Prokaryotic Cells

1. Have a nuclear envelope

2. Have two to hundreds of chromosomes per cell; DNA is double stranded

3. Have membrane-bound organelles (e.g., plastids, mitochondria)

4. Have 80S ribosomes1

5. Have asexual reproduction by mitosis

6. Have sexual reproduction by fusion

1. Lack a nuclear envelope

2. Have a single, closed loop of double-stranded DNA plus (usually) several to 40 plasmids

3. Lack membrane-bound organelles

4. Have 70S ribosomes1

5. Have asexual reproduction by fission

6. Sexual reproduction unknown

1. S is a Svedberg unit, which is used to measure the rate at which a particle suspended in a fluid settles to the bottom when centrifuged. An 80S ribo-some is larger than a 70S ribosome.

Chapter 17

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A. B. C.

Figure 17.1 Duplication of a nucleoid in a bacterial cell. The duplication begins (A) at a single point or bubble (upper right) that expands (B) until the two new rings of DNA separate (C), x150,000. (Electron micrographs courtesy Tsuyoski Kakefuda)

Figure 17.1 Duplication of a nucleoid in a bacterial cell. The duplication begins (A) at a single point or bubble (upper right) that expands (B) until the two new rings of DNA separate (C), x150,000. (Electron micrographs courtesy Tsuyoski Kakefuda)

Tem Bacterial Cells Ribosomes Nucleoid

remain attached to each other in chains. This simple form of asexual reproduction, fission, is found so universally in bacteria that they were in the past called schizomycetes, which means "fission fungi" (Fig. 17.2). Bacteria, however, are not currently believed to be closely related to the fungi, as was the case when the name was originally applied.

Under ideal conditions of moisture, food supply, and temperature, a bacterium may undergo fission every 10 to 20 minutes. If a single bacterium of average size were to duplicate itself every 20 minutes for as little as 36 hours, a mass of bacteria numbering 322,981,536,679,200,000,000,000,000,000,000 and weighing 126,464,618,590 metric tons (137,438,953,472 tons) would be produced. If this mass were not compacted, its volume would exceed that of the earth. Of course, bacteria do not continue to reproduce at their maximum rate for very long because several factors prevent it. These factors include the exhaustion of food supplies and the accumulation of toxic wastes.

Bacteria do not produce gametes or zygotes, nor do they undergo meiosis. However, at least three forms of genetic recombination occur.

1. In conjugation, a plasmid and/or part of the DNA strand is transferred from a donor cell to a recipient cell, usually through a tiny, hollow, tubelike pilus while the two cells are in contact (Fig. 17.3). Once in the recipient cell, the DNA becomes part of the new cell. Bacterial


Locomotion Pili
Figure 17.3 Conjugation in bacteria. Part of the DNA strand of the donor cell migrates through the hollow, tubelike pilus to the recipient cell, where it is incorporated into the DNA of its new cell. Shorter pili cover the surfaces of both cells.

cells that are capable of conjugating have different sexual forms of DNA, and usually only cells of different mating types undergo conjugation.

2. In transformation, a living cell acquires fragments of DNA released by dead cells into the medium in which it occurs and incorporates them into its own cell.

3. In transduction, fragments of DNA are carried from one cell to another by viruses (discussed toward the end of this chapter in the section entitled "Viruses").

Size, Form, and Classification of Bacteria

Except for the cyanobacteria, which are discussed separately, and a few giant bacteria, which attain lengths of up to 60 micrometers, most bacteria are less than 2 or 3 micrometers in diameter, and a few of the smaller species approach 0.15 micrometer. The latter are so small that 6,500 of them arranged in a row would not quite extend across the head of a pin. Because of their small size, bacteria are not visible individually to the unaided eye and are studied in the laboratory with electron microscopes or with the highest magnifications of light microscopes.

The thousands of different kinds of bacteria occur primarily in three forms: cocci (singular: coccus), which are spherical or sometimes elliptical; bacilli (singular: bacillus), which are rod shaped or cylindrical; and spirilla (singular: spirillum), which are in the form of a helix, or spiral (Fig. 17.4). Further classification is based on various visible features including, for example, development of slimy or gummy capsulelike sheaths around cells; color; presence of hairlike or budlike appendages; development of internal thicker-walled endospores and other cell inclusions; mechanisms of movement, including gliding movements by which threadlike groups of bacteria appear to slide back and forth


Bacteria Cocci Bacilli Spirilla
Figure 17.4 Three basic forms of bacteria. A. Cocci. B. Bacilli. C. Spirilli. x1500.

Chapter 17

lengthwise against each other; and features of colonies. Biochemical characteristics, including energy sources, cellwall components, fermentation products, luminescence, optimum ranges of pH and temperature, oxygen relationships, amino acid sequences of proteins, and salt tolerance are also used in classification.

Some bacteria have slender flagella, usually about 5 to 10 micrometers in length, which propel them through fluid media. Others have somewhat shorter tubelike pili, which resemble flagella but do not function in locomotion. The pili apparently enable bacteria to attach themselves to surfaces or to each other, as in conjugation, which was discussed previously in the section "Cellular Detail and Reproduction of Bacteria."

True bacteria are also grouped into two large categories based on the reaction of their cell walls to a dye. After a heat-fixed smear of cells has been stained blue-black with a violet dye, a dilute iodine solution, alcohol, or acetone is added. When so treated, some species rapidly lose their color (but absorb pink safranin dye) and are called gram-negative. Others, called gram-positive, retain most of the blue-black color. Gram's stain, named after Christian Gram who discovered it in 1884, has many variations, but all the variations produce similar results. Other features used in classifying bacteria are indicated in the discussions that follow.


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  • Mandy
    How do acids work against bacteria?
    7 years ago
  • pedro
    Where is are dna strands in a plant cell?
    7 years ago

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