Unit 20 CELL REPRODUCTION LEARNING OBJECTIVES: 1. Be able to distinguish the differences between mitotic and meiotic cell division. 2. Learn the role that both mitotic and meiotic types of cell division play in the human life cycle. 3. Learn the sequence of events in the cell cycle. 4. Learn the events that must occur in order to assure that each of the cells resulting from mitotic or meiotic cell division end up with identical compliments of chromatin (chromosomes). Introduction One characteristic of all living organisms is that cells and organisms reproduce themselves. Organismic reproduction can be classified as either asexual or sexual. Sexual reproduction requires the bringing together of two different types of cells, thus uniting genetic material from two different sources. Asexual reproduction results from one organism splitting off a part of itself to produce a new organism; this is called vegetative reproduction. Or it can result from the process of sporulation; in this type, a small part of the parent organism is split off in a single cell called a spore. In both vegetative reproduction and sporulation, the genetic makeup of the new individual is identical to the original parent. There are two types of cell reproduction: one type results in the production of somatic or body cells, and results in an increase in size of the organism; and the other type results in the production of reproductive cells -- sperm and eggs, which when they combine results in a new individual. This is the means of sexual reproduction. Whereas, only the first type is necessary for an individual organism to increase in size and repair or replace tissue, the second type is necessary for the continuance of the species. The production and fusion of reproductive cells allow each parent to contribute half the genetic material for the new individual or offspring; this allows for new combinations of characteristics that may not be present in the parent organisms. Figure 20-1 illustrates the role of both types in the human life cycle. As seen in this figure, reproductive cells contain half the genetic material of somatic or body cells; 228
this condition is indicated by the notation N=1. The portion of the life cycle in which the cells contain half the normal amount of genetic material is called the haploid generation. The fusion of gametes each containing half the normal amount of genetic material is called fertilization. Fertilization restores the normal amount of genetic material found in somatic or body cells; the normal amount of genetic material in the somatic or body cells is represented by the notation N=2. The portion of the life cycle during which the cells contain N=2 genetic material is called the diploid generation. Figure 20-1. The human life cycle involves both mitotic and meiotic cell divisions. Mitotic cell division occurs in the production of somatic or body cells. Two cells are produced from one parent cell with each of the daughter cells having the same chromosome complement (same number and type) as the original parent cell. Starting with a fertilized egg, the zygote, mitotic division results in an embryo, fetus, and newborn. The newborn grows into a mature adult with most of the growth accomplished by an increase in number of cells using mitotic divisions. In addition to an increase in the 229
number of cells, growth also is accomplished by an increase in cell size and an accumulation of intercellular matrix (i.e. calcium and phosphorus salts in the hard matrix of bone). Meiotic cell division occurs in the production of reproductive cells -- sperm and eggs in animals and spores in plants. Each reproductive cell, gamete, will contain only half the chromosome complement of the original parent cell. The production of eggs is called oogenesis; the production of sperm is called spermatogenesis. Fertilization is the fusion of the sperm and egg nuclei, which restores the chromosome complement in the zygote; this process allow for each parent to contribute half the chromosomes for the new individual. The genetic material, chromatin, is DNA, deoxyribonucleic acid; it is a long strand of material and the cell, depending on the individual organism, has a specific number of chromatin strands. The human body has 46 strands. A chromosome consists of a folded and coiled strand of chromatin with associated protein and some RNA, ribonucleic acid (Fig. 20-2). 230
Figure 20-2. Illustrates the relationship between double stranded DNA and the chromosome. Along the length of the chromosome are genes. The gene is the basic unit of heredity and codes for proteins, trna, and rrna. As of the present time, it is thought that the human cell contains about 30,000 genes. The entire amount of chromatin, 231
genes, in a cell is called the genome. Thus the process or project of identifying all the genes is referred to as the Human Genome Project. Mitosis There are problems to overcome in mitosis. The cell must synthesize chromatin material for the daughter cells. This process is called DNA replication. Somehow, the cell must get the DNA out of the nucleus and make sure that each of the daughter cells gets an identical amount of chromatin. And finally the parent cell must provide each of the daughter cells with equal portions of its cytoplasmic organelles. Figure 20-3 illustrates the phases of the cell cycle showing the sequence of the required activities. The cycle can be divided into mitosis and interphase, the interval between successive mitotic divisions. Cells spend the majority of their time in interphase. Interphase can be separated into three phases. The first phase is G 1, growth; this a period of time during which general growth and replication of cytoplasmic organelles occur. The second phase labeled S is when chromatin, DNA, replication occurs. The third phase, G 2, is another growth phase, which is the final preparation for mitosis. Mitosis, M phase, then occurs. Figure 20-3. Showing the phases of the mitotic cell cycle. 232
Table 20-1 provides an example of the amount of time required for each phase of the cell cycle. This is only an example for human cells in tissue culture. Different types of cells can have longer or shorter time periods in each phase of the cycle, but the % of each in the cell s life cycle remains relatively constant. Table 20-1. Relative times for different phases of the mitotic cell cycle. Phase Time (hours) % of Cell Life Cycle G1 8.2 42 S 6.2 32 G2 4.6 23 M 0.6 3 Total 19.6 100 Mitotic cell division can result in either reproduction or growth (Fig. 20-4). Many simple organisms, such as bacteria and single-cell algae, reproduce by the one parent cell dividing and the two resulting cells separating from one another and thus producing two individual organisms. The two resulting organisms would be identical; production of two identical organisms is often termed cloning with each of the new, identical individuals termed a clone. Human beings employ this type of reproduction in the formation of identical twins. During a very early stage in the development of the zygote, the division results in two separate cells as in Figure 20-4. Each cell, which is genetically identical, will then develop into an individual fetus. Since the two cells are genetically identical, each fetus will be identical, thus identical twins. Identical refers to genetic material; the influence of environment will result in certain differences in the twins even though they possess identical genetic material. If the two daughter cells stay together, the result is growth. If the early embryo starts to split but does not complete the separation into two individual organisms, it may result in Siamese twins. Figure 20-4. Mitotic cell division resulting in reproduction (A) or growth (B). 233
Mitosis involves one duplication and one division of chromatin (Fig. 20-5). The duplication, replication, occurs during the S phase and the division during M phase. And although this process is a continuous one, it can be separated into several phases, each being given a phase name. Rather than memorize phase names, it is better to understand the sequence of events that must occur in order to get the duplication and an equal proportion of DNA to the two daughter cells. Figure 20-5. Showing the replication and division of chromatin during mitosis. The nuclear membrane breaks down, the duplicated chromosome line up across the middle, equator, of the cell, protein fibers, spindle fibers, which are organized by the centriole attach to the chromatids. The centriole lies outside the nucleus and divides and organizes the spindle fibers during mitosis and meiosis. The spindle fibers help guide the chromosomes to opposite polar positions in the cell. The cell then forms a nuclear membrane around each of the polar groups of chromosomes. A separate process called cytokinesis now occurs. This results in the division of the cytoplasm. In 234
animal cells this occurs by a furrow being produced around the equator of the cell. This furrow constricts, like pulling a belt tighter and tighter around ones middle, until the cell is divided into two equal parts. This is then followed by interphase-- G 1, S, and G2 phases in preparation for the next mitosis. Meiosis During meiotic cell division there is one duplication and two divisions of chromatin or chromosomes resulting in having half the chromosome complement of the parent cell (Fig. 20-6). This process results in 4 sperm cells from one parent cell but only one egg cell from one parent cell. The parent cell has 2 sets of chromosomes, 23 pairs in humans, which is called the diploid number or 2N. The sperm or egg receives a single set of chromosomes, the haploid number or 1N. The uniting of the sperm and egg during fertilization results in a 2N cell, the zygote. The first meiotic division results in the separation of the homologous chromosomes. The two chromosomes of each kind have the same structure and contain genes for the same traits and are called homologous chromosomes. The second division results in the separation of the original chromosome from its duplicate. Because the original chromosome and its duplicate share a common centromere, they are referred to as chromatids. The centromere is the condensed region on a chromosome where sister chromatids are attached to each other. It is the last part of the chromosome to separate. As soon as the chromatids separate, they each possess their own centromere and are chromosomes. 235
Figure 20-6. Showing one replication and two divisions of chromatin during meiosis. Figure 20-7A shows how 2 divisions result in the production of 4 sperm but only one egg. The first division in oogenesis results in half the chromosomes moving out of the egg into a polar body. The polar body contains mainly nuclear material and little, in any, cytoplasm. The second division results in the production of a second polar body with the same results. The one resulting egg ends up with all the original cytoplasm; whereas, in the production of the sperm, each of the four resulting sperm gets ¼ of the original cytoplasm. 236
Figure 20-7. Meiotic cell division results in the production of 4 sperm or 1 egg (A). The 2N chromosome number is restored by the process of fertilization and maintained by mitotic division in all cells of the embryo (B). Figure 20-7B illustrates the process of fertilization. During fertilization, the egg or ovum is penetrated by the sperm as the egg moves through the fallopian tube on its way from the ovary to the uterus. The nuclei from the sperm and egg fuse; since each nuclei contains half the normal complement of chromosomes, the resulting fusion nucleus contains a full complement of chromosomes. The fertilized egg cell undergoes mitotic cell division to develop into a blastocyst on its way to developing into the fetus. 237
In this unit we have examined the role of mitosis and meiosis in growth and reproduction of organisms. We have introduced the concept of genes along a chromosome as the units of heredity. In the next unit, we will continue this examination with a further examination of the structure and function of genes and chromosomes. 238
Unit 20 OBJECTIVE QUESTIONS OVER CELL REPRODUCTION 1. Mitotic cell division involves of chromatin material. A) one duplication and one division (B) one duplication and two divisions C) two duplications and one division (D) two duplications and two divisions 2. Meiotic cell division involves of chromatin material. (A) one duplication and one division (B) one duplication and two divisions (C) two duplications and one division (D) two duplications and two divisions 3. Chromosome (chromatin) duplication takes place during phase. (A) G1 (B) G2 (C) M (D) S 4. Spindle fibers are made of (A) protein (B) lipid (C) carbohydrate. 5. A is/are composed of DNA. (A) gene (B) chromatin (C) chromosome (E) chromatin fiber (E) two of the preceding (F) three of the preceding (G) all the preceding 6. Cells that contain half the normal amount of DNA are called (A) diploid (B) haploid (C) polyploidy (D) monoploidy 7. Fusion of gametes to form a zygote is termed (A) meiosis (B) cytokinesis (C) fertilization (D) oogenesis 8. The production of sperm is called (A) fertilization (B) oogenesis (C) spermatogenesis (D) morphogenesis 9. The portion of the life cycle of an organism during which the cell contains N=2 genetic material is called the generation. (A) haploid (B) diploid (C) bipolid (D) polyploid 10. The entire amount of chromatin in a cell is called the (A) centromere (B) histone (C) genome (D) zygote 11. The phase of the cell cycle during which general growth and replication of cytoplasmic organelles occur is called the phase (A) S (B) M (C) G1 (D) G2 12. The phase of the cell cycle during which final preparation for mitosis occurs is (A) S (B) M (C) G1 (D) G2 239
13. The division of the cytoplasm during cell division is term (A) cytokinesis (B) mitosis (C) microkinesis (D) meiosis 14. The condensed region on a chromosome where sister chromatids are attached to each other is called the (A) chromatin (B) chromomere (C) centromere (D) zygote 15. The original and duplicate chromosomes attached through a centromere are called (A) chromatids (B) centrioles (C) polar bodies (D) spindles 16. are produced during the first and second meiotic divisions of oogenesis and contain little, if any, cytoplasm. (A) oocytes (B) ootids (C) polar bodies (D) spindles 17. The fertilized egg cell undergoes mitotic cell division to develop into a hollow sphere called a (A) oocyte (B) blastocyst (C) ootid (D) ovum 18. Cell division that produces the sex cells or gametes used in sexual reproduction is called (A) mitosis. (B) meiosis. (C) cytokinesis. (D) parthenocarpy. 19. The end result of mitotic cell division can be either (A) growth (B) reproduction (C) both A and B (D) neither A or B. 20. Mitosis occurs in the production of cells and meiosis occurs in the production of cells. 21. Relate the symbols 1N and 2N to the cells below: a. body cell b. egg cell c. sperm cell DISCUSSION QUESTIONS OVER CELL REPRODUCTION 1. In single-celled organisms, mitotic division can result in reproduction. Does this ever occur in higher organisms such as humans? Explain. 2. Where does the material come from to duplicate, replicate, the chromosomes during mitosis and meiosis? 3. Relate the following terms: chromatid, chromatin, chromosome, DNA, nucleic acid. 4. What is the advantage of G 1 and G 2 being almost eliminated during early embryonic development? 240
5. Where does the nuclear membrane go during the early phases of mitosis? 6. What advantage is there to only one egg developing from a parent cell during oogenesis? 7. Do you have any cells or tissues in your body that are continuously undergoing mitotic division? Explain. 8. Do you have any of your cells continuously undergoing meiosis? Explain. 9. How does a chromatid differ from a chromosome? 10. What is cytokinesis? 241