Download One Gene - One Polypeptide

One Gene - One Polypeptide
Background
The genes for each of our traits are
located on our chromosomes, which in
turn are made up of DNA. Each
chromosome represents a single
molecule of DNA tightly wound around
sets of proteins called histones. Each
DNA molecule (chromosome) is made up
of sequences nucleotides in varying
orders and lengths. A molecule of DNA
may be hundreds of thousands of
nucleotides long, but is broken up into
sequences of several hundred to several
thousand nucleotides called genes that
each code for a single polypeptide.
Polypeptides are chains of amino acids
that are eventually folded or joined
together in the cell to form proteins.
Recall that most proteins usually
consist of between 2 and 4 polypeptide
chains bonded together. These proteins
form the molecular basis of our
phenotypes; structural proteins are the
building blocks of the body and enzymes
control all of our metabolic processes.
The process that a cell uses to convert a
sequence of nucleotides into a specific
polypeptide involves a variety of
enzymes, protein factors, ATP, amino
acids, and cellular organelles. This
process can be summarized in two
general steps; transcription and
translation. During transcription the
sequence of nucleotides in a gene in
DNA is copied into a sequence of
nucleotides in RNA. During translation
tRNA serves as an interpreter between
nucleotide language of nucleic acids and
the amino acid language of proteins.
We will be using paper models to
simulate transcription and translation and learn how proteins are built from the DNA code.
You will work with a partner to model the actual steps that a cell follows to carry out
transcription and translation. Remember, cellular enzymes like RNA polymerase that do
not have a brain, hands, or eyes are carrying out many of these steps. Thus, transcription
and translation must proceed in a step-by-step chemical process that adds one nucleotide or
one amino acid at a time.
Pre-Lab Questions
Answer the following questions in complete sentences.
1. Describe the relationship between DNA structure and genes.
2. What is the relationship between a gene and a protein (be specific)?
3. How does DNA ultimately determine our phenotype?
4. What are the base pairing rules that RNA polymerase follows during transcription?
5. Complete the table shown below.
Original instructions in
the form of :
Molecule that is being
synthesized:
Location where this
takes place:
Transcription
Translation
Procedures:
1. Today your desk represents a cell. To set up your cell for protein synthesis you will need
several structures and molecules. Get a piece of DNA and place it in the nucleus of your
cell. Place a ribosome in the cytoplasm of your cell. Scatter 9 tRNA molecules throughout
the cytoplasm. Finally scatter a set of amino acids throughout the cytoplasm. You will not
use all of the amino acids.
2. Obtain a strip of paper labeled mRNA .
3. One partner in the group will model the role of the mRNA molecule and should
transcribe the DNA code from the gene whose sequence is shown on the DNA, onto the
mRNA molecule. Using a dry erase marker, this partner will write down the codons onto
the spaces provided on the mRNA strip of paper, three letters per underlined space. The
transcribed mRNA will then leave the nucleus and travel to the ribosome and bind to the
small subunit of the ribosome.
4. Now you will begin translation. tRNA is the cellular interpreter. The other partner in
the pair will interpret the first mRNA codon and write a complimentary anticodon onto the
bottom of one laminated tRNA molecule using a dry erase pen. Meanwhile the mRNA
student uses the codons in the mRNA to look up the amino acid that the tRNA student will
get from the cytoplasm and attach to the tRNA with a small piece of tape.
5. Write the sequence of nucleotides from the DNA in the first row of the table below (3
letters per box). Write the corresponding mRNA codons in the second row of the table on
the next page. List the corresponding sequence of nucleotides that represent the anticodons
from each of the 9 molecules of tRNA you have created in the third row of the table. List
the corresponding amino acids, determined by the mRNA codons, in the fourth row of the
table.
1
2
3
4
5
6
7
8
9
10
DNA
mRNA
tRNA
Amino
Acid
.
5. The mRNA should still be positioned
in the ribosome. Now, the first tRNA
molecule with its attached amino acid
goes to the ribosome / mRNA complex
and the tRNA anti-codon pairs with the
first mRNA codon at the P site of the
ribosome.
7. The next tRNA along with its amino
acid molecule enters the A site of the
ribosome. Only the tRNA anti-codon
matching the next mRNA codon in the
sequence can bind to this site.
8. The original amino acid is then
transferred to the amino acid of the
tRNA in the A site and the two amino
acids are taped together. The tape
represents a peptide bond. The empty
tRNA leaves the P site and goes back
into the cytoplasm.
9. The tRNA, mRNA and the attached
chain of amino acids move to the left and
into the P site. The amino acids should point vertically away from the tRNA, with the first
amino acid being farther from the P site. The next matching tRNA arrives in the A site and
delivers its amino acid.
10. Continue repeating this process until you have linked together four of the amino acids
coded for by the gene on your DNA and have the fifth amino acid in the A site of the
ribosome waiting to join the polypeptide. When you and your partner reach this point, both
partners should make their own full page, detailed sketch of the model. You may use the
blank back page of this lab handout for the drawing. This sketch should use color and show
the relationship of components of protein synthesis including the following:
a. The small ribosomal subunit, clearly labeled
b. The large ribosomal subunit, clearly labeled, showing the P site and A site
c. mRNA, clearly labeled and with nucleotide sequence clearly represented
d. tRNA clearly labeled, showing the nucleotide sequence of its anticodon and with
the polypeptide attached.
e. Another tRNA, clearly labeled, showing the nucleotide sequence of its anticodon
and its amino acid ready to be added to the polypeptide
f. A labeled arrow showing the direction of mRNA / tRNA / polypeptide movement
through the ribosome during translocation
g. An arrow showing how and where the fifth amino acid and the growing
polypeptide will be joined together
10. Continue the process of protein synthesis until a stop codon is reached. When the
mRNA stop codon enters the A site of the ribosome the amino acid chain is released from
the tRNA in the P site. The amino acid chain is now called a polypeptide. The small and
large ribosomal subunits separate from each other and translation is complete.
11. Remove all tape from laminated pieces and use a paper towel to erase the dry erase
marker from the laminated pieces.
Post Lab Questions
Answer the following questions in complete sentences.
1. Explain the overall function and process of transcription including where it happens
in a cell and the purpose of any enzymes that are required.
2. Before mRNA can leave the nucleus it is chemically modified. We did not model this
today. What happens to the mRNA when it is chemically modified in the nucleus?
3. Transcription and DNA replication are different processes with different outcomes,
but they also have some similarities. Describe three similarities and three
differences between DNA replication and transcription?
4. Explain the overall function and process of translation including where it happens in
a cell and the purpose of any enzymes that are required.
5. tRNA is sometimes called the interpreter molecule of protein synthesis. Explain.