Saturday, August 18, 2012

TRANSCRIPTION & RNA PROCESSING

TRANSCRIPTION
  • Formation of the base sequence of a single stranded molecule of RNA from the base sequence of a dsDNA molecule.
  • Only one strand of DNA molecule (i.e, template strand) is copied by RNA polymerase as it synthesize RNA in 5’ to 3’ direction for any particular gene.
  • RNA product is antiparallel & complementary to the template strand.
  • RNA polymerase recognizes start signals (promoters) and stop signals (terminators).



TYPES OF RNA

Ribosomal RNA (rRNA): most abundant used as structural component of the ribosome associates with ribosomal     proteins to form the complete, functional ribosome.

Transfer RNA (tRNA): second most abundant RNA carry amino acids to the ribosome during protein synthesis.

                                        
Messenger RNA (mRNA): carries information specifying  the amino acid sequence of a protein to the ribosome.Only type of RNA that is translated. 
                                               

Heterogeneous nuclear RNA (hnRNA): found only in the nucleus of the eukaryotic cells.Precursors of mRNA, formed during posttranscriptional processing.
                                                                   


Small nuclear RNA (snRNA): found in eukaryotic nucleus major functional is to participate in splicing mRNA (removal of introns)

Ribozymes: RNA molecules with enzymatic activity. 

Found both in prokaryotes & eukaryotes.


CONCEPTS AND TERMINOLOGY : TRANSCRIPTION

RNA is synthesized by a DNA dependant RNA polymerase.
RNA polymerase locates gene in DNA and binds the promoter site. No primer is required.
RNA polymerase moves along the template strand in the 3’ to 5’ direction as it synthesizes the RNA product in the 5’ to 3’ direction. The RNA product is complementary and antiparallel to the template strand. Coding strand is identical in sequence to RNA product except that RNA contains U instead of T in DNA.




The numbering system is used to identify the location of important bases. The 1st base transcribed as RNA is defined as the +1 base of that gene region. To the left (upstream) of it, bases are -1,-2,-3,etc. To the right (downstream) are +1,+2,+3,etc.
Transcription ends when RNA polymerase reaches a termination signal.


RNA Polymerases 



Production of Prokaryotic mRNA


  • The mRNA produced by the gene is a monocistronic in the given figure. That is, it is transcribed from a single gene and codes for only a single protein.
  • Some bacterial operons produce polycistronic messages. The mRNA in this case contains information from several genes and codes for several different proteins.


Production of Eukaryotic RNA


Processing of Eukaryotic mRNA



  • Alternative Splicing of Eukaryotic mRNA
  • For some genes, the primary transcript is spliced differently to produce two or more types of a protein from same gene k/a alternative splicing. E.g : Troponin (T, C, I) Immunoglobulins.


Ribosomal RNA

Transfer RNA (tRNA)

The important regions in tRNA are:
 1. Acceptor arm: appropriate AA is attached.
 2. Anticodon arm: recognizes a specific codon ( for the AA it carries) in mRNA.
The CCA tail is a cytosine-cytosine-adenine sequence at the 3' end of the tRNA molecule. This sequence is important for the recognition of tRNA by enzymes and critical in translation.


Structure of tRNA


Summary:


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