Cell Division Questions
1. What is mitosis? What is the importance of mitosis?
Mitosis is the process in which one eukaryotic cell divides into two cells identical to the parent cell (generally identical, since alterations in genetic material can occur, more or less organelles may be distributed between the daughter cells, etc.)
Mitosis is fundamental for asexual reproduction of eukaryotes, for the embryonic development, for the growth of pluricellular beings and for tissue renewal.
2. Why in some cases is mitosis a synonym of reproduction?
In some living beings asexual reproduction occurs by many means: binary division, schizogony, budding, grafting, etc. In asexual reproduction of eukaryotes mitosis is the mechanism by which the constituent cells of the new beings are made.
The term mitosis does not apply to prokaryotes since it involves nuclear division and eukaryotic structures.
3. What is the importance of mitosis for the embryonic development?
Every embryo grows from a single cell that suffers mitosis and generates other cells that also divide themselves by mitosis forming tissues and complete organs. The perfect regulation and control of each of those cell divisions are fundamental for the creation of a normal individual. Without mitosis the embryonic development would be impossible.
4. What are some examples of organs and tissues where mitosis is more frequent, less frequent or practically absent?
Generally in vertebrates mitosis is more frequent in tissues that require intense renewing due to their functions, like epithelial tissues and the bone marrow. In plants the meristem tissue has numerous cells undergoing mitosis.
Mitosis take place with low frequency in tissues of slow renovation, like the bones in adults and the connective tissues.
In some adult tissues mitosis is almost absent, like the nervous tissue and the striated muscle tissue (skeletal and cardiac). The nervous tissue develops from stimulus by development of new electrical networks between cells and the striated muscle tissue grows by cellular hypertrophy.
5. How does mitosis participate in the growth of pluricellular organisms?
All pluricellular beings grow with the increase in quantity of their cells. This increase is produced by mitosis (although some types of growth occur by cellular hypertrophy or by deposition of substances in interstitial spaces).
6. What is the uncontrolled mitotic process that occurs as disease in pluricellular beings called?
Uncontrolled mitotic cell division is called neoplasia. Neoplasia (the formation of new strange tissues) occurs when a cell suffers mutation in its genetic material, loses the ability to control its own division and the failure is transmitted to its descendants.
Cancers are malignant neoplasias. The term malignant means that neoplastic cells can disseminate to distant sites invading other organs and tissues. Neoplasias whose cells cannot disseminate to distant sites are called benign neoplasias.
7. Is the internal epithelium of the bowel the same as it was one month ago?
The internal epithelial covering of the intestine acts as protective barrier and also as means of nutrient absorption. The traffic of ingested material inside the intestinal lumen is very intense and the consequent tissue damage requires incessant epithelial renovation through cell division. The tissue renovation is completed in two to three days and is made by mitosis.
8. What is cellular regeneration? How is mitosis related to this process?
Some tissues are able to regenerate when injured. The liver, for example, regenerates when small pieces of hepatic tissue are removed, bones make new tissues in fracture regions, etc. Some animals, like planarias, are capable of regenerating their bodies when sectioned. In tissue regeneration cellular proliferation happens by mitosis.
9. What is cell cycle?
Cell cycle, or mitotic cycle, is the time period that begins when the cell is created and finishes when it is divided by mitosis creating two daughter cells. The cell cycle is divided into interphase and the mitotic phase.
Cell Division Review - Image Diversity: cell cycle
10. Is cell division happening during the entire cell cycle? What is interphase?
Cell division properly occurs during the mitotic phase of the cell cycle. During interphase processes that are a preparation to cell division take place, like the duplication of DNA and centrioles. Interphase is the preceding phase and the mitotic is the following phase.
Cell Division Review - Image Diversity: interphase
11. What are the three periods into which interphase is divided?
Interphase is the preceding phase to the mitotic division. It is divided into three periods, G1, S and G2 (the letter G comes from “gap”, meaning interval or breach, and the letter S comes from “synthesis”, indicating the period in which DNA replicates).
In fact, “gap” is not totally appropriate for the periods immediately before and after the DNA synthesis. The idea of “growth” would be more adequate since in those periods (G1 and G2) the cell is growing to divide later in mitosis.
12. In general which phase of the cell cycle has longer duration?
The interphase comprises approximately 4/5 of the cell cycle and the mitotic phase has quite a shorter length.
13. What are the events that mark the beginning and the end of the first interphase period? What happens within the cell in this period?
The first interphase period is the G1. It begins with the end of the preceding cell division, i.e., with the formation of the new cell and it ends with the beginning of DNA replication. In the G1 period the cell is growing.
14. What are the events that mark the beginning and the end of the second interphase period? What happens in the cell in this period?
The second interphase period is the S. It starts with the beginning of DNA replication and finishes with the end of that process. The main event in this period is the synthesis of new polynucleotide chains, each bound to each DNA chain that served as a template, i.e., the duplication of the original set of DNA molecules.
15. What are the events that mark the beginning and the end of the third interphase period? What happens in the cell in this period?
The third interphase period is the G2. It begins with the end of DNA replication and ends with the beginning of the first period of the mitotic phase. On G2 the cell is growing too and the duplication of centrioles occurs (only in cells that have these structures).
16. Does mitosis properly occur before or after the interphase? Is it a mere “point of view” issue?
Mitosis must be considered a succeeding phase after interphase since this is a preparation step to mitosis. Thus it is not merely a point of view issue.
17. Into which periods is mitosis divided?
Mitosis is divided into four periods: prophase, metaphase, anaphase and telophase.
18. What are centrioles? In which type of cell are they present?
Centrioles are tiny cylindrical structures made of nine microtubule triplets. They appear in pairs in the cell. Centrioles participate in the making of cytoskeleton and of cilia and flagella. In cell division they play a role in the formation of the aster fibers.
Centrioles are structures present in animal cells, in most protists and in some primitive fungi. There are no centrioles in cells of superior plants and in general it is considered that plant cells do not have centrioles (although this is not entirely correct since some plants have centriole-containing cells).
The region where the centrioles are located is called the centrosome of the cell.
19. What are the main events of the first mitotic period?
The first mitotic period is prophase. During prophase the following events occur: migration of each centriole pair (centrioles were duplicated in interphase) to opposite cell poles; aster formation around the centriole pairs; formation of the spindle fibers between the two centriole pairs; end of chromosome condensation; disintegration of the nucleolus; breaking of the karyotheca; dispersion of condensed chromosomes in the cytoplasm; binding of chromosomes to the spindle fibers.
Cell Division Review - Image Diversity: mitosis prophase
20. What is the mitotic apparatus?
Mitotic apparatus is the set of aster fibers, radial structures around each centriole pair, plus the spindle fibers, fibers that extend across the cell between the two centriole pairs located in opposite cell poles. The mitotic apparatus appears in prophase and has important role in the orientation and gripping of chromosomes and other cellular elements causing them to separate and migrate to opposite cell poles.
Substances that disallow the formation of the mitotic apparatus, like colchicine, a molecule that binds to tubulin molecules and prevents the synthesis of microtubules, interrupt cell division. Colchicine is used to study chromosomes since it paralyzes mitosis when chromosomes are condensed and so are more easily viewed under the microscope.
21. What are the main events of the second mitotic period?
The second mitotic period is metaphase. In metaphase the following events occur: condensed chromosomes bind (in their centromere region) to the spindle fibers and get concentrated in the middle of the cell; the formation of the mitotic apparatus is completed. Metaphase ends with the breaking of the binding of identical chromatids and then anaphase begins.
Cell Division Review - Image Diversity: mitosis metaphase
22. What are the main events of the third mitotic period?
The third mitotic period is anaphase. In anaphase the following events occur: duplication and breaking of centromeres with separation of identical chromatids; traction (by the spindle fibers) of identical chromatids each to opposite cell poles; beginning of chromosome decondensation.
Cell Division Review - Image Diversity: mitosis anaphase
23. During mitotic anaphase is there separation of homologous chromosomes or separation of identical chromatids?
In the anaphase of mitosis the identical chromatids separate and complete pairs of homologous chromosomes continue to exist in each daughter cell. The separation of the homologous chromosomes occurs in the anaphase of the cell division by meiosis.
24. What are the main events of the final mitotic period?
The final mitotic phase is telophase. In telophase the following events occur: decondensation of chromosomes, each set located in opposite cell poles; karyotecha formation around each set of chromosomes forming two nuclei; destruction of the mitotic apparatus; reappearing of the nucleoli; beginning of cytokinesis (the division of cytoplasm to ultimately separate the new cells).
Cell Division Review - Image Diversity: mitosis telophase
25. What is the name of the cytoplasm division in the end of mitosis? What are the differences in this process between animal and plant cells?
Cytoplasm division occurs after telophase and it is called cytokinesis. In animal cells an invagination of the plasma membrane toward the cell center appears in the equator of the parent cell and then the cell is strangulated in that region and divided into two daughter cells. This type of division is called centripetal cytokinesis (from outside).
In plant cells the cytokinesis is not centripetal since the division happens from the inside. Membranous sacs full of pectin concentrate in the internal central region of the cell and propagate to the periphery toward the plasma membrane. The pectin-containing sacs fuse themselves and form a central structure called phragmoplast. On the phragmoplast cellulose deposition occurs and a true cell wall is created to separate the daughter cells. Plant cells thus present centrifugal cytokinesis.
The phragmoplast has “failures”, or pores, to permit cytoplasmic communications between the daughter cells. These openings are called plasmodesms.
Cell Division Review - Image Diversity: cytokinesis
26. Why is it important for chromosomes to be condensed during mitosis and decondensed during interphase?
During mitosis the main problem to be solved is the correct separation of chromosome sets between daughter cells. If chromosomes were decondensed long tiny fibers of DNA would be dispersed in cytoplasm after the karyotheca breaking and chromosomes could not be easily organized and pulled by the spindle fibers.
During interphase the function of chromosomes, i.e, of DNA molecules, is the synthesis of RNA and thus of proteins. For this task it is necessary for functional molecular regions to be decondensed (these regions form the euchromatin). During interphase in addition DNA replication occurs as a preparatory step for cell division. In this process it is fundamental for the exposition of DNA molecules to serve as templates to new DNA chains under production.
27. How does the quantity of genetic material vary within the cell during the sequential phases of the cell cycle?
The first period of the first phase (interphase) of the cell cycle is the G1, followed by S and G2 and then by the mitotic phase.
In G1 the ploidy (the quantity of DNA molecules in the cell) can be represented by the formula 2n (n is the number of DNA molecules in a gamete cell of a given species). In S DNA duplicates and the quantity of genetic material increases from 2n to 4n. In G2 that quantity is constant: 4n. After the mitotic phase the quantity of genetic material decreases to 2n in each daughter cell.
28. What are the differences between astral and anastral mitosis?
Astral mitosis is that in which there is formation of the aster, a structure made by the centrioles. Anastral mitosis is that in which there is no formation of the aster; it occurs in cells without centrioles, like plant cells (superior plants).
29. Can mitosis occur in haploid (n) cells? And in triploid cells?
The mitotic cell division can occur in haploid (n) cells, diploid (2n) cells, triploid (3n) cells, etc. Mitosis is a copying process that does not interfere with cell ploidy.
30. Concerning their final products (daughter cells and their ploidies) what are the differences between mitosis and meiosis?
In mitosis one cell, for example, with 2n chromosomes, duplicates its chromosomal set and divides generating two other cells, each with 2n chromosomes too. In meiosis, one diploid cell (2n) duplicates its chromosomes too, but four cells with n chromosomes are generated.
31. Concerning their biological function what is the difference between mitosis and meiosis?
The main biological function of mitosis is cellular multiplication, a fundamental process for the growth and development of multicellular organisms, tissue renewing, asexual reproduction, etc. The biological function of meiosis is gamete formation (in gametic meiosis) or spore formation (in sporic meiosis), i.e., the production of cells qualified for sexual reproduction with half the quantity of chromosomes compared to the original cell.
There is a special type of meiosis that happens in zygotes of some algae, protozoans and fungi. This meiosis, called zygotic meiosis, has the function of reducing to a half the number of chromosomes of adult individuals that will be formed from the zygote. In species with zygotic meiosis the adult individuals are haploid and they form gametes by mitosis. These gametes fuse in pairs with others and generate a diploid zygote that, then, undergoes meiosis to restitute the normal ploidy of adult individuals.
32. For the biological diversity is mitosis or meiosis the more important process?
Meiosis is the cell division process that allows the formation of gametes to sexual reproduction, with aleatory separation of each chromosome of the individual homologous pairs. These gametes can fecundate gametes from other individuals promoting combination of homologous chromosomes from different individuals. In that manner the chromosomal recombination provided by meiosis and sexual reproduction creates individuals with dissimilar genetic patrimony from their fathers and thus promotes biological diversity.
Some fungi species and plants, for example, present sporic meiosis, i.e., a structure where half of the chromosomes of the species is generated from meiosis. This structure, by mitosis, forms gametes. Even in this case diversity comes from meiosis. Meiosis then is the cell division process that in conjunction with genetic mutations is responsible for the biological diversity.
Even in species having zygotic meiosis the aleatory separation of homologous chromosomes in meiosis creates biological diversity.
33. What are the respective ploidies of gamete, zygote and somatic cells in a species with gametic meiosis?
Adopting as pattern an “x” quantity of chromosomes for gametes, zygotes will have 2x chromosomes and somatic cells will have 2x too.
34. Why is meiosis important for the maintenance of the normal quantity of chromosomes of a species with sexual reproduction?
A reduction to a half of the maximum normal quantity of chromosomes is mandatory in some phase of the life cycle of a species that reproduces sexually. If that could not happen in each generation, whenever a zygote is formed by fusion of gametes there would be duplication in the quantity of chromosomes in a geometric progression.
35. What is the difference between sexual spores and gametes? Do humans present sexual spores or gametes?
Sexual spores are structures generated from meiosis with ploidy (number of chromosomes) reduction to a half compared to the spore mother cell. Spores germinate and give existence to gametophytes, individuals that by mitosis form gametes. The meiosis that generates sexual spores is called sporic meiosis; it is, for example, the type of meiosis that occurs in plants.
Gametes are also cells with half the number of chromosomes of the normal cell of the species, but they are specialized in fecundation, the fusion with another gamete that generates the zygote, a cell with double the number of chromosomes than gametic cells. Gametes can appear from gametic meiosis or by mitosis in gametophytes originated from sexual spores.
In humans as well in most animals the meiosis is gametic. There are no spores nor alternation of generations. The male gamete is the sperm cell, and the female gamete is the egg cell.
36. Is the interphase of meiosis different from the interphase of mitosis?
The interphase that precedes meiosis is similar to the interphase that precedes mitosis. In them the main event is DNA replication (chromosome duplication).
37. What are the two divisions of meiosis? What are the main events that occur in those divisions?
Meiosis is divided into first meiotic division, or meiosis I, and second meiotic division, or meiosis II. During meiosis I the separation of homologous chromosomes occurs, with formation of two haploid cells. In meiosis II there is separation of identical chromatids of each of the two haploid cells created in meiosis I, giving birth to four haploid cells.
Meiosis II is a process identical to mitosis.
38. In which meiotic division does the separation of the homologous occur? What are the ploidies of the generated cells after the end of that process?
The separation of the homologous chromosomes occurs in the first division of meiosis, or meiosis I. After the end of this cell division two haploid cells are made, each having different chromosomes (with no set of homologous). Note that in the cells generated after meiosis I each chromosome is still duplicated since the homologous chromosomes and not the identical chromatids were separated.
Cell Division Review - Image Diversity: meiosis I
39. In which meiotic division does the separation of identical chromatids occur? After the end of this process what are the ploidies of the new cells?
The separation of identical chromatids occurs in the second meiotic division, or meiosis II. After this cell division (similar to mitosis and that does not alter ploidy) the cells are still haploid (they have become haploid after meiosis I).
Cell Division Review - Image Diversity: meiosis II
40. How many cells are made after meiosis I and meiosis II?
After meiosis I two cells with already separated homologous are created. After meiosis II four cells are created.
41. What are the periods of the first meiotic division?
Meiosis I is divided into prophase I, metaphase I, anaphase I and telophase I.
42. In which period of meiosis does the pairing of homologous chromosomes occur?
The pairing of homologous chromosomes is a vital step for meiosis because the rightness of the homologous separation depends on the process. This event occurs in prophase I of the cell division.
43. What is crossing over? In which period of meiosis does this event occur?
Crossing over is the eventual exchange of chromosomal fragments between homologous chromosomes. The phenomenon occurs in prophase I when homologous chromosomes are paired. Crossing over is of great importance for evolution and biodiversity since it provides recombination of alleles (of different genes) linked in the same chromosome during cell divison by meiosis.
Cell Division Review - Image Diversity: crossing over
44. What are the “chiasms” of homologous chromosomes seen in prophase I?
Chiasms are intersections of two tracts in the form of X.
The chiasms seen in prophase I are chromosome arms crossing over same arms of their homologous. In fact when chiasms are seen under the microscope chromatids are exchanging chromosomal segments with other chromatids of its homologous.
45. Is there interphase again between meiosis I and meiosis II?
There is no interphase nor DNA duplication between the divisions of meiosis. Only a short interval called diakinesis occurs.
46. What are the periods of the second meiotic division?
Meiosis II is divided into prophase II, metaphase II, anaphase II and telofase II.
47. What are the respective functions of the separation of homologous chromosomes and of the separation of identical chromatids in meiosis?
The separation of homologous chromosomes in meiosis I has two main functions: to reduce to a half the total number of chromosomes, generating haploid daughter cells at the end of the process, and to make possible genetic recombination since the separation is aleatory, i.e., each pair of daughter cells can be different from the other pair relating chromosomal combination from paternal and maternal origins. (And if crossing over is considered each of the four resulting cells can be different from the others.)
The separation of identical chromatids in meiosis II has the same function it has in mitosis: to separate the chromosomes already duplicated to the daughter cells.
48. During which meiosis division does ploidy reduction occur? Does ploidy reduction occur in mitosis?
In the cell division by meiosis ploidy reduction occurs in meiosis II. Initially, taking as example a 2n somatic cell, ploidy increases to 4n (duplication of DNA) during interphase. During meiosis I, since homologous chromosomes are separated, ploidy falls to 2n (the original number) and then during meiosis II ploidy finally falls to n in the resulting daughter cells.
Ploidy reduction does not occur in mitosis. This fact shows that, although in meiosis ploidy is decreased from its original number, in meiosis II, a process similar to mitosis, the cause of that reduction is what happens in meiosis I, i.e., the separation of the homologous chromosomes.
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