Transcript Features of the genetic code

Features of the genetic code:
• Triplet codons (total 64 codons)
• Nonoverlapping
• Three stop or nonsense codons UAA (ocher), UAG (amber) and
UGA (opal)
• Degenerate
• Open reading frame starting at the initiation codon (AUG)
• Each codon has 5’ base and a 3’ base e.g. 5’CGU3’
• Mutations that modify the genetic code are of 3 types: frameshift
(include deletions and insertions), missense (lead to an amino acid
replacement) and nonsense (mutation that generates any of the three
stop codons leading a a premature truncation of the polypeptide.
TRANSCRIPTION
General features:
• RNA polymerase (one kind in prokaryote containing five subunits
and several kinds in eukaryotes where each transcribes a different
type of genes)
• Promoter sequences immediately upstream of the gene and contain
sequences that have high affinity to bind RNA polymerase
• Synthesis of RNA is in the 5’ to 3’ direction. At the transcription
bubble, RNA forms a duplex with the template DNA strand where A
is complementary to U and C is complementary to G.
• Transcription is terminated at the the 3’ end of the gene at
sequences know as terminators which direct RNA polymerase to
stop synthesis.
Processing of RNA transcript in eukaryotes:
The immediate RNA transcript is known as primary transcript and is
subject to three modifications prior to moving to the cytoplasm:
• A capping enzyme adds a G to the first nucleotide in the transcript
in the unusual 5’-5’ direction (phosphate to phosphate bond). Then a
methyl thransferase adds methyl groups (-CH3) to the G and one or
more of the first few bases of the RNA transcript. Capping and
methylation is believed to be critical for efficient translation.
• Addition of a poly A tail (100-200 As) at the 3’ end of the primary
transcript by a poly-A-polymerase. Tailing is believed to stabilize
mRNA so it remains for longer time to be translated to many
polypeptide molecules before it is degraded and increase the
efficiency of the initial steps of translation.
• RNA splicing and removal of introns from the primary transcript
followed by a very precise joining of the exons.
Mechanism of splicing:
• Within each intron three sequences are present: splice donors (at the
5’ end of the intron), splice acceptors (at the 3’ end of the intron) and
the branch sites (sandwiched between the splice donors and
acceptors).
• Splicesome is needed to identify and catalyze the sequence of
events leading to removal of the intron and rejoining of the two
successive exons. The splicesome consists of snRNP (snRNA 100300 nucleotides long + proteins). Each splicesome is composed of
four snRNPs together and each snRNP is five snRNA plus about 50
proteins. Some snRNAs base pair with the splice donor and acceptor
in the primary transcript and this way can bring them together.
• Alternative splicing (as in the gene encoding the antibody heavy
chain) and trans-splicing (as in C. elegans) are means of controlling
gene expression in eukaryotes.