Elongation:
- RNA polymerase unzipped the DNA double helix and forms an open loop.
- It uses ribonucleoside triphosphates as substrate and polymerizes in a DNA template following the rule of complementarity.
- Only a short stretch of polymerized RNA remains binds with the enzyme.
- The process of polymerization continued till the enzyme reaches the terminator gene.
Termination:
- RNA polymerase recognizes the terminator gene by a termination-factor called rho (ρ) factor.
- The RNA polymerase separated from the DNA and also the transcribed RNA.
Additional complexities in eukaryotes:
- There are three different types of RNA polymerases in the nucleus:
- RNA polymerase I transcribes rRNA (28S, 18S, and 5.8S)
- RNA polymerase II transcribes heterogeneous nuclear RNA (hnRNA).
- RNA polymerase III transcribes tRNA, 5srRNA and snRNA.
- Post transcriptional processing: (occurs inside the nucleus)
(a) Splicing:
- The primary transcript (hn RNA) contain both exons and introns and required to be processed before translationally active (mRNA).
- The introns are removed and exons are joined in a defined order.
- This process is catalyzed by SnRNP, introns removed as spliceosome.
(b) Capping: an unusual nucleotide called methyl guanosine triphosphate is added to the 5’ end of hnRNA.
(c) Tailing: Adenylate residues (200-300) are added at 3’ end of hnRNA in a template independent manner.
The processed hnRNA is now called mRNA and transported out of the nucleus for translation.
GENETIC CODE:
Contribution to discovery:
- The process of replication and transcription based on complementarity.
- The process of translation is the transfer of genetic information form a polymer of nucleotides to a polymer of amino acids. There is no complementarity exist between nucleotides and amino acids.
- If there is change in the nucleic acid (genetic material) there is change in amino acids in proteins.
- There must be a genetic code that could direct the sequence of amino acids in proteins during translation.
- George Gamow proposed the code should be combination of bases, he suggested that in order to code for all the 20 amino acids, the code should be made up of three nucleotides.
- Har Govind Khorana enables instrumental synthesizing RNA molecules with desired combinations of bases (homopolymer and copolymers).
- Marshall Nirenberg’s cell – free system for protein synthesis finally helped the discovery of genetic code.
- Severo Ochoa enzyme (polynucleotide phosphorylase) was also helpful in polymerizing RNA with desired sequences in a template independent manner (enzymatic synthesis of RNA)
Salient features of genetic code:
- The codon is triplet. Three nitrogen base sequences constitute one codon.
- There are 64 codon, 61 codes for amino acids and 3 codons are stop codon.
- One codon codes for only one amino acid, hence it is unambiguous.
- Degeneracy: some amino acids are coded by more than one codon.
- Comma less: the codon is read in mRNA in a continuous fashion. There is no punctuation.
- Universal: From bacteria to human UUU codes for phenyl alanine.
- Initiation codon: AUG is the first codon of all mRNA. And also it codes for methionine (met), hence has dual function.
- Non-overlapping: The genetic code reads linearly
- Direction: the code only read in 5’ → 3’ direction.
- Anticodon: Each codon has a complementary anticodon on tRNA.
- Non-sense codon: UAA, GUA, and UAG do not code for amino acid and has no anticodon on the tRNA.
Mutation and Genetic code:
- Relationship between DNA and genes are best understood by mutation.
Point mutation:
- It occurs due to replacement nitrogen base within the gene.
- It only affects the change of particular amino acid.
- Best understood the cause of sickle cell anemia.
Frame shift mutation:
- It occurs due to insertion or deletion of one or more nitrogen bases in the gene.
- There is change in whole sequence of amino acid from the point of insertion or deletion.
- Best understood in β-thalasemia.
tRNA-the Adaptor molecule:
- The tRNA is called sRNA (soluble RNA)
- It acts as an adapter molecule.
- tRNA has an anticodon loop that base complementary to the codon.
- It has an amino acid accepter end to which it binds with amino acid.
- Each tRNA bind with specific amino acid i.e 61 types of tRNA found.
- One specific tRNA with anticodon UAC called initiator tRNA.
- There is no tRNA for stop codons. (UAA, UGA, UAG)
- The secondary structure is like clover-leaf.
- The actual structure of tRNA is compact, looks like inverted ‘L’.
TRANSLATION:
- It refers to polymerization of amino acids to form a polypeptide.
- The number and sequence of amino acids are defined by the sequence of bases in the mRNA.
- The amino acids are joined by peptide bond.
- Amino acids are activated in the presence of ATP and linked to their specific tRNA is called charging of tRNA or aminoacylation of tRNA.
- Ribosome is the cellular factory for protein synthesis.
- Ribosome consists of structural rRNA and 80 different proteins.
- In inactive state ribosome(70S) present in two subunits:-
- A large sub unit 50S.
- A small sub unit 30S.
CBSE Biology (Chapter Wise) Class XII ( By Mr. Hare Krushna Giri )
Email Id : [email protected]
