DNA TO PROTEIN
genotype to phenotype
Look deep into nature, and
then you will understand
everything better.
Albert Einstein
one-gene, one-enzyme hypothesis
• Many enzymes are composed of more
than one polypeptide chain, or subunit
(that is, they have a quaternary structure).
In this case, each polypeptide chain is
specified by its own separate gene. Thus,
it is more correct to speak of a one-gene,
one-polypeptide relationship: The function
of a gene is to control the production of a
single, specific polypeptide.
Expression of Hereditary Material
Information is useless without required machinery
CENTRAL DOGMA OF MOLECULAR
BIOLOGY. The central dogma, simply stated, is
that DNA codes for the production of RNA, RNA
codes for the production of protein, and protein
does not code for the production of
protein, RNA,
or DNA
2 entities modify the central dogma
• RNA VIRUSES: Synthesis of DNA from
RNA is called reverse transcription, and
not surprisingly, such viruses are called
retroviruses
• PRIONS:
Hypotheses of the Dogma
Messenger Hypothesis: RNA molecule forms a
complementary copy of one DNA strand and this
mRNA moves to cytoplasm to serve as a template
for protein synthesis.
Adapter Hypothesis: An adapter molecule binds a
specific amino acid with one region and recognizes
nucleotide sequence with a different region.
tRNA acts as an intermediary between nucleotide
sequence of mRNA and AA sequence in protein.
Transcription of a Eukaryotic Gene
DNA of a
eukaryotic
cell is
confined to
the nucleus,
but
proteins are
synthesized
in the
cytoplasm.
TRANSCRIPTION
DNA-Directed RNA Synthesis
Transcription
The synthesis of RNA molecules
using
DNA strands as the
templates so that the genetic
information can be transferred
from DNA to RNA.
• DNA regions that can be transcribed
into RNA are called structural genes.
• only one of the two strands of DNA—the
template strand—is transcribed.
• non-template strand, remains
untranscribed
Template
The template strand is the strand from
which the RNA is actually transcribed. It
is also termed as antisense strand.
The coding strand is the strand whose
base sequence specifies the amino acid
sequence of the encoded protein.
Therefore, it is also called as sense
strand.
5'
GCAGTACATGTC
3' coding
3'
CGTCATGTACAG
5'
strand
template
strand
transcription
5'
GCAGUACAUGUC
3'
RNA
Promoter
regulatory
sequences
5'
3'
promotor
RNA-pol
structural gene
3'
5'
Transcription Process
• Three phases: initiation, elongation, and
termination.
Transcription in Prokaryotes
• Initiation
phase:
RNA-polymerase
recognizes the promoter and starts the
transcription.
• Elongation phase: the RNA strand is
continuously growing.
• Termination phase: the RNA-polymerase
stops synthesis and the nascent RNA is
separated from the DNA template.
Transcription in Eukaryotes
Initiation
•Transcription initiation needs promoter, a special
sequence of DNA to which RNA polymerase binds very
tightly.
•There is at least one promoter for each gene
•Promoters are important control sequences that “tell”
the RNA polymerase three things:
•where to start transcription
•which strand of DNA to read
•the direction to take from the start
•
Part of each promoter is the initiation site,
where transcription begins
Transcription factors
• RNA-polymerase does not bind the
promoter directly.
• RNA-polymerase associates with six
transcription factors, TFII A - TFII H.
Elongation
• The elongation is similar to that in
prokaryotes.
• The transcription and translation do not
take place simultaneously since they
are separated by nuclear membrane
• RNA Pol moves 3’ to 5’,
synthesis 5’ to 3’, no primer,
Unwinds 20 bp at a time
Transcription Elongation
Transcript
antiparallel to
DNA template
strand
No proofreading
error rate 1 in
10,000-100,000
Transcription Termination
The termination sequence is AATAAA followed by
GT repeats
Translation starts before termination in prokaryotes
Pre-mRNA initial product of transcription in eukaryotes
Questions???