Answer Key
Protein Synthesis Paper Lab
Part A: RNA Structure
Besides ensuring the exact replication of
chromosomes, the order of the bases is a genetic
code of instructions for the cell. How does the
cell “read” the chemical message coded in its
DNA?
Part of the answer lies with a second molecule
present in cells called ribonucleic acid (RNA).
RNA is similar to DNA in that it is made of
nucleotides. However, there is no deoxyribose
or thymine in RNA. Deoxyribose is replaced
by ribose in RNA, and thymine is replaced by
uracil. If you look at their structural formulas
and models, you will see that ribose and uracil
are similar to the molecules they replace.
Part B: RNA Transcription
Open (unzip) one of your DNA models
between the base pairs and separate the
two halves. Using the left side of your DNA
model as a pattern, match up the proper
RNA nucleotides with the DNA nucleotides.
You have made a messenger RNA molecule.
4. Do the RNA half-rung bases pair exactly as
they would if this were DNA replication? not
exactly
Remove the RNA nucleotide series from the DNA
pattern. Close the DNA molecule back up with its
original right side. DNA molecules “unzip”
temporarily during RNA transcription and then
zip closed again.
mRNA is a single-stranded molecule. Thus, the
sequence of RNA nucleotides formed from DNA
represents a complete mRNA molecule. After its
formation, this mRNA leaves the nucleus and goes
to where the ribosomes are. It carries the DNA
message of base sequences in the exact same order
out of the nucleus and into the cytoplasm.
On this table compare DNA and RNA, check all
that apply:
DNA
Deoxyribonucleic Acid
Ribonucleic Acid
Contains the sugar deoxyribose
Contains sugar ribose
1. Which base is replaced by uracil in RNA?
____________thymine____________
2. What molecule replaces deoxyribose in
RNA?
____________ribose_____________
Contains a phosphate group
Contains adenine
Contains thymine
Contains uracil
Contains guanine
3. To which base in DNA do the following RNA
bases pair?
Guanine _____cytosine________
Adenine _____uracil__________
Cytosine _____guanine________
Uracil _______adenine________
C
T
G
A
G
C
Contains cytosine
Monomers are nucleotides
Double-strand molecule
Single-strand molecule
Leaves/moves out of the nucleus
Contains a chemical code
mRNA
Part C: Structure of tRNA
Build a molecule of RNA using the paper models.
Join the nucleotides in this order: (new strand!)
Guanine
Adenine
Cytosine
Uracil
Cytosine
Guanine
Remember that the molecules of messenger RNA
(mRNA) leave the nucleus and join the cell’s
ribosomes in the cytoplasm. Other molecules
called transfer RNA (tRNA) are also present in
the cytoplasm. Molecules of tRNA are composed
of many nucleotides, just like mRNA, but the
difference is this: tRNA folds so that a three base
sequence (called an anticodon) sticks out and can
match up with a three base sequence on the
mRNA. Look at the paper models provided.
1a. Name the four nitrogen bases present in the
nucleotides of the tRNA molecule:
Cytosine, Guanine, Adenine, Uracil
Any given tRNA molecule can only pick up one
kind of amino acid, depending on the three base
sequence (anticodon) that attaches to the mRNA.
The amino acid attaches to the tRNA at the
opposite end from the anticodon. Fit the proper
amino acids onto your two tRNA molecules.
4. Which amino acid connects to the tRNA
molecule with the anticodon: (see chart)
a. AGC
serine
b. CUG
aspartic acid
5. How many nucleotide bases on the mRNA
molecule are responsible for the coding of
one amino acid? three
Part D: Building a Protein (Translation)
As the amino acid molecules are brought to the
mRNA by the tRNA molecules, the amino acids
join to form a protein molecule. This process is
called translation, because the DNA message has
been translated into a protein.
Amino acid
1b. Do these bases differ from those in mRNA?
no, same bases
1c. How does the shape and structure of tRNA
compare to mRNA?
tRNA is a single strand in a T-loop form, mRNA
is a single linear strand
Join the tRNA molecules to the mRNA that you
built at the beginning.
2. What base in tRNA can only join with the:
a. adenine base of mRNA Uracil
b. uracil base of mRNA
Adenine
c. guanine base of mRNA
Cytosine
d. cytosine base of mRNA
Guanine
3. What order of bases on mRNA will match a
sequence on tRNA of:
a. UUA (uracil, uracil, adenine)
AAU
b. UCA (uracil, cytosine, adenine) AGU
c. UGA (uracil, guanine, adenine) ACU
d. AAA (adenine, adenine, adenine) UUU
serine
proline
leucine
glutamic acid
tyrosine
arginine
glutamine
phenylalanine
valine
lysine
aspartic acid
tRNA
anticodon
AGC
GGG
AAU
CUC
AUA
GCU
GUU
AAA
CAC
UUU
CUG
mRNA
codon
UCG
CCC
UUA
GAG
UAU
CGA
CAA
UUU
GUG
AAA
GAC
Use the table to answer the questions below.
6. What amino acid is attached to a tRNA
molecule having a base sequence of
a. UUU
lysine
b. GCU
arginine
7. What tRNA anticodon is needed to join with
the following amino acids?
a. phenylalanine
AAA
b. valine
CAC
c. glutamic acid
CUC
Depending on the number, type and order of
amino acids, an almost endless variety of proteins
can be produced. Because of the repeated
matching of the base sequences, the base sequence
in the DNA of chromosomes codes for and
controls the formation of proteins at the
ribosomes.
Part D: Base Sequence Errors and Mutations
Every now and then errors may occur in the
process of forming proteins from the DNA coded
instructions. An error is a mutation, which will
result in a different amino acid sequence. The
protein may be different in a good way or (more
frequently) a bad way.
8. A protein molecule contains the following
amino acid sequence: leucine, glutamine,
tyrosine, leucine, serine, serine. What would be
the sequence of tRNA bases responsible for
forming this protein?
AAU, GUU, AUA, AAU, AGC, AGC
Hemoglobin is the protein in red blood cells that is
responsible for carrying oxygen. There are 600
amino acids in this relatively small protein
molecule, and in most people the amino acids are
all arranged correctly. There is a very slight
change in the amino acid sequence that results in
the very serious disease known as sickle cell
disease. The normally soft and flexible red blood
cells become distorted into a rigid sickle shape and
they form “logjams” in the smallest capillaries,
with disastrous consequences.
9. A ribosome receives the following mRNA
message: AAA CGA GAA GUU.
a. What will be the sequence of tRNA bases
matching up with the mRNA molecule?
UUU, GCU, CUU, CAA
b. What will be the sequence of amino acids
formed from this code?
lysine, arginine, glutamic acid, valine
To review: You should now be able to transcribe
(decode) a message in DNA base code (triplet)
into mRNA, and then translate it into an amino
acid sequence. Let’s try it. A portion of DNA on
a chromosome has the sequence of bases along
one strand as indicted in the table below.
Transcribe this message first into mRNA code,
then into the tRNA code, and finally translate it
into the correct amino acid sequence.
DNA code
AAT
GGG
ATA
AAA
GTT
mRNA code
(codon)
UUA
CCC
UAU
UUU
CAA
tRNA code
(anticodon)
AAU
GGG
AUA
AAA
GUU
The following amino acid sequence represents a
portion of a normal hemoglobin molecule:
proline, glutamic acid, glutamic acid, lysine.
10. Translate the normal amino acid sequence
into:
a. tRNA base code GGG, CUC, CUC, UUU
b. mRNA base code CCC, GAG, GAG, AAA
c. DNA base code
GGG, CTC, CTC, TTT
In sickle cell disease, the sequence of amino acids
is different in just one spot. The sequence above
becomes: proline, valine, glutamic acid, lysine.
amino acids
leucine
proline
tyrosine
phenylalanine
glutamine
11. Translate the sickle cell amino acid sequence
into:
a. tRNA base code GGG, CAC, CUC, UUU
b. mRNA base code CCC, GUG, GAG, AAA
Now can you do it the other way around?
c. DNA base code
amino acids
12. In terms of base nucleotides, explain the only
difference between the DNA code for
normal hemoglobin and sickle cell disease.
proline
glutamic acid
lysine
serine
leucine
tRNA code
(codon)
GGG
CUC
UUU
AGC
AAU
Analysis Questions:
mRNA code
(anticodon)
CCC
GAG
AAA
UCG
UUA
DNA code
GGG
CTC
TTT
TGC
AAT
GGG, CAC, CTC, TTT
The DNA triplet code CTC had a point mutation
to CAC ( A replaced the T) and the amino acid
glutamic acid was replaced with valine.
1. a. What is the structure and function of mRNA?
mRNA is a long single strand of RNA nucleotides. mRNA “reads” or copies the DNA code and takes the
code out of the nucleus and into the cytoplasm to synthesize proteins.
b. Why is messenger RNA a good name?
Messenger RNA is a good name because it carries the DNA message
message out into the cytoplasm to make proteins
out of the nucleus and takes the
2. What happens during transcription?
During transcription mRNA “reads” or copies the DNA code in its own language of RNA nucleotides.
3. a. What is the structure and function of tRNA?
tRNA is a hairpin-shaped molecule made up of RNA nucleotides that has a 3-base anticodon on one end
and carries a specific amino acid on the other end.
b. Why is transfer RNA a good name?
Transfer RNA is a good name because tRNA transfers the code from a message of nucleotides to a message
that contains the correct order of amino acids.
4. What happens during translation?
The mRNA codon and the tRNA anticodon match up (with the help of a ribosome) so that the tRNA can
drop off the correct amino acid in the polypeptide chain that is being assembled to make a protein.
5. What does this mean?
DNA
makes
RNA
(during transcription)
PROTEINS
makes
(during translation)
On this table, compare DNA, mRNA and tRNA by checking all that apply:
DNA
Contains deoxyribose
Contains ribose
Contains a phosphate group
Contains adenine
Contains thymine
Contains uracil
Contains cytosine
Nucleotides are the monomers
Carries a chemical message or code
Stays only in the nucleus
Carries the message out of the nucleus into the cytoplasm
Always in the cytoplasm
Is a double strand
Is a single strand
Is a hairpin shape and carries an amino acid to a ribosome
mRNA
tRNA