Download Dusty Carroll Lesson Plan 6: DNA to RNA How Protein Synthesis

Dusty Carroll
Lesson Plan 6: DNA to RNA How Protein Synthesis Works
Target Audience
AP chemistry class. I’ll be assuming they have the appropriate background knowledge of the structure
of DNA and RNA and proteins from their previous biology course. This lesson will focus on the more
chemical aspects of the protein synthesis process.
Objectives
• Recall the basic mechanisms of protein synthesis
• Explain the chemistry of the translation mechanism
Introduction to the Lesson
Use the “Reference for Nucleic Acids” handout
• Review the basic structure of nucleic acids
• Recall that DNA and RNA are similar in structure, but differ in one of their bases
• Review the basic structure of nucleotides
• Show the phosphodiester link
• Point out that continued polymerization causes a backbone of alternating sugars and phosphates
with bases as side chains
Use the worksheet “Overview of Protein Synthesis”
• Review structure of DNA
• Recall the 5 bases from previous handout
• Students draw proposed orientation of the bases for optimum hydrogen bonding
• Show overhead with correct orientation
• Review basic steps of protein synthesis
o Note that the complementary bases are such because of the chemical structure
Use the transparency “Translation: Chemical Aspects I”
• Show how the DNA stays in the nucleus and the mRNA takes the information out of the
nucleus to the ribosome.
• Notice how the mRNA has bases exposed (single strand, not double like DNA)
• The ribosome moves over the mRNA
• Each time this happens, a new amino acid is added to the peptide chain
• These amino acids are brought to the mRNA by tRNA
• The slide says, “Every amino acid is coded by a sequence of three bases” What does this
mean?
Use the transparency “Translation: Chemical Aspects II”
• A “codon” is a set of three nucleotide bases.
• Just as DNA has complementary bases, the three bases in the mRNA codon will have a
complementary codon that can interact with them. This is called the anticodon.
• The anticodon is found on tRNA
o tRNA binds to specific amino acids on one end and contains an anticodon on the other
end
o Show transparency “tRNA structure” – note the anticodon and the site of the amino acid
•
When tRNA brings a new amino acid, an enzyme helps to form a peptide bond, attaching it to
the polypeptide chain
Use the transparency “Translation: Chemical Aspects III”
• This process is more clearly seen diagrammatically in this transparency
• Explain the process again and answer any questions that arise
Use the transparency “Translation: Chemical Aspects IV”
• This transparency shows the same process, but the diagram has some chemical structure to give
students an idea of how the amino acids continue to add to the polypeptide chain.
• Note that the bases are still drawn in cartoon form here, but remind students what they look like
(from handout and worksheet) and that they are complementary because of the hydrogen
bonding seen earlier
Summarize and Close
• Students previously accepted the explanation that the two strands of DNA stuck together
because of paired bases.
• They also accepted the explanation that mRNA magically copies the DNA information and
takes it away to make a protein.
• They now have a better understanding of the chemistry behind the magic!
References
Garrett, R. H., and Grisham, C.M., Principles of Biochemistry With a Human Focus. Brooks/Cole &
Thomson Learning, 2002.
McMurry, J. and Castellion, M.E., Fundamentals of General, Organic, and Biological Chemistry, 4th
ed. Pearson Education, Inc., 2003.
Image references noted within
Reference for Nucleic Acids
Remember that nucleic acids are polynucleotides.
A nucleotide is composed of three parts:
A sugar (5-membered monosaccharide)
CH2OH
CH2OH
OH
OH
O
O
or
OH
OH
OH
ribose
A phosphate group
2-deoxyribose
O
P
OO-
A nitrogen-containing cyclic compound that is a base
The 5 bases in DNA and RNA
NH2
N
N
N
H
N
N
H
N
Adenine
NH2
O
N
NH
N
NH2
Guanine
N
H
O
O
NH
NH
O
Cytosine
N
H
O
Thymine
N
H
O
Uracil
The nucleotides polymerize in a specific way
NH2
Two nucleotides connected through a phosphodiester link
N
N
HO
O
NH2
O
H
O
H
H
O
H
N
N
O-
P
O
N
H
Long chains of nucleotides form DNA and RNA
N
O
H
O
H
H
O
H
P
O-
O-
H
Thymine is only found in DNA; Uracil is only found in RNA
The other three bases are found in both
Overview of Protein Synthesis
DNA exists as a double helix. Each strand contains a backbone of alternating sugars and phosphates.
The bases of the nucleotides stick out from the backbone. In DNA, the bases on opposite strands are
complementary because they form hydrogen bonds which hold the strand together.
Image from http://www.biologycorner.com/bio1/DNA.html
The complementary bases are oriented in such a way as to maximize the
hydrogen bonding capabilities. Using the structures in the “Reference for
Nucleic Acids” handout, determine the orientation that each pair of bases
must assume in order to maximize hydrogen bonding. Use the following
parameters to assist you:
•
•
Guanine and Cytosine pair to form 3 hydrogen bonds
Thymine and Adenine pair to form 2 hydrogen bonds
Draw your structures in the space below:
Guanine and Cytosine
Thymine and Adenine
Steps for protein synthesis
• An enzyme breaks the hydrogen bonds between some base pairs in DNA in order to separate
the two strands
• Messenger RNA (mRNA) is assembled by enzymes to carry the DNA information out of the
nucleus and into the ribosome where protein synthesis can occur
• mRNA and transfer RNA (tRNA) work together along with enzymes to translate the DNA
information into a chain of amino acids (protein)
Overview of Protein Synthesis – Answer Key
DNA exists as a double helix. Each strand contains a backbone of alternating sugars and phosphates.
The bases of the nucleotides stick out from the backbone. In DNA, the bases on opposite strands are
complementary because they form hydrogen bonds which hold the strand together.
Image from http://www.biologycorner.com/bio1/DNA.html
The complementary bases are oriented in such a way as to maximize the
hydrogen bonding capabilities. Using the structures in the “Reference for
Nucleic Acids” handout, determine the orientation that each pair of bases
must assume in order to maximize hydrogen bonding. Use the following
parameters to assist you:
•
•
Guanine and Cytosine pair to form 3 hydrogen bonds
Thymine and Adenine pair to form 2 hydrogen bonds
Draw your structures in the space below:
Thymine and Adenine
Guanine and Cytosine
HN
H
O
O
H
NH
N
sugar
N
sugar
N
N
N
H
N
N
N
N
H
N
N
sugar
N
sugar
O
H
NH
O
Steps for protein synthesis
• An enzyme breaks the hydrogen bonds between some base pairs in DNA in order to separate
the two strands
• Messenger RNA (mRNA) is assembled by enzymes to carry the DNA information out of the
nucleus and into the ribosome where protein synthesis can occur
• mRNA and transfer RNA (tRNA) work together along with enzymes to translate the DNA
information into a chain of amino acids (protein)
Transparency
Translation: Chemical Aspects I
http://www.genomeeducation.ca/GEcurious/crashCourse/proteinsSynthesis.asp?l=e
Transparency
Translation: Chemical Aspects II
http://www.anselm.edu/homepage/jpitocch/genbio/translat.JPG
Transparency
tRNA
http://www.med.uottawa.ca/patho/devel/
Transparency
Translation: Chemical Aspects III
http://www.med.uottawa.ca/patho/devel/
Transparency
Translation: Chemical Aspects III
http://www.med.uottawa.ca/patho/devel/