Download Chapter 17

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
ribosome
translation
polypeptide
Transcription: synthesis of mRNA
under the direction of DNA
- one step in prokaryotic cells
Eukaryotic Cell
- Two steps in eukaryotic cells
1. creating a 1o transcript
transcription
mRNA
1o transcript
RNA Processing
2. RNA processing (editing) to create 2o
transcript
mRNA
2o transcript
translation
polypeptide
ribosome
Translation: synthesis of a
polypeptide under the direction of
mRNA
Directional Triplet-code Instructions
for a polypeptide
3’
A C C A A A C C G A G T
5’
DNA
3’
mRNA
Transcription
3’ to 5’ direction
on DNA
5’
U G G U U U G G C U C A
Translation
5’ to 3’ direction
on RNA
trp
phe
gly
ser
polypeptide
Central
Dogma
3 RNA
Players
Involved
tRNA
rRNA= =type
RNA
of that,
RNAcombined
that carries
with
theprotein,
amino acids
makes
to up
thethe
mRNA = DNA transcript
ribosome
structural component of a ribosome
Transcription
Initiation
3 steps
1) Initiation
2) Elongation
3) Termination
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
Termination
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. Transcription video
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
Termination
RNA Processing Continues
Primary transcript of mRNA is long and includes
introns (non-coding regions) and exons
(coding regions)
mRNA
Evolutionary Significance? Non-coding RNA
regions were the result of non-coding DNA
regions. Longer DNA increased chances of Xover during meiosis.
During RNA processing, introns must be cut out
(spliced) before a functional polypeptide can
be made
Termination
RNA 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
Aminoacyl-tRNA
coded by synthase
the mRNA
(enzyme)
catalyzes
the binding
to the
ribosome
for
of a specific amino acid to a free
polypeptide
tRNA.
(protein) synthesis
Activated t-RNA is now ready
for translation.
Structure of a Ribosome
P= Creates peptide bonds
P = Peptidyl-tRNA
between AA and
A
holds the growing polypeptide
binding chain
site
A=
= AminoacyltRNA brings
tRNA
site
in newbinding
AA
E= Free t-RNA detaches
E = Exit
Site
from
ribosome
Large subunit
E
P
A
tRNA
Anticodon
Small subunit
mRNA
codon
Ribosome=
rRNA + protein
Initiation
Translation
1) Initiation
2) Elongation
3) Termination
- 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
Free tRNA
exits and
can return to
bind with
other AA for
repeat
deliveries
Ribosome
slides down
the 3’ end of
the mRNA
New tRNA brings in
another amino acid,
based on the codon
on the mRNA. Basepairing occurs.
A peptide bond
forms between
the two
adjacent AA
Termination
Ribosome encounters
a “stop” codon:
UAA, UAG, or UGA
Release factor facilitates
the release of both
ribosomal subunits
Translation Video
Watch these realtime videos on DNA
transcription and
translation…Totally
Cool Dude!
http://www.dnai.org/a/in
dex.html
Redundancy in AA
- Notice…several codons
code for the same AA
 Evolutionary advantage?
Other ways of increasing
Redundancy
redundancy:
helps to
Although A pairs with U
minimize errors
and G pairs with C, U
protein pair
can in
sometimes
synthesis
due to
with
G  wobble
effect
mutations
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.