Transcript How cells use DNA, part 1: TRANSCRIPTION

Section 6.3
An overview:
Most commonly, what
comes to mind is the
process by which we
take ideas expressed
in one language, &
make them
intelligible in another
language.
Often this means a
change of script,
from one we don’t
understand to
another we can read.
In the process of
translation in a cell,
the transcribed
message of mRNA is
translated to a totally
different ‘language’,
that of protein.
DNA & RNA are
‘written’ in very
similar chemicals,
but protein is
‘written’ in an
entirely different
‘script’: amino
acids.
For Translation we need:
•
•
•
•
An ‘edited’ or ‘mature’ mRNA
Ribosomes
An unusual molecule, transfer or tRNA
Lots of available Amino Acids
The overall goal:
• Use the DNA
message that was
copied out into
mRNA to produce
a polypeptide or
protein.
• This is the second
part of the
CENTRAL DOGMA
• It relies on the
GENETIC CODE.
Translation
Stage
Initiation
Elongation
Termination
Events
A ribosome binds to the mRNA
Ribosome recognizes a specific three-base
sequence on the mRNA, and binds.
Ribosome moves along the strand ‘reading’ each
codon (3 nucleotides at a time)
tRNA delivers the appropriate amino acids to the
ribosome for addition to the growing
polypeptide chain
Ribosome catalyzes the formation of a peptide
bond between a.a.
Transcription ceases when ribosome reaches stop
codon, it falls off the mRNA (protein – release
factor) and the polypeptide is released
The tRNA:
• Acts as a ‘taxi’ for
Amino Acids
• Single stranded, but
folded upon itself into a
clover-like shape.
• Able to bind to Amino
Acids at one end, and to
mRNA at the other.
• The mRNA binding end
has an ANTICODON.
• Each Anticodon codes
for a different Amino
Acid.
The tRNA:
• Amino acids bind at
the 3’ end of tRNA.
• This requires some
ATP energy!
• The Anticodon binds
to a complementary
codon sequence on
the mRNA.
• i.e. AUG codon = UAC
anticodon
The Ribosome:
• Site of translation
• Can be free in the
cytoplasm, or
associated with the
R.E.R., Golgi Body, or
Nucleolus.
• Two Subunits Lg
(60S)/Sm (40S)
• Able to bind mRNA
• Binds tRNA at one of
three sites: E (Exit), P
(Peptidyl Aminoacyl) or
A (Acetyl Aminoacyl)
The Ribosome:
• The mRNA binds in the
groove between the
large & small subunits.
• The first tRNA binds to
the P Site.
• A second tRNA binds to
the A Site.
• This brings the amino
acids on each tRNA
close enough to form a
peptide bond.
• As the ribosome shifts
down the mRNA, the
first tRNA is bumped
into the E site & is
released.
http://highered.mcgraw-hill.com/olc/dl/120077/micro06.swf
The Amino Acids:
• During translation,
the Amino Acids
‘meet’ at the
ribosome
• When they are
brought close
together (on the
ribosome), the
Amino Group of one
reacts with the
other’s Carboxyl
Group.
• In a dehydration
synthesis reaction, a
peptide bond forms.
Initiating Translation:
• mRNA binds to
the Ribosome
• tRNA’s carrying
amino acids
arrive, binding
anticodon to
codon
• Peptide bond
forms between
Amino Acids
Continuing the chain:
• The ribosome now
shifts 1 codon,
moving the first
tRNA into the E
Site, the second
into the P site, and
opening the A site
for a new tRNA to
bind.
Continuing the chain:
• Many ribosomes can bind to the same
mRNA & translate it simultaneously,
amplifying the amount of protein made.
The genetic code
How exactly does the base sequence of an
mRNA dictate the order of amino acids??
The genetic code
Reading the mRNA:
• Codons in the mRNA
are ‘read’ in threes
• Each three-base
combination
represents a
specific amino acid,
& matches a tRNA
anticodon
• Some amino acids
have only one code;
others have several
• Thus, the code is
redundant.
Different forms of the Code:
Different forms of the Code:
Different forms of the Code:
Summary
http://www.youtube.com/user/ndsuvirtualce
ll#p/a/44B161B3F290FC23/0/5bLEDdPSTQ
How the code works:
The DNA Sequence:
TAC AAA GCC TAG GAT ACA ATT
Is translated to the mRNA sequence:
AUG UUU CGG AUC CUA UGU UAA
Which in turn encodes the following sequence of
amino acids in a polypeptide:
MET—PHE—ARG—ILE—LEU—CYS—(stop)
Wrapping things up:
• There are three
mRNA codons that
signal the end of a
protein
• They are called STOP
CODONS: UAA, UAG,
UGA
• *in DNA, these are
ATT, ATC, & ACT.
• When it reaches a
stop codon, the
ribosome releases
the mRNA, &
translation ends.
Try your hand at this:
mRNA Sequence:
AUGCCUCGCAAAGGUUGCCACGUAUAA
Amino Acid Sequence:
MET PRO ARG LYS GLY CYS HIS VAL Stop
“Wobble” base pairing
Refers to the fact that the third base in the codon/
anticodon may not actually match
Wobble base pairing
• Allows some anticodons to bind to more than
one codon
• Recall the genetic code: Multiple codons for
one amino acid. Usually the third base in the
codon is what differs.
• Helps overcome errors in transcription
(no proofreading mechanism)
Practical Applications
“What makes a Firefly Glow?”
• demonstration of how protein synthesis is involved
in making a firefly glow
Foods and Protein Synthesis
• examples of foods that increase
protein synthesis to support
muscle building
Exercise and Protein Synthesis
• discuss the rate of protein
synthesis in relation to exercise
The Central Dogma:
Teaching Approach
• Protein Synthesis Virtual Lab
Transcription and Translation Virtual Lab
The Central Dogma:
Teaching Approach
• Protein Synthesis Role-Play
 Teacher ropes off a designated area as the nucleus where transcription must
occur. The rest of the classroom is the cytoplasm where translation will
occur.
 8 students are assigned to DNA sequences (24 nucleotides in each)
 8 students are assigned to complementary mRNA sequences
 8 students are assigned to complementary tRNA anticodons with
corresponding amino acids (polypeptide chain)
 8 special learning opportunity messages (Ex. “Ribosomes move along the
mRNA in a 5’ to 3’ direction, while reading the coding sequence.”) are
posted around the classroom corresponding to amino acid polypeptide chains
The Central Dogma:
Teaching Approach
• Protein Synthesis Role Play Continued
 DNA students and mRNA students remain in nucleus during
transcription. After transcription, mRNA students move into
cytoplasm, where tRNA students are waiting for translation.
 DNA students begin by writing down the complimentary RNA
sequence to their DNA sequence (transcription). They then search
the nucleus for their matching mRNA student.
 mRNA student then leaves the nucleus and uses the genetic code to
write down the corresponding amino acids to their RNA sequence
(translation). They then search the cytoplasm for their matching tRNA
student.
 The tRNA student then searches for the
special message associated with their
polypeptide chain, thus completing
the task.