Telophase I

Daughter cell

Telophase I

Daughter cell

Prophase II

Daughter cell

Metaphase II

Anaphase II

Telophase II

Daughter cells

Daughter cells

■ Figure 3.33 Meiosis, or reduction division. In the first meiotic division, the homologous chromosomes of a diploid parent cell are separated into two haploid daughter cells. Each of these chromosomes contains duplicate strands, or chromatids. In the second meiotic division, these chromosomes are distributed to two new haploid daughter cells.

(a) First meiotic prophase Chromosomes pairing Chromosomes crossing-over it 1

(b) Crossing-over

(b) Crossing-over

Figure 3.34 Crossing-over. (a) Genetic variation results from the crossing-over of tetrads, which occurs during the first meiotic prophose. (b) A diagram depicting the recombination of chromosomes that occurs as a result of crossing-over.

division occurs, each daughter cell will obtain a complement of twenty-three chromosomes that are randomly derived from the maternal or paternal contribution to the homologous pairs of chromosomes of the parent cell.

In addition to this "shuffling of the deck" of chromosomes, exchanges of parts of homologous chromosomes can occur at prophase I. That is, pieces of one chromosome of a homologous pair can be exchanged with the other homologous chromosome in a process called crossing-over (fig. 3.34). These events together result in genetic recombination and ensure that the gametes produced by meiosis are genetically unique. This provides additional genetic diversity for organisms that reproduce sexually, and genetic diversity is needed to promote survival of species over evolutionary time.

Test Yourself Before You Continue

1. Draw a simple diagram of the semiconservative replication of DNA using stick figures and two colors.

2. Describe the cell cycle using the proper symbols to indicate the different stages of the cycle.

3. List the phases of mitosis and briefly describe the events that occur in each phase.

4. Distinguish between mitosis and meiosis in terms of their final result and their functional significance.

5. Summarize the events that occur during the two meiotic cell divisions and explain the mechanisms by which genetic recombination occurs during meiosis.

HPer Links of Basic Cell Concepts to the Body Systems

Nervous System

• Regeneration of neurons is regulated by several different chemicals (p. 157)

• Different forms (alleles) of a gene produce different forms of receptors for particular neurotransmitter chemicals (p. 178)

• Microglia, located in the brain and spinal cord, are cells that transport themselves by amoeboid movement (p. 155)

• The insulating material around nerve fibers, called a myelin sheath, is derived from the cell membrane of certain cells in the nervous system (p. 156)

• Cytoplasmic transport processes are important for the movement of neurotransmitters and other substances within neurons (p. 153)

Endocrine System

• Many hormones act on their target cells by regulating gene expression (p. 292)

• Other hormones bind to receptor proteins located on the outer surface of the cell membrane of the target cells (p. 294)

• The endoplasmic reticulum of some cells stores Ca2+, which is released in response to hormone action (p. 296)

• Chemical regulators called prostaglandins are derived from a type of lipid associated with the cell membrane (p. 317)

• Liver and adipose cells store glycogen and triglycerides, respectively, which can be mobilized for energy needs by the action of particular hormones (p. 609)

• The sex of an individual is determined by the presence of a particular region of DNA in the Y chromosome (p. 635)

Muscular System

• Muscle cells have cytoplasmic proteins called actin and myosin that are needed for contraction (p. 330)

• The endoplasmic reticulum of skeletal muscle fibers stores Ca2+, which is needed for muscle contraction (p. 336)

Circulatory System

• Blood cells are formed in the bone marrow (p. 370)

• Mature red blood cells lack nuclei and mitochondria (p. 368)

• The different white blood cells are distinguished by the shape of their nuclei and the presence of cytoplasmic granules (p. 369)

Immune System

• The carbohydrates outside the cell membrane of many bacteria help to target these cells for immune attack . . . .(p. 446)

• Some white blood cells and tissue macrophages destroy bacteria by phagocytosis (p. 446)

• When a B lymphocyte is stimulated by a foreign molecule (antigen), its endoplasmic reticulum becomes more developed and produces more antibody proteins (p. 453)

• Apoptosis is responsible for the destruction of T lymphocytes after an infection has been cleared (p. 462)

Respiratory System

• The air sacs (alveoli) of the lungs are composed of cells that are very thin, minimizing the separation between air and blood (p. 480)

• The epithelial cells lining the airways of the conducting zone have cilia that move mucus (p. 483)

Urinary System

• Parts of the renal tubules have microvilli that increase the rate of reabsorption . .(p. 526)

• Some regions of the renal tubules have water channels; these are produced by the Golgi complex and inserted by means of vesicles into the cell membrane . .(p. 536)

Digestive System

• The mucosa of the digestive tract has unicellular glands called goblet cells that secrete mucus (p. 566)

• The cells of the small intestine have microvilli that increase the rate of absorption (p. 570)

• The liver contains phagocytic cells . .(p. 575)

Reproductive System

• Males have an X and a Y chromosome, whereas females have two

X chromosomes per diploid cell . .(p. 634)

• Gametes are produced by meiotic cell division (p. 634)

• Follicles degenerate (undergo atresia) in the ovaries by means of apoptosis (p. 656)

• Sperm cells are motile through the action of flagella (p. 650)

• The uterine tubes are lined with cilia that help to move the ovulated egg toward the uterus (p. 654)

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