Supplemental Lecture (97/02/16 update) by Stephen T. Abedon (abedon.1@osu.edu)

1. Chapter title: Meiosis
1. A list of vocabulary words is found toward the end of this document
2. "Laughter and joy and loneliness, and sex and sex and sex and sex." - The Rolling Stones
3. Sex and meiosis are all mixed up. Sex is the mixing together (recombination) of DNAs that come from two different parents. Meiosis describes the cell division during which (i) this recombination occurs and (ii) the total amount of DNA in a cell is cut in half (haploidy). Cutting the amount of DNA in half allows the fusion of two post-meiotic cells, resulting in a cell containing a full compliment of DNA (diploidy). In that familiar animal, the human being, the sexual cycle can be considered simply in terms of meiosis, mitosis, and a clever mechanism by which outcrossing is achieved.
4. Consider sperm. Sperm are cells containing only half the compliment of DNA as that found in the male parent by which they are produced (i.e., they are haploid). Sperm cells bind to egg cells thus fertilizing the egg. Prior to fertilization an egg cell too contains only half the compliment of DNA as that found in the parental (this time female) organism. The fusion of a sperm with an egg (a.k.a., fertilization) results in the formation of a cell which contains the full compliment of DNA (i.e., is diploid). Fertilized eggs undergo repeated rounds of mitosis, ultimately resulting in the formation of what is known as soma, all the cells in an organism except the germ line.
5. It is from the germ line that sperm and egg cells (gametes) are made. However, as noted above, these gametes contain only half the compliment of DNA as is found in typical somatic cells, as well as in germ line progenitor cells. What happens to the rest of the DNA in the course of the production of gametes? In addition, I started out this introduction with mention of recombination, but never got back to it. What exactly is recombination and how and where does it occur? The answer to both questions is meiosis. In this lecture we will consider these questions as well as continue our consideration of many of the themes we began discussing in the previous lecture on mitosis.
2. Overview (meiosis)
1. Sex consists of outcrossing and recombination. That is, recombination occurs between two chromosomes found in a single cell, each of which has come from the cell of a different parent. Fertilization is the fusion between two cells thus creating a single cell carrying chromosomes sourced from different parents. This cell is said to be diploid. That is, it carries two copies of each type of chromosome normally found in such a cell. Note that this implies that the two cells that fused upon fertilization each must have carried half as many chromosomes, i.e., one copy of each type of chromosome. This condition is known as haploidy. If mitosis of the diploid cell follows fertilization, then sexual reproduction is said to have occurred.
2. A typical human sexual cycle may be defined as follows: Fertilization results in diploidy. Mitotic division of diploid cells follows. Diploid cells may then undergo a reduction division. Concurrently, recombination occurs, as well as an additional mechanism by which chromosomes (and, therefore, genes) are assorted called independent assortment. Haploid cells are then formed, each a genetic form intermediate between that of each of its parent's two parents. Finally, this haploid cell may fuse (fertilization) with a haploid cell sourced from a second individual, so long as that second individual is not of the same gender (and appropriate and timely intimacy has been achieved).
3. Note that the human sexual cycle is not representative of all sexual cycles. Numerous variations exist on the sexual cycle, including no sex at all (asexual reproduction).
4. Meiosis is the process by which variable haploid cells are produced. Meiosis is a form of nucleus duplication which in many ways resembles the simpler mitotic nuclear duplication, though with a few major difference between the two mechanisms. In particular, with meiosis true nuclear duplication is not achieved. Instead, in the course of meiosis there is a reduction division (decrease in chromosome number from diploid to haploid), recombination between parental chromosomes, and an independent assortment of recombined parental chromosomes to opposite cell poles. In addition, meiosis consists of two nuclear duplication-like events termed, in order of their occurrence, meiosis I and meiosis II. In fact, it is during meiosis I that the most interesting events occur including recombination, reduction division, and one round of independent assortment.
5. Meiosis II mechanistically resembles mitosis much more than does meiosis I. However, two differences are relevant. First, there is no S phase prior to meiosis II and thus the products of meiosis II are haploid rather than diploid (this is equivalent, actually, to the post S phase mitosis which occurs among haploid cells). Second, because the sister chromatids which separate during meiosis II are products of the recombination which occurred during meiosis I (this is in addition to a second round of independent assortment). That is, the two haploid cells which are the products of meiosis II are not identical (as too the two cells which were the product of meiosis I were not identical--in mitosis occurring among haploid cells this would not be the case ). Thus completes meiotic cell division and with it this one aspect of the sexual cycle.
3. Sex
1. recombination between DNAs sourced from different parents (i.e., recombination plus outcrossing).
2. Note, do not confuse sex with gender, they are not the same thing (this latter point is something which personally drives me crazy though I'm afraid my reaction to it is very much in the minority).
4. Sexual reproduction
1. Reproduction with sex:
1. The production of progeny organisms by a mechanism involving sex.
2. Note that this not a statement of the obvious since reproduction in many organisms occurs without a requirement for sex.
3. Sexual reproduction generally requires the participation of two individuals, though even this is true in only the strictest sense.
2. Sex without reproduction:
1. Note, however, that sex and sexual reproduction are not completely analogous since sex may occur without the occurrence of reproduction. Yes, I'm sure that seems obvious to all of you, given our society of facile birth control, but that is not what I mean.
2. Instead, two free-living haploid cells could fuse, go through a round of recombination, then part without undergoing any mitotic divisions.
3. In such as case, sex, but not reproduction, has occurred, i.e., sexual reproduction in a strict sense has not occurred.
5. Asexual reproduction
1. Reproduction without sex:
1. The production of progeny organisms by a mechanism that does not involve sex.
2. Asexual reproduction generally can be accomplished by only a single individual. Asexual reproduction is quite common.
2. Quite common:
1. To accept asexual reproduction this one has to get over one's anthropocentric bias that outcrossing somehow is an absolutely necessary requirement of reproduction.
2. The majority of the world's species would beg to differ, if only they were they were capable of articulating such a concept (see binary fission, see mitosis, see budding, etc.).
6. Gender [male, female]
1. The differentiation of parent organism into one of at least two types.
2. One type contributes chromosomes and cytoplasmic contents to progeny (the female) and the other type contributes only chromosomes (the male).
7. Outcrossing
1. The sourcing of DNAs from different parents.
2. The outcrossing aspect of sex is the familiar part.
8. Haploid
1. A description of the number of sets of chromosomes a cell contains. If the cell contains one of every type of chromosomes, then it is said to be haploid.
2. Note that this name comes from the amount of chromosomes being half of what is normally present in, for example, the cells of humans and most other animals (i.e., "half"-ploid).
9. Diploid
1. A cell containing two complete sets of chromosomes. In other words, a diploid cell contains two haploid sets of chromosomes.
2. Near identity:
1. It is important to note that duplicate chromosomes in a diploid cell are not so much identical as nearly so (i.e., there are differences in DNA sequences).
2. Thus, merely copying a haploid set of chromosomes so that there are now two identical sets of chromosomes is not what one normally is referring to when one speaks of diploid.
3. See independent assortment illustration above.
10. Reduction division
1. The actual meiotic step in which a diploid cell is converted to a haploid cell.
2. Reduction division occurs during meiosis I when the total number of chromosomes is cut in half thus forming a cell which genetically resembles a post S phase haploid cell. A lack of S phase between meiosis I and meiosis II is actually less important in this regard than that fact that meisosi I occurs with an absence of centromere division.
11. Gamete [sperm and egg]
1. Sperm and egg (or their equivalents in non-animals).
2. Products of meiosis:
1. Gametes are the products of meiosis.
2. Sperm, of course, is the gamete produced by males and eggs are the gamete produced by females.
3. Gametes are what is sourced from different parents in the process of sex.
12. Germ line
1. The cells in a body from which gamete are derived.
13. Somatic cells [soma]
1. The rest of the cells in a body, i.e., all the cells less the germ line and the products of the germ line.
2. Note that when you see an individual, what you see is their somatic cells (a.k.a., soma). Only under the most intimate of circumstances are you likely to come in contact in any manner with cells of an individual other than those of the soma.
14. Fertilization
1. The fusion of gametes.
2. Familiarly, this would be the fusion between a sperm cell and an egg.
15. Sexual cycle
1. Alternating haploid-diploid:
1. In the sexual cycle haploidy and diploidy alternate.
2. Meiosis constitutes the transition between the diploid mitotic state and the haploid state.
3. Fertilization, in turn, constitutes the transition from the haploid state to the diploid state.
2. Alternating meiosis-mitosis:
1. The sexual cycle can also be considered as a a multi-generational series of cell divisions in which meiosis and mitosis alternate.
2. Multiple rounds of mitosis, particularly, can occur between each round of meiosis (depending upon the species involved and other factors).
3. In some organisms, however, no mitosis occurs following fertilization. Instead, meiosis occurs followed by haploid mitosis.
4. That is, depending upon species, etc., the mitotic stage may coincide either the haploid stage, the diploid stage, or both.
3. Meiosis cannot follow meiosis:
1. Meiosis cannot follow meiosis.
2. This is because meiosis involves a reduction division, thus necessitating a fertilization step between meiotic steps.
4. Mitosis and meiosis in humans:
1. In humans, the diploid stage and mitosis coincide, but the haploid stage and mitosis do not.
2. What we call our bodies though in terms of descent the important players are the germ line. Germ line cells are diploid and undergoe mitosis, but can also undergoe meiosis to produce gametes.
3. The haploid gametes do not divide in humans (i.e., they do not undergo mitosis).
16. Recombination [crossing over]
1. Recall that a stand of DNA consists of two linear molecules of DNA wound around each other. As an analogy, consider both strands to make up a string (such as that used to tie packages). Now take two of these strings and place them side by side. Cut both strings at the same point. Now take the string on the left and attach it to the string on the right, and the string on the right and attach it to the string on the left (literally cross over the string from one chromosome to the other). The result is two hybrid strings, each made up of itself and a part of the other.
2. The molecular process that is analogous to this is called recombination and in eucaryotes recombination occurs during meiosis. See illustration below.
17. Illustration, recombination



18. Independent assortment
1. A crude recombination equivalent which does not involve the cutting and splicing of chromosomes.
2. Inevitable consequence of reduction division:
1. Independent assortment occurs because most organisms contain more than one type of chromosome. Given two, not necessarily identical copies of each type, one can envisage that upon reduction division a daughter cells could receive any number of combinations of chromosomes.
2. However, the meiotic machinery assures that each daughter cell receives only one of each type of chromosomes. Therefore, variation between same types of chromosomes that is randomly assorted between daughter cells.
3. Statistical independence:
1. This random assortment is called independent assortment because the segregation of variation found within same types of chromosomes occurs with statistical independence.
2. This process is equivalent to recombination because recombination acts to increase the statistical independence of variation within chromosomes (i.e., independent assortment effects the statistical independence of variation between chromosomes of the same type).
4. See illustration below.
19. Illustration, independent assortment



20. Meiosis
1. Reduction division/recombination/indepedent assortment:
1. A series (two) of cell divisions that results in the formation of haploid cells (gametes) whose DNA has undergone recombination.
2. These haploid cells may then fuse with a second haploid cell thus forming a diploid cell. Diploid cells are the starting material of meiotic cell divisions in most animals.
2. Meiosis consists of two rounds of mitosis-like cell divisions called meiosis I and meiosis II (i.e., Meiosis = meiosis I + meiosis II).
3. See figure 1115.1 for an overview of meiosis I and meiosis II as well as mitosis.
4. As an interesting aside: Because of the way this system works, you and your mate's DNA are first found in the same cells in your children, and make up half of the DNA found in your grandchildren. Most of the DNA in your children is not the product of recombination between you and your mate's DNA (in other words, any recombination going on in your child would be between your and your mate's DNA, but recombination only occurs in the cells making up the germ line). Half of the DNA in your grandchildren is the product of the recombination between your and your mate's DNA (the other half is the product of recombination of your child's mate's parent's DNA). Once these statements start making sense you may consider yourself to be on the verge of having an understanding of meiosis and some of its consequences.
21. Meiosis I
1. This is the first round of meiotic division.
2. Meiosis I consists of:
1. prophase I
2. metaphase I
3. anaphase I
4. telophase I
3. It is during this first round of division that recombination occurs.
4. Reduction division occurs:
1. Segregation of chromosomes differs from that which occurs in mitosis; particularly, sister chromatids are not separated but instead end up at the same cell poles. This variation on the mitotic theme is called a reduction division.
2. The cell's chromosomes still consist entirely of pairs of sister chromatids, but the cell now has half as many pairs of sister chromatids (and half as many chromosomes since the centromeres have not separated).
5. See illustration on the genetic consequences of meiosis I below.
22. Illustration, genetic consequences of meiosis I



23. Meiosis II
1. Second round of meiosis:
1. Meiosis II is the second round of meiotic division.
2. Meiosis II consists of:
1. prophase II
2. metaphase II
3. anaphase II
4. telophase II
2. Meiosis II consists of a more mitosis-like division in that separation of sister chromatids occurs.
3. No preceding S phase:
1. However, no DNA synthesis occurs subsequent to meiosis I or prior to meiosis II.
2. Thus, following meiosis II a haploid cell is produced, i.e., it has half as much DNA as its diploid ancestral cells had following their last mitotic division.
3. See illustration on the genetic consequences of meiosis II (below).
24. Illustration, genetic consequences of meiosis II



25. Prophase I
1. Prophase I is analogous to prophase of mitosis in a number of ways including:
1. follows DNA synthesis
2. nuclear membrane remains intact
3. centrioles begin to move
4. etc.
2. Recombinative phase:
1. Prophase I of meiosis also has the distinction of being the period during which recombination occurs.
2. Note that recombination does not occur between the chromosomal progeny of an individual chromosome.
3. Such recombination would accomplish nothing in terms of increasing variation. Indeed, given the hypothesis that recombination exists in order to repair double stranded DNA damage, such recombination also would not be expected to occur.
3. Thus, recombination does not occur within attached sister chromatid pairs, but between attached sister chromatid pairs of the same type of chromosome.
4. As a consequence of recombination occurring in this manner, sister chromatids come to no longer identically resemble each other.
5. See illustration on the genetic consequences of meiosis I above.
26. Metaphase I [bivalent, tetrad]
1. Prelude to chromosomal segregation:
1. As with mitosis, one of the goals of the meiosis I is the segregation of a complete set of chromosomes to each of two daughter cells.
2. In both mitosis and meiosis I this process is initiated during the various metaphases. However, there are differences.
2. Lining up of pairs of sister chromatid pairs:
1. In mitosis, sister chromatids line up such that one sister chromatid may end up in one daughter cell and the other sister chromatid in the other daughter cell.
2. In meiosis I sister chromatids are not separated. Instead, one daughter cell receives one pair of attached sister chromatids and the other daughter cell receives the other pair of attached sister chromatids.
3. In other words, sister chromatid pairs do not separate during meiosis I.
4. Consequently, in metaphase I pairs of sister chromatids line up in the middle of the cell for a total of four chromosomes per chromosome type, instead of the two chromosomes per chromosome type which lines up in the metaphase of mitosis.
3. This grouping of four chromosomes during metaphase I is called a bivalent or tetrad.
27. Anaphase I
1. One round of independent assortment occurs during anaphase I as bivalent sister chromatid pairs are separated, one pair going to one pole of the cell and the other pair going to the opposite pole of the cell.
2. Anaphase I is otherwise equivalent to anaphase of mitosis.
28. Telophase I
1. Equivalent to telophase of mitosis, including the reformation of the nuclear envelope, except that chromosomes remain bound as sister chromatids.
29. Prophase II
1. Equivalent to prophase of mitosis.
2. However, that no DNA synthesis step occurs prior to prophase II and thus the total number of sister chromatid pairs is exactly one-half that found during the prophase I of mitosis of diploid cells.
30. Metaphase II
1. Equivalent to metaphase of mitosis except that the total number of sister chromatid pairs is exactly one-half that found during the prophase I of mitosis of diploid cells.
31. Anaphase II
1. Equivalent to anaphase of mitosis except that the total number of sister chromatid pairs is exactly one-half that found during the prophase I of mitosis of diploid cells.
2. It is only at anaphase II of meiosis that sister chromatid pairs finally separate.
32. Telophase II
1. Equivalent to telophase of mitosis except that the total number of sister chromatid pairs is exactly one-half that found during the prophase I of mitosis of diploid cells.
33. Vocabulary
1. anaphase I
2. anaphase II
3. asexual reproduction
4. diploid
5. female
6. fertilization
7. gamete
8. gender
9. Genetic consequences of meiosis I, illustration
10. Genetic consequences of meiosis II, illustration
11. germ line
12. haploid
13. independent assortment
14. Independent assortment, illustration
15. male
16. meiosis
17. meiosis I
18. meiosis II
19. metaphase I
20. metaphase II
21. outcrossing
22. prophase I
23. prophase II
24. recombination
25. Recombination, illustration
26. reduction division
27. sex
28. sexual cycle
29. sexual reproduction
30. somatic cells
31. telophase I
32. telophase II
34. Practice questions
1. Sex, in a cell biological sense, is?[PEEK]
2. A name of a process which must occur during the transition from a diploid state to a haploid state is? [PEEK]
1. fertilization
2. adiploidy
3. meiosis
4. mitosis
5. all of the above
6. none of the above
3. A cell from a diploid individual which has 64 chromosomes per haploid has how many chromosomes during anaphase II? [PEEK]
1. 16
2. 32
3. 64
4. 128
5. all of the above
6. none of the above
4. In the absence of recombination, during what phase of meiosis does some process involved in independent assortment occur? [PEEK]
1. G1 phase
2. anaphase I
3. prophase II
4. telophase II
5. all of the above
6. none of the above
35. Practice question answers
1. The recombination of two (or more) chromosomes sourced from different individuals.
2. i, meiosis
3. iv, 128
4. ii, anaphase I
36. References
1. Raven, P.H., Johnson, G.B. (1995). Biology (updated version). Third Edition. Wm. C. Brown publishers, Dubuque, Iowa. pp. 218-230.