NEET Biology Notes Molecular Basis of Inheritance Genetic code
The triplet sequence of nucleotides on mRNA, which stores information for linking amino acid in a definite sequence during protein synthesis is called genetic code. The term ‘genetic code’ was given by George Gamow. The first clue to codon assignment was given by MW Nirenberg and JH Mathaei. Later on, HG Khorana deciphered genetic code by developing a new technique.
FHC Crick (1965), proposed Wobble hypothesis, according to which, the third base of a codon is not very important and the specificity of a codon is particularly determined by the first two bases.
The third base pair loosely (Wobble) in pairing with the corresponding base in anticodon of fRNA. There are 64 codons, out of which AUG (rarely GUG) is called initiation codon and UAA, UGA, UAG are called termination or non-sense codons because non-sense codons do not code for any amino acids, so they stop the growing of polypeptide chain.The genetic code is triplet (AAG, GUU, etc), degenerate, non-overlapping, commaless, non-ambiguous and universal.
Mutations explain relationships between genes and DNA.
These are of different types as:
- Point mutation It involves a change in single base pair. It causes a disease called sickle-cell anaemia.
- Frame-shift mutation It occur where addition/insertion or deletion of one or two bases changes the reading from the site of mutation, resulting in a protein with a different set of amino acids.
- Silent mutation It occurs, if a base change in a codon does not alter the amino acid coded.
- Germinal or somatic mutations It occurs in germ cell and can be transferred to progeny. So, are called as germinal mutation.
- Sex-linked and autosomal mutation Mutations occurring is the genes located on sex chromosome are called sex-linked mutations and mutations occur other than sex. Sex chromosomes are called autosomal mutations.
- Deleterious mutations These are harmful mutations, which result in loss of structure or functioning of traits.
- Advantageous mutations These are useful to the organism, m which, they occur.
- Dominant mutations These are able to immediately produce their phenotypic effect since, they are able to suppress the normal or wild type of genes.
- Invisible mutations These are those, which do not produce any effect on the phenotype even when they occur in homozygous state.
- Macro mutations These are large mutations, which show a marked phenotypic effect (e.g. Ancon sheep).
- Micro mutations These are small mutations, which show only a minor phenotypic effect.
- Forward and reverse mutations Mutation of a wild type of gene into a new type of gene is forward mutation and conversion of mutated gene into original type through second mutation is reverse mutation.
- Silent mutation A change in single nucleotide may not change the amino acid specificity of the new codon, e.g. AGA, AGG, AGC. And AGT is called same sense or silent mutation.
- Mis-sense mutation When a change in nucleotide changes only one codon that produces a new amino acid, but at a specific site in polypeptide is called mis-sense mutation.
- Non-sense mutations Mutations bring about chain terminator earlier; so that short polypeptides are produced.
One Gene-One Enzyme Hypothesis
‘One gene-one ehzyine’ hypothesis’ was proposed by Beadle and Tatum (1948), according to this, particular gene controls the synthesis of specific enzyme.
Beadle and Tatum conducted experiment on pink bread mould [Neurospora crassa).
Beadle and Tatum stated that each gene has the information to produce one enzyme. One gene-one enzyme hypothesis is discarded because some regions of DNA produce fRNA or rRNA rather than mRNA (which produces a protein).
Thus, Yanofsky et. al (1965), proposed a modified hypothesis known as one gene-one polypeptide hypothesis because he observed that the enzyme tryptophan synthetase of E. coli consists of two separate polypeptides.
The decoding of the message of mRNA in the form of protein is purely unidirectional process called translation. It takes place in ribosomes.
tRNA the Adapter Molecule
Transfer RNA (tRNA) has an anticodon loop that has bases complementary to the code, and it also has an amino acid acceptor end to which it binds to amino acids.
tRNAs are specific for each amino acid. For initiation, there is another specific fRNA that is rpferred to as initiator tRNA. There are no fRNAs for stop codons.
Initiation of Peptide Bond Sequence
mRNA binds to small unit; of/ribosome in the presence of initiation factor. Protein synthesis mostly starts with methionine (formlylated methionine in case of prokaryotes). The tRNA first binds to P site of ribosome. Then the large subunit of ribosome gets attached to the small subunit forming a complex.
Second tRNA with amino acid forms hydrogen bonds with the second codon on mRNA. This fRNA comes on site ‘A’ of ribosome. Peptide bond is formed between the first amino acid and second amino acid. The enzyme peptidyl transferase catalyses the reaction.
Now the second amino acid shifts to ‘P’ site. The process of shifting of amino acid from A’ site to ‘P’ site is called translocation.
When the code UAA, UAG or UGA comes, it is the indication of termination. UAA, UAG and UGA are also called as non-sense codons. The process of initiation, elongation and termination involve the mediation of several factors and GTP for energy.
It is the process by which information contained in genes is decoded to produce other molecules that determine the phenotypic traits of organisms. The process is initiated, when the information contained in the base sequence of DNA is copied into a molecule of RNA.
The processed RNA molecule is used to specify the order in which amino acids are joined together to form a polypeptide chain.
Johanssen in 1909 introduced a term ‘gene’ as a elementary unit of inheritance. Gene can be defined by
- Cistron – Functional unit of DNA.
- Recon – Unit of recombination.
- Muton – Unit of mutation.
It is the mechanism by which the expression of different genes controlled.
Gene regulation or regulation of gene expression takes place at the following steps
- Gene amplification, destruction or distribution
- Post transcription
- Post translation
Two French scientists, Jacob and Monod (1961) proposed an operon model for regulation of gene activity in prokaryotes. Operon is a part of genetic material (or DNA), which acts as a single regulated unit having one or more structural genes, an operator gene, a promoter gene, a regulator gene, a repressor and an inducer or corepressor (from outside).
Repressor is a proteinaceous substance synthesised by the regulator gene, which blocks the operator gene, so that the structural genes are unable to form mRNA by transcription and protein (enzyme) formation is halted.
Inducer is a chemical substance (substrate, hormone or other metabolite), which binds to repressor and changes the repressor into non-DNA binding state so as to free the operator gene. Lactose acts as inducer in lac operon of E. coli.
Regulator gene controls the activity of operator gene by producing repressor protein, which gets attached to operator gene and checking mRNA synthesis (transcription is stopped). Promotor is that sequence of an operon, where RNA polymerase binds and initiates transcription.
Operator gene controls the activity of structural genes taking part in the synthesis of proteins. Operator gene is situated between the promoter gene and first structural gene, where it serves as binding site for repressor protein.The control of protein synthesis by regulator proteins may be inducible or repressible.