Part 1: Modeling Transcription (14 points)
You will first build a model of a segment of DNA. This segment is part of the gene for beta globin, a polypeptide found in normal hemoglobin. You will make both complementary strands of DNA. Then you will use your DNA model to make an mRNA model by transcription.
1. The sequence of nitrogenous bases in the template DNA strand is CAC GTA GAC TGA GGA CTC CTC TTC. This sequence is given in the table below as a series of triplets. Use the rules for base pairing to determine the sequence of bases for the complementary DNA strand. Record this sequence in the table. Then determine the sequence of bases in the segment of mRNA that is complementary to the template DNA strand. Remember that RNA contains uracil (U) in place of thymine (T). Use the table to record the sequence of mRNA codons. (2 points)
(on bottom)
2. Assign a color to each of the five nitrogenous bases. Record this information in the table below. Then count the number of times each base occurs in the two strands of DNA and the single strand of mRNA. Record the totals in the table. (2 points)
Colors and Numbers of Model DNA and RNA Bases
3. Make enough sticky-note strips of each color to represent the total number of times that base occurs. (For example, if guanine is assigned blue and occurs 10 times, you should make 10 blue strips.) Label the non-sticky side of each strip with the letter that represents the base.
4. To start making your DNA model, place the labeled sticky notes and two 60 cm strips of paper on a flat working surface. Label one strip "Template DNA strand." Label the left-hand end of this strip 3′ and the right-hand end 5′. Starting at the 3′ end, follow the DNA sequence for the template strand given in Step 1, sticking the bases to the paper so that they hang off the bottom of the strip. Leave about 0.5 cm between the bases. Verify the sequence of your model template strand after it is complete.
5. Label the second strip of paper "Complementary DNA strand." This time, label the left-hand end 5′ and the right-hand end 3′. Starting at the 5′ end, use the DNA sequence for this strand that you determined in Step 1. This time, stick the bases to the paper so that they extend off the top of the strip.
6. Place the complementary strand below the template strand to model a double-stranded segment of DNA that is not twisted into a double helix.
7. Use a long piece of string or yarn to represent the nuclear membrane. Encircle the model DNA segment.
8. Draw or attach a photo of your finished model in the space below. (6 points)
9. Repeat Step 5 to model the process of transcription, but this time, label the strip of paper "mRNA strand." Construct the complementary strand of mRNA. Think about what happens to the original DNA and where transcription occurs.
10. Draw or attach a photo of your finished mRNA model in the space below. Include what the DNA looks like and use the string to model where the DNA and mRNA are located immediately after transcription is complete. (4 points)
Part 2: Modeling Translation of mRNA (6 points)
In this part, you will use the mRNA you made in Part 1 to model translation.
11. Copy the sequence of mRNA codons you determined in Step 1 into the table below. Then use the mRNA codon table to determine the corresponding sequence of amino acids. Write the first three letters of each amino acid in the table. (2 points)
12. Write the abbreviation for each amino acid on a paper circle. Arrange your model amino acids into the sequence you listed in Step 11 to build a portion of the polypeptide in normal hemoglobin. Use tape to attach the amino acids to one another.
13. Draw or attach a photo of your finished polypeptide model in the space below. Use the string to show the locations of the polypeptide, the mRNA, and the DNA with respect to the nucleus. (4 points)
