DNA (Deoxyribonucleic acid)

Deoxyribonucleic acid and ribonucleic acid are two chemical substances involved

in transmitting genetic information from parent to offspring. It was known early

into the 20th century that chromosomes, the genetic material of cells, contained

DNA. In 1944, Oswald T. Avery, Colin M. MacLeod, and Maclyn McCarty concluded

that DNA was the basic genetic component of chromosomes. Later, RNA would be

proven to regulate protein synthesis. (Miller, 139)

DNA is the genetic material found in most viruses and in all cellular organisms.

Some viruses do not have DNA, but contain RNA instead. Depending on the organism,

most DNA is found within a single chromosome like bacteria, or in several

chromosomes like most other living things. (Heath, 110) DNA can also be found

outside of chromosomes. It can be found in cell organelles such as plasmids in

bacteria, also in chloroplasts in plants, and mitochondria in plants and animals.

All DNA molecules contain a set of linked units called nucleotides.

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Each

nucleotide is composed of three things. The first is a sugar called deoxyribose.

Attached to one end of the sugar is a phosphate group, and at the other is one

of several nitrogenous bases. DNA contains four nitrogenous bases. The first two,

adenine and guanine, are double-ringed purine compounds. The others, cytosine

and thymine, are single-ringed pyrimidine compounds. (Miller, 141) Four types of

DNA nucleotides can be formed, depending on which nitrogenous base is involved.

The phosphate group of each nucleotide bonds with a carbon from the deoxyribose.

This forms what is called a polynucleotide chain. James D. Watson and Francis

Crick proved that most DNA consists of two polynucleotide chains that are

twisted together into a coil, forming a double helix. Watson and Crick also

discovered that in a double helix, the pairing between bases of the two chains

is highly specific. Adenine is always linked to thymine by two hydrogen bonds,

and guanine is always linked to cytosine by three hydrogen bonds. This is known

as base pairing. (Miller, 143)

The DNA of an organism provides two main functions. The first function is to

provide for protein synthesis, allowing growth and development of the organism.

The second function is to give all of it's descendants it's own protein-

synthesizing information by replicating itself and providing each offspring with

a copy. The information within the bases of DNA is called the genetic code. This

specifies the sequence of amino acids in a protein. (Grolier Encyclopedia, 1992)

DNA does not act directly in the process of protein synthesis because it does

not leave the nucleus, so a special ribonucleic acid is used as a messenger

(mRNA). The mRNA carries the genetic information from the DNA in the nucleus out

to the ribosomes in the cytoplasm during transcription. (Miller, 76)

This leads to the topic of replication. When DNA replicates, the two strands of

the double helix separate from one another. While the strands separate, each

nitrogenous base on each strand attracts it's own complement, which as mentioned

earlier, attaches with hydrogen bonds. As the bases are bonded an enzyme called

DNA polymerase combines the phosphate of one nucleotide to the deoxyribose of

the opposite nucleotide.

This forms a new polynucleotide chain. The new DNA strand stays attached to the

old one through the hydrogen bonds, and together they form a new DNA double

helix molecule. (Heath, 119) (Miller, 144-145)

As mentioned before, DNA molecules are involved in a process called protein

synthesis. Without RNA, this process could not be completed. RNA is the genetic

material of some viruses. RNA molecules are like DNA. They have a long chain of

macromolecules made up of nucleotides.