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Liver cell

FIGURE 2.20 Somatic Cells versus Germ Line

After an egg is fertilized and begins its development into an animal embryo, cells have two fates. A small number of cells form the germ line, which gives rise to the gametes (eggs or sperm) that give rise to future generations. However, most cells are part of the somatic cell line, which forms the remainder of the organism. These somatic cells will die either before the organism as a whole, or with it, as part of the natural life cycle.

Plants—Possess both mitochondria and chloroplasts and are photosynthetic. Typically they are non-mobile and have rigid cell walls made of cellulose.

Fungi—Possess mitochondria but lack chloroplasts. Once thought to be plants that had lost their chloroplasts, it is now thought they never had them. Their nourishment comes from decaying biomatter. Although fungi are non-mobile, they lack cellulose and their cell walls are made of chitin. They may be more closely related to animals than plants.

Animals—Lack chloroplasts but possess mitochondria. Differ from fungi and plants in lacking a rigid cell wall. Typically mobile. They are divided into 20 to 30 phyla (singular,phylum), depending somewhat on personal taste. Some phyla include:

Porifera—sponges

Cnidaria—sea anemones and jellyfish Platyhelminthes—flatworms Nematoda—roundworms Arthropoda—insects, crustaceans, etc. Annelida—segmented worms, such as earthworms Mollusca—snails, squids, etc.

phylum (plural phyla) Major groups into which animals are divided, roughly equivalent in rank to the divisions of plants or bacteria

Biotechnology is a new word but not a new occupation. Brewing and baking both use yeast and date back to the earliest human civilizations.

Echinodermata—starfish, sea urchins Chordata—vertebrates and their relatives. Phyla are divided into classes, such as mammals. Classes are divided into orders, such as primates. Orders are divided into families, such as hominids. Families are divided into genera, such as Homo. Genera are divided into species, such as Homo sapiens

Some Widely Studied Organisms Serve as Models

Biologists have always concentrated their attention on certain living organisms, either because they are convenient to study or are of practical importance. Inevitably, model organisms are atypical in some respects. For example, few bacteria grow as fast as E. coli and few mammals breed as fast as mice. Nonetheless, information discovered in such model systems is assumed to apply also to related organisms. In practice this often proves to be true, at least to a first approximation. As discussed above, the basic principles of molecular biology have been investigated in simple single-celled prokaryotes. However, to obtain knowledge that is useful in medicine and agriculture, researchers need model organisms that are much more closely related to humans and to crop plants, respectively. Even these models have their limitations; ultimately, human cells and agriculturally useful animals and plants have to be studied directly.

Yeast Is a Widely Studied Single-Celled Eukaryote

Yeast is widely used in molecular biology for many of the same reasons as bacteria. It is the eukaryote about which most is known and the first whose genome was sequenced—in 1996. Yeasts are members of the fungus kingdom and are about equally related to animals and plants. A variety of yeasts are found in nature, but the one normally used in the laboratory is brewer's yeast, Saccharomyces cerevisiae (Fig. 2.21).This is a single-celled eukaryote that is easy to grow in culture. Even before the age of molecular biology, yeast was widely used as a source of material for biochemical analysis. The first enzymatic reactions were characterized in extracts of yeast and the word enzyme is derived from the Greek for "in yeast".

Although it is a "higher organism", yeast measures up quite well to the list of useful properties that make bacteria easy to study. In addition, it is less complex genetically than many other eukaryotes:

a. Yeast is single-celled microorganism. Like bacteria, a yeast culture consists of many identical cells. Although larger than bacteria, yeast cells are only about a tenth the size of the cells of higher animals.

b. Yeast has a haploid genome of about 12 Mb of DNA with about 6,000 genes, as compared to E. coli, which has 4,000 genes, and humans, who have approximately 25,000.

c. The natural life cycle of yeast alternates between a diploid phase and a haploid phase. Thus it is possible to grow haploid cultures of yeast, which, like bacteria, have only a single copy of each gene, making research interpretations easy.

d. Unlike many higher organisms, yeast has relatively few of its genes—about 5%—interrupted by intervening sequences, or introns.

e. Yeast can be grown under controlled conditions in chemically defined culture medium and forms colonies on agar like bacteria.

f. Yeast grows fast, though not as fast as bacteria. The cell cycle takes approximately 90 minutes (compared to around 20 minutes for fast growing bacteria).

g. Yeast cultures can contain around 109 cells per ml of culture media, like bacteria.

A Roundworm and a Fly Are Model Multicellular Animals 41

FIGURE 2.21 Yeast Cells

Colored scanning electron micrograph (SEM) of budding yeast cells (Saccharomyces cerevisiae). The larger mother cells are budding off smaller daughter cells. Magnification: ¥4,000. Courtesy of: Andrew Syred, Science Photo Library.

FIGURE 2.21 Yeast Cells

Colored scanning electron micrograph (SEM) of budding yeast cells (Saccharomyces cerevisiae). The larger mother cells are budding off smaller daughter cells. Magnification: ¥4,000. Courtesy of: Andrew Syred, Science Photo Library.

Yeast illustrates the genetic characteristics of higher organisms in a simplified

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