Microbiology and Immunology

WORLD OF MICROBIOLOGY AND IMMUNOLOGY Cell cycle and cell division

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logues, consist of two chromosomes that carry genetic infor-

mation for the same traits, although that information may hold

different messages (e.g., when two chromosomes carry a mes-

sage for eye color, but one codes for blue eyes while the other

codes for brown). The fertilized eggs (zygotes) of all sexually

reproducing organisms receive their chromosomes in pairs,

one from the mother and one from the father. During synapsis,

adjacent chromatids from homologous chromosomes “cross

over” one another at random points and join at spots called

chiasmata. These connections hold the pair together as a tetrad

(a set of four chromatids, two from each homologue). At the

chiasmata, the connected chromatids randomly exchange bits

of genetic information so that each contains a mixture of

maternal and paternal genes. This “shuffling” of the DNA pro-

duces a tetrad, in which each of the chromatids is different

from the others, and a gamete that is different from others pro-

duced by the same parent. Crossing over does explain why

each person is a unique individual, different even from those

in the immediate family. Prophase I is also marked by the

appearance of spindle fibers (strands of microtubules) extend-

ing from the poles or ends of the cell as the nuclear membrane

disappears. These spindle fibers attach to the chromosomes

during metaphase I as the tetrads line up along the middle or

equator of the cell. A spindle fiber from one pole attaches to

one chromosome while a fiber from the opposite pole attaches

to its homologue. Anaphase I is characterized by the separa-

tion of the homologues, as chromosomes are drawn to the

opposite poles. The sister chromatids are still intact, but the

homologous chromosomes are pulled apart at the chiasmata.

Telophase I begins as the chromosomes reach the poles and a

nuclear membrane forms around each set. Cytokinesis occurs

as the cytoplasm and organelles are divided in half and the one

parent cell is split into two new daughter cells. Each daughter

cell is now haploid (n), meaning it has half the number of

chromosomes of the original parent cell (which is diploid-2n).

These chromosomes in the daughter cells still exist as sister

chromatids, but there is only one chromosome from each orig-

inal homologous pair.

The phases of meiosis II are similar to those of meiosis

I, but there are some important differences. The time between

the two nuclear divisions (interphase II) lacks replication of

DNA (as in interphase I). As the two daughter cells produced

in meiosis I enter meiosis II, their chromosomes are in the

form of sister chromatids. No crossing over occurs in prophase

II because there are no homologues to synapse. During

metaphase II, the spindle fibers from the opposite poles attach

to the sister chromatids (instead of the homologues as before).

The chromatids are then pulled apart during anaphase II. As

the centromeres separate, the two single chromosomes are

drawn to the opposite poles. The end result of meiosis II is that

by the end of telophase II, there are four haploid daughter cells

(in the sperm or ova) with each chromosome now represented

by a single copy. The distribution of chromatids during meio-

sis is a matter of chance, which results in the concept of the

law of independent assortment in genetics.

The events of meiosis are controlled by a protein

enzyme complex known collectively as maturation promoting

factor (MPF). These enzymesinteract with one another and

with cell organelles to cause the breakdown and reconstruction

of the nuclear membrane, the formation of the spindle fibers,

and the final division of the cell itself. MPF appears to work

in a cycle, with the proteins slowly accumulating during inter-

phase, and then rapidly degrading during the later stages of

meiosis. In effect, the rate of synthesis of these proteins con-

trols the frequency and rate of meiosis in all sexually repro-

ducing organisms from the simplest to the most complex.

Meiosis occurs in humans, giving rise to the haploid

gametes, the sperm and egg cells. In males, the process of

gamete production is known as spermatogenesis, where each

dividing cell in the testes produces four functional sperm cells,

all approximately the same size. Each is propelled by a prim-

itive but highly efficient flagellum (tail). In contrast, in

females, oogenesis produces only one surviving egg cell from

each original parent cell. During cytokinesis, the cytoplasm

and organelles are concentrated into only one of the four

daughter cells—the one that will eventually become the

female ovum or egg. The other three smaller cells, called polar

bodies, die and are reabsorbed shortly after formation. The

concentration of cytoplasm and organelles into the oocyte

greatly enhances the ability of the zygote (produced at fertil-

ization from the unification of the mature ovum with a sper-

matozoa) to undergo rapid cell division.

The control of cell division is a complex process and is

a topic of much scientific research. Cell division is stimulated

by certain kinds of chemical compounds. Molecules called

cytokinesare secreted by some cells to stimulate others to

begin cell division. Contact with adjacent cells can also con-

trol cell division. The phenomenon of contact inhibition is a

process where the physical contact between neighboring cells

prevents cell division from occurring. When contact is inter-

rupted, however, cell division is stimulated to close the gap

between cells. Cell division is a major mechanism by which

organisms grow, tissues and organs maintain themselves, and

wound healing occurs.

Cancer is a form of uncontrolled cell division. The cell

cycle is highly regulated by several enzymes, proteins, and

cytokines in each of its phases, in order to ensure that the

resulting daughter cells receive the appropriate amount of

genetic information originally present in the parental cell. In

the case of somatic cells, each of the two daughter cells must

contain an exact copy of the original genome present in the

parental cell. Cell cycle controls also regulate when and to

what extent the cells of a given tissue must proliferate, in order

to avoid abnormal cell proliferation that could lead to dyspla-

sia or tumor development. Therefore, when one or more of

such controls are lost or inhibited, abnormal overgrowth will

occur and may lead to impairment of function and disease.

See alsoAmino acid chemistry; Bacterial growth and division;

Cell cycle (eukaryotic), genetic regulation of; Cell cycle

(prokaryotic), genetic regulation of; Chromosomes, eukary-

otic; Chromosomes, prokaryotic; DNA (Deoxyribonucleic

acid); Enzymes; Genetic regulation of eukaryotic cells;

Genetic regulation of prokaryotic cells; Molecular biology and

molecular genetics

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