter cells are identical in all respects. They receive the same number and kind of chromosomes as were in the mother cells.
- It is the process by which growth takes place in animals by constantly adding more and more cells.
- It also plays a role in repair by growth, example in wound healing, regeneration of damaged parts and replacement of cells lost during normal wear and tear (as the surface cells of the skin or the red blood cells).
Figure 13- Mitosis
- Meiosis (meioun = make smaller, sis = action)
This division is also known as ‘reduction division’.
But why this name? This is because, in this kind of cell division the normal chromosome number of the mother cell is reduced to half in daughter cells. The normal chromosome number in human being is 46 (23 pairs), but as a result of meiosis this number is halved to 23 in daughter cells.
Where does it occur? It occurs in reproductive cells, e.g. in the testes of male and in the ovaries of female animals.
Why does it occur? In meiosis the chromosome number is reduced to half so that when doubled at fertilisation (zygote formation) during reproduction it once again becomes full or normal.
How does meiosis occur?
Meiosis is characterized by two successive divisions of the nucleus (meiosis I and II) and cytoplasm, while the chromosomes divide only once. The phases of meiotic division are given in the flow chart drawn here.
- The interphase which precedes the onset of meiosis is similar to the interphase which precedes mitosis. At S-phase, the DNA molecule of each chromosome duplicates to give two DNA molecule and hence two chromatids are found in one chromosome.
Meiosis-I and meiosis-II are continuous and have sub-stages.
Like mitosis, meiosis also consists of four stages; prophase, metaphase, anaphase and telophase.
The prophase of meiosis-I is much longer as compared to the prophase of mitosis.
It is further sub-dividied into the following five sub-stages :
- Leptotene (‘leptos’ – thin; ‘tene – thread)
The chromosomes become distinct and appear as long and thin threads due to condensation and thickening of chromosomes.
Each chromosome consist of two chromatids held together by a centromere but these are not easily visible.
- Zygotene (‘Zygos’-pairing)
Similar or homologus chromosomes start pairing from one end. This pairing is known as synapsis.
Each pair of homologous chromosomes is called a bivalent.
- Pachytene (‘pachus’ – thick)
The chromosome become shorter and thicker due to contraction.
Each paired unit called a bivalent consists of four chromatids (hence bivalents are also known as tetrads.
Crossing-over occurs at the end of pachytene i.e. break and exchange of parts (genes) occurs between non-sister chromatid (chromatids of a homologous pair).
The point of interchange and rejoining appears X-shaped and is known as chiasma or points of crossing over.
- Diplotene (‘Diplous’-double)
The homologous chromosomes begin to separate.
The two non-sister chromatids of a homologous pair remain, attached at one or two points, the chiasmata.
It is at the chiasmata that exchange of segments of chromatids (genes) between homologous chromosomes has taken place. The proces of gene exchange is known as genetic recombination.
- Diakinesis (dia = through, in different directions, kinesis = motion)
The homologous chromosomes of a bivalent move apart from each other.
Nuclear membrane and nucleolus disappear.
Spindle formation is completed.
- The bivalents arrange themselves at the equator.
- The spindle fibres are attached at the centromere of the chromosomes.
- The spindle fibres shorten.
- The centromeres of homologous chromosomes are pulled along by the spindle fibres towards the opposite poles (no division of centromere).
- Thus, half of the chromosome (each with two chromatids) of the parent cell go to one pole and the remaining half to the opposite pole.
- Each set of chromosomes that moves to one pole consists of a mixture of paternal and maternal chromosome parts (new gene combination).
- The separated chromosomes form nuclei.
- The daughter nuclei have half the number of the parent nucleus. The full set of chromosomes of a cell has paired chromosomes or a diploid set (2n).
- The daughter cells are now called haploid (n) or having 1 set of chromosomes.
- The nucleous reappears and nuclear membrane forms.
- The daughter nuclei begin the second meiotic division.,
B) Second Meiotic Division has the same four stages;
(i) Prophase II (ii) Metaphase II (iii) Anaphase II (iv) Telophase II
i. Prophase II
- The chromosomes shorten and reappear. The two chromatids are attached to the single centromere.
- Formation of spindle starts.
- Nucleolus and nuclear membrane begin to disappear.
ii. Metaphase II
- The chromosomes arrange themselves along the equator.
- Formation of spindle apparatus is completed.
- The centromere of each chromosome is attached to the spindle fibre.
iii. Anaphase II
- The centromere in each chromosome divides.
- The chromatids get their centromore and become daughter chromosomes and begin to move towards the opposite poles.
iv Telophase II
- On reaching the poles the chromo-somes organize themselves into haploid daughter nuclei.
- The nucleolus and the nuclear membrane reappear.
- This may occur in two successive stages, once after meiosis I and then after meiosis II, or in some instances it occurs only after meiosis II.
- Meiosis results in four haploid cells.
Significance of Meiosis
- It helps to maintain constant number of chromosomes in a species undergoing sexual reproduction.
- Meiosis occurs during gamete formation (gemetogenesis) and reduces the number of chromosomes from diploid (2n) to haploid (n) in the gametes. These haploid gametes fuse to form diploid zygote during fertilization. The diploid zygote develops into a normal diploid individual.
- Meiosis establishes new combination of characters due to (i) mixing of paternal and maternal chromosomes and (ii) crossing over during prophase I. As a result the progeny inherits the traits of both mother and the father in new gene combinations.
Figure 14- Meiosis I & Meiosis II
Comparision of Mitosis and Meiosis