10.3 and 10.4

When conditions are optimal, some prokaryotic cells can divide every 20 minutes. The generation times of eukaryotic cells are generally much longer, although the frequency of cell division varies widely among different species and among different tissues of the same species. Some skeletal muscle cells usually stop dividing after the first few months of life, whereas blood-forming cells, digestive tract cells, and skin cells divide frequently throughout the life of the organism. Under optimal conditions of nutrition, temperature, and pH, the length of the eukaryotic cell cycle is constant for any given cell type. Under less-favorable conditions, however, the generation time may be longer.

Certain regulatory molecules that control the cell cycle are common to all eukaryotes. Genetically programmed in the cell’s nucleus, these regulatory molecules are components of the cell cycle control system found in organisms as diverse as yeast, clams, frogs, humans, and plants. Regulatory molecules trigger a specific sequence of events during the cell cycle.

Because the cell cycle consists of hundreds of sequential events that proceed in an orderly manner, a failure to carefully control these events can have disastrous consequences. Control mechanisms in the genetic program, called cell-cycle checkpoints, temporarily block key events from being initiated during the cell cycle. Cell-cycle checkpoints ensure that all the events of a particular stage have been completed before the next stage begins (Fig. 10-13). The checkpoints are inactivated after they have done their job so the cell cycle can proceed.

The cell cycle consists of hundreds of sequential events. The red bars show three important checkpoints that determine that previous steps are completed so that the next steps may proceed. Each checkpoint is inactivated after it has performed its function, allowing the cell cycle to continue.

Genes that encode molecules involved in checkpoints are critically important to the cell cycle. If a checkpoint gene is defective, it can result in cancer or other serious diseases. Consider what might happen if the metaphase–anaphase checkpoint molecules were nonfunctional. In this case, anaphase might be initiated too early, before all chromosomes were properly attached to spindle fibers. The resulting daughter cells might have too few or too many chromosomes. An abnormal number of chromosomes is associated with Down syndrome as well as many cancers.

Figure 10-14 shows some of the key molecules involved in regulating the cell cycle. Among them are protein kinases, enzymes that activate or inactivate other proteins by phosphorylating (adding phosphate groups to) them. The protein kinases involved in controlling the cell cycle are cyclin-dependent kinases (Cdks). The activity of various Cdks increases and then decreases as the cell moves through the cell cycle. Cdks are active only when they bind tightly to regulatory proteins called cyclins. The cyclins are so named because their levels fluctuate predictably during the cell cycle (i.e., they “cycle,” or are alternately synthesized and degraded as part of the cell cycle).

Although the details of the reproductive process vary greatly among different kinds of eukaryotes, biologists distinguish two basic types of reproduction: asexual and sexual. In asexual reproduction a single parent splits, buds, or fragments to produce two or more individuals. In most kinds of eukaryotic asexual reproduction, all the cells are the result of mitotic divisions, so their genes and inherited traits are like those of the parent. Such a group of genetically identical organisms is called a clone. In asexual reproduction organisms that are well adapted to their environment produce new generations of similarly adapted organisms. Asexual reproduction occurs rapidly and efficiently, partly because the organism does not need to expend time and energy finding a mate.

In contrast, sexual reproduction involves the union of two sex cells, or gametes, to form a single cell called a zygote. Usually, two different parents contribute the gametes, but in some cases a single parent furnishes both gametes. In the case of animals and plants, the egg and sperm cells are the gametes, and the fertilized egg is the zygote.

Sexual reproduction results in genetic variation among the offspring. (How this genetic variation arises is discussed later in this chapter and in Chapter 11.) Because the offspring produced by sexual reproduction are not genetically identical to their parents or to each other, some offspring may be able to survive environmental changes better than either parent does. However, one disadvantage of sexual reproduction is that some offspring with a different combination of traits may be less likely to survive than their parents.

There is a potential problem in eukaryotic sexual reproduction: if each gamete had the same number of chromosomes as the parent cell that produced it, the zygote would have twice as many chromosomes. This doubling would occur generation after generation. How do organisms avoid producing zygotes with ever-increasing chromosome numbers? To answer this question, we need more information about the types of chromosomes found in cells.

Each chromosome in a somatic cell of a plant or animal normally has a partner chromosome. The two partners, known as homologous chromosomes, are similar in size, shape, and the position of their centromeres. Furthermore, special chromosome-staining procedures produce a characteristic pattern of bands evident in the members of each chromosome pair. In most species chromosomes vary enough in their structure that cell biologists can distinguish the different chromosomes and match up the homologous pairs. The 46 chromosomes in human cells constitute 23 homologous pairs.

The most important feature of homologous chromosomes is that they carry information about the same genetic traits, although this information is not necessarily identical. For example, each member of a homologous pair may carry a gene that specifies hemoglobin structure. One member, however, may have the information for the normal hemoglobin β chain (see Fig. 3-23a), whereas the other may specify the abnormal form of hemoglobin associated with sickle cell anemia (see Chapter 16). Homologous chromosomes can therefore be contrasted with the two members of a pair of sister chromatids, which are precisely identical to each other.

A set of chromosomes has one of each kind of chromosome; in other words, it contains one member of each homologous pair. If a cell or nucleus contains two sets of chromosomes, it is said to have a diploid chromosome number. If it has only a single set of chromosomes, it has the haploid number.

In humans the diploid chromosome number is 46 and the haploid number is 23. When a sperm and egg fuse at fertilization, each gamete is haploid, contributing one set of chromosomes; the diploid number is thereby restored in the fertilized egg (zygote). When the zygote divides by mitosis to form the first two cells of the embryo, each daughter cell receives the diploid number of chromosomes, and subsequent mitotic divisions repeat this. Thus, somatic cells are diploid.

An individual whose cells have three or more sets of chromosomes is polyploid. Polyploidy is relatively rare among animals but common among plants (see Chapter 20). In fact, polyploidy has been an important mechanism of plant evolution. As many as 80% of all flowering plants are polyploid. Polyploid plants are often larger and hardier than diploid members of the same group. Many commercially important plants, such as wheat and cotton, are polyploid.

The chromosome number found in the gametes of a particular species is represented as n, and the zygotic chromosome number is represented as 2n. If the organism is not polyploid, the haploid chromosome number is equal to n and the diploid number is equal to 2n; thus, in humans,

𝑛

and

𝑛

. For simplicity, in the rest of this chapter, the organisms used as examples are not polyploid. We use diploid and 2n interchangeably, and haploid and n interchangeably, although the terms are not strictly synonymous. It is important to note that while diploid organisms typically have two sets of chromosomes, haploid organisms have only one set, which is critical for understanding their reproductive strategies.

A cell division that reduces chromosome number is called meiosis. The term meiosis means “to make smaller,” and the chromosome number is reduced by one-half. In meiosis a diploid cell undergoes two cell divisions, potentially yielding four haploid cells. It is important to note that the haploid cells produced by this process do not contain just any combination of chromosomes, but one member of each homologous pair.

The events of meiosis are similar to the events of mitosis, with four important differences:

Meiosis involves two successive nuclear and cytoplasmic divisions, producing up to four cells.

Despite two successive nuclear divisions, the DNA and other chromosome components duplicate only once, during the interphase preceding the first meiotic division.

Each of the four cells produced by meiosis contains the haploid chromosome number, that is, only one chromosome set containing only one representative of each homologous pair.

During meiosis, each homologous chromosome pair is shuffled, so the resulting haploid cells each have a virtually unique combination of genes.

Meiosis typically consists of two nuclear and cytoplasmic divisions, designated the first and second meiotic divisions, or simply meiosis I and meiosis II (Fig. 10-15). Each includes prophase, metaphase, anaphase, and telophase stages. During meiosis I, the partner homologous chromosomes physically pair with each other and subsequently separate and move into different nuclei. In meiosis II, the sister chromatids that make up each duplicated chromosome separate from each other and are distributed to two different nuclei. The following discussion describes meiosis in an organism with a diploid chromosome number of 4. Refer to Fig. 10-16 as you read.

The events of meiosis are similar to the events of mitosis, with four important differences:

Meiosis involves two successive nuclear and cytoplasmic divisions, producing up to four cells.

Despite two successive nuclear divisions, the DNA and other chromosome components duplicate only once, during the interphase preceding the first meiotic division.

Each of the four cells produced by meiosis contains the haploid chromosome number, that is, only one chromosome set containing only one representative of each homologous pair.

During meiosis, each homologous chromosome pair is shuffled, so the resulting haploid cells each have a virtually unique combination of genes.

In addition to the unique processes of synapsis and crossing-over, events similar to those in mitotic prophase also occur during prophase I. A spindle forms, consisting of microtubules and other components. In animal cells one pair of centrioles moves to each pole, and astral microtubules form. The nuclear envelope disappears in late prophase I, and in cells with large and distinct chromosomes, the structure of the tetrads can be seen clearly with the microscope.

The sister chromatids remain closely aligned along their lengths. However, the centromeres (and kinetochores) of the homologous chromosomes become separated from one another. In late prophase I, the homologous chromosomes are held together only at specific regions, called chiasmata (sing., chiasma). Each chiasma originates at a crossing-over site, that is, a site at which homologous chromatids exchanged genetic material and rejoined, producing an X-shaped configuration (Fig. 10-18). At the chiasmata, cohesins hold homologous chromosomes together after the synaptonemal complex has been disassembled. Later, the cohesins dissociate from the chiasmata, freeing the homologous chromosome arms from one another. The consequences of crossing-over and genetic recombination are discussed in Chapter 11 (e.g., see Fig. 11-12).

Metaphase I occurs when the tetrads align on the midplane. Both sister kinetochores of one duplicated chromosome are attached by spindle fibers to the same pole, and both sister kinetochores of the other duplicated homologous chromosome are attached to the opposite pole. (By contrast, sister kinetochores of each duplicated chromosome are attached to opposite poles in mitosis.)

During anaphase I, the paired homologous chromosomes separate, or disjoin, and move toward opposite poles. Each pole receives a random combination of maternal and paternal chromosomes, but only one member of each homologous pair is present at each pole. The sister chromatids remain united at their centromere regions. Again, this process differs from mitotic anaphase, in which the sister chromatids separate and move to opposite poles.

During telophase I, the chromatids generally decondense somewhat, the nuclear envelope may reorganize, and cytokinesis may take place. Each telophase I nucleus contains the haploid number of chromosomes, but each chromosome is a duplicated chromosome (it consists of a pair of chromatids). In our example, 2 duplicated chromosomes lie at each pole, for a total of 4 chromatids; humans have 23 duplicated chromosomes (46 chromatids) at each pole.

An interphase-like stage called interkinesis usually follows. Interkinesis is not a true interphase: there is no S phase and therefore no DNA replication. Interkinesis is brief in most organisms and absent in some.

Because the chromosomes usually remain partially condensed between divisions, the prophase of the second meiotic division is brief. Prophase II is similar to mitotic prophase in many respects. There is no pairing of homologous chromosomes (indeed, only one member of each pair is present in each nucleus) and no crossing-over.

During metaphase II, the chromosomes line up on the midplanes of their cells. You can easily distinguish the first and second metaphases in diagrams; at metaphase I the chromatids are arranged in bundles of four (tetrads), and at metaphase II they are in groups of two (as in mitotic metaphase). This difference is not always so obvious in living cells.

During anaphase II, the chromatids, attached to spindle fibers at their kinetochores, separate and move to opposite poles, just as they would at mitotic anaphase. As in mitosis, each former chromatid is now referred to as a chromosome. Thus, at telophase II there is one representative for each homologous pair at each pole. Each is an unduplicated (single) chromosome. Nuclear envelopes re-form, the chromosomes gradually elongate to form chromatin fibers, and cytokinesis occurs.

The two successive divisions of meiosis yield four haploid nuclei, each containing one of each kind of chromosome. Each resulting haploid cell has a different combination of genes. This genetic variation has two sources:

DNA segments are exchanged between maternal and paternal homologues during crossing-over; and

during meiosis, the maternal and paternal chromosomes of homologous pairs separate independently, and the chromosomes are “shuffled” so that each member of a pair becomes randomly distributed to one of the poles at anaphase I.

Although mitosis and meiosis share many similar features, specific distinctions between these processes result in the formation of different types of cells. Mitosis is a single nuclear division in which sister chromatids separate from each other and are distributed to the two daughter cells, which are genetically identical to each other and to the original cell. A diploid cell that undergoes mitosis produces two diploid cells. Similarly, a haploid cell that undergoes mitosis produces two haploid cells. (Some eukaryotic organisms—e.g., certain yeasts—are haploid, as are plants at certain stages of their life cycles.) Homologous chromosomes do not associate physically at any time in mitosis.

In meiosis, a diploid cell undergoes two successive nuclear divisions, meiosis I and meiosis II. In prophase I of meiosis, the homologous chromosomes undergo synapsis to form tetrads. Homologous chromosomes separate during meiosis I, and sister chromatids separate during meiosis II. Meiosis ends with the formation of four genetically different, haploid daughter cells. The fates of these cells depend on the type of life cycle; in animals they differentiate as gametes, whereas in plants they become spores.