Mitosis

Mitosis (my-TOE-sis), in the sense used here, is the process by which a cell divides into two daughter cells with identical copies of its DNA. (Some define it as division of the nucleus only and do not include the subsequent cell division.) Mitosis has four main functions:

  1. formation of a multicellular embryo from a fertilized egg;
  2. tissue growth;
  3. replacement of old and dead cells; and
  4. repair of injured tissues.

Egg and sperm cells are produced by a combination of mitosis and another form of cell division, meiosis, described in chapter 27. Otherwise, all cells of the body are produced entirely by mitosis. Four phases of mitosis are recognizable—prophase, metaphase, anaphase, and telophase (fig. 4.13).

In prophase,2 at the outset of mitosis, the chromosomes supercoil into short, dense rods (fig. 4.14) which are

Chapter 4 Genetics and Cellular Function 143

easier to distribute to daughter cells than the long, delicate chromatin. A chromosome at this stage consists of two genetically identical bodies called sister chromatids, joined together at a pinched spot called the centromere. At prophase, there are 46 chromosomes, two chromatids per chromosome, and one molecule of DNA in each chromatid. The nuclear envelope disintegrates during prophase and releases the chromosomes into the cytosol. The centrioles begin to sprout elongated microtubules, which push the centrioles apart as they grow. Eventually, a pair of centrioles lies at each pole of the cell.

In metaphase,3 the chromosomes line up at random along the midline of the cell. Microtubules grow toward them from each centriole and some attach to the centromeres. This forms a football-shaped array called the mitotic spindle. Shorter microtubules also radiate from each centriole pair to form a star-shaped array called an aster.4 These microtubules anchor the centrioles to the nearby plasma membrane.

In anaphase,5 each centromere divides in two and chromatids separate from each other. Each chromatid is now a chromosome in its own right. These two daughter chromosomes migrate to opposite poles of the cell, with their centromeres leading the way and their arms trailing behind. There is some evidence that the spindle fiber acts a little like a railroad track, and a protein complex in the centromere called the kinetochore6 (kih-NEE-toe-core) acts as a molecular motor that propels the chromosome along the track. One of the kinetochore proteins is dynein, the same motor molecule that causes movement of cilia and flagella (see chapter 3). Since sister chromatids are genetically identical, and since each daughter cell receives one chromatid from each metaphase chromosome, you can see why the daughter cells of mitosis are genetically identical.

In telophase,7 the chromosomes cluster on each side of the cell. The rough ER produces a new nuclear envelope around each cluster, and the chromosomes begin to uncoil and return to the thinly dispersed chromatin form. The mitotic spindle breaks up and vanishes. Each new nucleus forms nucleoli, indicating it has already begun making RNA and preparing for protein synthesis.

Telophase is the end of nuclear division but overlaps with cytokinesis8 (SY-toe-kih-NEE-sis), division of the cytoplasm. Cytokinesis is achieved by the motor protein myosin pulling on microfilaments of actin in the membrane skeleton. This creates a crease called the cleavage furrow around the equator of the cell, and the cell eventually pinches in two. Interphase has now begun for these new cells.

3meta = next in a series

4 aster = star

5ana = apart

6kineto = motion + chore = place

7telo = end, final scyto = cell + kinesis = action, motion

Saladin: Anatomy & I 4. Genetics and Cellular I Text I © The McGraw-Hill

Physiology: The Unity of Function Companies, 2003 Form and Function, Third Edition

144 Part One Organization of the Body

Aster Mitotic Spindle Spindlechromosomes

Aster

Mitotic spindle Chromosomes Centrioles

Prophase

Chromatin condenses into chromosomes. Nucleoli and nuclear envelope break down. Spindle fibers grow from centrioles. Centrioles migrate to opposite poles of cell.

Prophase

Chromatin condenses into chromosomes. Nucleoli and nuclear envelope break down. Spindle fibers grow from centrioles. Centrioles migrate to opposite poles of cell.

Aster

Mitotic spindle Chromosomes Centrioles

Mitotic Spindle Fibers

Metaphase

Chromosomes lie along midline of cell. Some spindle fibers attach to kinetochores. Fibers of aster attach to plasma membrane.

Metaphase

Chromosomes lie along midline of cell. Some spindle fibers attach to kinetochores. Fibers of aster attach to plasma membrane.

Mitotic Spindle Fibers

  • w-'""- 11 V
  • H ■ - . .

I t ' J ' ih /"

Anaphase

Telophase

Centromeres divide in two.

Chromosomes gather at each pole of cell.

Spindle fibers pull sister chromatids to

Chromatin decondenses.

opposite poles of cell.

New nuclear envelope appears at each pole.

Each pole (future daughter cell) now has an

New nucleoli appear in each nucleus.

identical set of genes.

Mitotic spindle vanishes.

(Above photo also shows cytokinesis.)

Figure 4.13 Mitosis. The photographs show mitosis in whitefish eggs, where chromosomes are relatively easy to observe. The drawings show a hypothetical cell with only two chromosome pairs; in humans, there are 23 pairs.

Saladin: Anatomy & I 4. Genetics and Cellular I Text I © The McGraw-Hill

Physiology: The Unity of Function Companies, 2003 Form and Function, Third Edition

Chapter 4 Genetics and Cellular Function 145

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