Nucleic Acids

Although biochemistry arose as a scientific discipline from a concerted interest in the investigation of proteins and enzymes, modern biochemistry and molecular biology focus arguably more attention on nucleic acids and their role in directing cellular activities. Because nucleic acids encode the blueprints for protein design, thus controlling their activity, we will discuss nucleic acids first, and then proteins.

In the mid 1800s, a monk named Gregor Mendel postulated that each trait of a living organism (in his case, the pea plant) was regulated by tiny physical factors in the cells of the organism (later named "genes") that could be passed on to the next generation (6). As illustrated by the experiments of Hershey and Chase in 1952, nucleic acids contain the material of heredity (8). Later, it was found that, in fact, it is the nucleotide sequences which make up the nucleic acids that contain the genes. Thus, genes composed of specific nucleotide sequences are the material of heredity.

There are two basic types of nucleic acids, deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). Nucleic acids are composed of a five-carbon sugar, a phosphate group, and a nitrogenous base. The sugar in DNA is 2-deoxyribose. The sugar in RNA is ribose. The nitrogenous bases of nucleic acids are derived from either a purine (for adenine and guanine) or a pyrimidine (for cytosine, uracil, and thymine). Adenine, guanine, and cytosine are found in both DNA and RNA. Uracil is found only in RNA, and thymine is found only in DNA. In the DNA double helix, the base adenine (A) in one strand pairs with (i.e., hydrogen bonds to) a thymine base (T) in the opposite strand, while the base guanine (G) pairs with a cytosine (C) base. In DNA, the strands are anti-parallel, which means they run in opposite directions. In a DNA-RNA hybrid complex, the adenine bases in the DNA strand pair with cytosine bases in the RNA strand.

Nucleic acids are linear sequences of nucleotides. Nucleotides are nucleosides bound to a phosphate group through an ester linkage. The term "nucleoside" refers to a nitrogenous base linked to a five-carbon sugar. When bound to a sugar, the base becomes more water-soluble that it normally would be. To create nucleic acids, nucleotides join together to form ordered linear sequences in which the 5'-nucleoside monophosphate of one nucleotide is attached to the free 3'-OH group of the preceding nucleotide. Since the sequence originates at the 5'-end and extends to the 3'-end, base sequences are therefore represented in base shorthand (A, G, C, T, or U) written in a 5'-3' orientation. The key importance of nucleic acids is in their ability to form ordered linear sequences which allows for nucleic acid sequences to hold [chemical] information, similar to the way words contain information because of the correct orientation of letters in the words.

For most living organisms, genetic information is stored in the form of DNA, although some viruses are known to use RNA. While there are two main types of nucleic acids, there are several main types of RNA, such as messenger RNA (mRNA), ribosomal RNA (rRNA), transfer RNA (tRNA), and, in eukaryotes, small nuclear RNA (snRNA). mRNA is produced during transcription, and acts as a physical messenger to carry the genetic information stored in DNA from the nucleus to the cytosol, where translation activities take place. rRNA molecules are involved in translation activities, along with ribosomes. tRNAs shuttle specific amino acids to the ribosome for protein synthesis. snRNAs join to form ribonucleopro-teins that can edit (cut) mRNA sequences before they leave the nucleus.

DNA can be cut, or "cleaved," enzymatically at specific points, by restriction endo-nucleases. These restriction endonucleases recognize and cut at specific DNA sequences, usually between 4-6 bases in length. One of the first uses of these endonucleases was in restriction mapping of DNA. By this technique, specific sites on DNA molecules can be mapped. The technique involves creating fragments of various lengths, using restriction endonucleases. Then the orientation of the sites is deduced from the patterns formed after separating the fragments by electrophoresis through an agarose gel (explained later).

Gluten Free Living Secrets

Gluten Free Living Secrets

Are you sick and tired of trying every weight loss program out there and failing to see results? Or are you frustrated with not feeling as energetic as you used to despite what you eat? Perhaps you always seem to have a bit of a

Get My Free Ebook


Post a comment