Human Genetics: Concepts and Applications (Lewis), 9th Edition

Chapter 2: Cells

Chapter Outline

CHAPTER OVERVIEW

1. Cells comprising the body are termed somatic cells. They are diploid and contain two copies of the genome. Gametes, sperm and egg cells, are haploid. Stem cells, which are diploid, can reproduce themselves and differentiate for repair and replacement of specialized cells.

1. All cells must maintain basic functions for growth and reproduction, responding to the environment, and utilizing energy.
2. Specialized cells in humans and other multicellular organisms perform additional functions related to their role within tissues or organs.
3. There are three broad varieties of cells — Eubacteria (the more common forms of bacteria), Archaea (the extreme bacteria), and Eukaryotes (higher cells) – based on cellular complexity.
4. The Archaea and Eubacteria are similar in that they are single-celled organisms, but they differ in certain features of their RNA and membranes. These cells lack nuclei and other organelles and therefore are categorized as prokaryotes.
5. Eukaryotic cells are complex, with abundant and diverse organelles that compartmentalize biochemical reactions. Human cells are therefore eukaryotic.

1. Cells are constructed from numerous small molecules and macromolecules.
2. The building blocks of cells include carbohydrates (simple sugars and polysaccharides), amino acids and proteins, lipids (fats and oils), and nucleic acids (nucleotides, DNA, RNA).
3. Enzymes are proteins that catalyze the multitude of biochemical reactions that occur in the cell.
4. The study of nucleic acids is key to the field of genetics.

1. Organelles represent the compartments (and unique microenvironments) in the cell and are involved in a variety of functions (division of labor).
2. The nucleus (the storehouse of the majority of DNA in the cell) has a double membrane and nuclear pores, which allow macromolecular traffic in and out of the nucleus.
3. The rough endoplasmic reticulum (ER), smooth ER, and Golgi body function as a membrane network for the synthesis of proteins and lipids that are targeted for delivery to the plasma membrane, organelles, or for secretion. Secretions bud off in vesicles and leave the cell.
4. Lysosomes contain enzymes that degrade cellular debris.
5. Peroxisomes house enzymes that detoxify certain substances, break down lipids, and synthesize bile acids.
6. A mitochondrion has a double membrane whose inner folds carry enzymes that catalyze reactions that extract energy from nutrients.

1. The plasma membrane surrounds the cell and regulates which molecules enter and leave.
2. Biological membranes are composed of a bilayer of phospholipids.
3. Proteins, glycoproteins and glycolipids residing in the cell membrane function as enzymes, signal transduction receptors, transport proteins, and cell adhesion proteins.

1. The cytoskeleton gives a cell its specific architecture.
2. The major cytoskeleton components include microtubules (tubulin), microfilaments (actin), and intermediate filaments (a family of closely related proteins).
3. Microtubules also serve at the foundation of cilia and flagella.
4. Spherocytosis is a heredity defect in the cytoskeleton of the red blood cell. An abnormal cytoskeletal protein, ankyrin, lies beneath the cell membrane and causes the red blood cells to balloon out, blocking narrow blood vessels in organs.

1. The cell cycle consists of interphase, when a cell is not dividing, and mitosis.
2. During interphase, proteins, lipids, and carbohydrates are produced in the G₁ phase; DNA and proteins are made during S phase; and more proteins are produced in G₂. Replicated chromosomes have two sister chromatids attached at their centromeres. Non-dividing cells may become arrested during interphase and enter a quiescent phase (G₀).
3. In mitotic prophase, replicated chromosomes condense, a spindle forms, and the nuclear membrane breaks down. In metaphase, chromosomes align down the center of the cell (equator or metaphase plate). In anaphase, centromeres part, one chromatid from each pair is pulled to opposite ends of the cell. In telophase, the cell pinches in the middle (cytokinesis), and the two new cells separate.
4. The cell cycle is tightly controlled and regulated at several "checkpoints."
5. A cellular clock that limits the number of divisions is based on shrinking telomeres.
6. Crowding, hormones, and growth factors are extracellular influences on mitosis.
7. Within cells, kinases and cyclins activate the genes whose products carry out mitosis.

1. Mitosis (cell division) and apoptosis (cell death) are continuous processes that are both initiated by signals in the extracellular environment.
2. The balance between cell division and death maintains tissues in growth, development, and repair.
3. In prenatal development, coordination of these processes sculpts body form. After birth, mitosis and apoptosis protect and maintain the body.
4. Disruption of the balance between cell division and cell death can lead to cancer or other disorders.

1. In signal transduction, receptors get information from extracellular first messengers and trigger the release of second messengers inside the cell.
2. The second messengers cause a reaction within the cell.
3. Neurofibromatosis type I (NF1) is caused by faulty signal transduction. Nerve cells beneath the skin inappropriately transmit a growth factor signal, triggering cell division and a tumor forms.

1. Cell adhesion is a precise sequence of interactions between cell surface proteins that join cells.
2. In inflammation, cell adhesion molecules (CAMs) guide white blood cells to injury sites.
3. Leukocyte-adhesion deficiency and cancer are disorders that can result from abnormal cell adhesion.

1. Stem cells are unique non-specialized cells that retain the potential to differentiate and enable a tissue to grow or repair itself.
2. A fertilized egg is totipotent, capable of producing any cell type.
3. Later in development, pluripotent stem cells give rise to progenitor cells that are committed to a particular pathway.
4. Stem cells persist in many adult tissues and have the potential to replace injured or diseased tissue.

1. Researchers are investigating uses of stem cells to replace or rejuvenate injured or diseased tissue