1)You are given a yeast temparature-sensitive cell cycle mutant strain. The hapl
ID: 84403 • Letter: 1
Question
1)You are given a yeast temparature-sensitive cell cycle mutant strain. The haploid (1N) strain grows fine at the permissive temperature, but at the restrictive temperature, all of the yeast cells in the culture stop growing and accumulate as two large cells still joined together, with a 2N DNA content. What does this tell you about the gene that has been mutated?
a) it encodes a protein that is required for DNA replication
b) it encodes a protein that is required for mitosis c) it encodes a mutant protein that is inactive at both permissive and restrictive temperatures
d) it encodes a protein that is required for cytokinesisi
e) it encodes a protein required at the G1/S transition
2)Tumor-suppressor genes normally control
a) DNA polyermase processivity
b) the cell cycle or apoptosis
c) telomerase actvity
d) sister chromatid separation e) protein degradation
3)You discover a mutant in Caenorhabditis elegans that now grows wings as an adult and can fly. You identify the single gene in the genome that has been mutated. When you digest the gene from wild type animals with EcoRI and do a Southern blot you see four bands on a gel, of sizes 12, 8, 7 and 2 kilobases. When you digest the gene from mutant animals with EcoRI and do a Southern blot you see five bands on a gel, of sizes 12, 8, 4, 3 and 2 kilobases. Which of these can explain the origin of the mutant allele?
a) loss of one copy of the gene
b) a deletion of multiple bases
c) a single base change that creates an EcoRi site
d) a triplet expansion e) an insertion of multiple bases
4)You need to make a radioactive DNA probe for use in a Southern blot, and it is your job to order the radioactive nucleotide you will need for probe synthesis. You place your order for dATP with a radioactive phosphorus atom located at the gamma phosphate position (farthest from the sugar). When your shipment of radioactive nucleotide arrives, the boss of the lab berates you for wasting valuable research funds. Why is she mad at you before you even did the experiment?
a) DNA polymerase cannot use nucleotide triphosphates when it synthesizes a new DNA strand, you needed to order dAMP with a radioactive phosphorus
b) to make a radioactive probe, you need to use dideoxynucleotide triphosphate, you should have ordered ddATP
c) there is no phosphorus in dATP, so you clearly ordered the wrong compound
d) the gamma phosphate position is cleaved off when a nucleotide is added to a growing DNA strand, you needed to order dATP with the radioactive P at the alpha position (closest to the sugar)
e) I don’t know, she just always yells at me
Explanation / Answer
Answer:
1. b) it encodes a protein that is required for mitosis
Explaination: At the restrictive temperature, all of the yeast cells in the culture stop growing and accumulate as two large cells still joined together, with a 2N DNA content. It illustrating that the DNA has been duplicated in the synthesis phase of life cycle thus having 2N DNA. The cell mass is still joined together (at the restricted temperature) and would be divided through cytokinsis events, only after the nuclear division (in Mitosis phase of cell cycle) in which a 2N DNA divides into two equal parts (n DNA content). Thus, the gene required to encode proteins needed for mitosis events are mutated (thus, not encoding the proteins at restricted temperature).
2. B) the cell cycle or apoptosis
Explaination: Tumor suppressor genes are normal genes which are responsible for slowing down the cell division event (during cell cycle), OR guide the cells for the programmed cell death (apoptosis). Thus, to prevent the uncontrolled cell growth (leading to cancer), the functioning of Tumor suppressor genes are essential.
3. c) a single base change that creates an EcoRi site
Explaination: The gene of wild type animal when digested with restriction enzyme EcoRI, resulted to four bands i.e. of sizes 12, 8, 7 and 2 kilobases indicating that the gene have four restriction sites for the enzyme EcoRI. However, gene of mutant animal when digested with restriction enzyme EcoRI, resulted to five bands i.e. of sizes 12, 8, 4, 3 and 2 kilobases indicating that the gene have five restriction sites for the enzyme EcoRI. In both animals all the band are of same size except the band of 7 kb in wild type (found as 4 and 3 kb in mutant) when digested with EcoRI.
If their would be a) loss of one copy of the gene b) a deletion of multiple bases d) a triplet expansion or e) an insertion of multiple bases, the other band size (produced by EcoRI) would be also affected. But they are not affected, illustrating the single base change in the gene of mutant creates an EcoRI site.
4. d) the gamma phosphate position is cleaved off when a nucleotide is added to a growing DNA strand, you needed to order dATP with the radioactive P at the alpha position (closest to the sugar).
Explaination: During polymerization reaction of nucleotides (for example, during DNA probe synthesis), the 3' OH group of one nucleotide joined with the 5' phosphate group of the next adjacent nucleotide (For example, dATP). During this polymerization reaction, the sugar-phosphate backbone of DNA incorporate the more stable nucleotide monophosphate (alpha-phosphate group) into the chain, however, the beta and gamma phosphates comes out as pyrophosphate. Thus, if the gamma phosphate of dATP was radioactively labelled, it would not serve the purpose and you needed to order dATP with the radioactive P at the alpha position, so that the incorporation of this syntheized dATP (with radioactive P at the alpha position) resulting to radiolabelled DNA probe.
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