DNA and RNA

Deoxynucleic acids have been isolated from all types of living cells and it was established that their main function is to carry genetic information [57]. These are very high molecular weight polymeric materials. Some were found to be as high as 100 million. Analyses of DNA structures show that the numbers of adenine bases are always the same as the number of thymine bases. Also, the numbers of guanine bases always equal the numbers of cytosines. Based on the information from various analyses and an X-ray investigation of the structure, Watson and Crick concluded that the secondary structure of DNA must be a double helix [58]. Two separate right-handed helical chains wind around each other and are held together by hydrogen bonding between base pairs. The bases that are paired off are adenine with thymine and guanine with cytosine:

The base pairs are extended perpendicularly toward the center and the deoxyribose–phosphate ester chains are located on the outside of the helix. The two strands are antiparallel to each other. One turn of the helix corresponds to ten nucleotide pairs, 34 A ˚ in length. The width of the helix is 20 A ˚ . Evidence was presented that some DNAs in their native forms are cyclic [59] and may even occur as two interlocking rings. While most known DNA molecules form a right-handed helix, a left-handed helix can be prepared synthetically in the laboratory [52]. It was speculated that in some instances left-handed helixes may exist in nature and have a biological function [53]. These DNA conformations were named Z-DNAs because the backbones zigzag down the molecule.

There is less information about the secondary structures of RNAs. It is known that the RNA molecules are lower in molecular weight than are the DNA molecules. In addition, it is known that there are three main types of RNAs in living cells. These are ribosomal RNA (r-RNA), transfer RNA (t-RNA), and messenger RNA (m-RNA). The molecular weights of the three forms on the average are about 1,000,000, 25,000, and 500,000, respectively. RNA molecules, with the help of hydrogen bonding, take three-dimensional cloverleaf structures [54]. The molecule’s three-dimensional shape also assumes an L-shape, into which the cloverleaf is bent.

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مواضيع ذات صلة

Polyalkanoates

Synthetic Methods for the Preparation of Nucleic Acids

Nucleic Acids

Chemical Modification of Proteins

Synthetic Methods for the Preparation of Polypeptides

Structures and Chemistry of Proteins

Proteins

Polyisoprene

Lignin

Derivatives of Cellulose

Regenerated Cellulose

Cellulose