Cell Reproduction

Growth and reproduction of organisms proceeds primarily by increasing the number of cells rather than by increasing cell size. The cell cycle is a sequence of growth, duplication, and division. Cell reproduction in prokaryotes is called binary fission. Cytoplasm is produced during growth and the cell enlarges. The single loop of DNA and other structures, such as the ribosomes, are duplicated and collect at opposite ends of the cell. A partition grows between the two ends, which then separate into two new cells, called daughter cells. Under optimum conditions of temperature, nutrients, and so on, bacteria can double as often as once every 10 minutes. For example, under ideal conditions, E. coli doubles in as little as 17 minutes, and Bacillus sterothermophilus can double in 10 minutes.

Cell division in eukaryotes is more complex. In eukaryotic cells, the chromosome is a complex structure consisting of a single linear DNA molecule wrapped up around a group of proteins. The eukaryotic chromosomes are large enough that at some stages of the cell cycle they can be made visible in a light microscope by staining. At this point it is worth reviewing the organization of DNA in a eukaryotic cell. DNA consists of two complementary strands of a linear polymer of nucleic acids, which forms a double helix. Each three units of the polymer codes for a single amino acid. A sequence of these units contains the code for a single protein. A gene is a sequence of DNA that codes for a single protein. One DNA molecule has many genes. Each DNA molecule is wrapped up with proteins in a chromosome structure. Most eukaryotic cells have their chromosomes in functional pairs. Thus, there will be two of each gene in the nucleus, both of which code for a protein

Helical coil DNA double helix

Figure 4.4 Chromosome structure in eukaryotes. (Based on Postlethwait and Hopson, 1995.)

Helical coil DNA double helix

Figure 4.4 Chromosome structure in eukaryotes. (Based on Postlethwait and Hopson, 1995.)

having the same function. One chromosome of each pair comes from the organism's father, the other from its mother. It is important to note that the proteins are not necessarily identical, just that they have the same function. Humans, for example, have 23 pairs of chromosomes, 46 in all.

There are two main phases of eukaryotic cell reproduction. In interphase, the cell produces new protein, ribosomes, mitochondria, and so on, and this is followed by replication (copying) of the cell's chromosomes. Each chromosome is linked with its new copy at a point along its length, so they have the appearance of two sausagelike shapes tied together. This results in a number of X- or Y-shaped structures, depending on where they are linked (see Figure 4.4). The cell then enters the division phase. The division phase itself is composed of two stages. Mitosis is the separation of the replicated DNA into two new nuclei, and cytokinesis is the distribution of the cytoplasm components and a physical separation into daughter cells. The eukaryotic cell cycle may take much longer than prokaryotic binary fission. Red blood cells are produced by division of cells in the bone marrow, which divide every 18 hours. The events of mitosis are the most dramatic of the cell cycle, and can be readily observed under the microscope with suitable staining.

Figure 4.5 shows that mitosis is also separated into a number of stages of development. At the beginning of mitosis, the chromosomes are dispersed in the nucleus and not readily visible under the light microscope. As mitosis proceeds, the nuclear envelope disperses and a spindle forms, consisting of a web of microtubules. By this time the chromosomes can be made visible under a light microscope when stained, displaying the well-known X and Y shapes. (The X, for example, is actually two replicated chromosomes linked along their length, due to be separated into daughter cells. The Y shape is the same thing except that the chromosomes are joined near one end.) The chromosomes migrate into positions aligned along the center between the two poles of the spindle. The spindle then physically separates the chromosomes to the poles of the now-elongated cell. Finally, the spindle dissolves and the nucleus re-forms, completing mitosis. Cytokinesis continues, forming new plasma membranes to complete the cell division.

Figure 4.5 Stages of mitotic division. (From Fried, 1990. © The McGraw-Hill Companies, Inc. Used with permission.)

There are two salient facts regarding mitosis. First, mitosis results in two cells with identical genetic composition, barring errors in reproduction. Thus, each of our "body" cells are genetically identical. Skin cells have the same genes as pancreas cells, so skin cells have the genes to produce insulin, although they do not actually do so. The remarkable variety in cell structure and function despite their having the same control codes is one of the greatest mysteries in biology. How, during the formation of the body as well as later, does a cell "know" that it is a bone marrow cell and not a liver cell?

The second salient fact about mitosis is that it both starts and ends with cells having two copies of each chromosome. Such cells are called diploid cells (Figure 4.6). The "body" cells in most multicellular organisms are diploid and are called somatic cells. This is to distinguish them from cells that are destined to produce offspring by way of sexual reproduction, called germ cells, which are also diploid.

Germ cells produce gametes, which are reproductive cells such as the egg and sperm cells in animals, and the egg cells and pollen spores in flowering plants. Gametes have only one copy of each chromosome, a condition that is called haploid. In the process of fertilization the haploid germ cells from two parents fuse to create a new diploid

cell, the zygote, which then grows into a new organism. Thus, the life cycle of complex multicellular organisms such as animals and higher plants consists of growth by mitotic cell division, meiosis, and fertilization.

Meiosis is a form of cell division that converts a diploid cell into a haploid cell. Meiosis actually consists of two cell divisions, resulting in four cells that have chromosomes that are not only different from each other, but are different from those in the parent cell.

The process will be illustrated using the example of a cell with two sets of chromosomes (Figure 4.7). This diploid cell has two short chromosomes: one from the organism's father (shown in black) and one from the mother (white). Similarly, there are two longer chromosomes, one from each parent. As in mitotic division, the chromosomes replicate. But a critical difference is that the chromosome pairs come together and may intertwine themselves. At the points where the chromosomes cross each other, the DNA molecules can break and reconnect with the fragment from its complement. This results in new chromosomes that can have both maternal and paternal genes. This process is called crossing over, and the resulting exchange of genetic material between chromosomes is called recombination. The crossing-over point can occur at many locations on a single chromosome, although only one is shown in the figure. Thus, a huge number of possible combinations can result.

Recombination is an important source of genetic variability in organisms. In addition, it is important to science for several reasons. It is the basis of chromosome mapping, a technique that determines the location of genes on the chromosomes. Artificial control of recombination has become one of the most important procedures of genetic engineering (discussed further in Chapter 6). It has inspired a mathematical optimization method called appropriately the genetic algorithm, which has applications far beyond biology.

After crossing over, the replicated chromosomes line up and are separated by a spindle apparatus, just as in mitosis. However, note that this could occur in four different ways. The two daughter cells could have all paternal chromosomes in one cell and maternal chromosomes in the other; or one cell could have a long paternal and a short maternal, and the other would then have a long maternal and a short paternal. If there were more

chromosomes, more combinations would be possible. In all, there would be 2N, where N is the number of chromosome pairs. For humans with 23 pairs, this results in 223 = 8.4 x 106 possible combinations. If crossing over did not occur, this is the number of genetically unique offspring that a single human couple could possible produce. The combinations occur randomly, depending on how the chromosomes align before being pulled apart by the spindle. The random distribution of chromosomes to the daughter cells in the first stage of meiosis is called independent assortment.

When random assortment is combined with crossing over, the number of possible daughter cells that can be formed is extremely large. This is an important source of the random variation that drives evolution. Even without mutations, crossing over can create new genes by combining pieces of two old ones. The great variation in offspring that results can help a species adapt rapidly to changing environmental conditions. This is one of the great evolutionary advantages of sexual reproduction.

The meiotic process just described is actually only the first step, called meiosis I. After that step, two daughter cells are produced that although already haploid, contain two of each type of chromosome that originated in the replication process. In meiosis II, these are separated in another division process, resulting in a total of four haploid cells, each of which probably has a unique genetic complement.

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