Genetic Code, it's properties and Post-translational modification of polypeptide

Genetic code:

The genetic code is the interrelationship between nucleotide sequence in DNA, codons of mRNA and amino acid sequence of proteins, genetic code. Genetic code transfers the genetic message of DNA to protein. The message of DNA is copied in the form of mRNA by transcription then the message of mRNA is copied in the form of protein by translation. DNA holds the message in the form of codon and protein holds the message in the form of a sequence of amino acid. Four types of nucleotides are regarded as 'letters' of the genetic code and codons of mRNA acts as 'words' of genetic code. Codons are assembled to form 'sentence' of genetic code. The sequence of codons is translated into the sequence of amino acids in proteins. Simply, genetic code refers to the means by which DNA and RNA molecules carry genetic information in living cells.

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Properties of genetic code:

Some properties of genetic code include following points:

1.Triplet nature:

Each of the 20 amino acids in proteins is specified by one or more nucleotide triplets (codons) in mRNA. Among 64 codons, 61 specify amino acids and 3 signal chain termination.

2.Non-overlapping:

The genetic code is read in words of 3 nucleotides. After reading one triplet, the "reading frame" shifts over three letters, not just one or two.

Example: GACUGACUGACU.............

The genetic code wouldn't be read GAC CUG GAC...................... Rather, code would be read GAC UAG....................

3.The genetic code is comma free (commaless):

The genetic code has no punctuation. In the following examples: GACUGACUGACU............. The code wouldn't be read GAC GAC........ rather the code would be read GAC UGA.............

4.The genetic code has degenerated:

one codon always codes only one amino acid, but the same amino acid can be coded by more than one codons. This is called degeneracy of genetic code.

5.Genetic code is universal:

Each codon specifies a single amino acid, however, there are expectations like

-UGA is stopped codon but encodes selenocysteine in few cases and tryptophan in Mycoplasm.

-AUG and GUG encode formylmethionine if they are in beginning (acts as a start codon) but they may encode methionine and valine.

6.The genetic code is ordered (5' to 3' direction):

All the processes of genetic code i.e. DNA replication, transcription and translation proceed in 5' to 3' direction.

7.The genetic code contains start and stop codon:

In bacteria the start codon is usually 5'-AUG-3' but 5'-GUG-3' and sometimes even 5'-UUG-3' are also used. Eukaryotic cells always use 5'-AUG-3' as a start codon. Stop codons (5'-UAA-3', 5'-UGA-3', 5'-UAG-3') define the end of ORF (open reading frame) and signal termination of translation.

Codon:

Codon is a sequence of three nucleotides of mRNA that codes particularly amino acids. There are four types of nitrogenous bases in mRNA. Therefore,total types of the codon are 64. Out of 64 codons, 61 codons particularly are amino acids, such codons are called sense codons. However, remaining three codons i.e. UAA, UAG and UGA do not code any amino acids. So they are also called nonsense codon or stop codon or terminating codon.

Codon recognized anticodon present in tRNA during the process of protein synthesis. Only when anticodon of tRNA recognized by the codon, then tRNA adds amino acids and growing polypeptide chain. But cell does not contain the tRNA having anticodon complementary to the stop codon. Therefore, amino acids cannot be added at the site of stop codon and protein synthesis process stops.

Sense codon and Non-sense codon:

The codons which specify specific amino acids are sense codon. They can be read by t-RNA. They are responsible for elongation of translation. There are 61 sense codons.

The codons which cannot specify any amino acids are non-sense codon. They cannot be read by t-RNA. They are responsible for termination of translation. UAA,UAG,UGA are non-sense codons.

Post-translational modification of polypeptide:

Modification of newly synthesized polypeptide in order to make it biologically functional is called post-translational modification. In other words, it refers to the covalent and enzymatic modification of proteins in the course of protein biosynthesis. Proteins are synthesized by ribosomes translating mRNA into polypeptide chains, which may undergo post-translational modification to form the mature protein product. PTM_s are the important components in cell signal transduction. They occur on the amino acids side chains. Phosphorylation is a very common mechanism for regulating the activities of enzymes and is the most common post-translational modification.

Types of post-translational modification of polypeptide:

Some types of post-translational modifications include:

1.Trimming:

Proteolytic cleavage of the polypeptide with endoprotease enzyme in order to make it biologically active is called trimming. Zymogens are the inactive precursor of secreted enzymes that require cleavage of the extra peptide from the active site to make it functional. For example, pepsinogen is an inactive precursor of pepsin and trypsinogen is an inactive precursor of trypsin.

2.Covalent modifications:

Both enzymatic and structural proteins may be activated or inactivated by the covalent attachment of chemical groups. Examples of this modification include phosphorylation, hydroxylation, glycosylation, etc.

3.Protein degradation:

Defective proteins are rapidly degraded by the proteasome (a cellular component).