The following contrasting differences reveals that telophase is reverse of prophase, in cell division
| Prophase | Telophase |
|---|---|
| I'st stage of (karyokinesis ) in cell division, Viscosity of cytoplasm increases. The indistinct and interind DNA condense to form elongated chromosomes. The chromatin disappears and chromosome fibres get shortened and thickened. Spindle fibres appears (awards the poles from the centriole connected in animals with astral rays and in plants without asters Nucleolus degenerate completely. Cell organelles such as ER, Golgi complex disorganise, and difference between cytoplasm and nucleoplasm disappears.  |
Last stage of karyokinesis in cell diveision. Viscodity of cytoplasm deereses. Chromosome groups reorganise themselves into nuclei. Chromosomes elongate and overlap each other to form chromatin. Spindle fibres disappear around the poles. Astral rays also disappear in plants. Nuclear envelope appears and two daughter nuclei are formed at the poles. Cell organelles, i.e., ER and Golgi complex are reformed in the cell. Nucleoplasm also appears in the chromatin area. Making it distinct from rest of cytoplasmic area.  |
Prophase-I occurs over a long duration and involves several complicated changes in meiotic cell division. It is important because genetic recombination and variation in sexually reproducing organism occurs due to the events of this phase.
Leptotene
(i) The chromatin network opens out and threads become clear.
(ii) The chromosomes are thin, slender and long.
(iii) Chromosome number is diploid.
Zygotene
(i) Corresponding chromosomes become intimately associated.
(ii) The process of pairing is known as synapse. It is so exact that pairing is not merely between corresponding chromosomes but between corresponding individual units.
(iii) The chromosomes become shorter and thicker.
Pachytene or Pachynema
(i) The synaptic chromosomes become very intimately associated.
(ii) The pair of chromosomes becomes short and thick.
(iii) Crossing over occurs at this stage. Chiasmata are clearly seen.
Diplotene
(i) Homologous chromosomes start separating from one another.
(ii) Chiasmata tend to slip out of the chromosomes. This is known as terminalisation of chiasmata.
(iii) Chromosomes start separating out but the separation is not complete.
(iv) Nuclear membrane and nucleolus start disappearing.
Diakinesis
(i) The bivalents condense further and get randomly distributed.
(ii) The separation of paired chromosomes is almost complete.
(iii) Terminalisation of chiasmata is almost complete.
(iv) Nuclear membrane and nucleolus disappear.
Mitotic cell division results into the increase in the number of cells that have same genetic composition whereas meiosis has its importance in the life cycle of sexually reproducing organisms.
| Mitosis | Meiosis |
|---|---|
| Event Prophase $\bullet$ Chromomeres are not conspicuous. $\bullet$ Prophase is of shorter duration. $\bullet$ Prophase is simpler and is hardly distinguishable into substages. $\bullet$ Each chromosome has two distinct chromatids. $\bullet$ No bouquet stage is recorded. $\bullet$ Chiasmata are absent. |
$\bullet$ Chromomeres are quite conspicuous. $\bullet$ Prophase-I is of longer duration while prophase-II is very brief. $\bullet$ Prophase-I is complicated and is divisible into five substages. Prophase-II is, however, very simple. $\bullet$ Chromosomes of prophase-I do not show distinct chromatids. $\bullet$ Chromosomes of animals and some plants show convergence towards one side during early prophase-l. It is known as bouquet stage. $\bullet$ Chiasmata or visible connections between homologous chromosomes of bivalents are observed during diplotene, diakinesis (prophase-I) and metaphase-I. |
| Metaphase $\bullet$ Centromeres produce a single metaphasic plate. $\bullet$ Chromosomes are independent and do not show connections. $\bullet$ Only the centromeres lie at the equator. The limbs of chromosomes are oriented in various direction. $\bullet$ A centromere is connected with both the spindle poles. $\bullet$ Two chromatids of a chromosome are genetically similar. |
$\bullet$ A double metaphasic plate is formed by centromeres in metaphase-I but only one in metaphase-II. $\bullet$ Homologous chromosomes are interconnected. Hence, the chromosomes occur in pairs or bivalents in metaphase-l. They are however, free in metaphase-II. $\bullet$ Limbs of the chromosomes mostly lie at the equator while the centromeres project towards the poles in metaphase-l. $\bullet$ A centromere is connected to one spindle pole in metaphase-I, but both in metaphase-II. $\bullet$ The two chromatids of a chromosome are often genetically different due to crossing over. |
| Anaphase $\bullet$ A centromere splits length-wise to form two centromeres in the beginning of anaphase. $\bullet$ Anaphasic chromosomes are single stranded. $\bullet$ Similar chromosomes move towards the opposite poles in anaphase. |
$\bullet$ Centromeres do not divide during anaphase-I but do so in anaphase-II. $\bullet$ Chromosomes are double-stranded in anaphase-l, but single stranded in anaphase-II. $\bullet$ Dissimilar chromosomes move toward the opposite poles both in anaphase-l and anaphase-II. |
| Telophase Telophase is longer and produces interphase nuclei. |
$\bullet$ Telophase-l is shorter and nuclei now enter the interphase. |
| Cytokinesis Cytokinesis follows every mitosis. It produces two new cells. |
$\bullet$ Cytokinesis often does not occur after first or reductional division. It is then simultaneous after second division resulting in four new cells. |
Write brief note on the following
(a) Synaptonemal complex
(b) Metaphase plate
(a) Synaptonemal complexes are zipper-structures which are assembled between homologous chromosomes during the prophase of the first meiosis. Their asssembly and disassembly correlate with the successive chromatin rearrangements of meiotic prophase namely the condensation, pouring, recombination and dysfunction of homologous chromosomes.
They are considered to be the structures that control the number and distribution of reciprocal exchanges between homolgous chromosomes. They also are known to convert cross over into functional chiasmata.
(b) In metaphase, the centromeres of the chromosomes assemble themselves on the metaphase plate (equatorial plate), an imaginary line that is equidistant from two centrosome poles. This even alignment is due to the opposing kinetochore microtubules. At this plate, chromosomers, especially sister chromatids are attached to the bundle of four to eight spindle fibres.
Multicellular organisms grow and develop with the involvement of mitotic cell division. Meiosis occurs in them to form gametes in their reproductive phase of life cycle.
Significance of Mitosis
(i) Multicellular plants and animals start life as single cells. The process of mitosis gives rise to many cells which differentiate to form tissues, organs and organ-systems of the organism.
(ii) It results in increase in size and growth of an organ.
(iii) Cell reproduction is used to form new cells to renew certain tissues and to replace worn out cells.
(iv) Mitosis is also involved in asexual reproduction in some organisms like in unicellular. Amoeba and multicellular Hydra as well as in vegetative reproduction in plants.
Significance of Meiosis
(i) Meiosis is the mechanism by which conservation of specific chromosome number of each species is achieved across generations in sexually reproducing organisms.
(ii) Meiosis also increases the genetic variability in the population of organisms from one generation to the next. Variations are very important for the process of evolution.