EXTRA CONCEPTS: Cell Reproduction or Cell Division
1. Cell Division was first studied by Strasburger (1875) in plants, W. Flemming (1882) in animal cells and Prevost and Dumas (1824) in frog egg.
2. Any agent that stimulates cell division is called mitogen. Temperature, cytokinin,
auxin, gibberllin, insulin, steroids and mitogens.
3. The continuation of species from one generation to next is governed by two processes; syngamy (union of gametes) and division of cells (meiosis and mitosis).
4. A cell divides to have high surface area per unit of volume and high nucleocytoplasmic ratio. The smaller the size of cell, more the surface area and nucleo-cytoplasmic ratio it has.
5. Genetic continuity is due to duplication of DNA in cell division that occurs in S-phase.
6. Mitotic poisons are inhibitors of cell divisions. Azides and cyanides inhibit prophase; colchicines checks spindle formation; chalones inhibit cell division in vivo and in vitro both; ribonuclease blocks prophase; heat shocks prevent cell division and Mustard gas agglutinate all chromosomes.
7. Animal cytokinesis is centripetal and plant cytokinesis is centrifugal.
8. In fungi, spindle is formed inside nucleus (intranuclear division); nuclear membrane
remains intact; nucleus divides by furrow (karyochoriosis).
9. Endomitosis is duplication of chromosomes without division of nucleus.
10. Non-disjunction is failure of migration of chromatids at anaphase; discovered by
Bridges (1961).
11. Brachymeiosis: It is believed by some mycologists that in some ascomycetes,
fertilization takes place in single celled stage resulting in a diploid nucleus which
then undergoes free nuclear divisions followed by pairing (dikaryon formation). These dikaryons then fuse and thus become a tetraploid nucleus. This is ascus mother cell. If it has to form haploid ascospores it must now undergo two reductional & one
equational division. This is knows as brachymeiosis.
12. Acetocarmine is made by dissolving carmine dye (obtained from cochineal Coccus insect) in acetic acid. It gives purple red colour to chromosomes.
13. C-mitosis is colchicines induced mitosis. Colchicine is an alkaloid, obtained from
underground corms of autumn crocus – Colchicum autumnale. It was discovered by
Dustin (1934) and used by Blakeslee (1937) to induce polyploidy. Granosan is similar
to colchicines in action. Both inhibit spindle formation.
14. In Cyperus, one meiosis produces one pollen grain instead of four.
15. Cell doubles in size and then stops growing in G1 phase. G1 is longest, most variable phase in which maximum growth occurs. Circumstances which induce a cell to divide arise in G1 under the influence of some cytoplasmic clock. Decision for cell division also occurs here.
16. Size of nucleus increases in interphase; size of nucleolus increases in first four
substages of Prophase-I of meiosis.
17. Aristolochia (duck weed) has all types of tetrads.
18. Protein (histone) for DNA synthesis is formed in S-phase while tubulin protein requiredfor spindle is synthesized in G2.
19. All organelles (organoids) including centrioles are doubled in G2.
20. Amount of DNA doubles in S-phase.
21. Interphase is most active phase followed by prophase. This interphase takes 70-95%
of total time of cell cycle. M-phase takes very less time.
22. Amitosis is called direct or incipient cell division and is found in yeasts, protozoans,
monerans (prokaryotes), cartilage and degenerated/old tissues.
23. Mitosis forms 2 daughter cells that are morphologically and genetically similar. It
distributes chromosomes equally both quantitatively and qualitatively. Term mitosis was given by W. Flemming (1870). Meristems, cells of bone marrow, base of nails and skin are used to study mitosis.
24. Spindle is astral (amphiastral) and arises from centriole in animal cell and in plantbcell; it is anastral and arise from cytoplasmic proteins by gelation. It consists of microtubules made up of sulphur rich tubulin protein (95-97%), RNA (3-5%) and
ATPase. Spindle is seen with polarizing microscope only.
25. Prophase is of longest duration.
26. Metaphasic chromosomes are least coiled. The structure of chromosomes is best studied at metaphase while shape of chromosomes is best studied at
anaphase.
27. Anaphase is of shortest duration; centromere divides and disjunction occurs here. About 30 molecules of ATP are needed to move one chromosome from equator to
pole.
28. Telophase is reverse of prophase. Nuclear membrane reappears from ER and remnants of original nuclear membrane.
29. Meiosis was first discovered by Boveri (1892), studied by Strasburger (1883) and
Winiwarter (1990); term by Farmer and Moore (1905). It is double division in which nucleus divides twice but chromosomes only once. It is antithesis of fertilization
and havles the number of chromosomes. It maintains number of chromosomes
constant through successive generations.
30. Meiosis occurs in diploid reproductive cells (meiocytes) at the time of reproduction.
31. Anthers of unopened young flowers and testes of grasshopper are widely used to
study meiosis.
32. Zygotene is zipping or synapsis or pairing of homologous chromosomes.
33. Tetrad formation occurs at pachytene stage.
34. Diplotene is of longest duration and involves chiasmata formation very distinctly. Crossing over begins at pachytene but chiasmata becomes distinct at diplotene, hence we can say crossing over occurs at diplotene.
35. Transportation is exchange and rejoining of chromatids parts during crossing over.
36. In metaphase-I, migrating chromosomes are dyad, i.e., each chromosome has 2
chromatids.
37. Reduction in number of chromosomes occur in anaphase-I but haploidy (reduction) in terms of DNA occur during anaphase-II.
38. In Trillium, anaphase-I directly enters into metaphase-II.
39. Tetrad is a group of 4 haploid cells formed during meiosis. It can be tetrahedral,isobilateral, linear, decussate or T-shaped but tetrahedral tetrad is most common in plants.
40. Kinetochore is a proteinaceous region of the centromere in chromosome to which
spindle fibres attach.
41. Nucleoprotein complex present between synapsed chromosomes is called synaptinemal complex.
42. Karyokinesis is division of nucleus. It was first studied by Schleiden.
43. Chromatids move towards the pole at a speed of 1 µm per minute.
44. Chiasmata are the result of crossing over and first observed by Janseens (1909).
45. He La cells are human cancer cells of a patient Henrietta Lack; maintained in
tissue culture since 1953. They divide and double their number in every 24 hrs &
widely used in research.
46. During G2, a cell contains double the amount of DNA (4n) as compared to original diploid cell (2n).
47. Repair of damaged DNA also takes place in the interphase.
48. In plants, mitosis occur in meristematic tissues (shoot & root tips). Root tip is the
most preferred regions to observe mitosis.
49. Anaphase is a rapid phase lasting only 2-3 minutes. It starts abruptly. The
centromere splits into two; each chromatid is pulled slowly towards a spindle pole
(each chromatid with own centromere now becomes a separate single stranded (1
DNA) chromosome. The chromatids are moved (towards the pole they face) at a
speed of 1µm/minute. The separation of the chromatids starts at the centromeres
while the arms trail behind it. As a result, the chromosomes are pulled into V, J and
T shapes.
50. The telophase lasts for an hour or so.
51. Stimulation of mitosis: Kinetin (6-furturyl amino purine) increases the mitotic rate in meristems of Allium. At low concentration, it reduces the duration of interphase and increases the mitotic rate.
52. In human males, meiosis starts after puberty.
53. In human females, meiosis starts at the end of 3rd month of prenatal life. In the fifth month of prenatal life, the oocytes reach the diplotene stage and remain arrested at this stage for many (About 12) years, when ovulation occurs.
54. Number of meiosis required to form n number of seeds/grains = n + n/4 (for all cases except cyperus); in cyperus, it is = n + n.
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