Notes to Instructors
Chapter 17 From Gene to Protein
What is the focus of this activity?
It is very difficult for many introductory biology students to sort out and visualize active
What is this particular activity designed to do?
Activity 17.1 Modeling transcription and translation: What processes produce RNA
from DNA and protein from mRNA?
This activity allows students to build a visual model of transcription in the nucleus and
translation of the transcript in the cytoplasm. Building an active model gives students a
better understanding of these dynamic processes.
What misconceptions or difficulties can this activity reveal?
Activity 17.1
Students tend to encounter a number of difficulties, misconceptions, and missing
conceptions as they model both transcription and translation. Here are several possible
problems:
1. Most students are able to give the definition of a gene (for example, a gene is the
2. Many students want to know “how we know which strand is the template strand.”
We often answer this by saying, “the one that makes the mRNA transcript.”
3. In translation, many students have difficulty understanding what occurs at each of
4. Many students don’t understand “where the amino acids in the cell come from” or
“where they are in the cell.” It’s often useful to remind them that the food they eat is
5. Students often ask how the various aminoacyl tRNA synthetases are able to find the
correct amino acids and tRNA molecules. Again, it is useful to remind them that
6. The modeling activity helps students to better understand why a stop codon actually
stops polypeptide formation and allows the mRNA and ribosome to dissociate.
Answers
Activity 17.1 Modeling transcription and translation: What
processes produce RNA from DNA and protein from mRNA?
Create a model of the processes of transcription and translation. Your model should be a
dynamic (working or active) representation of the events that occur first in transcription
in the nucleus and then in translation in the cytoplasm.
Activity 17.1 115
Building the Model
• Use chalk on a tabletop or a marker on a large sheet of paper to draw a cell’s
plasma membrane and nuclear membrane. The nucleus should have a diameter of
about 12 inches.
• Draw a DNA molecule in the nucleus that contains the following DNA sequence:
Template strand 3⬘TAC TTT AAA GCG ATT 5⬘
Nontemplate strand 5⬘ATG AAA TTT CGC TAA 3⬘
Use your model of transcription and translation to answer the questions.
1. How would you need to modify your model to include intron removal? Your explanation
should contain definitions or descriptions of the following terms and structures:
pre-mRNA exons
RNA splicing spliceosome
introns
116 Activity 17.1
gene
DNA
nucleotides: A, T, G, and C versus A, U,
G, and C
RNA modification(s) after transcription
mRNA
RNA polymerase
start codon (methionine)
aminoacyl-tRNA synthetase
amino acids (see Figure 17.4, page 329, in
Campbell Biology, 9th edition)
peptidyl transferase
polypeptide
energy
2. If 20% of the DNA in a guinea pig cell is adenine, what percentage is cytosine?
Explain your answer.
If 20% is adenine, then 20% is thymine. The remaining 60% is composed of
3. A number of different types of RNA exist in prokaryotic and eukaryotic cells. List
the three main types of RNA involved in transcription and translation. Answer the
questions to complete the chart.
Activity 17.1 117
a. Types of RNA b. Where are they
produced?
c. Where and how do they
function in cells?
mRNA In the nucleus, from
specific genes (often called
mRNA functions in the
cytoplasm, where it is
tRNA Other genes in the nuclear
DNA code for tRNA
molecules
tRNA molecules function in
the cytoplasm in translation
Each tRNA molecule can
combine with a specific
amino acid. Complementary
base pairing of tRNA
molecule with a codon in
4. Given your understanding of transcription and translation, fill in the blanks below
and indicate the 5⬘and 3⬘ends of each nucleotide sequence. Again, assume no RNA
processing occurs.
5. Scientists struggled to understand how four bases could code for 20 different
amino acids. If one base coded for one amino acid, the cell could produce only four
different kinds of amino acids (41). If two bases coded for each amino acid, there
would be four possible choices (of nucleotides) for the first base and four possible
choices for the second base. This would produce 42or 16 possible amino acids.
a. What is the maximum number of three-letter codons that can be produced using
only four different nucleotide bases in DNA?
b. How many different codons could be produced if the codons were four bases long?
Mathematical logic indicates that at least three bases must code for each amino acid.
This led scientists to ask:
• How can we determine whether this is true?
• Which combinations of bases code for each of the amino acids?
Nontemplate strand of DNA: 5⬘A T G T A T G C C A A T G C A 3⬘
Template strand of DNA: –⬘T – – – – – – – – – – – – – – –⬘
118 Activity 17.1
Assume a scientist makes three artificial mRNA strands:
(x) 5⬘AAAAAAAAAAAAAAAAAAAAAAAAAA 3⬘
When he analyzes the polypeptides produced, he finds that:
xproduces a polypeptide composed entirely of lysine.
yproduces a polypeptide that is 50% phenylalanine and 50% proline.
zproduces a polypeptide that is 50% isoleucine and 50% tyrosine.
c. Do these results support the three-bases-per-codon or the four-bases-per-codon
hypothesis? Explain.
Only if there were three bases per codon would both yand zproduce only two
d. This type of experiment was used to discover the mRNA nucleotide codons for
each of the 20 amino acids. If you were doing these experiments, what sequences
would you try next? Explain your logic.
There are many possible ways to answer this question. One possibility follows:
Continue as above and make the remaining three types of mRNA made up of
6. Now that the complete genetic code has been determined, you can use the strand of
DNA shown here and the codon chart in Figure 17.4 on page 329 in Campbell
Biology, 9th edition, to answer the next questions.
Original template strand of DNA: 3⬘TAC GCA AGC AAT ACC GAC GAA 5⬘
a. If this DNA strand produces an mRNA, what does the sequence of the mRNA
read from 5⬘to 3⬘?
Activity 17.1 119
b. For what sequence of amino acids does this mRNA code? (Assume it does not
contain introns.)
c. The chart lists five point mutations that may occur in the original strand of DNA.
What happens to the amino acid sequence or protein produced as a result of each
mutation? (Note: Position 1 refers to the first base at the 3⬘end of the transcribed
strand. The last base in the DNA strand, at the 5⬘end, is at position 21.)
Original template strand: 3⬘TAC GCA AGC AAT ACC GAC GAA 5⬘
Mutation Effect on amino acid sequence
i. Substitution of T for G at
position 8.
This changes the codon in mRNA to a stop
codon; translation stops at this point. A
shorter (truncated) polypeptide is pro-
duced and this shortened polypeptide is
likely to be nonfunctional.
iv. Substitution of T for C at
position 18.
The original mRNA codon, CUG, and the
one resulting from the substitution, CUA,
both code for leucine, so no change occurs
in the polypeptide sequence.
vi. Which of the mutations produces the greatest change in the amino acid
sequence of the polypeptide coded for by this 21-base-pair gene?
The addition of T between positions 8 and 9 still leaves the third amino
120 Activity 17.1
7. Sickle-cell disease is caused by a single base substitution in the gene for the beta
subunit of hemoglobin. This base substitution changes one of the amino acids in the
hemoglobin molecule from glutamic acid to valine. Look up the structures of
glutamic acid (glu) and valine (val) on page 79 of Campbell Biology, 9th edition.
What kinds of changes in protein structure might result from this substitution? Explain.
Glutamic acid is polar, and valine is nonpolar. Being polar, the glutamic acid
8. Why do dentists and physicians cover patients with lead aprons when they take
mouth or other X-rays?
As noted in this and other chapters, X-rays, UV light, and many chemicals can
Activity 17.1 121
17.1 Test Your Understanding
During DNA replication, which of the following would you expect to be true? Explain
your answers.
T/F3. More helicase would be associated with the lagging strand than with the
leading strand.
False—Both strands would need to be unwound equally.
T/F 4. DNA ligase links the 3⬘end of one Okazaki fragment to the 5⬘end of the
another Okazaki fragment in the lagging strand.
6. You obtain a sample of double-stranded DNA and transcribe mRNA from this DNA.
You then analyze the base composition of each of the two DNA strands and the one
mRNA strand, and get the following results. The numbers indicate percentage of
each base in the strand:
a. Which of these strands must be the mRNA? Explain.
b. Which one is the template strand for the mRNA? Explain.
7. In a new experiment, you obtain a different sample of double-stranded DNA and
transcribe mRNA from this DNA. You then analyze the base composition of each of
the two DNA strands and the one mRNA strand, and get the following results. The
numbers indicate percentage of each base in the strand:
b. Which one must be the template strand for the mRNA? Explain.
Strand 2 must be DNA and it must also be the template strand for the mRNA
A G C T U
strand 1 40.1 28.9 9.9 0.0 21.1
strand 2 21.5 9.5 29.9 39.1 0.0
strand 3 40.0 29.0 9.7 21.3 0.0
A G C T U
strand 1 29.1 39.9 31.0 0.0 0.0
strand 2 0.0 30.0 39.8 30.2 0.0
strand 3 29.4 39.4 31.2 0.0 0.0
122 Activity 17.1
8. Cystic fibrosis transmembrane conductance regulator (CFTR) proteins function in
cell membranes to allow chloride ions across cell membranes. Individuals with
cystic fibrosis (CF) have abnormal CFTR proteins that do not allow Cl⫺to move
across cell membranes. Chloride channels are essential to maintain osmotic balance
inside cells. Without properly functioning Cl⫺channels, water builds up inside the
cell. One result is a thickening of mucous in lungs and air passages.
You are doing research on a different disease, and you hypothesize that it may also
be due to a defect in an ion channel in the cell membrane.
a. Diagram or model production of a normal membrane ion channel.
b. Based on you understanding of cell membrane structure and function, propose at
least three different alterations that could result in a nonfunctional or missing ion
channel.
c. What questions would you need to answer to determine which of these may be
correct?
This question is designed to allow students to integrate their understanding of
general cell structure and function with that of genetic mutation and control.
a. Briefly, the diagram should include:
i. DNA/gene(s) for the channel protein
Activity 17.1 123