Mitosis and Meiosis

David S. Buchanan

Oklahoma State University, Stillwater, Oklahoma, U.S.A.

INTRODUCTION

Multicelled organisms are composed of millions or billions of cells. Although these cells perform numerous diverse functions, almost all of them contain the genetic information necessary for operation of the entire organism. This information is stored in deoxyribonucleic acid (DNA). The structure and organization of DNA were discovered only 50 years ago.[1]

Two unique processes must be present for these cells to contain the proper DNA. These processes are called mitosis and meiosis. Mitosis is ordinary cell division. Meiosis is the process by which sperm and egg cells are formed in a manner that allows them to join together to start a new life. Detailed descriptions of these can be found in any standard textbook on genetics.

MITOSIS

DNA is organized into genes that reside on chromosomes. In animals, the chromosomes are linear DNA-protein complexes that reside in the nucleus of cells. Each species has a characteristic number of chromosomes (Table 1).[2]

Mitosis is the process by which one cell becomes two cells. Each life begins as a single cell and billions of mitoses enable the organism to grow and mature. New cells replace those that are no longer viable or are lost. Yet each of those cells, with only a few exceptions, contains the same genetic information. The process of mitosis allows this to happen properly.

Each cell goes through a process called the cell cycle. This cycle is roughly divided into four phases that total approximately 24 hours. One of those phases is mitosis, which takes approximately one hour. The other three phases presynthesis gap, synthesis of DNA, and postsyn-thesis gap compose interphase. The key event that occurs during interphase is the replication of DNA. During this time each chromosome is precisely duplicated.

The cell goes through a very precise four-step process during mitosis. These steps are called prophase, metaphase, anaphase, and telophase (Fig. 1). During prophase the chromosomes shorten and thicken, such that they become visible by examination under a microscope. Spindle fibers begin to form at the ends of the cell and they attach to the centromeres (point of junction between the two chromatids). The chromosomes are composed of two chromatids as they enter mitosis because they have undergone replication during interphase. During metaphase the chromosomes migrate to and line up along the central plane of the cell. Anaphase is characterized by the splitting of the centromeres and migration of the chromosomes toward the poles. This is accomplished with the assistance of the spindle fibers. The final phase is telophase, during which the cell will undergo cytokinesis (division of the cytoplasm or cell contents) to become two cells.

At this point there are two new cells where previously only one existed. The genetic material in the nucleus of the two new cells is identical. Furthermore, even if these are cells that have become specialized (muscle, nerve, skin, cardiac, etc.), each of these new cells contains all of the genetic material for the organism, not just the genetic material pertaining to the nature of the specialized cell.

MEIOSIS

In organisms that reproduce sexually, there must be a mechanism by which cells are formed that can combine to form a new member of the species. These specialized cells are called gametes. In animals, the male gamete is called the sperm and the female gamete is called the egg. Such cells must contain one member of each pair of chromosomes so that, when combined with the cell from the other parent, they will form a zygote (fertilized egg), which is the first cell of the new organism. The mechanism by which such cells are formed is called meiosis.

In animals, meiosis occurs only in those cells that are designed to produce the sperm or egg cells. Such cells exist in the testes in the male and in the ovaries of the female. These cells undergo normal mitoses until they are prepared to start a process during which they will reduce the number of chromosomes by half and produce gametes.

Meiosis involves two successive cell divisions (Fig. 2). Each division includes a prophase, metaphase, anaphase, and telophase. There is no interphase between the two divisions. Prophase I is divided into several specific

Table 1 Chromosome numbers of selected species

Organism

Chromosome number (2N)

Human

46

Cattle

60

Goat

60

Sheep

54

Swine

38

Chicken

78

Turkey

82

Horse

64

Donkey

62

Dog

78

Cat

38

Fruit fly

8

Mouse

40

phases: leptotene (thin thread), during which the chromosomes start to condense; zygotene (paired thread), when the homologous chromosomes (members of the same pair) pair together (synapsis); and pachytene (thick thread), in which the homologous pairs form tetrads (each pair of chromosomes is composed of four chromatids). The genetic diversity-increasing process of crossing over (sharing of genetic material between homologous chromosomes) begins during pachytene. Finally, diplotene (double thread) is characterized by more condensation of the chromosomes and clearly distinguished chiasma (points of crossing over). Prophase I is completed when the homologous chromosomes begin to pull apart, except at the points of chiasma in a process called diakinesis. Metaphase I is characterized by the homologous pairs of chromosomes lining up along the central plane. During anaphase I the homologous chromosomes separate, but the chromatids stay together and during telophase I the

Fig. 1 The four stages of mitosis. (From Refs. 3 6.)

Fig. 1 The four stages of mitosis. (From Refs. 3 6.)

cellular material divides so that there are now two cells. At this point each of the two cells contains one replicated chromosome from each pair. The member of each homologous pair that is contained in each cell is chosen randomly. There is no interphase, and therefore no replication, prior to prophase II. Indeed, the events of telophase I and prophase II blend together so that there are not two separate pictures in Fig. 2 illustrating the two phases. During metaphase II the chromosomes line up along the metaphase plate. Anaphase II is characterized by a splitting of the centromeres and movement of the separated chromatids (now called chromosomes again) to the poles. During telophase II, the cellular material splits and daughter cells are formed. There are now four cells, each with one member of each homologous pair of chromosomes. Genetic diversity has been created by the Mendelian law of segregation, which describes the fact that the member of each pair of chromosomes that resides in the gamete is random. The principle of independent assortment describes the fact that the segregation of chromosomes for any one pair is independent of the segregation of chromosomes for any other pair and crossing over.

GAMETOGENESIS

Meiosis, when combined with additional processes to form functional gametes, is called gametogenesis (formation of gametes). In the males, this is called spermato-genesis and is referred to as oogenesis in the female. Spermatogenesis occurs continuously throughout the reproductive life of the male. Oogenesis is typically arrested during prophase I until the female begins to ovulate. At that time, oogenesis continues to develop one egg for release at each ovulation. For some species, like cattle, there is typically one ovulation each estrous cycle. Litter-bearing species may have several ovulations during each estrous cycle. Each spermatogenesis results in four sperm cells that have the potential to be viable. Each oogenesis results in only one potentially viable egg cell. The other products of meiosis are called polar bodies. Polar bodies contain the chromosomes in the nucleus but little additional cytoplasm (nonnucleus cell contents). The resulting larger size means that the egg has more nutrients for the developing embryo than for the cells that become the polar bodies.

CONCLUSION

Mitosis is ubiquitous in life. The machinery for mitosis is essentially the same in almost all organisms. One cell becomes two cells. Each of those becomes two cells and so on and so on. Single-celled organisms undergo mitosis as the mechanism of reproduction, whereas multicelled organisms undergo mitosis to build bodies. In either case, the daughter cells have the same genetic material and have all the genetic material for any type of cell, even if differentiation has occurred and the cells in question now have a specific function. Species with sexual reproduction also undergo meiosis in a manner that is highly uniform across a wide range of species. In meiosis, gametes are formed that contain one member of each chromosome pair. Processes in meiosis create genetic diversity through a number of mechanisms such that no two gametes produced by any one individual contain precisely the same genes. These mechanisms must work properly an exceptionally high proportion of the time in order for life to continue.

Mitosis and meiosis are similar processes but differ in some important ways. Mitosis involves only a single division, whereas meiosis involves two divisions. Mitosis results in two identical daughter cells, whereas meiosis results in four unique daughter cells. The daughter cells from a mitotic division contain the full complement of chromosomes, whereas the daughter cells from meio-sis contain just one representative from each pair of chromosomes.

REFERENCES

1. Watson, J.D.; Crick, F.H.C. Molecular structure of nucleic acids. Nature 1953, 171, 737 738.

2. Hutt, F.B.; Rasmusen, B.A. Animal Genetics, 2nd Ed.; John Wiley & Sons Inc.: New York, 1982; 108 121.

3. Hartl, D.L.; Jones, E.W. Genetics: Analysis of Genes and Genomes, 5th Ed.; Jones and Bartlett: Sudbury, MA, 2001; 134 148.

4. Klug, W.S.; Cummings, M.R. Concepts of Genetics, 7th Ed.; Prentice Hall Pearson Education, Inc.: Upper Saddle River, NJ, 2003; 19 44.

5. Russell, P.J. Genetics; Benjamin Cummings: San Francisco, 2002; 9 24.

6. Snustad, D.P.; Simmons, M.J. Principles of Genetics, 2nd Ed.; John Wiley & Sons, Inc.: New York, 2000; 23 40.

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