Download PROTEIN SYNTHESIS LAB

PROTEIN SYNTHESIS LAB
PREPARATION FOR THE EXERCISE
Read chapter 3 & 12 in your textbook, and answer the following review questions
thoroughly in order to be prepared for a lab quiz.
1. What is a protein?
2. Give some examples of proteins found in living organisms.
3. Name the subunits that are bonded together to compose a protein.
4. How many amino acids occur in nature?
5. Describe the appearance of the following types of proteins:
primary
secondary
tertiary
quaternary
6. Compare DNA and RNA characteristics.
7. What is the function of a ribosome?
8. Describe the process of transcription.
9. Describe the process of translation.
10. What is the role of tRNA in protein synthesis?
PROTEIN SYNTHESIS
Protein synthesis is the process whereby cells construct proteins based on the
information encoded in their DNA. Transcription and translation account for the two
major activities of protein synthesis. In the process of transcription, mRNA is produced
based on the sequence of bases encoded in the DNA molecule. This single stranded
molecule (mRNA) is transferred to the cytoplasm where it is read by the ribosome
(rRNA). Ribosomes establish a codon reading frame and translate the information
encoded in the sequence of mRNA into a protein by way of translation. Transfer RNA
(tRNA) is used extensively in the process of translation.
PROTEIN 1
The gene coding for protein 1 unwinds and RNA polymerase transcribes mRNA.
Examine the mRNA 1 molecule, and list the bases of DNA which correspond with those
found on mRNA 1.
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mRNA 1
1. Place the contents of bag 1 on the table. Be careful not to mix the contents of this
bag with other bags to be opened later in the exercise.
2. mRNA is read by the ribosome when the 5' AUG codon is in the peptidyl tRNA
binding site thus initiating translation.
3. Arrange mRNA and insert into ribosome so that 5' AUG codon is in the peptidyl tRNA
binding site (note: mRNA should be upside down relative to the ribosome).
4. Bring appropriate tRNA and amino acid into peptidyl tRNA binding site.
5. Next position the proper tRNA with amino acid into aminoacyl tRNA binding site.
6. Form a peptide bond between the two amino acids by removing the methionine from
the tRNA and attaching it to the adjacent amino acid. Transfer RNA now is removed
from codon and returns to the cytoplasm.
7. Now slide the ribosome (rRNA) down so that the next unread codon is occupying the
aminoacyl tRNA binding site, and repeat the activities of tRNA.
8. Continue the translation process until a polypeptide chain of amino acids is formed.
9. Give the sequence of amino acids produced by the translation of mRNA 1 into a
protein.
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10. DO NOT DISASSEMBLE THE PROTEIN UNTIL INSTRUCTED TO DO SO!
PROTEIN 2
mRNA 2
1. Translate mRNA 2 as you did mRNA 1.
2. List the sequence of amino acids produced by mRNA 2.
________________________________________________________________________
3. How does this amino acid sequence compare to the one produced with mRNA 1?
__________
4. List the codons of mRNA 1 and 2 below and circle any different codons.
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5. Examine the codons circled that specify ARG. Using the Genetic Code given below
or in your text, determine the number of codons that code for ARG (arganine)
_____________; for VAL (valine) ____________.
6. Study the codons in the Genetic Code chart that code for valine. How many mRNA
bases must be in place to translate valine? 1 of 3, 2 of 3, or 3 of 3 (circle correct
answer)
7. The genetic code is degenerative; what does this mean?_____________________________________________________________________________________________
PROTEIN 3
Mutations are alterations in the base sequence of DNA. Small mutations are typically
of two types: base phase (frame) shift and base substitution mutations. In phase shift
mutations one (or a few) bases are added to the DNA. Such an addition affects the
reading frame of the ribosome - throwing off the translation of the mRNA. Mutations that
result in deletion of one or more bases will also create a novel protein that may not be
functional.
MUTATION BY BASE PHASE (FRAME) SHIFT
mRNA 3
1. A duplication of the base T occurred in a DNA molecule; the segment of this DNA
containing the extra T was transcribed into mRNA 3.
2. Given that a T was added to DNA, what base will be transcribed into the
mRNA?______
Compare mRNA 3 to the normal mRNA 1 to see where the added base is located.
_______
3. Once the duplicated T of DNA is transcribed into mRNA, it may not be removed and
the protein may be produced even if it is non-functional. Describe the manner in which
DNA polymerase works during DNA replication.
________________________________________________________________________
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PROTEIN 4
Base substitution mutations occur when DNA polymerase is ineffective at detecting and
excising incorrect bases that are incorporated into the DNA. When such bases are
present they may or may not cause a change in the sequence of amino acids coded for
by the mRNA.
MUTATION BY BASE SUBSTITUTION
mRNA 4
1. The mRNA 4 is different from mRNA 1 in the following ways: 4th base changed from
C to U; 14 and 15th bases changed from C to A.
2. Synthesize the protein of mRNA 4 and compare to the one coded by mRNA 1. List
the protein sequence for both and circle any changes.
________________________________________________________________________
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3. In this case did a base substitution result in an altered protein? ____________
4. If the 14th base of mRNA 4 read C (entire codon CCA), would it code for the same
amino acid as its counterpart on mRNA 1? ______________
Explain.________________________________________________________________