Learn about the comparison between Mitosis and Meiosis.
Comparison # Mitosis:
1. Nuclear division and chromosome duplication:
Mitosis involves one nuclear division. It is the chief process of reproduction in many unicellular organisms. Each division ensures duplication of chromosomes.
2. Chromosome number:
As a consequence of mitosis daughter nuclei maintain the same number of chromosomes as before the division (diploid chromosome set).
3. Occurrence in the body:
Mitosis is the general mode of division of body cells. The resultant cells are similar.
4. Purpose of division:
The general purpose of mitotic division is the material increase of cell number.
5. Time of DNA synthesis:
Synthesis of DNA occurs strictly in the interphase. In addition, synthesis must occur in every interphase.
6. Stages:
A. Prophase:
Prophase is the longest phase. Chromosomes become visible and in due time each appears as paired threads of chromatids. Nuclear membrane disappears and the spindles appear. Usually there occurs no bivalent, chiasma and crossing-over though there are recorded instances of somatic crossing- over with the pairing of chromosomes. But formation of chiasma is lacking in those cases.
B. Metaphase:
Chromosomes become most distinct and take the equatorial position. Each attaches to the spindle fibre at the centromeric region.
C. Anaphase:
Centromere divides. Chromosomes are pulled lengthwise to the opposite poles and since each chromosomal material consists of two chromatids, one chromatid of each chromosome moves towards one pole of the spindle.
D. Telophase:
Chromosomes start reassuming the thread-like state by the gradual process of uncoiling. Spindles disintegrate. Daughter nuclei (and cells after cytokinesis) result.
7. Duration:
Cells pass through the interphase state until further division occurs in the same manner. Duration of the entire mitotic division ranges from 30 minutes to several hours.
Comparison # Meiosis:
1. Nuclear division and chromosome duplication:
Nuclear division occurs twice in close succession but the chromosomes are duplicated once only.
2. Chromosome number:
Meiotic division results into four daughter cells. The number of chromosomes in each of these cells are reduced to half (haploid chromosome set).
3. Occurrence in the body
Meiosis occurs in the specialised tissues like testis and ovary resulting into sperms and ova (reproductive cells) respectively in sexually reproducing organisms. The resultant cells may be dissimilar (as in the case of oogenesis).
4. Purpose of division:
Meiosis is a significant condition for sexual reproduction. The haploid set of chromosomes in gametes fuses to form the diploid zygote and thus the usual chromosome number of the species is maintained.
5. Time of DNA synthesis:
Synthesis may extend even up to early prophase (I).
Transient interphase after the first meiotic division is devoid of synthetic phase.
6. Stages:
A. Prophase (1st):
This is also the longest phase as in mitosis but is much more complicated.
The entire phase has been subdivided into the following:
(i) Leptotene or Leptonema,
(ii) Zygotene or Zygonema,
(iii) Pachytene or Pachynema,
(iv) Diplotene or Diplonema and
(v) Diakinesis.
The characteristic events are:
Chromosomes become discrete, visible and oriented in a polarised way. The homologous chromosomes become entwined. These paired chromosomes are now termed bivalents. Now each chromosome in the bivalent splits into two chromatids. This is followed by breaking and re-joining of chromatid segments resulting into exchange of genetic material between the homologous chromosomes (crossing-over).
This is accompanied by chiasma formation. Chiasma is the cytological equivalent of crossing-over. The bivalents now begin to orient themselves on the spindle. Each has two centromeres. Interchange of chromosomal material involving genetic exchange renders evolution in its dynamicity.
B. Metaphase (1st):
Centromeres get attached with spindle fibres. Bivalents orient themselves at the equator in such a way that each of the homologous chromosomes can move to the reverse pole.
C. Anaphase (1st):
Characteristic feature of meiotic anaphase (I) is that it results in the segregation of homologous chromosomes to the opposite poles.
Each of these parting members of the homologous set (with its new genetic make-up resulting from crossing-over) may eventually participate in the genetic constitution of two individuals of the next generation through fertilisation of the resultant sex cells. The phenomenon has definite importance from the point of view of evolution.
D. Telophase (1st):
Two daughter nuclei (and eventually cells) are formed in the same way or the two sets of chromosomes may enter directly into the second meiotic division.
7. Duration:
Duration of the entire meiotic division is usually much longer in comparison to that of mitosis. The second meiotic division involves two daughter nuclei or cells.
The characteristics are:
(i) Centromere splits so that at anaphase (II) the chromatids can move to the two poles.
(ii) At telophase (II) nuclear membrane forms around each mass of chromosomal materials. Four nuclei (and cells after cytokinesis) result, each containing only one set of homologous chromosomes.