Lab 9: Transcription and Translation
- To build a DNA molecule
- To simulate DNA replication
- To simulate transcription by building an mRNA molecule from DNA
- To simulate translation by building a polypeptide chain from the mRNA transcript
- DNA, RNA, and protein synthesis kit (supplied to you)
Description of the problem
Using the supplied kit, you will build a DNA and RNA molecule and simulate the processes of DNA replication, transcription, and translation.
Building a DNA molecule
DNA is composed of monomers of nucleotides. Each nucleotide contains a 5-carbon sugar (= deoxyribose), a phosphate, and a nitrogenous base (adenine, cytosine, guanine, or thymine). Since DNA is a double-stranded molecule, adenine (A) on one strand always pairs with thymine (T) on the other strand, whereas cytosine (C) always pairs with guanine (G).
Build a DNA molecule that’s 6 base pairs long. Use the figures below as a guide (though you don’t have to use the same sequence of bases). Note the color of the four bases in your kit and make sure you pair the colors properly.
1. What molecules make up the backbone of DNA? What four molecules make up the “rungs” of the DNA “ladder” (be specific)?
2. For the molecule you just built, write the sequence of bases (A, C, G, T) you used on one strand and the complementary bases on the other strand.
Simulating DNA replication
DNA undergoes semi-conservative replication. Before this begins, DNA must first “unzip” (right).
3. What bonds are broken in order to unzip the DNA?
Unzip your DNA molecule. Attach new complementary nucleotides to each strand using complementary base pairing.
4. What is meant by semi-conservative replication?
Building an RNA molecule
Like DNA, RNA is composed of monomers of nucleotides. Each nucleotide contains a 5-carbon sugar (= ribose), a phosphate, and a nitrogenous base. Unlike DNA, RNA is a single-stranded molecule.
Build an RNA molecule that’s 6 nucleotides long, using the picture at left as a guide. Notice the kit has a different colored sugar to represent ribose (purple) rather than the black colored sugar that represents deoxyribose.
5. What are the 4 bases in RNA? DNA serves as a template to make mRNA. Adenine (A) in DNA always pairs with what base in mRNA?
One strand of DNA serves as the template to make mRNA in the process known as transcription. Transcription is the first step in expressing a gene.
Extend your original DNA molecule by making it 12 (instead of 6) base pairs long. Again, feel free to use whatever sequence of bases you want, making sure to use the appropriate colors for complementary base pairing. Also, at this point your DNA molecule is long enough to twist into a double helix. Do this so you get an idea of its three-dimensional shape (below).
6. What is the base sequence of your two DNA strands (they should be complementary with one another)?
Make an mRNA transcript that’s 9 nucleotides long. Before mRNA can be made, DNA must first unzip. Unzip the DNA, choose one strand of DNA as the template, and make an mRNA transcript using complementary base pairing (right). When making the mRNA transcript, be sure to use the appropriate color sugar (purple, which represents ribose) and uracil instead of thymine.
7. What is the sequence of bases in your mRNA transcript? Make sure the sequence is complementary to one of the strands in question #6.
Once you’ve transcribed the mRNA transcript, separate it from the DNA (right). The transcript will eventually leave the nucleus and go to the ribosomes in the cytoplasm. The two DNA strands will also reattach to one another (not shown).
A sequence of three bases in mRNA codes for a particular amino acid. The process of converting the sequence of bases in RNA into a sequence of amino acids in a protein is called translation. Translation occurs on the ribosomes in the cytoplasm of the cell.
8. What is the name of each three-base sequence in mRNA?
Your kit contains three purple tRNA molecules and three black amino acids. Notice that the amino acids can only fit onto specific tRNA molecules. In cells, tRNA with an anticodon complementary to the codon will transport its attached amino acid to the ribosome. The next codon is “read” by the ribosome, whereby another tRNA with the correct anticodon will attach to the codon. The two amino acids are joined together with a peptide bond.
Simulate the cell’s cytoplasm by putting the three amino acids onto the appropriate tRNA molecules. Additionally, place the plastic ribosome and the mRNA transcript in your work area (left).
Simulate translation by placing the ribosome on the first codon and attaching a tRNA molecule onto the codon. Move the transcript along the ribosome so that the second codon is read. Attach another tRNA molecule onto the codon and combine the two amino acids with a peptide bond. Repeat this process with the last codon (see below).
The end result will be a polypeptide chain composed of three amino acids.
9. Using the mRNA transcript you generated in #7, what are the three amino acids of your polypeptide chain? (See Fig. 22.6 in your book.)
10. What happens to the mRNA transcript after translation is complete?