Name: _____________________________
Discussion Section (circle one): D10, D11, D12
Bio 93 Discussion Week 8 Homework: DUE DECEMBER 4
ANSWER KEY
1. You are provided with an actively dividing culture of E. coli bacteria to which radioactive thymine has
been added – what would happen if a cell replicates once in the presence of this radioactive base?
A.
B.
C.
D.
E.
One of the daughter cells, but not the other, would have radioactive DNA
Neither of the daughter cells would be radioactive
All four bases of the DNA would be radioactive
Radioactive thymine would pair with nonradioactive guanine
DNA in both daughter cells would be radioactive
Radioactive thymine would be incorporated into all newly synthesized DNA. This would lead both
daughter cells to be radioactive but would not alter Watson and Crick base pairing rules or make the
other bases radioactive.
2.
In the diagram below draw the leading and lagging strands for each strand of DNA.
Origin of replication
Lagging
3’
Leading
3’
5’
5’
5’
3’
5’
3’
The leading strand is
5’ to 3’ from OR
The lagging strand is
3’ to 5’ from OR
Origin of replication
A. Name the molecule involved in untwisting the double helix at the replication fork
Helicase
B. Name the molecule involved in correcting overwinding ahead of the replication fork.
Topoisomerase
C. Which strand has the Okazawi fragments? Draw it in.
Lagging strand
3.
What is the difference between DNA polymerase I and DNA polymerase III?
Pol I- Replaces RNA nucleotides of primer with DNA
Pol III- Synthesizes new DNA strand by adding DNA molecules
4. What are some differences between transcription and translation in eukaryotes versus transcription
and translation in prokaryotes?
Prokaryotes- one OR, no post-translational modifications, transcription end is signal terminator,
because bacteria lack nuclei, transcription and translation are coupled, ribosomes attach to mRNA
molecule while transcription is still in progress.
Eukaryotes- multiple ORs, post-translation modification, transcription end is poly A tail, Almost all
transcription occurs in the nucleus, translation occurs mainly at ribosomes in the cytoplasm.
5. Based on the following sequence of DNA 5’ GCG TTA
ATT ACG CAT CCC 3’ answer the following questions:
I.
Provide the primary transcript (label the 3’ and 5’
ends).
mRNA 3’ CGC AAU UAA UGC GUA GGG 5’
II.
Would the transcript be synthesized from left to
right or right to left? Why?
Right to left because RNA Polymerase always adds
to the 3’ end
III.
Translate the transcript
5’ GGG AUG CGU AAU UAA CGC 3’
Met-Arg-Asn
6. You are given the anti-codon sequence 5’ UUCGUCAUCUACC 3’. Provide the DNA sequence which
gave rise to this anti-codon sequence (include the 5’ and 3’).
3’ tRNA anticodon 5’
5’ mRNA codon 3’
3’-CCAUCUACUGCUU-5’
5’-GGUAGAUGACGAA-3’
7. In 1961 the biochemists Nirenberg and Khorana made synthetic mRNA to try to understand the
genetic code. When they produced mRNA containing only uracil nucleotides, it coded for one type of
amino acid, phenylalanine. When mRNA was produced with alternating uracil and guanine nucleotides,
two types of amino acid were coded for, valine and cysteine. Alternating U’s and C’s produced Serine
and Leucine.
Nucleotide sequence in mRNA
UUUUUUUUUUUUUUUUUUU
UGUGUGUGUGUGUGUGUG
UCUCUCUCUCUCUCUCUCU
Amino acids coded for
Phenylalanine
Valine and Cysteine
Serine and Leucine
A. For the amino acid phenylalanine what is the tRNA anti-codon sequence? Make sure to indicate the
5’ and 3’
3’-AAA-5’
A tRNA anticodon contains 3 bases. Since the mRNA codon is 5’-UUU-3’, the tRNA
Codon must be complementary and run in the opposite direction: 3’-AAA-5’.
B. What are the potential mRNA codons for Valine predicted by this experiment? Make sure to
indicate the 5’ and 3’?
5’-UGU-3’ and 5’-GUG-3’
The two possible codons with correct prime end labels are listed above .
These are the only possible codons in the given mRNA strand.
C. What are the potential mRNA codons for Leucine? Make sure to indicate the 5’ and 3’?
5’-UCU-3’ and 5’-CUC-3’.
8. What is transcription? Identify and describe the 3 steps of transcription. Make sure to name key
players and their roles in transcription (example: RNA polymerase).
Transcription: Synthesis of mRNA from DNA template
1. Initiation: Promoters (TATA box), transcription factors, RNA polymerase (the transcription initiation
complex
2. Elongation: RNA nucleotides
3. Termination: Prokaryotes- terminator, eukaryotes- poly A tail
9. Which of the following STATEMENTS is false?
A. A single DNA template strand can give rise to different proteins due to alternative splicing
B. The TATA box is a region on the mRNA that allows RNA polymerase to attach (on DNA)
C. A gene codes for a polypeptide
D. An organism’s genotype is carried in its sequence of bases
E. The phenotype is a consequence of the proteins that are expressed