Chapter 17
Central Dogma of Molecular Biology
From Genes to ProteinOne gene – one polypeptide hypothesisOne gene dictates the production of a single polypeptide
transcription
transcription
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
translation
translation
RNA Processing
Central Dogma of Molecular BiologyProkaryotic Cell
Eukaryotic Cell
mRNA
1o transcript
mRNA
polypeptide
polypeptide
ribosome
ribosome
mRNA
2o transcript
Directional Triplet-code Instructions for a polypeptide
Transcription
3’ to 5’ direction on DNA
Translation
5’ to 3’ direction on RNA
A C C A A A C C G A G T
U G G U U U G G C U C A
trp phe gly ser
3’
3’
5’
5’
DNA
mRNA
polypeptide
3 RNA Players Involved
mRNA = DNA transcripttRNA = 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
Central Dogma
Transcription3 steps
1) Initiation
2) Elongation
3) Termination
Initiation
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
Elongation
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
Termination1) 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.
mRNA
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
Transcription video
TerminationRNA Processing Continues
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
mRNA
TerminationRNA Processing Continues
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
t-RNA Activation for Translation
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.
Structure of a Ribosome
Large subunit
Small subunit
Ribosome=
rRNA + protein
E P A
A = Aminoacyl-tRNA binding site
P = Peptidyl-tRNA binding site
E = Exit Site
tRNA Anticodon
mRNA codon
A = tRNA brings in new AA
P= Creates peptide bonds between AA and holds the growing polypeptide chain
E= Free t-RNA detaches from ribosome
Translation
1) Initiation
2) Elongation
3) Termination
Initiation
- 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.
Elongation
New tRNA brings in another amino acid, based on the codon on the mRNA. Base-pairing occurs.
A peptide bond forms between the two adjacent AA
Ribosome slides down the 3’ end of the mRNA
Free tRNA exits and can return to bind with other AA for repeat deliveries
Termination
Ribosome encounters a “stop” codon:
UAA, UAG, or UGA
Release factor facilitates the release of both ribosomal subunits
Translation Video
http://www.dnai.org/a/index.html
Watch these real-time videos on DNA
transcription and translation…Totally
Cool Dude!
Redundancy in AA
- Notice…several codons code for the same AA
Evolutionary advantage?
Redundancy helps to minimize errors in protein synthesis due to
mutations
Although A pairs with U and G pairs with C, U can sometimes pair with G wobble effect
Other ways of increasing redundancy:
Endo-membrane System
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.