Understanding Genetics - The Cell Is The Basic Unit Of Life

Since Matthias Schleiden and Theodor Schwann put forth their theories in 1838 that all plants and animals are composed of cells, there has been continuous refinement of cell theory. The early view that cells were made up of protoplasm (a jelly-like substance) has given way to the more sophisticated understanding that cells are highly complex organizations of even smaller molecules and substructures. Cytology—the study of the formation, structure, and function of cells—has benefited from ever-improving technology, including powerful microscopes that enable researchers to identify the organelles (component parts of the cell) and determine their roles in inheritance.

FIGURE 2.8
Codon
SOURCE: "Codon," in Talking Glossary of Genetic Terms, U.S. Department of Health and Human Services, National Institutes of Health, National Human Genome Research Institute, http://www.genome.gov/Pages/Hyperion//DIR/VIP/Glossary/Illustration/codon.shtml (accessed February 3, 2005)

Cells are the basic units and building blocks of nearly every organism. (One exception is viruses, which are simple organisms that are not composed of cells.) Each cell of an organism contains the same genetic information, which is passed on faithfully when cells divide. Different types of cells arise because they use different parts of the information, as determined by the cell's history and immediate environment. Different cell types may be organized into tissues and organs.

Cell Structure and Function

Plants and animals, as well as other organisms such as fungi, are composed of eukaryotic cells, or eukaryotes, because they have nuclei and membrane-bound structures known as organelles. In eukaryotic cells the organelles within the cell sustain, support, and protect it, creating a FIGURE 2.9
Ribosome
SOURCE: "Ribosome," in Talking Glossary of Genetic Terms, U.S. Department of Health and Human Services, National Institutes of Health, National Human Genome Research Institute, http://www.genome.gov/Pages/Hyperion//DIR/VIP/Glossary/Illustration/ribosomes.shtml (accessed February 3, 2005) FIGURE 2.10
Gene
SOURCE: "Gene," in Talking Glossary of Genetic Terms, U.S. Department of Health and Human Services, National Institutes of Health, National Human Genome Research Institute, http://www.genome.gov/Pages/Hyperion//DIR/VIP/Glossary/Illustration/gene.shtml (accessed February 3, 2005) FIGURE 2.11
Chromosome
SOURCE: "Chromosome," in Talking Glossary of Genetic Terms, U.S. Department of Health and Human Services, National Institutes of Health, National Human Genome Research Institute, http://www.genome.gov/Pages/Hyperion//DIR/VIP/Glossary/Illustration/chromosome.shtml (accessed February 3, 2005) barrier between the cell and its environment, acting to build and repair cell parts, storing and releasing energy, transporting material, disposing of waste, and increasing in number.

Each organelle functions like an organ system for the cell. For example, the nucleus is the "command center," masterminding protein synthesis within the cell. The ribosomes work as protein factories, the Golgi apparatus is a protein sorter, and the endoplasmic reticulum operates as a protein processor. Lysosomes and peroxisomes serve as the cell's digestive system, and mitochondria convert energy in the cell. The surface membrane of the cell acts like skin, selectively permitting molecules in and out of the cell.

The nuclei of eukaryotes contain the chromosomes, chains of genetic material coded in DNA. The thread-like chromosomes are contained in the nucleus of a typical animal cell. Genes are segments of DNA that carry a basic unit of hereditary information in coded form. (See Figure 2.10.) They contain instructions for making proteins.

FIGURE 2.12
Karyotype
SOURCE: "Karyotype," in Talking Glossary of Genetic Terms, U.S. Department of Health and Human Services, National Institutes of Health, National Human Genome Research Institute, http://www.genome.gov/Pages/Hyperion//DIR/VIP/Glossary/Illustration/karyotype.shtml (accessed February 3, 2005)

The eukaryotic chromosome is composed of chromatin (a combination of nuclear DNA and protein) and contains a linear array of genes. It is visible just prior to and during cell division. (See Figure 2.11.) Human cells normally contain twenty-three pairs of chromosomes, or a total of forty-six chromosomes, that may be examined using a process known as karyotyping, the organization of a standard picture of the chromosomes. Figure 2.12 is a karyotype—a photo of an individual's chromosomes.

Cells without nuclei, such as bacteria and blue-green algae, are called prokaryotic cells or prokaryotes. Prokaryotic cells are smaller than eukaryotic cells, contain less genetic information, and are able to grow and divide more quickly. They perform these functions without organelles. A prokaryotic cell's DNA is not contained in one location; it floats in different regions of the cell. The DNA of prokaryotes also contains "jumping genes" that are able to bind to other genes and transfer gene sequences from one site of a chromosome to another.