Transcription and Translation
GENE EXPRESSION - How the genes are
expressed is a function of the protein produced.
DNA (gene)
Ribosome
tRNA & rRNA
Protein
m RNA
Trait (expression)
There are two processes that make
this happen…
Transcription
• mRNA strand is built from
the code on the DNA strand
(gene)
Translation
• mRNA strand is used at the
ribosome to help join
amino acids to form a
protein.
TRANSCRIPTION
Let’s watch a video of transcription…
Requirements for Transcription
a gene segment on the DNA
many free floating RNA nucleotides
the enzyme RNA polymerase
PREDICT: What do you think the RNA polymerase will do?
located
Free floating
RNA
nucleotides
bond with
complimentary
DNA
nucleotides to
form a strand
of mRNA
4
Weak
hydrogen
bonds break
between
the 2 DNA
strands
3
DNA double
helix
unwinds at
the area of
the DNA
where the
gene is
2
1
Steps in transcription
When the
mRNA is
completely
formed, it
breaks away
from the DNA.
The DNA
strands
reconnect and
the mRNA is
free to travel to
the cytoplasm
These stages are catalyzed by the enzyme RNA polymerase
Each triplet code on a DNA molecule is transcribed
into a triplet codon on the mRNA molecule.
• If the DNA codes for a polypeptide is
T-A-C—C-C-G—T-A-G—C-T-T—A-C-T
• What would the codons on the complimentary
strand of mRNA codons look like?
A-U-G – G-G-C – A-U-C – G-A-A – U-G-A
• DNA codes:
T-A-C—C-A-T—C-C-C—A-A-A—A-C-T
• mRNA codons:
A-U-G – G-U-A – G-G-G – U-U-U – U-G-A
Think…think…..think….
• What does each letter in the code or codon stand for
represent?
That’s right….a nucleotide
• What do the particular codes and codons represent?
Amino Acids….good job!
• Because each base triplet on the mRNA stands for an
amino acid, each mRNA molecule must contain how
many times the number of nucleotides as the
number of amino acids making up the protein
product.
3X….Wow! You guys are on a roll!
Engage your BRAIN….The
intersection of science and math
• An mRNA strand with 66 nucleotides codes for a
polypeptide how many amino acids in length?
22…you all are Math whizzes
• A protein 300 amino acid units in length was
synthesized from an mRNA strand how many
nucleotides in length?
900…Woo Hoo!
• A DNA strand 700 nucleotides in length will be
transcribed into an mRNA strand how many
nucleotides in length.
700…Ah! This one is tricky!
How do we read mRNA codons?
•
•
•
•
There are 4 different bases
in DNA (A, C, G, T) which
can combine in different
triplets to form 64 possible
triplet codes (43).
When these codes are
transcribed into mRNA,
there are 64 triplet codons
which can be formed.
Only 20 amino acids are
used so sometimes more
than one codon represents
an amino acid.
We can use the chart to the
right to determine the
exact sequence of amino
acids that will make up a
protein.
•
How many different mRNA codons are there
on the chart?
43 = 4 x 4 x 4 = 64
different codons (using A, G, C and U in
triplets)
• How many different amino acids are there?
Only 20
• Which amino acids only have ONE triplet
code assigned to them?
Methionine and Trytophan
• With the exception of Methionine and
Tryptophan,
more than one nucleotide triplet codes for
each amino acid.
• Give an example of an amino acid which has
more than one codon.
valine, alanine, isoleucine, threonine, and
many more
• What do you notice about the codons that
code for the same amino acid?
They are very similar and usually only the last
nitrogen base is different.
Special Codons
Start
Codon
• AUG-codes for the amino acid
methionine. It signals the ribosome
that it is the beginning of the mRNA
sequence.
Stop
Codons
• UAA, UAG, UGA- Don’t code for an
amino acid!!!! Signal the ribosome
that it is the end of the mRNA
sequence and triggers the release of
the mRNA from the ribosome.
• What is the DNA code for the mRNA start
codon, methionine?
TAC
• What are the DNA codes for the stop codons
UAA, UAG and UGA?
ATT, ATC, ACT
• After transcription has taken place, the mRNA
moves out of the nucleus to link with a
ribosome in the cytoplasm. Translation will
take place here…..
TRANSLATION
Let’s Watch a video on Translation…
Requirements for Translation
Steps in Translation
mRNA joins with the
ribosome (in
cytoplasm or near the
Rough Endoplasmic
Reticulum
the mRNA slides
through the ribosome
to expose the next
codon
the next tRNA with
amino acid joins the
mRNA
tRNA (with an amino acid
attached) joins the mRNA
codon to ANTI-codon
(The anticodon on the
tRNA is complimentary to
the codon on the mRNA.)
the first tRNA is
released from the
mRNA amino acids
stay bonded to each
other
Sequence is repeated
Repeat steps 4-8 until
a stop codon is
reached
a second tRNA with
attached amino acid
joins the mRNA
a peptide bond forms
between the first two
amino acids
mRNA and protein is
released from the
ribosome
The Final Products of Protein
Synthesis
Let’s Watch a video to sum up Transcription and Translation…