What kind of information do the genes have?
Answer to this question was very properly given by George Beadle and Edward Tatum in 1941. They exposed spores of the fungus Neurospora crassa to mutagenic agents like X-rays causing mutations in DNA. They obtained some mutant strains of Neurospora crassa which were lacking in specific enzymes. They concluded that a gene is a segment of DNA which codes for an enzyme. It was named as One gene- One enzyme hypothesis. Later in 1957 this hypothesis was modified by Vernon Ingram as One-gene-One polypeptide hypothesis. It is because of the fact that many genes code for proteins which are not enzymes.
Protein synthesis is the most important, essential and significant metabolic activity of the living world taking place in every living cell continuously.
For actual phenotypic expression of any character in a living cell or in the body of an organism, biochemical reactions are essential. Each biochemical reaction needs a specific enzyme for its initiation and completion. Almost all the enzymes are proteins. The cell also needs many other structural proteins. Thousands of different types of structural and catalytic proteins are continuously required within the cell at any moment.
The Genetic Code :
DNA contains all the genetic information of an organism. The genes control, regulate and express the characters. The expression of a character takes place through a specific protein.
The gene is a segment of DNA, mRNA molecule is formed on DNA which later helps in construction of a polypeptide chain. Thus, the mRNA acts as an agent in conveying information form DNA to cytoplasm.
A sequence of three nucleotides on the mRNA strand is called codon. Each codon codes for a specific amino acid. This was suggested by George Gamow in 1954 while the direct evidence for this was provided by Crick in 1961, Subsequently Marshall Nirenberg, Heinrich Matthaei and Har Gobind Khorana deciphered (cracked) complete genetic code by using artificial mRNA templates, (homopolymers and copolymers), and cell free system of protein synthesis. For this work Khorana shared Nobel Prize in 1968 with Nirenberg and Holley.
In RNA, there are four types of nitrogen bases (A, U, G, C). They can form 64 possible combinations of triplets, called codons. Among these 3 codons serve as termination codons and remaining 61 codons are sense codons. They code for 20 essential amino acids required for protein synthesis.
Characteristics of genetic code :
Genetic code is triplet and commaless i.e. on the mRNA strand the triplet codons are arranged one after the other without any gap or space.
It is non-ambiguous. Each codon will specify a particular amino acid. (However, there is an exception. AUG codes for methionine and GUG codes for valine but if AUG is not available then GUG codes for methionine, as a start codon in protein synthesis).
The genetic code is degenerate, as 61 codons are available for 20 amino acids. Two or more codons can specify the same amino acid. Hence it is said to be degenerate e.g. GGG, GGA, GGC and GGU code for glycine.
Polarity is one of the important feature of the genetic code. It can be read only in 5' to 3' direction of m-RNA.
The genetic code has start and stop signals. The codon AUG acts as start/initiation codon which codes for methionine, while the codons UAG (amber), UAA (ochre) and UGA (opal) serve as stop codons or termination codons.
The genetic code is universal i.e. it is similar in all the organisms, from simple bacteria to complex organisms. (However, some exceptions are found in mitochondria of yeast and Mycoplasma
Degeneracy of Code is many condons for one amino acid. There are 61 condos for 20 amino acids. One would expect presence of 61 types of tRNAs, each with a specific anticodon for recognition of a particular codon. However, the actual types of tRNAs is much less than 61. This indicates that the anticodon of tRNA can read two or more codons on mRNA.
In 1966 Crick proposed Wobble hypothesis to explain the same.
According to this hypothesis, in codon-anticodon pairing the third base may not be complementary. The third base of the codon is called wobble base and this position is called wobble position. The actual base pairing occurs at first two positions only.
In the above example though the codon and anticodon do not match perfectly then also the required amino acids is brought perfectly. This enable the economy of tRNA. GUU, GUC, GUA and GUG code for amino acid - Valine
Central Dogma of Protein Synthesis
DNA Transcription> mRNA Translation> Protein
In 1958 Crick proposed that DNA determines the sequence of amino acids in a polypeptide (protein) through mRNA, This is the main principle or central dogma of protein synthesis which involves transcription and translation.
Central Dogma of Protein Synthesis This Process Needs :
i) DNA to provide the base sequence.
ii) 20 types of amino acids.
iii) Non genetic types of RNA (mRNA, rRNA, tRNA).
iv) Ribosomes as site of protein synthesis.
v) Various enzymes/factors and
vi) ATP/GTP as source of energy.