Rna

is transcribed into RNA

FIGURE 2.05 Ribosomes Make Protein

The information stored in DNA is transported to the ribosome where proteins are synthesized from components known as amino acids.

Ribosome

Ribosome

FIGURE 2.05 Ribosomes Make Protein

The information stored in DNA is transported to the ribosome where proteins are synthesized from components known as amino acids.

which interacts with the ribosomes where amino acids are assembled into protein.

forming protein which interacts with the ribosomes where amino acids are assembled into protein.

forming protein

FIGURE 2.06 Prokaryotic and Eukaryotic Cells

A comparison of prokaryotic and eukaryotic cells shows that the eukaryotes have a separate compartment called the nucleus that contains their DNA.

Prokaryote

Chromosome (DNA)

Cytoplasm

Eukaryote

Cell membrane

Based on differences in compartmentalization, living cells may be divided into two types, the simpler prokaryotic cell and the more complex eukaryotic cell. By definition, prokaryotes are those organisms whose cells are not subdivided by membranes into a separate nucleus and cytoplasm. All prokaryote cell components are located together in the same compartment. In contrast, the larger and more complicated cells of higher organisms (animals, fungi, plants and protists) are subdivided into separate compartments and are called eukaryotic cells. Figure 2.06 compares the design of prokaryotic and eukaryotic cells.

eukaryote Higher organism with advanced cells, which have more than one chromosome within a compartment called the nucleus nucleus An internal compartment surrounded by the nuclear membrane and containing the chromosomes. Only the cells of higher organisms have nuclei.

prokaryote Lower organism, such as a bacterium, with a primitive type of cell containing a single chromosome and having no nucleus

Prokaryotic Cells Lack a Nucleus 27

Prokaryotic Cells Lack a Nucleus

Cells are separated from their environments by membranes. In the more complex cells of eukaryotes, the genome is separated from the rest of the cell by another set of membranes.

Ribosome

Chromosome

FIGURE 2.07 Typical Bacterium

Ribosome

Chromosome

FIGURE 2.07 Typical Bacterium

The components of a bacterium are depicted.

Bacteria (singular, bacterium) are the simplest living cells and are classified as prokary-otes. By definition, prokaryotes lack a nucleus and their DNA is therefore in the same compartment as the cytoplasm. Bacterial cells (Fig. 2.07) are always surrounded by a membrane (the cell or cytoplasmic membrane) and usually also by a cell wall. Like all cells, they contain all the essential chemical and structural components necessary for life. Typically, each bacterial cell has a single chromosome carrying a full set of genes providing it with the genetic information necessary to operate as a living organism. [Occasional bacteria are known that have more than one chromosome, however this is relatively uncommon.] Typically, bacteria have 3,000-4,000 genes, although some have as few as 500. The minimum number of genes to allow the survival of a living cell is uncertain. Experiments are presently in progress to successively delete genes from certain very small bacterial genomes in an attempt to create a truly minimal cell.

A typical bacterial cell, such as Escherichia coli, is rod shaped and about two or three micrometers long and a micrometer wide. A micrometer (mm), also known as a micron, is a millionth of a meter (i.e., 10-6 meter). Bacteria are not limited to a rod shape (Fig. 2.08); spherical, filamentous or spirally twisted bacteria are also found. Occasional giant bacteria occur, such as Epulopiscium fishelsoni, which inhabits the surgeonfish and measures a colossal 50 microns by 500 microns—an organism visible to the naked eye. Typical eukaryotic cells are 10 to 100 microns in diameter.

A smaller cell has a larger surface-to-volume ratio. Smaller cells transport nutrients relatively faster, per unit mass of cytoplasm (i.e., cell contents), and so can grow more rapidly than larger cells. Because bacteria are less structurally complex than animals and plants, they are often referred to as "lower organisms." However, it is important to remember that present-day bacteria are at least as well adapted to modern conditions as animals and plants, and are just as highly evolved as so-called "higher organisms." In many ways, bacteria are not so much "primitive" as specialized for growing more efficiently in many environments than larger and more complex organisms.

FIGURE 2.08 False Color TEM of Staphylococcus aureus

Colored transmission electron micrograph (TEM) of a cluster of Staphylococcus aureus seen dividing. S. aureus may cause boils, usually by entering the skin through a hair follicle or a cut. They are also responsible for internal abscesses and most types of acute suppurative infection. Magnification: x24,000. Provided by Dr Kari Lounatmaa, Science Photo Library.

chromosome Structure containing the genes of a cell and made of a single molecule of DNA Escherichia coli A bacterium commonly used in molecular biology

FIGURE 2.09 Hot spring in Ethiopia

Hot springs are good sites to find archaebacteria. These springs are in the Dallol area of the Danakil Depression, 120 metres below sea level. The Danakil Depression of Ethiopia is part of the East African Rift Valley. Hot water flows from underground to form these pools. The water is heated by volcanic activity and is at high pressure, causing minerals in the rock to dissolve in the water. The minerals precipitate out as the water cools at the surface, forming the deposits seen here. Provided by Bernhard Edmaier, Science Photo Library.

At a fundamental level, three domains of life, eubacteria, archaebacteria and eukaryotes, have replaced the old-fashioned division into animal and vegetable.

Eubacteria and Archaebacteria Are Genetically Distinct

There are two distinct types of prokaryotes, the eubacteria and archaebacteria, which are no more genetically related to each other than either group is to the eukaryotes. Both eubacteria and archaebacteria show the typical prokaryotic structure—in other words, they both lack a nucleus and other internal membranes. Thus, cell structure is of little use for distinguishing these two groups. The eubacteria include most well known bacteria, including all those that cause disease. When first discovered, the archaebacteria were regarded as strange and primitive. This was largely because most are found in extreme environments (Fig. 2.09) and/or possessed unusual metabolic pathways. Some grow at very high temperatures, others in very acidic conditions and others in very high salt.The only major group of archaebacteria found under "normal" conditions are the methane bacteria, which, however, have a very strange metabolism. They contain unique enzymes and cofactors that allow the formation of methane by a pathway found in no other group of organisms. Despite this, the transcription and translation machinery of archaebacteria resembles that of eukaryotes, so they turned out to be neither fundamentally strange nor truly primitive when further analyzed.

Biochemically, there are major differences between the eubacterial and archae-bacterial cells. In all cells, the cell membrane is made of phospholipids, but the nature and linkage of the lipid portion is quite different in the eubacteria and archaebacteria (Fig. 2.10). The cell wall of eubacteria is always made of peptidoglycan, a molecule unique to this group of organisms. Archaebacteria often have cell walls, but these are made of a variety of materials in different species, but peptidoglycan is never

Archaebacteria (or Archaea) Type of bacteria forming a genetically distinct domain of life. Includes many bacteria growing under extreme conditions Eubacteria Bacteria of the normal kind as opposed to the genetically distinct Archaebacteria species A group of closely related organisms with a relatively recent common ancestor. Among animals, species are populations that breed among themselves but not with individuals of other populations. No satisfactory definition exists for bacteria or other organisms that do not practice sexual reproduction.

transcription Process by which information from DNA is converted into its RNA equivalent translation Making a protein using the information provided by messenger RNA

Bacteria Were Used for Fundamental Studies of Cell Function 29

FIGURE 2.10 Lipids of Archaebacteria

In eubacteria and eukaryotes, the fatty acids of phospholipids are esterified to the glycerol. In archaebacteria, the lipid portion consists of branched isoprenoid hydrocarbon chains joined to the glycerol by ether linkages (as shown here). Such lipids are much more resistant to extremes of pH, temperature and ionic composition.

Phosphate

Phosphate

FIGURE 2.10 Lipids of Archaebacteria

In eubacteria and eukaryotes, the fatty acids of phospholipids are esterified to the glycerol. In archaebacteria, the lipid portion consists of branched isoprenoid hydrocarbon chains joined to the glycerol by ether linkages (as shown here). Such lipids are much more resistant to extremes of pH, temperature and ionic composition.

present. Thus the only real cellular structures possessed by prokaryotes, the cell membrane and cell wall, are in fact chemically different in these two groups of prokaryotes. The genetic differences will be discussed later when molecular evolution is considered (see Ch. 20).

Biologists have always been pulled in two directions. Studying simple creatures allows basic principles to be investigated more easily. And yet we also want to know about ourselves.

Bacteria Were Used for Fundamental Studies of Cell Function

Most of the early experiments providing the basis for modern day molecular biology were performed using bacteria such as Escherichia coli (see below), because they are relatively simple to analyze. Some advantages of using bacteria to study cell function are:

1. Bacteria are single-celled microorganisms. Furthermore, a bacterial culture consists of many identical cells due to lack of sexual recombination during cell division. In contrast, in multi-cellular organisms, even an individual tissue or organ contains many different cell types. All the cells in a bacterial culture respond in a reasonably similar way, whereas those from a higher organism will give a variety of responses, making analysis much more difficult.

2. The most commonly used bacteria have about 4,000 genes as opposed to higher organisms, which have up to 50,000. Furthermore, different selections of genes are expressed in the different cell types of a single multicellular organism.

3. Bacteria are haploid, having only a single copy of most genes, whereas higher organisms are diploid, possessing at least two copies of each gene. As discussed in Ch. 1, the multiple gene copies may differ in a variety of ways, making research results more complex.

diploid Possessing two copies of each gene haploid Possessing only a single copy of each gene

FIGURE 2.11 Graph of Exponential Growth of Bacterial Culture

The number of bacteria in this culture is doubling approximately every 45 minutes. This is typical for fast growing bacteria such as Escherichia coli that are widely used in laboratory research. The bacterial population may reach 5 x 109 cells per ml or more in only a few hours under ideal conditions.

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