Cell Cycle: Cell Division and Cytoplasmic Division

Cell Cycle

The life cycle of each cell is regulated via stimulation from hormones or growth factors. Disruption of the cycle can affect the health of the body. Most human cells divide from 40 to 60 times before they die. The life cycle of a cell includes the following steps:

■■ Interphase: The cell obtains nutrients to grow andduplicate. This is actually the period from cell for mation to cell division. This step may be betterunderstood as being a metabolic or growth phase.

■■ Cell division (mitosis): The nucleus divides.

■■ Cytoplasmic division (cytokinesis): The cytoplasmdivides.

■■ Differentiation: The cell becomes specialized.

Cell Division and Cytoplasmic Division

The two types of cell division are meiosis and mitosis/cytokinesis. Meiosis is part of gametogenesis (the for-mation of egg or sperm cells depending on ­gender). Meiosis reduces by half the number of chromo-somes, from 46 to 23, in eggs and sperm, so when they unite the fertilized egg will have the proper total of 46 ­chromosomes. Mitosis is characteristic of the somatic cells. Two new daughter cells result from cell division, receiving the same number of chromosomes present in the parent cell.

Mitosis

Mitosis occurs in the somatic cells, but not in everymature cell such as cardiac muscle and nerve cells. Cell numbers increase via this process, in which cell nuclei divide. In cytokinesis, the cytoplasm of a cell divides. All cells, except egg and sperm cells, can be divided by mitosis. When the nucleus divides, it must be pre-cise so an accurate copy of the DNA information can be made by the new cell. Connective tissue and liver cells are examples of cells that divide as needed to heal injury or to replace lost or damaged cells. Cells of the digestive tract and bone marrow divide continually. Overall, the rate of cell division is controlled by the body so that excess cells are not produced.

Growth-promoting substances called growth factors are secreted by nearby cells. They bind totarget cell membrane receptors, activating them. The activated receptors transmit signals that cause the cell to divide, assisted by genes. The effects of genes include promotion of cell growth by producing cell surface receptors to which growth factors attach. Other genes create signals that suppress cell growth and division. Normal cells divide enough to be func-tional and to replenish cellular loss from aging or injury. Normal cells cannot continue to divide for-ever, but have a limited number of cell divisions before they die. The process of neoplasia or dysreg-ulated cell growth is linked to defective regulation of cell division, and may lead to cancer.

A cell’s DNA chains are duplicated, forming new chromosomal material known as the “S” phase. The chromosomes and their duplicates are located next to each other. The two members of the pair are known as chromatids. In mitosis, the chromatidsseparate. During cell division, when chromosomes condense, each of them actually consists of two sep-arate chromosomes that are partially joined where their ­spindle fibers attach. The term “chromatid” describes these still joined chromosomes. Once they separate, they are again called chromosomes. There are several stages of mitosis, including prophase,metaphase, anaphase, andtelophase.

Prophase

In prophase, each chromosome becomes thicker and shorter (FIGURE 3-18). The centrioles move to opposite poles of the cell. They form the mitotic spindle, consist-ing of small fibers that radiate in many directions and form the centrioles. Some spindle fibers attach to the chromatids. The nuclear membrane breaks down near the end of prophase.

Metaphase

In metaphase, the chromosomes line up near the mid-dle portion (the equator of the cell) between the cen-trioles. The chromatids are partially separated but still joined, with spindle fibers attached to them at a con-stricted section called the centromere.

Anaphase

In anaphase, the chromatids of each chromosome are pulled apart to become individual homologous chro-mosomes. Pulled by the spindle fibers, they move toward opposite ends (poles) of the cell.

Telophase

In telophase, the spindle fibers disappear and the chromosomes lengthen and unwind. A nuclear enve-lope forms around them and nucleoli appear in each newly formed nucleus. The nuclear membranes of the two daughter cells reform. The cytoplasm divides to form two daughter cells that are exact duplicates of the parent cell.

Cytoplasmic division (cytokinesis) actually begins during anaphase when the cell membrane constricts down the middle portion of the cell. However, this continuous process is completed through telophase to divide the cytoplasm. The two newly formed nuclei are then separated, and nearly half of the organelles are distributed into each new cell.

Gametogenesis

The gonads, consisting of the testes and ovaries, con-tain precursor cells known as germ cells. These can develop into mature sperm or ova. When mature, germcells are called gametes. The process in which they form is known as gametogenesis. Two similar processes occur in males and females: sperm develop during spermatogenesis and ova develop during oogenesis.

Spermatogenesis

In the testicular tubules, precursor cells are called spermatogonia. They each contain 46 chromo-somes, and divide via mitosis forming primary ­spermatocytes . Like precursor cells, spermatocytes also contain 46 chromosomes. Primary spermato-cytes then divide by meiosis. In the first division, each primary spermatocyte forms two secondary sper-matocytes, with each containing 23 chromosomes. The secondary spermatocytes complete the second meiotic division forming two spermatids . These also contain 23 chromosomes and eventually mature to become sperm. Spermatogenesis takes about 2 months. Sperm are continually produced after the male reaches sexual maturity.

Oogenesis

Precursor cells of the ova are known as oogonia. They each contain 46 chromosomes, but divide repeatedly in the fetal ovaries prior to birth. This forms primary oocytes, also containing 46 chromosomes. A singlelayer of granulosa cells then surround the oocytes. Also called follicular cells, the granulosa cells form the primary follicles. Inside these follicles, the primary oocytes begin, but do not complete prophase of the first meiotic division during fetal life. Large numbers of primary follicles are formed, with many degener-ating during infancy and childhood. As many as 20% of oocytes have chromosome complement or aneu-ploidydefects. Even so, about 500,000 primary folliclespersist into adolescence. The loss of primary follicles continues throughout a female’s reproductive years. In every reproductive cycle, several oocytes start to mature. Usually, only one oocyte is ovulated, while the others degenerate. In menopause, only several thousand oocytes are left. Their numbers decline until there are no more oocytes left in the postmenopausal female’s ovaries.

The ovaries and their primary follicles are inactive until puberty. The cyclic ovulation starts, influenced by the pituitary gonadotrophic hormones, which include follicle-stimulating hormone (FSH) and luteinizing hormone (LH). In every menstrual cycle, a number of primary follicles start to grow. Usually, only one follicle reaches full maturity and is ovulated. When the oocyte is discharged, the first meiotic division is completed. Two daughter cells develop of ­different sizes. One daughter cell receives half of the chromo-somes, which is one member of each homologous pair. It also receives nearly all of the cytoplasm. This daughter cell is called secondary oocyte and contains 23 chromosomes. The other daughter cell receives theremaining 23 chromosomes, but nearly no cytoplasm. It is called the first polar body and is eventually dis-carded. The new secondary oocyte quickly begins its second meiotic division. This leads to the formation of a mature ovum and a second polar body. Each of these contains 23 chromosomes. The meiotic division is not completed unless the ovum is fertilized.

Meiosis

In meiosis, cell division reduces the amount of chro-mosomes by half. There is a mixing of genetic mate-rial between homologous chromosomes. This is a type of recombination process, which is referred to as ­crossing over. There are two separate meiotic divisions:

■■ First meiotic division: Like mitosis, every chro-mosome is duplicated prior to cell division. Two chromatids are formed. In prophase, each pair of homologous chromosomes lie next to each other over their entire length called a synapse. Some interchange of segments occurs called a crossover, a characteristic feature of meiosis. In females, the two X chromosomes synapse exactly like autosomes. In males, the X and Y chromosomes synapse end to end, with no segments being exchanged. Crossing over is faster in females than in males. In metaphase, paired chromosomes are arranged in a plane in the middle of the cell. In anaphase, the chromosomes separate and move to opposite poles in the cell. The chromosomes each consist of two chromatids that do not yet sepa-rate. In telophase, two new daughter cells form, each containing only one member of each pair of homologous chromosomes. Therefore, the chro-mosomes in each daughter cell are reduced by one-half. These chromosomes are different from those of the parent cell, due to the interchange of genetic material during synapse.

■■ Second meiotic division: Similar to a mitotic divi-sion, the two chromatids making up each chro-mosome separate. Two new daughter cells are formed. Each of them contains half of the normal number of chromosomes.

1. Explain the steps in cell division.

2. Why must division of DNA during mitosis be precise?

3. Describe gametogenesis.

4. Compare mitosis and meiosis.