Microbiology and Immunology

Transgenics WORLD OF MICROBIOLOGY AND IMMUNOLOGY

550

•

Haemophilis, Neisseria,and Streptococcus,possess natural

competence because their cells do not require special treatment

to take up DNA. This process is transient and occurs only in

special growth phases, typically toward the end of log phase.

The demonstration of DNA transformation was a land-

mark in the history of genetics. In 1944, Oswald Avery, Colin

MacLeod, and Maclyn McCarty conducted famous

Streptococcus pneumoniae transformation experiments.

Bacterial pneumoniais caused by the S strain of S. pneumo-

niae.The S strain synthesizes a slimy capsule around each

cell. The capsule is composed of a polysaccharide that protects

the bacterium from the immune response of the infected ani-

mal and enables the bacterium to cause the disease. The

colonies of the S strain appear smooth because of the capsule

formation. The strain that does not synthesize the polysaccha-

ride, hence does not have the capsule, is called R strain

because the surface of the colonies looks rough. The R strain

does not cause the disease. When heat-killed S strain was

mixed with live R strain, cultured, and spread on to a solid

medium, a few S strain colonies appeared. When S cell extract

was treated with RNase or proteinase and mixed with the live

R strain, R colonies and a few S colonies appeared. When the

S strain cell extract was treated with DNase and mixed with

live R strain, there were only R strain colonies growing on the

agarplates. These experiments proved fundamentally that

DNA is the genetic material that carries genes.

Transformation is widely used in DNA manipulation in

molecular biology. For most bacteria that do not possess natu-

ral competency, special treatment, such as calcium chloride

treatment, can render the cells competent. This is one of the

most important techniques for introducing recombinant DNA

molecules into bacteria and yeastcells in genetic engineering.

See alsoCell membrane transport; Microbial genetics

TTransgenicsRANSGENICS

The term transgenics refers to the process of transferring

genetic information from one organism to another. By intro-

ducing new genetic material into a cell or individual, a trans-

genic organism is created that has new characteristics it did not

have before. The genes transferred from one organism or cell

to another are called transgenes. The development of biotech-

nological techniques has led to the creation of transgenic bac-

teria, plants, and animals that have great advantages over their

natural counterparts and sometimes act as living machines to

create pharmaceutical therapies for the treatment of disease.

Despite the advantages of transgenics, some people have great

concern regarding the use of transgenic plants as food, and with

the possibility of transgenic organisms escaping into the envi-

ronment where they may upset ecosystem balance.

Except for retroviruses that utilize ribonucleic acid

(RNA), all of the cells of every living thing on Earth contain

DNA(deoxyribonucleic acid). DNA is a complex and long

molecule composed of a sequence of smaller molecules, called

nucleotides, linked together. Nucleotides are nitrogen-contain-

ing molecules, called bases, that are combined with sugar and

phosphate. There are four different kinds of nucleotides in

DNA. Each nucleotide has a unique base component. The

sequence of nucleotides, and therefore of bases, within an

organism’s DNA is unique. In other words, no two organisms

have exactly the same sequence of nucleotides in their DNA,

even if they belong to the same species or are related. DNA

holds within its nucleotide sequence information that directs

the activities of the cell. Groups, or sets of nucleotide

sequences that instruct a single function are called genes.

Much of the genetic material, or DNA, of organisms is

coiled into compact forms called chromosomes. Chromo-

somes are highly organized compilations of DNA and protein

that make the long molecules of DNA more manageable during

cell division. In many organisms, including human beings,

chromosomes are found within the nucleusof a cell. The

nucleus is the central compartment of the cell that houses

genetic information and acts as a control center for the cell. In

other organisms, such as bacteria, DNA is not found within a

nucleus. Instead, the DNA (usually in the form of a circular

chromosome) chromosome is free within the cell. Additionally,

many cells have extrachromosomal DNA that is not found

within chromosomes. The mitochondria of cells, and the

chloroplasts of plant cells have extrachromosomal DNA that

help direct the activities of these organelles independent from

the activities of the nucleus where the chromosomes are found.

Plasmidsare circular pieces of extrachromosomal DNA found

in bacteria that are extensively used in transgenics.

DNA, whether in chromosomes or in extrachromosomal

molecules, uses the same code to direct cell activities. The

genetic codeis the sequence of nucleotides in genes that is

defined by sets of three nucleotides. The genetic code itself is

universal, meaning it is interpreted the same way in all living

things. Therefore, all cells use the same code to store informa-

tion in DNA, but have different amounts and kinds of infor-

mation. The entire set of DNA found within a cell (and all of

the identical cells of a multicellular organism) is called the

genome of that cell or organism.

The DNA of chromosomes within the cellular genome

is responsible for the production of proteins. The universal

genetic code simply tells cells which proteins to make.

Proteins, in turn have many varied and important functions,

and in fact help determine the major characteristics of cells

and whole organisms. As enzymes, proteins carry out thou-

sands of kinds of chemical reactions that make life possible.

Proteins also act as cell receptors and signal molecules, which

enable cells to communicate with one another, to coordinate

growth and other activities important for wound healing and

development. Thus, many of the vital activities and character-

istics that define a cell are really the result of the proteins that

are present. The proteins, in turn, are determined by the

genome of the organism.

Because the genetic code with genes is the same for all

known organisms, and because genes determine characteris-

tics of organisms, the characteristics of one kind of organism

can be transferred to another. If genes from an insect, for

example, are placed into a plant in such a way that they are

functional, the plant will gain characteristics of the insect. The

insect’s DNA provides information on how to make insect

womi_T 5/7/03 11:02 AM Page 550