Anatomy & Physiology 34A
DNA Replication & Protein Synthesis Lab Activity
I. Objectives
A. Visualize how DNA replication takes place
B. Explain how protein synthesis occurs, including
1. Transcription from DNA to mRNA
2. Translation from mRNA to amino acids
C. Explain how a point mutation in DNA can affect a polypeptide
II. DNA Replication occurs in the cell’s nucleus
A. Double-stranded DNA molecules in chromosomes create exact copies of themselves during
the S-phase of interphase in the cell cycle. This replication ensures that the daughter cells
produced by mitosis are genetically identical to the original mother cell.
B. Instructions for DNA replication (work in groups of 4):
1. Remove the paper “nucleotides” from the envelope provided. Note that each nucleotide is
composed of a sugar (S), phosphate (P), and a nitrogenous base (A, T, U, G, or C).
2. Place the numbered DNA nucleotides in numerical sequence in a vertical column on the
desk top “nucleus.” This is the DNA sense strand.
3. Attach the complementary DNA nucleotides to the assembled DNA sense strand. Note
that DNA nucleotides have deoxyribose sugar, which is represented by a green S on the
nucleotides. Which DNA bases are always paired with each other? __________________
4. Separate your two assembled strands from each other by a distance of about one foot. This
step represents DNA unwinding, breaking the hydrogen bonds between the
complementary bases, and separating. Note that there are three hydrogen bonds between
guanine and cytosine, but only two hydrogen bonds between thymine and adenine.
5. Use the DNA nucleotides from another group to make complementary DNA strands for
each of the 2 strands you separated from each other. How do the two new DNA molecules
compare to the original double-stranded DNA molecule? __________________________
Why is this process called semi-conservative replication? __________________________
6. Return the borrowed DNA nucleotides to the other group, and reassemble the original
double-stranded DNA molecule.
III. Protein Synthesis includes the processes of transcription (DNA → mRNA) and
translation (mRNA → amino acids carried by tRNAs)
A. Transcription involves “rewriting” a segment of DNA (gene) into the form of a mRNA
molecule, which contains the assembly instructions to form a polypeptide. This occurs in
the cell’s nucleus. Instructions for transcription:
1. Separate the two strands of DNA about a foot apart. This represents DNA unwinding and
separating at a gene.
2. Attach the complementary RNA nucleotides to the DNA sense strand (the numbered
nucleotides). Note that the RNA nucleotides have a different sugar (ribose, represented by
a pink S). This is the formation of the mRNA molecule from the DNA template. Are there
any thymine nucleotides in RNA? _____. If not, what replaced thymine? _________
3. Carefully separate the mRNA strand from the DNA sense strand, and move the mRNA
from the desktop nucleus into the cytoplasm, taking care to maintain the same sequence of
the mRNA strand.
4. Reassemble the original DNA molecule in the nucleus.
B. Translation involves tRNAs with 3 base anticodons bringing specific amino acids to the
mRNA molecule in the sequence dictated by the mRNA codons (3 consecutive bases).
When all of the amino acids are bonded together (by peptide bonds), they form a
polypeptide. This occurs on ribosomes in the cytoplasm of the cell.
1. Align the mRNA molecule horizontally on the desktop, with the sugar-phosphate end
facing you and the nitrogenous bases facing the center of the table. Be sure to maintain the
original sequence of the mRNA molecule.
2. Take the tRNA clear plastic cups provided and match their anticodons to the
complementary codons of the mRNA. (Note: there is one extra tRNA cup; it will be used
later.)
3. Take the 4 resulting tRNA cups to the plastic pitchers containing amino acids (fruit juice)
on the side counter. Use the anticodon (3 base sequence) on each cup, the chart provided,
and the mRNA codons to determine which amino acid to pour into each cup. Pour about a
quarter cup of “amino acid” into each cup. This represents tRNAs picking up specific
amino acids in the cytoplasm.
3. Return the tRNAs with their amino acids to your desktop and match their anticodons to the
complementary codons of the mRNA molecule, from left to right. After matching them up,
pour each amino acid into the larger red cup. This represents the formation of the
polypeptide from the bonded amino acids.
4. Pour equal amounts of the “polypeptide” into 4 small paper cups and allow each member
of the group to taste the product. What does it taste like? __________________________
5. Wash all of the cups out in the sink when finished.
IV. DNA Point Mutation
1. Return to the DNA sense strand and exchange nucleotide #11 for the M11 (mutation)
nucleotide. This represents a point mutation in the DNA.
2. Use the mRNA nucleotides from part III to form a mRNA strand that is complementary to
the new DNA sense strand. Does this mRNA strand have the same nucleotide sequence as
that of the mRNA before the point mutation? _______.
3. Align the new mRNA molecule horizontally on the desktop, as done previously.
4. Match up the plastic tRNA cups’ complementary anticodons to the new mRNA’s codons.
Do the new mRNA codons call for same tRNAs as in part III of the exercise? __________
5. Take the resulting tRNA cups to pitchers on the side counter and use the chart to determine
which “amino acid” to pour into each cup.
6. Return the tRNAs with their amino acids to your desktop and match their anticodons to the
complementary codons of the mRNA molecule, from left to right.
7. After matching them up, pour each amino acid into the larger red cup.
8. Pour equal amounts of the resulting “polypeptide” into 4 small paper cups and allow each
group member to taste the product. What effect, if any, did the point mutation in the DNA
have on the final polypeptide? _______________________________________________
You have just simulated what actually happens in the synthesis of hemoglobin in someone with
sickle cell anemia. A point mutation in the sixth codon of the DNA gene for hemoglobin
changes the corresponding mRNA codon, which then calls for the amino acid valine rather than
the normal glutamate. The result is that the hemoglobin molecule does not fold properly and
cannot carry oxygen as efficiently as it would have without the mutation.