Subcommittee B: Subsection 1

DNA: Its Structure and Role in the Synthesis of Protein

By Michael Greenfield

DNA, or deoxyribonucleic acid, is an organic chemical of complex molecular structure that is found in all prokaryotic and eukaryotic cells and in many viruses. DNA codes genetic information for the transmission of inherited traits. As a nucleic acid, DNA directs the course of protein synthesis, thereby regulating all cell activities.

In February of 1953, Francis Crick and James D. Watson built a model of DNA that established its general structure. The structure of DNA can be summarized in four points. First, it is a double helix; second, the two strands run antiparallel; third, the double helix has an equal diameter, meaning that the two strands are always equal distance from each other; fourth, the helix twists to the right1 . As a polymer of nucleotides, or a polynucleotide, DNA consists of monomer nucleotides. Each nucleotide consists of a molecule of a deoxyribose sugar, a phosphate group, and a nitrogen-containing base. The two strands of nucleotides are what bond together and form the double helix. There are four different nucleotides that differ only because of their bases. There are two Purines, which are called Adenine and Guanine. The remaining two are Pyrimidines, which are called Cytosine and Thymine. The sugar-phosphate backbones of the polynucleotide chains coil around the outside of the helix, and the nitrogenous bases point toward the center. The two chains are held together by hydrogen bonding between specifically paired bases. Adenine pairs with Thymine with two hydrogen bonds, and Guanine pairs with Cytosine with three hydrogen bonds

The double helix is said to run antiparallel because by looking at the bonds (called phosphodiester bonds) between the nucleotides, one can see that the ending bond of one strand has a free 3’ carbon and the corresponding bond on the other strand has a free 5’ carbon. At the other end of the DNA helix, one can that the situation has been reversed.

Sequences of DNA that code for a specific protein are called genes. These DNA sequences produce protein through the help of RNA (ribonucleic acid), which is the other nucleic acid. RNA uses DNA as a template to create itself so it can then go produce amino acids, the building blocks of protein. Because it is crucial that the RNA begin creating itself off the DNA template at the right spot, there are promoters which turns the gene on or off. The enzyme that incites this whole reaction is RNA polymerase.

In the initiation stage, RNA polymerase binds to promoters and starts to unwind the DNA strands. In the elongation stage, RNA polymerase reads the DNA template stand from 3’ to 5’ and produces the RNA transcript from 5’ to 3’. The nucleotides always are added at the 3’ end of the growing RNA. In the final stage, the RNA polymerase reaches the termination site and the RNA transcript, i.e. messenger RNA (mRNA) is released from the template1

The mRNA is then transported to the ribosome, which is a tiny particle that is the site of protein synthesis2 . The ribosome is located in the cytosol of the cell (cytosol consists mostly of water that contains dissolved ions, small molecules, and soluble macromolecules such as enzymes). The transfer RNA (tRNA), an amino acid shuttle, determines the sequence of the mRNA and then binds it to amino acids. This process is done through the interaction of codons and anticodons. The mRNA contains nucleotide triplets that are called codons. Each mRNA codon specifies one of the twenty amino acids to be sequenced in a polypeptide chain, which is a large molecule made up of many amino acids joined by peptide linkages (large polypeptides are called proteins). tRNA also has nucleotide triplets called anticodons. These anticodons bind to the complementary codon on mRNA, thus assuring that amino acids are arranged in the sequence prescribed by the transcription from DNA.

Thus a nucleotide sequence in the DNA specifies a protein provided that a mRNA molecule is produced from that DNA sequence. Once again, each region of the DNA sequence specifying a protein in this way is called a gene.

Protein is a highly complex substance that is present in all living organisms. Protein are of great nutritional value and are directly involved in the chemical processes essential to life. The protein content of animal organs is usually higher than that of the blood plasma. Muscles, for example, contain about 30 percent protein, the liver 20 to 30 percent, and the red blood cells 30 percent. Higher percentages of protein are found in hair, bones, and other organs and tissues with low water content.

The importance of proteins is related principally to their function. All enzymes that have identified thus far are proteins. Enzymes, which are the catalysts of all metabolic reactions, enable an organism to build up the chemical substances necessary for life. For example, in vertebrates, the respiratory protein hemoglobin acts as an oxygen carrier in the blood. A large group of structural proteins maintains and protects the structure of the animal body.


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