ESP Essays 24 Apr 2024 Updated: 

Cellular Reproduction

Classical Genetics, III

Robert J. Robbins

When Mendel published his ground-breaking findings in 1866 the notion that all living things were composed of cells was still relatively new and the mechanisms of cellular reproduction — mitosis and meiosis — were completely unknown. Even chromosomes had not yet been described and named. between 1866 (Mendel's paper), and 1900 (Mendel's rediscovery), much was learned about cellular reproduction. In this document we will describe the basic concepts of cellular reproduction, especially as they relate to an understanding of a Mendelian model of heredity.

 

Background

Mendel's work may have gone unrecognized and neglected for nearly 34 years following its publication because biological science at that time could provide no evidence for any physical intracellular structures that might be the equivalent of Mendel's hereditary factors. Indeed, even the final tenet of the CELL THEORY — the idea that cells are the fundamental unit of living things and that all cells arise through the division of preexisting cells — had been proposed by Virchow less than twelve months before Mendel began his study of peas. Thus, when Mendel presented his results in 1865 the details of cellular division processes were completely unknown. In fact, the word chromosome was not even invented until 1888 — the year of Mendel's death.

However, between the time of Mendel's work and its rediscovery in 1900, research on cellular division and early developmental processes made remarkable progress. During this period it was established that:

1.The nucleus of the cell must be the location of the hereditary information.

2.Chromosomes are the most obvious structures in cell nuclei.

3.The behavior of chromosomes during cell division, gamete production, and fertilization makes them ideal candidates to be the physical carriers of the Mendelian factors.

A brief chronology of the important nineteenth century work is given in Table 3.1. Instead of examining this table as yet another unpalatable list of names and dates, you should consider it as you might a mystery novel's set of clues — clues that point inexorably toward the chromosomes as the culprit in the transmission of hereditary information.

The relationship between the findings of cell biology with the ultimate acceptance of Mendelism is best appreciated through a consideration of processes that affect the assortment of chromosomes. Therefore, this chapter will examine the behavior of chromosomes (a) during the type of cell division associated with asexual reproduction (MITOSIS), (b) during the cell division associated with sexual reproduction (MEIOSIS), and (c) during the process of cellular fusion (FERTILIZATION) that characterizes sexual reproduction. In doing so, we will present only a generalized overview of the processes, leaving a detailed treatment of them for texts dealing specifically with cell biology. Because (as we shall see in Chapter 6) the chromosomes have been shown to be the carriers of the hereditary information, you should read the following material with special attention toward the behavior and assortment of the chromosomes since their assortment is the assortment of the hereditary material.

Mitosis

First, let us consider a population of cells that are undergoing simple asexual reproduction (e.g., the skin cells in your hand). As each cell grows and matures it prepares for and ultimately carries out a division process, resulting in the production of two daughter cells. For this to occur, the cell must synthesize and then carefully distribute copies of the hereditary material to each of these daughter cells. With our modern knowledge, we recognize that this division process must involve a precise sorting of chromosomes into each of the daughter cells. To appreciate how this can occur, let us follow a cell through the entire MITOTIC CELL CYCLE — the process of cellular growth, maturation, and asexual reproduction. (Following tradition, we will break our consideration of the cycle into a series of separate phases. The process is summarized in Figure 3.1.)

INTERPHASE is the name given to the resting phase of the cycle — that is, the phase when the cell is not actively involved in division. This is, of course, a misnomer — the cell is not resting at all, but is instead metabolically very active. During interphase, the cell acquires nutrients, grows, and matures. From our point of view (that emphasizing the behavior of the hereditary material), there are several structures of interest in the interphase cell. The cell can be divided into the CYTOPLASM and the membrane bound NUCLEUS, which contains one or more NUCLEOLI (singular: nucleolus). Near the nucleus, pairs of CENTRIOLES occur, often oriented at right angles to each other. In interphase, these are no obvious structures, except nucleoli, in the nucleus itself, although with proper staining it can be seen to contain copious quantities of a darkly staining, gritty looking material.

As the cell begins to divide, it enters PROPHASE. In early prophase, the centrioles begin to move apart toward opposite poles of the cell and the gritty nuclear material begins to condense into a darkly staining thread like material referred to as CHROMATIN. As prophase progresses, the centrioles arrive at the opposite poles of the cell and seem to be connected to each other via a network of fibers known as the SPINDLE. (Centrioles do not occur in plant cells, but spindle formation occurs nonetheless.) Meanwhile, the chromatin continues to condense, until the mass of thread like material begins to appear as individual darkly staining bodies, now known as CHROMOSOMES. Throughout prophase, the nucleoli become less and less distinct. Finally, as prophase nears completion, the nucleoli can no longer be seen and the nuclear membrane breaks down completely.

With the breakdown of the nuclear membrane, the cell enters METAPHASE. During metaphase the chromosomes become attached to spindle fibers and begin to move (apparently as a result of their spindle fiber attachment) to a central plane of the cell oriented perpendicularly to the spindle. This central plane is sometimes referred to as the EQUATORIAL PLANE or as the METAPHASE PLATE. During metaphase, the chromosomes are in their most condensed and most distinct configuration.

Returning to the process of cellular division, the end of metaphase and the beginning of ANAPHASE is marked by the apparent division of the centromere and the separation of each double stranded chromosome into two SINGLE STRANDED CHROMOSOMES. A closer examination of the centromere shows that the apparent division of the centromere is actually accomplished by the detachment and separation of the two kinetochores. If one metacentric chromosome were followed through this process, it would appear as in Figure 3.3.

Once the centromeric division has occurred and the two chromatids have separated, each of them is considered to be a chromosome in its own right. Thus, we see that the boundary between mitotic metaphase and anaphase is marked by a process in which one double stranded chromosome becomes two single stranded chromosomes. As anaphase progresses, the two single stranded chromosomes that result from each double stranded chromosome are apparently pulled apart from each other and are moved to the opposite poles of the cell via their attachment to spindle fibers. (It can be easy, at first, to become confused about determining the number of chromosomes in a cell, since sometimes two chromatids are counted as one double stranded chromosome and sometimes the same two chromatids are counted as two single stranded chromosomes. However, there is a simple rule that always gives the correct number — always count chromosomes by counting the number of independent centromeres.)

When the two sets of single stranded chromosomes have each gathered at the respective end of the cell, TELOPHASE begins. Telophase is characterized by the actual division of the entire cell into two daughter cells (this process of cytoplasmic cleavage is known as CYTOKINESIS) and by the apparent reversal of the processes associated with prophase. That is, the distinct, single stranded chromosomes begin to decondense away back into the less distinct chromatin and finally into an indistinct gritty material, while at the same time the nuclear membranes begin to reform and the nucleoli reappear. When these reversal processes and cytokinesis are completed, two daughter cells, each in its own interphase, have been produced.

Recall that our original interest in this process was to see how the cell could insure that both of its two progeny cells would receive exactly matching sets of the cell's hereditary material. Notice that mitosis served to insure that the two daughter cells each received matching sets of single stranded chromosomes in that every double stranded chromosome of the original cell was distributed equally, with one chromatid going to each daughter cell. Because the two chromatids of every double stranded chromosome are in fact produced as exact chemical duplicates of each other (we'll see how in Chapter **), the mitotic assortment of chromosomes distributes precisely equivalent sets of the hereditary material to the two daughter cells. (In truth, the two chromatids are not produced as exact chemical duplicates of each other — errors do occur as the cell assembles the molecular subunits of the chromatids. This process is known as mutation and will be discussed in more detail in Chapters ** and **. However, as the error rate is usually less than one error per 100,000,000 incorporated molecular subunits, and as this would correspond roughly to one miscopied word per every ten volumes of the Encyclopedia Britannica, we are not exaggerating too much if we claim that the two chromatids are almost exact chemical duplicates of each other.)

Notice, however, that the chromosome sets of the daughter cells are not identical in every way to the original chromosomes — when the chromosomes condensed during prophase they condensed as double stranded structures, whereas they decondensed during telophase as single stranded structures. This means that, for mitosis to continue for more than one cell generation, during interphase every cell must replicate its hereditary material and in the process change its single stranded chromosomes into double stranded chromosomes. A study of this process has found that CHROMATID REPLICATION occurs during a distinct period in the middle of interphase. Thus, interphase itself can be subdivided into three periods: the prereplication period, the replication period, and the postreplication period. These three periods are designated as G1, S, and G2.

With all of this in mind, we can now summarize the mitotic cell cycle (Figure 3.4). In examining this figure, consider how this cell cycle acts to produce two daughter cells that carry sets of chromosomes that are exactly equivalent to those carried in the original parental cell. This is accomplished by two key events.

1.During the S period of interphase, chromatid replication acts to synthesize a double stranded chromosome from a single stranded chromosome. This process occurs such that the two chromatids are in fact rather precise chemical copies of the original single stranded chromosome.

2.During mitosis, the chromosomes behave so that each daughter cell receives precisely one chromatid from each double stranded chromosome.

This alternation of the synthesis and the distribution of precisely matching chromatids allows cells to grow and to divide, while at the same time insuring that the hereditary material is passed down unchanged from the original parental cell to the two daughter cells. However, this process does not allow hereditary material from two cells to be combined in a single progeny cell — the process of cellular fusion, or fertilization, that is the defining event in sexual reproduction. Thus, mitosis is the cellular division process associated with asexual cellular reproduction.

Meiosis

Although mitosis serves perfectly well for asexual reproduction, it is readily apparent that some other form of cell division must be involved in sexual reproduction. The key event of sexual reproduction — the joint contribution of hereditary material from two parents into one off spring — is accomplished at fertilization when two cells (one contributed from each parent) fuse to form a single cell. For the hereditary information to be pooled, the chromosomes from the two parental cells must also be pooled. Therefore, the cell produced by the union of these cells must contain as many chromosomes as in both gametes combined. Consequently, for sexual reproduction to occur, one of the following statements must be true. Either (a) the number of chromosomes in cells must double every generation, or (b) there must be some process that can balance the increase of chromosome number at fertilization by reducing the chromosome number at some other point in the life cycle.

The first of these logical possibilities simply cannot be true — if the number of chromosomes doubled every generation, the total number of chromosomes would be astronomically large in not very many generations. However, the second of these can be and is true, and the process that carries out this reduction of chromosomal number is known as meiosis, or as the MEIOTIC CELL CYCLE. The meiotic cell cycle actually involves two sequential cell divisions, so that whenever a cell enters meiosis, four daughter cells are ultimately produced. To distinguish between the two different divisions of meiosis, the first is referred to as MEIOSIS I and the second as MEIOSIS II. Similarly, the phases of the two divisions are known as prophase I, prophase II, etc.

We will now follow a typical animal cell through meiosis. For consistency we will illustrate it (Figure 3.5) with a cell that has the same karyotype as did the cell in the illustration of mitosis.

Before meiosis begins, the cell must pass through an interphase containing the S, or synthesis, stage so that it possesses all double stranded chromosomes in its nucleus. Then, the cell begins meiosis by entering PROPHASE I. This is somewhat similar to mitotic prophase in that a spindle is organized, the nucleoli disappear, and the nuclear membrane breaks down. However, there is a fundamental difference between prophase I and mitotic prophase. In mitosis, the double stranded chromosomes condense individually, whereas in meiosis I the double stranded chromosomes condense as pairs. This process of pair formation, also known as SYNAPSIS, does not occur at random — rather, each chromosome has a specific partner. During a normal prophase I, pairing always occurs so that each chromosome pairs only with its appropriate specific partner. Because these specific partners usually have similar morphology (that is, they look alike) and carry similar hereditary information, the pairs that condense together during prophase I are referred to as HOMOLOGOUS PAIRS of chromosomes, and the individual members of a given pair are said to be HOMOLOGS of each other. Since the chromosomes are condensing as pairs, they are sometimes referred to as BIVALENT CHROMOSOMES. Additionally, because the homologous pairs consist of two double stranded chromosomes, the pair itself involves four chromatids held in close physical approximation. Consequently, the process of pairing is also sometimes known as TETRAD FORMATION, and the grouping of four chromatids is referred to as a TETRAD.

With the breakdown of the nuclear envelope, the cell enters METAPHASE I and the chromosomes become attached to the spindle fibers and move to the equatorial plane of the cell. At this point, there is another fundamental difference between mitosis and meiosis, in that in meiotic metaphase I the chromosomes attach to the spindle such that both kinetochores from each individual double stranded chromosome attach only to spindle fibers oriented toward the same pole of the spindle (Figure 3.6). However, every pair of homologous chromosomes is oriented so that the two double stranded chromosomes are attached to spindle fibers oriented toward opposite poles of the spindle.

The final fundamental difference between mitosis and meiosis I occurs when the cell enters ANAPHASE I and the members of the homologous pairs begin to move apart from each other toward opposite poles of the cell, with no centromeric division having occurred. If one pair of metacentric homologs were followed through this process, it would appear as in Figure 3.6. Thus, we see that meiosis I is characterized by the separation, or DISJUNCTION, of the two members of each homologous pair of chromosomes. It is important to note that in meiosis I the two chromatids of a single double stranded chromosome do not separate from each other.

Anaphase I continues and the two sets of double stranded chromosomes are carried to the opposite poles of the cell. Then, TELOPHASE I begins and, as in mitotic telophase, the nuclear membranes reappear, the chromosomes decondense away, and cytokinesis occurs. Notice, however, that although mitosis and meiosis I both yield two daughter cells, there are profound differences between the results of the two processes.

1.In mitosis the daughter cells receive single stranded chromosomes, one from each double stranded chromosome of the original cell, whereas in meiosis I the daughter cells receive double stranded chromosomes, one from each homologous pair of double stranded chromosomes.

2.In mitosis (following the convention of counting chromosomes by counting centromeres) the daughter cells receive the same number of chromosomes as in the original cell, whereas in meiosis I the daughter cells each receive only half as many chromosomes as in the original cell.

Because the daughter cells of meiosis I receive double stranded chromosomes immediately at the close of telophase I, there is no need for chromatid replication to occur prior to the beginning of meiosis II. Therefore, after telophase I, the daughter cells enter a brief resting phase that does not include chromatid replication. This resting phase is called INTERKINESIS to distinguish it from mitotic interphase, which does include chromatid replication. Following interkinesis the daughter cells begin meiosis II, which is rather similar in terms of chromosome behavior to a mitotic division.

During PROPHASE II, the double stranded chromosomes condense one at a time — not as pairs. During METAPHASE II, they attach to spindle fibers in the same manner as in mitosis (i.e., the two kinetochores of one double stranded chromosome are attached to spindle fibers oriented in opposite directions) and move to the equatorial plane. Metaphase II terminates when the double stranded chromosomes undergo an apparent centromeric split and the two resulting single stranded chromosomes begin to move toward the opposite poles of the cell. This movement is completed during ANAPHASE II. Finally, the nuclear membranes reform, the chromosomes decondense, and cytokinesis occurs during TELOPHASE II. Since meiosis I produces two daughter cells from the original cell and meiosis II produces two daughter cells from each of these, the meiotic cell cycle results in the production of four daughter cells — the PRODUCTS OF MEIOSIS. According to the convention where we count chromosomes by counting centromeres, the first meiotic division reduces the number of chromosomes, whereas the second meiotic division does not. Therefore, the first meiotic division is sometimes called the REDUCTIONAL DIVISION and the second meiotic division the EQUATIONAL DIVISION.

In many organisms (e.g., higher animals) the PRODUCTS OF MEIOSIS themselves would undergo some specialization and become the gametes. At this point, however, we will not follow this process any further because there is some variation from species to species and even from males to females (e.g., in males, all four products of meiosis become functional spermatozoa whereas in females only one becomes a functional egg and the other three are discarded) and we are primarily interested in the general behavior of chromosomes during meiosis. Some of these differences, however, are discussed in Supplements at the end of the chapter.

The entire two division process of meiosis is summarized in Figure 3.7. In examining this figure, consider how the meiotic cell cycle acts to produce four products of meiosis that each carry sets of chromosomes that are equivalent to half of the chromosomes contained in the original, pre meiotic cell. This is accomplished by two key events.

1.During prophase I the chromosomes condense as homologous pairs of double stranded chromosomes.

2.In metaphase I the kinetochores attach to the spindle fibers such that both kinetochores from one homolog are connected to spindle fibers oriented toward one pole whereas both kinetochores from the other homolog are connected to spindle fibers oriented toward the other pole. Thus, at the end of metaphase I, the members of the homologous pairs separate from each other (with no centromeric division) and move to the opposite poles of the cell.

This pairing, followed by disjunction, of the homologs results in the production of daughter cells that each carry only half as many chromosomes as the original cell. Note, however, that this reduction in chromosome number is carried out in a very precise manner — each daughter cell receives exactly one member of each pair of homologous chromosomes. Because such a reduction is a necessary prerequisite for fertilization, meiosis is the cellular division process associated with sexual reproduction. However, sexual reproduction is not accomplished until cellular fusion has restored the number of chromosomes to their pre meiotic value.

Fertilization

The fusion of two cells (the GAMETES) whose chromosome sets have been reduced by meiosis produces a cell (the ZYGOTE) carrying the pre meiotic number of chromosomes. And, because each gamete is carrying precisely one member of each homologous pair, the zygote carries a completely balanced and reconstituted pre meiotic set of chromosomes. That is, the zygote contains not merely the same number of chromosomes as the pre meiotic cell, but also exactly two members from each pair of homologous chromosomes found in the pre meiotic cell. (Figure 3.8 illustrates this process using cells with the same karyotype as those used in the illustration of mitosis and meiosis.)

Notice that although the fusion of the two gametes generates a cell carrying the full pre meiotic number of chromosomes (remember, we count chromosomes by counting independent centromeres), these chromosomes are still single stranded. Thus, the newly formed zygote can be considered to be in the G1 period of interphase and incapable of undergoing any immediate cellular division. However, as soon as the zygote undergoes chromatid replication (i.e., as soon as it goes through the S period and into the G2 period) it can begin to undergo further divisions. In multi cellular animals, for example, the zygote quickly passes through S and G2 and begins a rapid sequence of mitotic divisions that results in the development of the embryo.

We have noted that the zygotes of a given species will have twice as many chromosomes as will the gametes of that species. We have also noted that individual species have a characteristic set of chromosomes (the species' karyotype). Therefore, we see that any species can be typified, to some extent, by two numbers — the number of chromosomes in each of its gametes and the number of chromosomes in its zygotes (which will be twice that of the gametes). For example, humans have 23 chromosomes per gamete and 46 per adult cell, whereas pea plants have seven per gamete and 14 per adult cell. Table 3.2 gives some characteristic numbers for a variety of different organisms.

Table 3.2 The numbers of chromosomes carried in the gametes and the zygotes of a variety of different species. Notice that although all normal members of a given species will carry the same number of chromosomes, all organisms with the same number of chromosomes do not necessarily belong to the same species. For example, both peas and petunias carry seven chromosomes in their gametes. Also, notice that there is no necessary relationship between an organism's complexity and the number of chromosomes it contains. Thus, sweet potatoes have 90 chromosomes in their zygotes, whereas humans have 46.

To distinguish between the number of chromosomes in a gamete and in a zygote, we say that a gamete contains a HAPLOID NUMBER of chromosomes and the zygote contains a DIPLOID NUMBER. Because the diploid number will be twice the haploid number, we can symbolize the haploid number as 1n and the diploid number as 2n (to be interpreted as one times n or as two times n, where n is the typical number of chromosomes in the gamete of that species — e.g., n = 23 for humans and seven for peas — notice that n is also equal to the number of pairs of homologous chromosomes found in the pre meiotic cell). Furthermore, we can characterize any cell of any given species by the PLOIDY it carries. For example, an original cell beginning the meiotic cell cycle would be considered a DIPLOID cell, whereas the two daughter cells produced by meiosis I would be considered HAPLOID.

In multicellular organisms in which the zygote divides mitotically to yield the growing embryo and thus ultimately the adult organism, all of the cells of the adult organism will be diploid, since the cellular division process of mitosis always produces daughter cells with the same number of chromosomes as the original cell — thus, we could also say that mitosis has no effect upon ploidy, whereas meiosis I acts to reduce ploidy and fertilization acts to increase ploidy.

Cellular Reproduction and Mendelism

Although we will defer a full presentation of this topic until later, we will take a moment here to consider briefly some aspects of the relationship of cellular reproduction to Mendelism. Thus, let us simply assume that genes are carried on chromosomes (cf. Chapter 6) and follow one pair of chromosomes through one generation of reproduction to see if the pattern of inheritance produced by chromosomal assortment is equivalent to the pattern predicted by the Mendelian model. Figure 3.9 follows a single homologous pair of chromosomes from the parental gametes through the development of the F₁.

Note in Figure 3.9 that the haploid gametes each carry only one copy of one single stranded member of the homologous pair, but the fusion of the gametes in fertilization yields a diploid zygote with both members of the pair. Then, chromatid replication occurs, followed by mitosis, followed by chromatid replication. Because chromatid replication produces two chromatids that are chemical duplicates of the original single stranded chromosome (details in Chapter **), following the S period of interphase there are extra copies of the alleles available to be passed into the daughter cells. And, because mitosis always divides double stranded chromosomes so that each daughter cell receives one and only one chromatid from each double stranded chromosome, mitosis yields two daughter cells carrying sets of chromosomes and alleles that are precisely equivalent to those of the original cell. Thus, the process of mitosis can continue indefinitely to yield, ultimately, an adult F₁ pea plant in which every cell carries exact copies of the chromosomal, and hence genetic, makeup of the zygote.

For this F₁ to reproduce, it will have to make gametes via meiosis. Figure 3.10 illustrates the behavior of chromosomes during meiosis in the F₁ plant. Notice that meiosis in the heterozygous F₁ plant yields equal numbers of two different kinds of gametes — one carrying the R allele and the other carrying r.

In considering Figures 3.9 and 3.10 you should note that they merely illustrate the normal behavior of chromosomes through the processes of fertilization, chromatid replication, mitosis, and meiosis — no special modifications have been made to adapt the processes to the rules of Mendelian inheritance. Yet, the combination of this normal chromosomal behavior with the simple assumption that genes are carried on chromosomes yields a pattern of allelic assortment that matches exactly with the pattern required by the Mendelian model for a monohybrid cross. (Of course, the situation becomes a little more complex with a dihybrid cross — e.g., should we assume that the two loci are carried on the same pair or on a different pair of homologous chromosomes — but a consideration of this will be deferred until Chapter 7.]

In considering Figures 3.9 and 3.10 you should also note that the standard Mendelian pattern of inheritance follows necessarily from the processes of mitosis, meiosis, and fertilization only in those organisms in which these processes occur in a certain pattern in the organism's life cycle. Specifically, the life cycle must be such that

1.The products of meiosis develop directly into gametes.

2.The diploid zygote divides mitotically to give rise to the multicellular diploid adult.

Although this type of life cycle is characteristic of all higher animals (and is approximately characteristic of all higher plants), it is most certainly not true of all living things. Therefore, to understand fully the process of inheritance you must recognize that the patterns of inheritance will vary in different groups of organisms, depending upon the timing of meiosis in the life cycle, and that the methods of genetic analysis and interpretation must be modified appropriately to deal with these variations. Supplement 3.2 provides a discussion of the different basic life cycle patterns that are known to occur.

Summary and Overview

Classical cytological and developmental studies of the nineteenth century established that the hereditary material in a cell must be carried in the nucleus of the cell and that chromosomes are the most prominent structures in the nucleus. Because it has since been shown that the chromosomes are in fact the carriers of the hereditary material (Chapter 6), an understanding of genetics requires an understanding of the assortment of chromosomes during cellular reproduction.

Asexual reproduction is accomplished on a cellular level through the process of mitosis. In the mitotic cell cycle, chromatids are replicated as precise chemical duplicates during the S period of interphase and then during mitosis one chromatid from each double stranded chromosome is distributed to each daughter cell. Thus, the mitotic cell division acts to insure that both daughter cells produced by the division contain sets of chromosomes — and thus sets of hereditary material — that are exactly equivalent to the set contained in the original dividing cell. Because mitosis does not change the genetic content of a cell, any cell can undergo mitosis, whether it is haploid, diploid, or some other ploid (cf. Chapter **).

Sexual reproduction involves the pooling of genetic information from two individuals through the fusion of two cells in fertilization. If fertilization is to occur at some point in an organism's life cycle, and if the number of chromosomes in the cells of individuals of that species is to remain constant from generation to generation, meiosis must occur at some other point in the organism's life cycle. In the first division of the meiotic cell cycle, homologous pairs of double stranded chromosomes pair with each other and then disjoin from each other without undergoing centromeric division. In the second division, the centromeres divide so that each daughter cell receives one chromatid from each double stranded chromosome. Thus, the meiotic cell cycle acts to insure that each of the four products of meiosis receives precisely one single stranded chromosome from each pair of double stranded homologous chromosomes that were contained in the original cell at the beginning of meiosis. Thus, each product of meiosis receives a precisely partitioned set of chromosomes containing exactly half as many chromosomes as contained in the pre meiotic cell. Because meiosis does change the genetic content of a cell through the pairing and disjunction of homologous chromosomes, it can occur only in cells that are in fact carrying homologous pairs of chromosomes — i.e., in cells that are diploid.

A comparison of the process of mitosis with the combined processes of meiosis and fertilization (Figure 3.11) shows that ultimately both act to maintain karyotypic stability in the cells of a given species. For example, as Figure 3.11 shows, if we start with a diploid cell in G2 that has two metacentric and two acrocentric chromosomes and follow it through mitosis until its daughter cells have completed their own G2, we see that the daughter cells also have sets of chromosomes containing two metacentric and two acrocentric chromosomes. Similarly, if we start with the same diploid cell in G2 and follow it through meiosis and then follow the products of meiosis through fertilization and finally follow the zygote through S and into G2 we see that the zygote produced also carries a set of chromosomes that have two metacentric and two acrocentric chromosomes. Thus, karyotypic stability is maintained.

Because (as we shall see in Chapter 7) the alleles for a given gene are carried at specific locations on specific homologous pairs of chromosomes, this maintenance of karyotypic stability also serves to maintain a balanced set of genetic information in the members of the species. That is, it serves to insure that each gamete carries one and only one allele for every locus and that each zygote receive two and only two alleles for each locus.