Write briefly the significance of mitosis and meiosis in multicellular organism.
Significance of mitosis is multicellular organisms are as follows:
(i)Growth. Somatic cells are formed by mitosis. Therefore, mitosis is essential for growth and development of a multicellular organism. Plants are able to grow throughout their life due to mitotic divisions in their apical and lateral meristems.
(ii)Nucleocytoplasmic’ Ratio : An efficient cell has a high nucleocytoplasmic ratio. Increase in size lowers the ratio. It is brought back to efficient level through division.
(iii)Maintenance of Chromosome Number : Mitosis involves replication and equitable distribution of all the chromosomes so that all the cells of a multicellular organism have the same number and type of chromosomes. This helps in proper co-ordination among different cells.
(iv)Regeneration : Mitosis keeps all the somatic cells of an organism genetically similar, resembling the fertilised egg. They, therefore, are able to regenerate part or whole of the organism.
(v) Repair and Healing: It is a mechanism for replacing old or worn out cells. An injury or wound is healed by repeated mitotic divisions of the surrounding healthy cells.
(vi)Opportunity for differentiation: Mitosis produces multicellular condition. It provides opportunity for differentiation.
Significance of meiosis in multicellular organism are as follows:
(i) Formation of gametes: Meiosis forms gametes that are essential for sexual reproduction.
(ii)Maintenance of chromosome number :
Meiosis maintains the fixed number of chromosomes in sexually reproducing organisms by halving the same. It is essential since the chromosome number becomes double after fertilisation.
(iii)Assortment of Chromosomes : In meiosis paternal and maternal chromosomes assort independently. It causes reshuffling of chromosomes and the traits controlled by them. The variations help the breeders in improving the races of useful plants and animals.
(iv)Crossing over : It introduces new combination of traits or variation.
Name a cell that is found arrested in diplotene stage for months and years. Comment in 2-3 lines how it completes cell cycle.
Given that the average duplication time of E.coli is 20 minutes, how much time will two E.coli cells take to become 32 cells?
A cell has 32 chromosomes. It undergoes mitotic division. What will be the chromosome number (n) during metaphase? What would be the DNA content (C) during anaphase?
Two key events take place during S phase in animal cells, DNA replication and duplication of centriole. In which parts of the cell do events occur?
lt is said that the one cycle of cell division in human cells (eukaryotic cells) takes 24 hours. Which phase of the cycle, do you think occupies the maximum part of cell cycle?
Mitosis results in producing two cells which are similar to each other. What would be the consequence if each of the following irregularities occur during mitosis?
(a) Nuclear membrane fails to disintegrate
(b) Duplication of DNA does not occur
(c) Centromeres do not divide
(d) Cytokinesis does not occur
In which phase of meiosis are the following formed? Choose the answers from hint points given below.
(a) Synaptonemal complex________
(b) Recombination nodule________
(c) Appearance/activation of________
enzyme recombinase
(d) Termination of chiasmata________
(e) Interkinesis________
(f) Formation of dyad of cells________
Hints: (1) Zygotene, (2) Pachytene, (3) Pachytene, (4) Diakinesis, (5) After Telophase-l/before Meiosis-ll, (6) Telophase-I /After Meiosis-I.
Mitochondria and plastids have their own DNA (genetic material). What is known about their fate during nuclear division like mitosis?
The following events occur during the various phases of the cell cycle, name the phase against each of the events.
(a) Disintegration of nuclear membrane________
(b) Appearance of nucleolus________
(c) Division of centromere________
(d) Replication of DNA________
lt is observed that heart cells do not exhibit cell division. Such cells do not divide further and exit______ phase to enter an inactive stage called_______of cell cycle. Fill in the blanks.
If a tissue has at a given time 1024 cells, how many cycles of mitosis had the original parental single cell undergone?
The diagram shows a bivalent at prophase-l of meiosis. Which of the four chromatids can crossover?
Both unicellular and multicellular organisms undergo mitosis. What are the differences, if any, observed in the process between the two?
While examining the mitotic stage in a tissue, one finds some cells withl 6 chromosomes and some with 32 chromosomes. What possible reasons could you assign to this difference in chromosome number. Do you think cells with 16 chromosomes could have arisen from cells with 32 chromosomes or vice versal.
An organism has two pair of chromosomes (i.e., chromosome number = 4). Diagrammatically represent the chromosomal arrangement during different phases of meiosis-ll.
Comment on the statement – Meiosis enables the conservation of specific chromosome number of each species even though the process per se, results in reduction of chromo-some number.
An anther has 1200 pollen grains. How many pollen mother cells must have been there to produce them?
What are the various stages of meiotic prophase-l? Enumerate the chromosomal events during each stage.
