Cell Division

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Starting with a single fertilized egg, the first cell division produces 2 cells. When these daughter cells divide, they each produce 2 cells, giving a total of 4. These 4 cells produce a total of 8, and so on. Thus, starting from a single cell, 3 division cycles will produce 8 cells (23), 10 division cycles will produce 210 = 1024 cells, and 20 division cycles will produce 220 = 1,048,576 cells. If the development of the human body involved only cell division and growth without any

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

Genetic Information and Protein Synthesis CHAPTER FIVE

Genetic Information and Protein Synthesis CHAPTER FIVE

cell death, only about 46 division cycles would be needed to produce all the cells in the adult body. However, large numbers of cells die during the course of development, and even in the adult many cells survive only a few days and are continually replaced by the division of existing cells.

The time between cell divisions varies considerably in different types of cells, with the most rapidly growing cells dividing about once every 24 h. During most of this period, there is no visible evidence that the cell will divide. For example, in a 24-h division cycle, changes in cell structure begin to appear 23 h after the last division. The period between the end of one division and the appearance of the structural changes that indicate the beginning of the next division is known as interphase. Since the physical process of dividing one cell into two cells takes only about 1 h, the cell spends most of its time in interphase, and most of the cell properties described in this book are properties of interphase cells.

One very important event related to subsequent cell division does occur during interphase, namely, the replication of DNA, which begins about 10 h before the first visible signs of division and lasts about 7 h. This period of the cell cycle is known as the S phase (synthesis) (Figure 5-12). Following the end of DNA

Checkpoint Mitosis

G2-M

20 h

DNA synthesis (replication)

Checkpoint Mitosis

G2-M

20 h

Cell Cycle With Time Elapsed

Checkpoint Gr

FIGURE 5-12

Phases of the cell cycle with approximate elapsed time in a cell that divides every 24 h. A cell may leave the cell cycle and enter the G0 phase where division ceases unless the cell receives a specific signal to reenter the cycle.

Checkpoint Gr

FIGURE 5-12

Phases of the cell cycle with approximate elapsed time in a cell that divides every 24 h. A cell may leave the cell cycle and enter the G0 phase where division ceases unless the cell receives a specific signal to reenter the cycle.

synthesis, there is a brief interval, G2 (second gap), before the signs of cell division begin. The period from the end of cell division to the beginning of the S phase is the Gj (first gap) phase of the cell cycle.

In terms of the capacity to undergo cell division, there are two classes of cells in the adult body. Some cells proceed continuously through one cell cycle after another, while others seldom or never divide once they have differentiated. The first group consists of the stem cells, which provide a continuous supply of cells that form the specialized cells to replace those (such as blood cells, skin cells, and the cells lining the intestinal tract) that are continuously lost. The second class includes a number of differentiated, specialized cell types, such as nerve and striated-muscle cells, that rarely or never divide once they have differentiated. Also included in this second class are cells that leave the cell cycle and enter a phase known as G0 (Figure 5-12) in which the process that initiates DNA replication is blocked. A cell in the G0 phase, upon receiving an appropriate signal, can reenter the cell cycle, begin replicating DNA, and proceed to divide.

Cell division involves two processes: nuclear division, or mitosis, and cytoplasmic division, or cytokinesis. Although mitosis and cytokinesis are separate events, the term mitosis is often used in a broad sense to include the subsequent cytokinesis, and so the two events constitute the M phase (mitosis) of the cell cycle. Nuclear division that is not followed by cytokinesis produces multinucleated cells found in the liver, placenta, and some embryonic cells and cancer cells.

When a DNA molecule replicates, the result is two identical chains termed sister chromatids, which initially are joined together at a single point called the centromere (Figure 5-13). As a cell begins to divide, each chromatid pair becomes highly coiled and condensed, forming a visible, rod-shaped body, a chromosome. In the condensed state prior to division, each of the 46 chromosomes, each consisting of 2 chro-matids, can be identified microscopically by its length and position of its centromere.

As the duplicated chains condense, the nuclear membrane breaks down, and the chromosomes become linked in the region of their centromeres to spindle fibers (Figure 5-13c). The spindle fibers, composed of microtubules, are formed in the region of the cell known as the centrosome. The centrosome, which contains two centrioles (described in Chapter 2) and associated proteins, is required for microtubule assembly.

When a cell enters the mitotic phase of the cell cycle, the two centrioles divide, and a pair of centri-oles migrates to opposite sides of the cell, thus establishing the axis of cell division. One centrosome will pass to each of the daughter cells during cytokinesis. Some of the spindle fibers extend between the two

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

PART ONE Basic Cell Functions

I. Basic Cell Functions

5. Genetic Information and Protein Synthesis

© The McGraw-Hill Companies, 2001

Interphase — cell nucleus

Spindle fiber -

-Centriole

Chromosome-,

Chromatin

DNA replication

Sister chromatids

DNA replication

Sister chromatids

Centromere

Interphase -

Centromere

Chromatid-

Cytokinesis -

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Responses

  • fulvus
    Which human cells rarely or never divide?
    8 years ago
  • oliviero rizzo
    Where is cell division most rapid in the body?
    8 years ago
  • Ralf
    What phase lasts from the end of one cell division to the beginning to the beginning of the next?
    8 years ago
  • sara
    Where in the human body is there a large number of dividing cells?
    8 years ago
  • ANNE
    What is the division of body cells?
    8 years ago
  • anke
    How many rounds of mitosis are required to produce 1024 cells?
    8 years ago
  • jouko
    What 2 cells in the body never divide?
    8 years ago
  • christina mackenzie
    Why do skeletal and neuron cells never leave the G0 phase of interphase?
    8 years ago

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