Genetic Code
Supplemental Instruction
Iowa State University
Leader:
Course:
Instructor:
Date:
Lilli Howard
BIOL/GEN 313
Dr. Rodermel/Dr. Tuggle
10/20/14
1. The genetic code has 4 letters which are: A, C, T, G (and sometimes U)
2. The genetic code is a triplet code. Meaning each amino acid is encoded by 3 nucleotides.
a. Describe the in vitro translation of nucleotides experiment. What was the
conclusion from this experiment?
Polynucleotide phosphorylase synthesizes RNA from nucleotides (phosphodiester bond
formation) without a template; nucleotides are inserted randomly
UUU codes for phe
AAA codes for lys
CCC codes for pro
GGG (could not determine: unspecified technical problems)
1.
2.
3.
4.
5.
b. Describe the ribosome-bound tRNA experiment. What was the conclusion from
this experiment?
Procedure
synthesize a codon in vitro of known sequence
attach it to a ribosome
add a mixture of tRNAs that have been “charged” with their amino acid
determine which tRNA with its attached amino acid binds to the codon (via
codon/anticodon interactions), hence a ribosome-bound tRNA
Conclusion: 5’- GUU – 3’ is codon for Val
3. The codon table is comprised of 64 codons and is read 5’3’ in the mRNA.
a. Using this table, what are the possible codons for serine? AGU, AGC, UC[A or U
or C or G]
b. Using this table, what does the codon CAU code for? His
4. The genetic code is degenerate. Describe what that means.
There are more codons than there are amino acids
5. Match the following terms to their definitions:
a. Sense codon(s) 2, 5
b. Termination codon(s) 1, 3, 7
c. Initiation codon(s) 2, 4, 6, 8
1.
2.
3.
4.
5.
6.
7.
8.
signal end of the protein
specify amino acids
do not encode an amino acid
codon that specifies Met
61 different codons
1 codon
3 different codons
Sets reading frame
6. Synonymous codons are codons that specify the same amino acid.
7. Because any given nucleotide is only included in one codon, we say that the genetic code
is non-overlapping.
a. What is a reading frame? How many different reading frames are there?
-each way of reading a sequence. There are 3.
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8. The initiation codon sets the reading frame.
9. Crick’s proposal of an adaptor molecule included two requirements:
1. interacted with codons in the mRNA
2. transferred an amino acid to the ribosome
a. What is the adapter molecule known as now? Describe it. How many are there?
-tRNA: 30-40 depending on organism
-cloverleaf structure with 5 arms; the arms are formed by complementary base
pairing (stem-loops)
-have several types of modified bases that are added by tRNA modifying enzymes
after the tRNA has been transcribed: dihydrouracil (DHU) and pseudouridine ()
-Cloverleaf folds on itself to generate an “L-shaped” structure (Structure/function:
this structure fits well into conformation of ribosome)
The arms include the:
1. acceptor arm (CCA at 3’ end is invariable); it binds the amino acid
2. anticodon arm; it base pairs with the codon in the mRNA
3. DHU arm; it usually contains DHU
4. TC arm; it contains this sequence
5. Variable arm
10. What does the Wobble Hypothesis predict?
a. Think of an example of an anticodon. Would wobble apply in that case?
11. The code is universal which means that each codon specifies the same amino acid in all
organisms. (There are very few exceptions)
Concept Checks
1. Through wobble, a single ___________ can pair with more than one _________.
a. Codon, anticodon
b. Group of 3 nucleotides in DNA, codon in mRNA
c. tRNA, amino acid
d. anticodon, codon
2. Do the initiation and termination codons specify an amino acid? If so, which ones?
a. Initiation Codon: In euikaryoites encodes Methionine. In prokaryotes encodes N-fMet
b. Termination Codon: do not encode amino acids
3. Amino acids bind to which part of the tRNA?
a. Anticodon
b. DHU arm
c. 3’ end
d. 5’ end
4. What are isoaccepting tRNAs? different tRNAs that accept the same amino acid
Example: the tRNAs containing CUU, CUC, CUA, and CUG all accept Leucine
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5. A series of tRNAs have the following anticodons. Consider the wobble rules and give all
possible codons with which each tRNA can pair.
a.
b.
c.
d.
e.
5’- GGC -3’
5’- AAG -3’
5’- IAA -3’
5’- UGG -3’
5’- CAG -3’
3’- CCG – 5’ or 3’- UCG – 5’
3’- UUC – 5’
3’- AUU – 5’ or 3’- UUU – 5’ or 3’- CUU – 5’
3’- ACC – 5’ or 3’- GCC – 5’
3’- GUC – 5’
6. An anticodon on a tRNA has the sequence 5’ – GCA - 3’
a. What amino acid is carried by this tRNA?
Codon: 3’ – CGU – 5’  Cysteine or Codon: 3’ – UGU – 5’  Cysteine
b. What would be the effect if the G in the anticodon were mutated to U?
Anticodon: 5’ – UCA - 3’  Codon: 3’ – GGU – 5’  Tryptophan
1060 Hixson-Lied Student Success Center  515-294-6624  sistaff@iastate.edu  http://www.si.iastate.edu
1060 Hixson-Lied Student Success Center  515-294-6624  sistaff@iastate.edu  http://www.si.iastate.edu