MOLECULAR GENETICS 10

 Process of transcription

In prokaryotes

Transcription takes place in three steps

• Initiation
• RNA polymerase binds to promoter and initiates transcription.
• Initiation factor or sigma (σ) recognizes the promoter of the DNA.
• Elongation
• RNA polymerase facilitates opening of the helix and continues elongation.
• RNA polymerase uses nucleoside triphosphates as substrate and polymerizes in a template depended fashion following the rule of complementarity.
• Only a short stretch of RNA remains bound to the enzyme.
• Termination
• Once the polymerases reaches the terminator region RNA polymerase binds with the termination-factor (ρ) to terminate transcription.
• The nascent RNA falls off with the RNA polymerase which results in termination of transcription.

• The transcription and translation can be coupled in bacteria as the mRNA does not require any processing to become active, 
• Since only exons are there.
• and also transcription and translation take place in the same compartment

In eukaryotes

• There are two additional complexities in eukaryotes.

1. The first complexity is that there are at least three RNA polymerases in the nucleus.

• The RNA polymerase I transcribes rRNAs (28S, 18S, and 5.8S)
• The RNA polymerase III is responsible for transcription of tRNA, 5srRNA, and snRNAs (small nuclear RNAs).
• The RNA polymerase II transcribes precursor of mRNA, the heterogeneous nuclear RNA (hnRNA).

1. The second complexity is that the primary transcripts contain both the exons and the introns and are non-functional.
2. 
   

• Primary transcripts are subjected to a process called splicing where the introns are removed and exons are joined in a defined order.

• hnRNA undergo two additional processing called as capping and tailing.
• In capping an unusual nucleotide (methyl guanosine triphosphate) is added to the 5'-end of hnRNA.
• In tailing, adenylate residues (200-300) are added at 3'-end in a template independent manner and the fully processed hnRNA is called mRNA

• mRNA is transported out of the nucleus for translation.

Significance of complexities

• The split-gene arrangements represent probably an ancient feature of the genome.
• The presence of introns is reminiscent of antiquity, and the process of splicing represents the dominance of RNA-world.

Genetic code

• The sequence of nucleotides on DNA which determines the sequence of amino acids in a polypeptide chain is termed as Genetic code.

• The process of translation requires transfer of genetic information from a polymer of nucleotides to a polymer of amino acids
•  but there is no complementarity between nucleotides and amino acids which led to the proposition of a genetic code that could direct the sequence of amino acids during synthesis of proteins.

(Minimum 20 amino acids,

4 bases,

1 bases /codon - only 4 amino acids

2 bases/ codon- combinations- 16 amino acids

So, 3 bases / codon - 64 combinations - easily code 20 proteins) -- means codon - triplet

• The salient features of genetic code are as follows:

• The codon is triplet, 61 codons code for amino acids and 3 codons do not code for any amino acids, hence they function as stop codons. (UAG, UAA, UGA)
• One codon codes for only one amino acid thus it is unambiguous and specific.

• Some amino acids are coded by more than one codon, hence the code is degenerate.

• The codon is read in mRNA in a contiguous fashion, there are no punctuations.

• The code is nearly universal. For example, from bacteria to human UUU would code for Phenylalanine (phe).

• AUG has dual functions, it codes for Methionine (met) , and it also act as initiator codon.

Mutation and genetic code

• Point mutation is the insertion or deletion of a single gene in the structural gene.
• 2 types - missense mutation - wrong amino acid added
• Nonsense mutation - stop codon is formed so peptide chain stopped here.

Example- point mutation is a change of single base pair in the gene for beta globin chain that results in the change of amino acid residue glutamate to valine, which results into a diseased condition called as sickle cell anaemia.

• Frame shift mutation is the insertion and deletion of three or its multiple bases which insert or delete one or multiple codon hence one or multiple amino acids, and reading frame remains unaltered from that point onwards.

Example- cystic fibrosis.