Jan 2006
PROTEIN SYNTHESIS
THE LINK BETWEEN GENES AND PROTEINS
1.
Jan 2006
2.
Gene -portion of DNA that codes
for a protein that controls a trait
Two types of proteins
a. Structural proteins: determine
how we look
 EX: Hair & skin pigments;
ear lobe shape
b. Enzymes: determine how we
function
 Control every chemical
reaction in the body
 EX: Sucrase breaks down
sucrose (table sugar)
Detached
ear lobes
Attached
ear lobes
OVERVIEW
Sequence of bases determines the “genetic code”
2. DNA NEVER leaves the nucleus
3. RNA copies DNA code of a gene.
a. RNA carries the copied code out of the nucleus to
the ribosome where protein is made
1.
DNA code
Jan 2006
TYPES OF RNA
Messenger RNA = mRNA
a. Copies instructions from
DNA code; carries code out
of nucleus to ribosome
2. Ribosomal RNA = rRNA
a. Ribosomes are made of
protein and RNA
3. Transfer RNA = tRNA
a. Carries and transfers a
specific amino acid to the
ribosome
b. T-shape
1.
mRNA
rRNA
tRNA
Step 1: Transcription
1.
Transcription: a single strand of
mRNA is copied from DNA in the
nucleus
a. DNA unwinds – helicase
b. RNA polymerase uses DNA
template to make mRNA strand
 If DNA is
TACCAGTTT
 mRNA will be
AUGGUCAAA
c.
mRNA is removed and DNA
strands will join & recoil
Slide # 8
Step 2: mRNA editing
1.
2.
3.
mRNA editing: cutting and
splicing mRNA before it leaves
the nucleus
a. Introns- (interfere) are cut
out by enzymes
b. Exons- (expressed) are left
 spliced together to
create an edited version
Poly-A tail added to end
Edited mRNA is sent to
ribosome
Introns are removed and exons
are spliced together. This is the
message that is translated.
Slide # 9
Step 3: Translation
1.
How the mRNA is read:
a. Every 3 bases on mRNA
represents a code for
an amino acid (codon)
b.
Amino acids are
abbreviated (usually
first 3 letters).
 Met = methonine
(start)
 Leu = leucine
Slide # 11
Step 3: Translation
4.
Occurs at a ribosome
5.
Codon on mRNA specifies which tRNA
brings the designated amino acid
 Anticodon on tRNA “reads” the codon
on mRNA
 Ribosome joins the amino acids
together with peptide bonds to build
a polypeptide (protein)
6.
DNA (gene)  RNA  Protein Trait
Slide # 12
TRANSLATION

Lysine
Phenylalanine
t RNA
Methionine
 Anticodon
Ribosome
mRNA 
Start codon
Go to
Section:
Jan 2006
Nucleus
mRNA
TRANSLATION
Slide # 13
Jan 2006
Growing polypeptide chain
The Polypeptide “Assembly Line”
Ribosome
tRNA
Lysine
tRNA
mRNA
Completing the Polypeptide
mRNA
Ribosome
Go to
Section:
Translation direction
Slide # 10
READING THE CODON CHART
Examples:
AUG = Methionine
CAU = Histidine
UAG = Stop
First
Position
Try
these:
Answers:
GCU:
Alanine
UAC:
Tyrosine
CUG:
Leucine
UUA:
Leucine
Jan 2006
Third
Position
This chart only works for mRNA codons.
Jan 2006