Cell Division

Complete Q&A Study Guide to Cell Division

The Importance of Mitosis

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, and a smaller or larger number of organelles may be distributed between the daughter cells, etc.)

Mitosis is fundamental for the asexual reproduction of eukaryotes, for embryonic development, for the growth of pluricellular organisms and for tissue renewal.

Cell Division - Biology Questions and Answers

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2. Why is mitosis a synonym of reproduction in some cases?

In some living organisms, asexual reproduction occurs by many means: binary division, schizogony, budding, grafting, etc. In the asexual reproduction of eukaryotes, mitosis is the mechanism by which the cells that make up the new organisms are produced.

The term mitosis does not apply to prokaryotes since, it involves nuclear division and eukaryotic structures.

3. What is the importance of mitosis in embryonic development?

Every embryo grows from a single cell that undergoes mitosis and generates other cells that also divide via mitosis, forming tissues and complete organs. The perfect regulation and control of each of those cell divisions is fundamental in the creation of a normal individual. Without mitosis, embryonic development would be impossible.

4. What are some examples of organs and tissues in which mitosis is more frequent, less frequent or practically absent?

Generally, in vertebrates, mitosis is more frequent in tissues that require frequent renewal due to their function, such as epithelial tissues and bone marrow. In plants, meristem tissue contains numerous cells undergoing mitosis.

Mitosis is less frequent in tissues that are renewed slowly, such as bones in adults and connective tissue.

In some adult tissues, mitosis is almost absent, such as in the nervous tissue and striated muscle tissue (skeletal and cardiac). Nervous tissue develops through stimulus via the development of new electrical networks between cells, and striated muscle tissue grows via cellular hypertrophy.

5. What is the role of mitosis in the growth of pluricellular organisms?

All pluricellular organisms grow as their number of cells increases. This increase is produced by mitosis (although some types of growth occur through cellular hypertrophy or through the depositing of substances in interstitial spaces). 

6. What is the name for the disease caused by uncontrolled mitosis in pluricellular organisms?

Uncontrolled mitotic cell division is called neoplasia. Neoplasia (the formation of new strange tissues) occurs when a cell undergoes a mutation in its genetic material, loses the ability to control its own division and passes on this failure 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 a person's bowels 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 flow of ingested materials inside the intestinal lumen is very intense and the resulting tissue damage requires constant epithelial renewal via cell division. This tissue renewal is completed in two to three days and carried out via 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 tissue in fracture regions, etc. Some animals, like planaria, are capable of regenerating their bodies when cut in half. In tissue regeneration, cellular proliferation occurs via mitosis.

The Cell Cycle

9. What is the cell cycle?

The cell cycle, or the mitotic cycle, is the time period that begins when the cell is created and ends when it is divided by mitosis, creating two daughter cells. The cell cycle is divided into interphase and the mitotic phase.

10. Does cell division happen during the entire cell cycle? What is interphase?

Cell division normally occurs during the mitotic phase of the cell cycle. During interphase, processes that prepare for cell division take place, such as the duplication of DNA and centrioles. Interphase is the first phase and the mitotic phase is the second phase.

  • Cell Division Review - Image Diversity: interphase

11. What are the three stages of interphase?

Interphase is the phase preceding mitotic division. It is divided into three stages: G1, S and G2 (the letter G comes from “gap”, meaning interval or breach, and the letter S comes from “synthesis”, indicating the stage in which DNA replicates).

In fact, “gap” is not completely appropriate to describe the stages immediately before and after DNA synthesis. The idea of “growth” would be more adequate, since during those stages (G1 and G2), the cell is growing to later divide via mitosis.

12. In general, which phase of the cell cycle takes more time?

Interphase makes up approximately 4/5 of the cell cycle. The mitotic phase is much shorter.

13. What events mark the beginning and the end of the first stage of interphase? What happens within the cell during this stage?

The first stage of interphase is the G1 stage. It begins with the end of the preceding cell division; that is, with the formation of the new cell. It ends with the beginning of DNA replication. During the G1 stage, the cell is growing.

14. What events mark the beginning and the end of the second stage of interphase? What happens in the cell during this stage?

The second stage of interphase is the S stage. It starts with the beginning of DNA replication and finishes with the end of that process. The main event during this period is the synthesis of new polynucleotide chains, each bound to the DNA chain that served as its template, that is, the duplication of the original set of DNA molecules.

15. What events mark the beginning and the end of the third stage of interphase? What happens in the cell during this stage?

The third stage of interphase is the G2 stage. It begins with the end of DNA replication and ends with the beginning of the first period of the mitotic phase. During G2, the cell is growing and the duplication of centrioles occurs (only in cells that have these structures).

16. Does mitosis occur before or after interphase? Is this merely an issue of “point of view”?

Mitosis must be considered subsequent to interphase, since interphase is carried out in preparation step for mitosis. Therefore, this is not merely a point of view issue. 

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Stages of Mitosis

17. What are the stages that make up mitosis?

Mitosis is divided into four stages: prophase, metaphase, anaphase and telophase. 

18. What are centrioles? In which types of cells are they found?

Centrioles are tiny cylindrical structures made of nine microtubule triplets. They appear in pairs in cells. Centrioles are involved in the production of the cytoskeleton and of cilia and flagella. In cell division, they play a role in the formation of aster fibers.

Centrioles are structures present in animal cells, in most protists and in some primitive fungi. There are no centrioles found in the cells of higher plants and, in general, plant cells are considered to not have centrioles (although this is not entirely correct, since some plants have centriole-containing cells).

The region where the centrioles are found is called the centrosome of the cell.

19. What are the main events of the first stage of mitosis?

The first stage of mitosis is called prophase. During prophase, the following events occur: the migration of each centriole pair (centrioles were duplicated in interphase) to opposite cell poles; the formation of aster around the centriole pairs; the formation of spindle fibers between the two centriole pairs; the end of chromosome condensation; the disintegration of the nucleolus; the breaking down of the karyotheca; the dispersion of condensed chromosomes in the cytoplasm; and the binding of chromosomes to spindle fibers. 

20. What is the mitotic apparatus?

The mitotic apparatus is a set of aster fibers and spindle fibres. Aster fibers are radial structures around each centriole pair. Spindle fibers are fibers that extend across the cell between the two centriole pairs located at opposite cell poles. The mitotic apparatus appears in prophase and plays an important role in the orientation and holding of chromosomes and other cellular elements, so as to cause them to separate and migrate to opposite cell poles.

Substances that prevent the formation of the mitotic apparatus, such as 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, making them easier to view under a microscope.

21. What are the main events of the second stage of mitosis?

The second stage of mitosis is called metaphase. During metaphase, the following events occur: condensed chromosomes bind (in their centromere region) to the spindle fibers and become concentrated in the middle of the cell; and the formation of the mitotic apparatus is completed. Metaphase ends with the breaking of the bond between identical chromatids. It is followed by anaphase.

22. What are the main events of the third stage of mitosis?

The third stage of mitosis is called anaphase. During anaphase, the following events occur: centromeres are duplicated and identical chromatids are separated; identical chromatids are drawn to each pole of the cell by the spindle fibers; and chromosomes begin to decondense. 

Cell Division Review - Image Diversity: mitosis anaphase

23. During mitotic anaphase, which are separated, homologous chromosomes or identical chromatids?

During the anaphase stage of mitosis, identical chromatids separate while complete pairs of homologous chromosomes continue to exist in each daughter cell. The separation of homologous chromosomes occurs during the anaphase I stage of meiosis.

24. What are the main events of the last stage of mitosis?

The last stage of mitosis is called telophase. During telophase, the following events occur: chromosomes, each set located at opposite poles of the cell, are decondensed; the karyotheca is formed around each set of chromosomes, forming two nuclei; the mitotic apparatus is destroyed; the nucleoli reappear; and cytokinesis (the division of cytoplasm to ultimately separate the new cells) begins. 

25. What is the name given to the division of the cytoplasm at the end of mitosis? How is this process different in plant and animal cells?

The division of the cytoplasm occurs after telophase and is called cytokinesis. In animal cells, an invagination of the plasma membrane toward center of the cell appears at the equator of the parent cell and then the cell is trapped in that region and divided into two daughter cells. This type of division is called centripetal cytokinesis (from outside).

In plant cells, 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, pushing themselves outwards toward the plasma membrane. The pectin-containing sacs fuse themselves and form a central structure called the phragmoplast. Cellulose is deposited on the phragmoplast and a true cell wall is created to separate the daughter cells. The process of cytokinesis in plant cells is called centrifugal cytokinesis.

The phragmoplast contains flaws, or pores, that permit cytoplasmic communication between the daughter cells. These openings are called plasmodesms.

Chromosomal condensation

26. Why is it important for chromosomes to be condensed during mitosis and decondensed during interphase?

During mitosis, the main problem 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 is broken and chromosomes could not be easily organized and moved by the spindle fibers.

During interphase, the function of chromosomes, or rather, of DNA molecules, is the synthesis of RNA and therefore proteins. For this task, it is necessary for functional molecular regions to be decondensed (these regions form the euchromatin). Furthermore, during interphase, DNA replication occurs to prepare for cell division. In this process it is necessary for DNA molecules to serve as templates for the new DNA chains being produced.

Ploidy in Mitosis

27. What is the variation in the quantity of genetic material within the cell during the phases of the cell cycle?

The first period of the first phase (interphase) of the cell cycle is G1, followed by S and G2, and finally by the mitotic phase.

During 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). During S, DNA duplicates and the quantity of genetic material increases from 2n to 4n. During G2, the quantity remains the same: 4n. After the mitotic phase, the quantity of genetic material decreases to 2n in each daughter cell.

28. Can mitosis occur in haploid (n) cells? What about in triploid cells?

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.

Astral and Anastral Mitosis

29. What are the differences between astral and anastral mitosis?

Astral mitosis is the process in which aster, a structure produced by the centrioles, is formed. Anastral mitosis is the process in which aster is not formed. It occurs in cells without centrioles, such as plant cells (higher plants).

30. What is the difference between mitosis and meiosis in terms of their final products (their daughter cells and their ploidies)?

In mitosis one cell, for example, with 2n chromosomes, duplicates its chromosomal set and divides, generating two other cells, each with 2n chromosomes. In meiosis, one diploid cell (2n) duplicates its chromosomes as well, but four cells with n chromosomes are generated.

The Importance of Meiosis

31. What is the difference between mitosis and meiosis in terms of their biological function?

The main biological function of mitosis is cellular multiplication, a process fundamental in the growth and development of multicellular organisms, tissue renewal, asexual reproduction, etc. The biological function of meiosis is gamete formation (in gametic meiosis) or spore formation (in sporic meiosis); that is, the production of cells used for sexual reproduction with half the quantity of chromosomes compared to the original cell.

There is a special type of meiosis that happens in the zygotes of some algae, protozoans and fungi. This meiosis, called zygotic meiosis, has the function of cutting in half the number of chromosomes of adult specimens that will be formed from the zygote. In species with zygotic meiosis, the adult specimens are haploid and form gametes by mitosis. These gametes fuse in pairs with others and generate a diploid zygote that undergoes meiosis to regain the normal ploidy of adult specimens.

32. Which process is more important for biological diversity, mitosis or meiosis?

Meiosis is the cell division process that allows for the formation of gametes for sexual reproduction, with the random separation of each chromosome of the individual homologous pairs. These gametes can fertilize gametes from other individuals, promoting the combination of homologous chromosomes from different individuals. In this way, the recombination of chromosomes provided by meiosis and sexual reproduction creates individuals with genetic heritage different from that of their parents, thus promoting biological diversity.

Some species of fungi and plants, for example, undergo sporic meiosis; that is, a structure in which half of the chromosomes of the species are generated from meiosis. This structure forms gametes via mitosis. Even in this case, diversity comes from meiosis. Therefore, meiosis is the cell division process that, in conjunction with genetic mutations, is responsible for biological diversity.

Even in species that undergo zygotic meiosis, the random 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?

Letting  "x” be the quantity of chromosomes present in gametes, zygotes will have 2x chromosomes and somatic cells will also have 2x.

34. Why is meiosis important for the maintenance of the normal quantity of chromosomes in a species with sexual reproduction?

For species that reproduce sexually, it is necessary at some point in the life cycle to reduce the normal quantity of chromosomes to half.  Without this happening in each generation, whenever a zygote is formed by the fusion of gametes, the quantity of chromosomes would be doubled in a geometric progression.

Spores and Gametes

35. What is the difference between sexual spores and gametes? Do humans have sexual spores or gametes?

Sexual spores are structures generated from meiosis with ploidy (the number of chromosomes) reduced to a half compared to the mother cell of the spore. Spores germinate and give birth to gametophytes, organisms that form gametes via mitosis. The meiosis that generates sexual spores is called sporic meiosis. It is the type of meiosis that occurs in plants, for example.

Gametes are also cells that contain half the number of chromosomes of the normal cell of the species, but they are specially designed for fertilization, the fusion with another gamete to generates a zygote, a cell with double the number of chromosomes of gamete cells. Gametes can be produced through gametic meiosis or by mitosis in gametophytes originated from sexual spores.

The process used in humans as well in most animals is gametic meiosis. There are no spores or alternation of generations. The male gamete is the sperm cell, and the female gamete is the egg cell.

The Phases of Meiosis

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. During them, the main event is DNA replication (chromosome duplication).

37. What are the two phases of meiosis? What are the main events that occur during those phases?

Meiosis is divided into the first meiotic division, or meiosis I, and the second meiotic division, or meiosis II. During meiosis I, the separation of homologous chromosomes occurs, resulting in the formation of two haploid cells. In meiosis II, the separation of the identical chromatids of each of the two haploid cells created in meiosis I occurs, giving birth to four haploid cells.

The process of meiosis II is identical to mitosis. 

38. During which phase of meiosis does the separation of homologous occur? What are the ploidies of the cells generated after the end of that process?

The separation of homologous chromosomes occurs during  the first phase of meiosis, or meiosis I. After the end of this cell division, two haploid cells are produced, each of which have different chromosomes (with no set of homologous chromosomes). 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. During which phase of meiosis 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 during the second phase of meiosis, or meiosis II. After this cell division (similar to mitosis, and which does not alter ploidy), the cells are still haploid (they became haploid after meiosis I). 

  • Cell Division Review - Image Diversity: meiosis II

40. How many cells are produced following meiosis I and meiosis II?

Following meiosis I, two cells with separated homologous chromosomes are created. After meiosis II, four cells are created.

41. What are the stages of the first phase of meiosis?

Meiosis I is divided into prophase I, metaphase I, anaphase I and telophase I.

Crossing Over

42. During which phase of meiosis does the pairing of homologous chromosomes occur?

The pairing of homologous chromosomes is a vital step in meiosis because the correctness of the homologous separation depends on this process. This event occurs during prophase I, therefore, during meiosis I.

43. What is crossing over? During which phase of meiosis does this event occur?

Crossing over is the casual exchange of chromosomal fragments between homologous chromosomes. This phenomenon occurs during prophase I (meiosis I), when homologous chromosomes are paired. Crossing over is of great importance for evolution and biodiversity since it provides the recombination of alleles (different genes) linked in the same chromosome during cell division by meiosis.

44. What are the “chiasms” of homologous chromosomes found in prophase I?

Chiasms are intersections between two tracts in the form of an X.

The chiasms seen in prophase I are chromosome arms crossing over the same arms of their homologous chromosome. When chiasms are seen under a microscope, chromatids can be seen exchanging chromosomal segments with the chromatids of their homologous chromosome.

The Second Meiotic Division

45. Does interphase occur once again between meiosis I and meiosis II?

There is no interphase or DNA duplication between the phases of meiosis. Only a short stage called diakinesis occurs. 

46. What are the stages of the second phase of meiosis?

Meiosis II is divided into prophase II, metaphase II, anaphase II and telophase II.

47. What are the respective functions of the separation of homologous chromosomes and of the separation of identical chromatids during 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 the recombination of genetic material possible, since the separation is random, that is, each pair of daughter cells can be different from the other pair, carrying different combinations of chromosomes from its progenitors. (Furthermore, if crossing over is considered, each of the four resulting cells can be different from the others.)

The separation of identical chromatids during meiosis II has the same function as it has in mitosis: to separate the chromosomes already duplicated into the daughter cells.

48. During which phase of meiosis does ploidy reduction occur? Does ploidy reduction occur in mitosis?

In cell division by meiosis, ploidy reduction occurs during meiosis II. Initially, taking as an example a 2n somatic cell, ploidy increases to 4n (duplication of DNA) during interphase. During meiosis I, since homologous chromosomes are separated, ploidy drops to 2n (the original number) and then, during meiosis II, ploidy finally drops 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 the processes that occur during meiosis I, specifically, the separation of homologous chromosomes.

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