Genetic Code

The particular sequence of bases in each polynucleotide chain is known as primary sequence of DNA. This primary sequence forms the genetic code, i.e., the information re­quired by the cell to synthesize all its specific proteins it needs. The primary sequence of DNA shows an unlimited variation, and this is the basis of huge variety seen in the living world.

Sense and Antisense Strands. The genetic information exists in the base sequence of one of the two chains of DNA molecule. This chain is often called sense chain. Its complementary chain is termed antisense or missense chain. The missense chain is important in the replication of DNA molecule, but does not take part directly in transcription. It is copied from the sense chain.

Cistron. A part of sense strand that codes for (has information for) a polypeptide is called a cistron. Each cistron consists of many codons, and a codon comprises three consecutive nitrogenous bases. A codon specifies a single amino acid in a polypeptide chain.

Types. Many types of DNA are known.

(/) Linear and Circular DNA. DNA molecules are of 2 types: linear and circular. Linear DNA has free ends. It is found in the nuclei of eukaryotic cells. It is associated with proteins. It is organized into a number of chromosomes, each containing one long DNA double helix. Circular DNA has its ends joined together. It is found in prokaryotic cells, and in the mitochondria and plas-tids of eukaryotic cells. It is not associated with proteins. The prokaryotic DNA exists as a single chromosome. DNA molecule is linear in some and circular in other DNA viruses. Both linear and cir­cular DNA are coiled to get accommodated in a small space.

(«) Trophic and Genetic DNA. The ciliates have separate trophic and genetic DNA located in the macronucleus and micronucleus respectively. The macronucleus controls the vegetative functions and the micronucleus controls the sexual reproduc­tion.

(Hi) A-, B-, C-, D-, and Z-DNA. Five forms of DNA have been reported : A, B, C, D and Z. These forms largely conform to the structure given by Watson and Crick, but show minor varia­tions such as the number of nucleotides per turn, the distance between successive base pairs, and direc­tion of coiling. The A, B, C and D forms are right- handed double helices. The B — DNA occurs under the physiological conditions in the living cells. It has the specifications given in the foregoing description of DNA. The A —DNA contains 11 base pairs per turn of the helix. C — DNA has 9 base pairs per turn of the helix and D — DNA has only 8 base pairs. The Z—DNA is left-handed, contains 12 base pairs per turn of the helix, each turn is 45 A long, and its sugar-phosphate backbones follow a zig-zag path along the helix. The last feature gives the DNA its name. The Z—DNA retains the most important features of the Watson-Crick model, viz., the double helix with antiparallel strands and the A —T and C —G base pairing. The Z-DNA was discovered and named by Alexander Rich and associates in 1979. It is present in the salivary gland polytene chromosomes of Drosophila. It has been found that the Z—DNA functions in the regulation of gene transcription (expression).

Right handed DNA can temporarily assume left-handed form over a short length and this can affect gene expression.

(iv) Coding and Noncoding DNA. DNA in a

cell may be functional or nonfunctional.

(a) Noncoding DNA. Greater part of DNA in eukaryotic cells does not code for RNAs. This "extra" or noncoding DNA seems to have no func­tion. The noncoding DNA has many base sequences repeated several times. The repeated sequences are collectively called repetitious DNA. Some repetitive DNA sequences are not found at fixed sites in the DNA of different individuals of the same species. Such "mobile" DNA segments are often referred to as "jumping genes". A jumping gene can make a copy of itself but the copy becomes incorporated in another part of the genome. The original gene remains in place and does not "jump". Bacteria have little nonfunctional DNA.

The existence of nonfunctional DNA indi­cates that the eukaryotes use only a small fraction of their total DNA.

(b) Coding DNA. The coding DNA sequences (genes) lie as islands in the sea of nonfunctional DNA sequences. It is further of two types —

(a) Protein-coding DNA. It contains most of the genetic information. It codes for all the proteins other than hist ones.

(ft) DNA Coding for rRNA, tRNA and His-tones. The DNA sequences that code for rRNA, tRNA and histones are repeated because these sub­stances are needed in large amounts in the cells. Human genome contains 400 copies of ribosomal RNA genes.