Chapter 17. Central Dogma of Molecular Biology From Genes to Protein. One gene – one polypeptide hypothesis. One gene dictates the production of a single polypeptide. Prokaryotic Cell. Central Dogma of Molecular Biology. transcription. mRNA.
Central Dogma of Molecular Biology
From Genes to Protein
One gene – one polypeptide hypothesis
One gene dictates the production of a single polypeptide
Central Dogma of Molecular Biology
Transcription: synthesis of mRNA under the direction of DNA
- one step in prokaryotic cells
- Two steps in eukaryotic cells
1. creating a 1o transcript
2. RNA processing (editing) to create 2o transcript
Translation: synthesis of a polypeptide under the direction of mRNA
A C C A A A C C G A G T
3’ to 5’ direction on DNA
U G G U U U G G C U C A
5’ to 3’ direction on RNA
tRNA = type of RNA that carries the amino acids to the ribosome
rRNA = RNA that, combined with protein, makes up the structural component of a ribosome
mRNA = DNA transcript
Transcription begins at a “Promotor” region of DNA, recognized by a TATA box sequence
A transcription factor binds to help RNA polymerase (RNA pol.) to bind to the DNA
RNA pol. binds and begins to unwind DNA
RNA pol unzips 10-20 DNA bases at a time in a 3’ 5’ direction and base pairs with the DNA using RNA nucleotides
Base-pair occurs at a rate of 60 nucleotides/second
A-U; G-C base-pair rules
New 1o transcript (initial copy) of mRNA peels away from DNA template
1) RNA sequence, not DNA sequence, is used to end transcription.
Once RNA pol. creates the AAUAAA termination sequence, the primary mRNA transcript peels away from the DNA template.
2) RNA processing occurs: (creating of 2o transcript)
- 5’ cap of guanine nucleotides are added to protect mRNA from hydrolytic enzymes and to provide a starting site for ribosomes in translation
- 3’ poly A tail is added to protect mRNA from hydrolytic enzymes and to help with mRNA export from the nucleus
Primary transcript of mRNA is long and includes introns (non-coding regions) and exons (coding regions)
Evolutionary Significance? Non-coding RNA regions were the result of non-coding DNA regions. Longer DNA increased chances of X-over during meiosis.
During RNA processing, introns must be cut out (spliced) before a functional polypeptide can be made
3) RNA splicing
A “spliceosome” complex recognizes intron sequences and deletes them
Spliceosomes are made up of special RNA called snRNA + various proteins
2o mRNA transcript is now ready
tRNA = brings in the corresponding AA coded by the mRNA to the ribosome for polypeptide (protein) synthesis
Aminoacyl-tRNA synthase (enzyme) catalyzes the binding of a specific amino acid to a free tRNA.
Activated t-RNA is now ready for translation.
P= Creates peptide bonds between AA and holds the growing polypeptide chain
P = Peptidyl-tRNA binding site
A = Aminoacyl-tRNA binding site
A = tRNA brings in new AA
E= Free t-RNA detaches from ribosome
E = Exit Site
E P A
rRNA + protein
- Small ribosomal subunit attaches near the 5” end of mRNA at AUG start codon
- tRNA carrying AA Methionine attaches to AUG start codon
- Large ribosomal subunit attaches to mRNA w/ tRNA occupying the “P” site.
New tRNA brings in another amino acid, based on the codon on the mRNA. Base-pairing occurs.
Free tRNA exits and can return to bind with other AA for repeat deliveries
A peptide bond forms between the two adjacent AA
Ribosome slides down the 3’ end of the mRNA
Ribosome encounters a “stop” codon:
UAA, UAG, or UGA
Release factor facilitates the release of both ribosomal subunits
Watch these real-time videos on DNA transcription and translation…Totally Cool Dude!
Redundancy helps to minimize errors in protein synthesis due to mutations
Other ways of increasing redundancy:
Although A pairs with U and G pairs with C, U can sometimes pair with G wobble effect
Recall that glyco-proteins (made by attached ribosomes) are processed by the ER and golgi before being incorporated within vesicles (ie. lysosomes) , exported from the cell, or incorportated into the plasma membrane.