The 5' end of the mRNA of prokaryotes has a special sequence which precedes the initial AUG start codon of mRNA. This ribosome binding site is called the Shine - Dalgamo equence or S-D sequence.

One of the alternative ways. of dividing up a sequence of bases in DNA or RNA into codon is called reading frame. Any sequence of DNA or RNA, beginning with a start codon and which can be translated into a protein is known as an open reading frame (ORF).

(I) The third base of the codon is called wobble base and this portion is called wobble position. The actual base pairing between codon of mRNA and anticodon of tRNA occurs at first two positions only. The pairing at the third base need not be complementary. This is called Wobbling.
(II) It allows more than one tRNA to code for an amino acid.

There are three nucleotides, called 'anticodon' found on the anticodon arm of tRNA. The anticodon pairs with a codon in mRNA to facilitate picking of correct amino acid from cytoplasm for synthesis of the polypeptide chain.

A sequence of there mRNA bases represents a triplet codon. It is the sequence relationship between nucleotide in genes (mRNA) and the amino acids in the proteins they encode. There are 64 possible triplets.

In some retroviruses that contain RNA as the genetic material (e.g. HIV), the flow of information is reversed. RNA synthesizes DNA by reverse transcription, then transcribed into mRNA by transcription and then into proteins by translation. This is called reverse transcription.

During replication in DNA, in the template strand, replication is continuous and is called the leading strand. In the coding strand with polarity 5' ➝ 3', replication is discontinuous and DNA is synthesized in fragments called okazaki fragments which are joined by the enzyme DNA ligase. This is called the lagging strand.

The clusters of gene with related functions are called operon. They usually transcribe single mRNA molecules. Eg Lac operon in E.coli

The RNase is an enzyme that destroys protein. Therefore DNA, RNA and proteins from heat killed S-strand of bacteria when added to 'R'strain changed surface character and also made them pathogenic. The DNA is the transforming principle and not the proteins.

The assembly of the ribosomal subunits, mRNA and tRNA represent the initiation complex.

The experiments of George Beadle and Edward Tatum in the early 1940's on Neurospora crassa (the red bread mould) led them to propose one gene-one enzyme hypothesis, which states that one gene controls the production of one enzyme. This is called one gene one enzyme hypothesis.

According to the classical concept of gene introduced by Sutton in 1902, genes have been defined as discrete particles that follow Mendelian rules of inheritance, occupy a definite locus in the chromosome and are responsible for the expression of specific phenotypic character. They show the following properties:
(I) Number of genes in each organism is more than the number of chromosomes, hence several genes are located on the same chromosome.
(II) The genes are arranged in a single linear order like beads on a string.
(Ill) Each gene occupies a specific position called locus.
(IV) Genes may exist in several alternate forms called alleles.