Download (codons) make a specific amino acid

Transcription and Translation
9/7
• Why does DNA take a double helical
orientation?
• How is an mRNA sequence translated to
produce a specific sequence of specific amino
acids?
• What happens if mutations occur during
semiconservative DNA replication?
• Why are ribosomes important for translation?
• Could you describe all steps and organelles that
permit the gene on DNA for insulin to become a
protein called insulin that leaves a cell.
Don’t Forget: DNA forms a double helix because the two base
types on the two sides can hydrogen bond to each other! The
helix forms in “antiparallel” orientation. The presence of uracil in
RNA prevents a helix from forming in RNA.
Semiconservative replication means that each strand of the
original pair is used to make a new strand. “Semi” means the
new cell has half the original strands and half “new” strands.
Semiconservative replication also means that a mistake
(mutation) in the original is passed on to all further
replications of the original DNA sequence.
• Consider This: It is possible that one or two of the original stands of
DNA from your mom or dad could still exist in your body!
(semiconservative DNA replication)
– Math: mom23chromosomes (23dsDNA)46ssDNA
– Dad23 chromosomes (23ds DNA)—46ssDNA
– You: sum of several trillion cells with 92 original strands still possible
floating around unchanged since inception! 
• Fatal mutations can kill a cell immediately and end the cell genetic
contribution of the cell to the body.
• Deleterious mutations to DNA can change the proteins produced by
a cell and cause cellular dysfunction….i.e. Cancer
– Why are they passed to cell in same body but not next generation?
• Why can mutations to your DNA can be passed on to future
generations if they occur in the cells that become an Egg or Sperm
DNA makes mRNA and mRNA bases (codons) code for/are
translated into amino acids in the growing protein chain (peptide
bonds link aa). tRNA (with an anticodon tRNA) carries each new
amino acid to the new (nascent) growing chain of amino acids.
Key Codons on
mRNA:
#1: AUG
“start codon”
This is where
protein is
started!
#2: a.a. to
protein chain
#3: Stop signal
is created by
UAG, UAA
and UGA
The DNA sequence that codes for a protein is called a “GENE”.
A series of special transfer RNA molecules (tRNAs) carry the
amino acids to the new protein chain being created. Anticodons of tRNAs (i.e. UUA below) bind the codons of the
mRNA (would be AAU on mRNA), when tRNA-aa bind to
mRNA, amino acids are linked together by peptide bonds.
How do we convert regular old cholesterol into testosterone? To
do this we need to make enzymes from DNA/mRNA that make the
chemical reactions possible. How do we make these enzymes?
There are 20 different amino acids (see below) to choose from. Each
A.A. has at least one 3-base codon, some amino acids have several
different codons.
A total of 64 different mRNA codons exist.
Don’t Memorize this list! Just realize there are 64 different possible combinations
for a 3-base codon given four different kinds of base. (Do remember AUG is the
start codon and that UAA, UAG and UGA are stop codons).
What happens to mRNA with respect to making codons and
a protein from the codons? How do I translate mRNA into a
specific amino acid sequence? Great Test Question
Remember these codons:
•
AUG= starts a protein with the amino acid methionine
•
UAG, UAA and UGA stop protein synthesis from mRNA
•
All other base triplets (codons) make a specific amino acid
1)
Identify where the protein starts (AUG)?
2)
Every three bases on mRNA after this makes an amino acid in the
protein chain being produced.
3)
Any time a triplet of bases (codon) is UAG, UAA and UGA protein
synthesis stops
4)
All proteins are made from 20 different amino acids
5)
Each amino acid has a specific mRNA codon of three bases on the
mRNA that signal that it should be placed at the growing end of the
protein chain.
6)
Each codon on the mRNA is read by an anti-codon on tRNA. Each
special tRNA carries its unique amino acid to the new protein.
7)
A protein that contains 10 amino acids had to be made from 10 codons,
each codon is from 3 bases in the mRNA. The stop codon does not
make an amino acid, it simply says “stop”.
8)
A 10 amino acid protein would need how many bases?
[10 codons X 3 bases/codon] + 3 bases for the stop codon= 33 bases
30 bases for codons for the 10 amino acids plus 3 bases from the stop codon
Lets look at how the base sequence of mRNA is translated into an amino acid
sequence before (original) and after we change the mRNA base sequence.
Ultimately mutations to DNA are what change the mRNA sequence!
1) How many amino acids long is this protein?
UUCGAUG-GCC-UCU-UGC-AUG-GCG-UAG-UUU-AG
1
5go
10
stop
30
2) If Base #5 is removed does the base sequence down stream change?
Where does synthesis start after the change?
A-removed
UUCGUGGCCUCUUGC-AUG-GCG-UAG-UUU-AG
1
5
10
go
stop
29
3) If an extra base (G) is ADDED immediately after #7 the total mRNA
becomes 31 bases, how many amino acids are produced?
UUCGAUG-GGC-CUC-UUG-CAU-GGC-GUA-GUU-UGC...
1
5go
10
31
“Additional” nucleotide added to mRNA sequence
Is a stop codon present? Will protein production stop?
For more help go to Supplemental Instruction!
Ribosomes are large protein-RNA structures that hold the
mRNA during translation so tRNA can add new amino acids
to the nascent end of the peptide chain!
Facts:
-Ribosomes are large particles made of many proteins and
pieces of rRNA
-Many ribosomes can translate a single mRNA at the same
time to make several proteins at the same time
-Ribosomes can be free floating in the cytosol
-Ribosomes can attach to the endoplasmic reticulum giving
it a studded appearance (Rough ER)
Insulin can now diffuse into blood stream!
Proteins are sent into vesicles that are created at the Golgi
apparatus. Secretory vesicles release their contents when
they fuse with the plasma membrane.
Protein is packaged into
vesicles and secreted out of
the cell after your meal.
mRNA
Protein is created and modified
How do we get insulin into the blood after a meal?
-mRNA associates with a ribosome and the rough side of the
endoplasmic reticulum
-The protein (insulin) is pushed into the E.R.
-Insulin can then be modified in the E.R.
-After modification in the ER they are sent to the Golgi
Apparatus for additional modification
-The Golgi Apparatus loads proteins into vesicles for secretion
-The vesicle fuses with the plasma membrane (exocytosis) and
the insulin can now pass on to the blood and the body.
-Insulin is secreted into the blood when second messengers tell
the pancreatic beta-cells you have just eaten your meal and
need to focus on anabolic reactions!