Microbiology and Immunology

WORLD OF MICROBIOLOGY AND IMMUNOLOGY Translation

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proteins within the plant because the genetic code is inter-

preted in the same way. That is, the insect genes give new

characteristics to the plant. This very process has already been

performed with firefly genes and tobacco plants. Firefly genes

were spliced into tobacco plants, which created new tobacco

plants that could glow in the dark. This amazing artificial

genetic mixing, called recombinant biotechnology, is the crux

of transgenics. The organisms that are created from mixing

genes from different sources are transgenic. The glow-in-the-

dark tobacco plants in the previous example, then, are trans-

genic tobacco plants.

One of the major obstacles in the creation of transgenic

organisms is the problem of physically transferring DNA from

one organism or cell into another. It was observed early on that

bacteria resistant to antibioticstransferred the resistance char-

acteristic to other nearby bacterial cells that were not previ-

ously resistant. It was eventually discovered that the resistant

bacterial cells were actually exchanging plasmid DNA carry-

ing resistance genes. The plasmidstraveled between resistant

and susceptible cells. In this way, susceptible bacterial cells

were transformed into resistant cells.

The permanent modification of a genome by the exter-

nal application of DNA from a cell of different genotypeis

called transformation. Transformed cells can pass on the new

characteristics to new cells when they reproduce because

copies of the foreign transgenes are replicated during cell divi-

sion. Transformation can be either naturally occurring or the

result of transgenics. Scientists mimic the natural uptake of

plasmids by bacterial cells for use in creating transgenic cells.

Certain chemicals make transgenic cells more willing to take-

up genetically engineered plasmids. Electroporation is a

process where cells are induced by an electric current to take

up pieces of foreign DNA. Transgenes are also introduced via

engineered viruses. In a procedure called transfection, viruses

that infect bacterial cells are used to inject the foreign pieces

of DNA. DNA can also be transferred using microinjection,

which uses microscopic needles to insert DNA to the inside of

cells. A new technique to introduce transgenes into cells uses

liposomes. Liposomes are microscopic spheres filled with

DNA that fuse to cells. When liposomes merge with host cells,

they deliver the transgenes to the new cell. Liposomes are

composed of lipids very similar to the lipids that make up cell

membranes, which gives them the ability to fuse with cells.

With the aid of new scientific knowledge, scientists can

now use transgenics to accomplish the same results as selec-

tive breeding.

By recombining genes, bacteria that metabolize petro-

leum products are created to clean-up the environment, antibi-

otics are made by transgenic bacteria on mass industrial

scales, and new protein drugs are produced. By creating trans-

genic plants, food crops have enhanced productivity.

Transgenic corn, wheat, and soy with herbicide resistance, for

example, are able to grow in areas treated with herbicide that

kills weeds. Transgenic tomato plants produce larger, more

colorful tomatoes in greater abundance. Transgenics is also

used to create influenzaimmunizations and other vaccines.

Despite their incredible utility, there are concerns

regarding trangenics. The Human Genome Project is a large

collaborative effort among scientists worldwide that

announced the determination of the sequence of the entire

human genome in 2000. In doing this, the creation of trans-

genic humans could become more of a reality, which could

lead to serious ramifications. Also, transgenic plants used as

genetically modified food is a topic of debate. For a variety of

reasons, not all scientifically based, some people argue that

transgenic food is a consumer safety issue because not all of

the effects of transgenic foods have been fully explored.

See alsoCell cycle (eukaryotic), genetic regulation of; Cell

cycle (prokaryotic), genetic regulation of; Chromosomes,

eukaryotic; Chromosomes, prokaryotic; DNA (Deoxyribo-

nucleic acid); DNA hybridization; Molecular biology and

molecular genetics

TTranslationRANSLATION

Translation is the process in which genetic information, car-

ried by messenger RNA(mRNA), directs the synthesis of pro-

teins from amino acids, whereby the primary structure of the

protein is determined by the nucleotide sequence in the

mRNA. Although there are some important differences

between translation in bacteriaand translation in eukaryotic

cells the overall process is similar. Essentially, the same type

of translational control mechanisms that exist in eukaryotic

cells do not exist in bacteria.

A molecule known as the ribosome is the site of the pro-

tein synthesis. The ribosome is protein bound to a second

species of RNA known as ribosomal RNA (rRNA). Several

ribosomesmay attach to a single mRNA molecule, so that

many polypeptide chains are synthesized from the same

mRNA. The ribosome binds to a very specific region of the

mRNA called the promoter region. The promoter is upstream

of the sequence that will be translated into protein.

The nucleotide sequence on the mRNA is translated into

the amino acid sequence of a protein by adaptor molecules

composed of a third type of RNA known as transfer RNAs

(tRNAs). There are many different species of tRNAs, with

each species binding a particular type of amino acid. In pro-

tein synthesis, the nucleotide sequence on the mRNA does not

specify an amino acid directly, rather, it specifies a particular

species of tRNA. Complementary tRNAs match up on the

strand of mRNA every three bases and add an amino acid onto

the lengthening protein chain. The three base sequence on the

mRNA are known as “codons,” while the complementary

sequence on the tRNA are the “anti-codons.”

The ribosomal RNA has two subunits, a large subunit

and a small subunit. When the small subunit encounters the

mRNA, the process of translation to protein begins. There are

two sites in the large subunit, an “A” site, and a “P” site. The

start signal for translation is the codon ATG that codes for

methionine. A tRNA charged with methionine binds to the

translation start signal. After the first tRNA bearing the amino

acid appears in the “A” site, the ribosome shifts so that the

tRNA is now in the “P” site. A new tRNA molecule corre-

sponding to the codon of the mRNA enters the “A” site. A pep-

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