Pre-reading and reading tasks

1. Make sure you know the following words:

trail	[treIl]	след
to decompose	["di:kqm'pqVz]	разлагаться, гнить
corpse	[kO:ps]	труп
to procure	[prq'kjVq]	добывать
to thrive (throve,thriven)	[TraIv]	пышно расти, разрастаться
to coil	[kOIl]	закручивать(ся)
to loop	[lu:p]	перекручиваться, образовывать петли
versatility	["vE:sq'tIlItI]	многосторонность
tuft	[tAft]	пучок
conjugation	["kPndZV'geIS(q)n]	соединение
dormant	['dO:mqnt]	находящийся в состоянии покоя
desiccation	["desI'keIS(q)n]	высушивание, сушка

2. Read and translate the text.

MONERA

Unlike viruses and subviruses, which are not cellular, the members of the kingdom Monera, including bacteria and blue-green bacteria (sometimes called cyanobacteria, or blue-green algae), are composed of true cells. Monerans are all prokaryotic; that is, their cells lack most organelles, they do not have a membrane-bound nucleus, and most occur as single-celled organisms.

Of the other 15,000 described species, many exist as a series of cells occurring in long filaments or as more complex colonies. Scientists are discovering bacteria that form complex communities, hunt prey in groups, and secrete chemical trails for the directed movement of thousands of individual bacterial cells.

In comparison to most single-celled eukaryotes, individual bacterial cells are smaller and far more abundant, representing a remarkably important component of nearly all ecosystems. Without bacteria, life on earth could not exist as we know it. Bacteria represent some of the most important groups of decomposers; without them, dead organisms would not decay properly. Many nutrients would remain locked up in corpses forever. Geochemical recycling of the earth’s nitrogen, carbon, and sulfur, which are critical to life, would not occur without bacteria. Chemicals such as nitrates, which certain plants use for protein synthesis, are produced by some species of bacteria. Certain bacteria are heterotrophic; that is, they procure their food by feeding on organic material formed by other organisms. Other species of bacteria are photosynthetic, capable of synthesizing their organic molecules from inorganic components, using the energy from the sun. One group of bacteria, the mycoplasmas, are the smallest known cells that grow and reproduce without needing a living host. Their diameters range from 0.12 to 0.25 micrometers.

Probably because of the small size of most types of bacteria, their rapid rate of cell division, remarkable metabolic versatility, and ability to live practically anywhere, they are the most numerous organisms on earth. Under optimal conditions, a population can double in size every 20 or 30 minutes. Species of bacteria are found thriving on icebergs, in hot springs, at the bottom of the oceans, in freshwater, on land, in the soil, and even in aviation fuel.

Although most bacteria use oxygen in their metabolic processes, there are many species that use alternative pathways, surviving perfectly well without any oxygen. Some species have the ability to form spores, which are inactive, thick-walled forms that survive for long periods without water or nutrients in what otherwise would be unfavorable conditions.

Bacteria were first discovered in 1676, but it was not possible to learn very much about them. In the nineteenth century, Louis Pasteur went as far as was possible without the aid of the subsequently developed electron microscope or advanced biochemical techniques, which enabled later researchers to study these small organisms in considerably more detail.

Being prokaryotes, bacteria have cells that differ from eukaryotes in the following ways.

1. Cell walls. Prokaryotic cell walls are composed of a polymer of glucose derivatives attached to amino acids. This substance is termed a mucocomplex. Some bacteria have an additional outer layer of a polymer composed of lipid and sugar monomers, which is termed lipopolysaccharide. Many bacteria can secrete polysaccharides that allow them to stick to things. Cell walls of cyanobacteria (blue-green bacteria) tend to be covered with gelatinous material.

2. Plasma membrane. Inside the cell wall of some bacteria is a plasma membrane that coils and loops, creating a unique structure known as the mesosome, which may be important in cell division.

3. Other internal membranous structures. Some prokaryotes have internal membranous structures containing photosynthetic pigments and related enzymes. Of the aerobic bacteria, those that use molecular oxygen for cellular respiration, some have internal membrane systems containing respiratory enzymes.

4. Ribosomes. The only organelles that consistently occur in prokaryotes are ribosomes, on which messenger RNA (mRNA) is found. The mRNA carries instructions from the genes to the ribosomes, where protein synthesis occurs. Prokaryotic ribosomes are smaller than those found in eukaryotic cells.

5. Flagella. Some bacteria are flagellated, meaning, they have whiplike appendages, extending singly or in tufts, that propel the cells. The flagella of higher organisms consist of a hollow cylinder containing nine pairs of fibrils surrounding two central fibrils. A bacterial flagellum consists of a single fibril of contractile protein.

Prokaryotic DNA (deoxyribonucleic acid) differs from eukaryotic DNA in that it is associated with different proteins. It also differs from eukaryotic DNA in that it is not paired, but is circular. Circular DNA molecules consist of only about one thousandth the DNA found in eukaryotic cells.

Most bacterial cells reproduce by the simple cell division, binary fission. Neither mitosis nor meiosis ever occurs in prokaryotic cells; however, some prokaryotes have a sexual process that transfers material between cells. Occasionally these bacterial cells will transfer DNA to another cell, after which some of the new DNA will replace the recipient's DNA. To date, nothing analogous to a sexual system has been observed in any of the cyanobacteria.

There are three methods by which genetic material may be transferred between bacteria.

1. Transformation. One bacterial cell breaks; its DNA can be taken up by another bacterial cell.

2. Conjugation. Two bacterial cells come together and are joined by a protein bridge, a pilus, through which DNA fragments pass from cell to cell.

3. Transduction. A bacteria-attacking virus, known as a bacterial virus, or bacteriophage, carries bacterial DNA from one bacterial cell to another.

Each of the three methods can result in the transfer of DNA fragments from one bacterial cell to another. During the transfer, sometimes homologous DNA fragments, those containing the same type of genetic information, are substituted in the recipient's circular DNA without a net increase or decrease in the total amount of circular DNA.

It is not certain how important genetic recombination is for prokaryotic evolution. However, despite the fact that mutations (inheritable changes in the organism's genetic material) occur infrequently, prokaryotes do have a high degree of genetic variability and therefore evolve quickly. When it exists, their rapid rate of evolution is usually attributed to their great numbers, and their incredible reproductive rate, as well as mutations and genetic recombinations. Knowledge of such DNA recombination led to research using viruses that transmit DNA fragments to other types of organisms. This research is expected to lead to human gene therapy.

Many prokaryotes are also capable of producing a dormant stage known as a spore. Unlike the spores of other organisms, this is not a reproductive unit. Rather, bacterial spores function wholly as units that contain stored food and are highly resistant to desiccation, as well as to extremely hot and cold temperatures. Bacterial spores have been shown to survive temperatures as cold as -252°C, and some may be able to live for thousands of years. When conditions become favorable, the bacterial spore germinates into a new cell.

Дата добавления: 2015-11-16