Exam 2-Txt Q's Flashcards
How were we able to determine that DNA, and not some other molecule serves as the genetic material in bacter?
Avery McClead and McCarty experiment
extracted bacteria cell contents and treated them with different inactivating enzymes like proteases, RNucleases, DNucleases to see which one would affect the transforming principle. DNase inactivated the transforming principle proving that it is made of DNA.
The Hershey and Chase experiments further proved the DNA theory because they labelled P with a radioactivity and saw that bacteriophages grew colonies with the same radioactive P, the DNA was carried from one colony to the next.
How do we know that in DNA G pairs with C and that A pairs with T as complementary strands are formed?
Chargaff’s Rules were determined through chromatographic experiments to separate the 4 bases and it was found that the amount of A was proportional to the amount of T and the same for G and C
Contrast the contributions made to an understanding of transformation by Griffith and by Avery and his colleagues.
Griffith discovered that there was a transformation principle through his work with rough and smooth bacteria.
Avery built on this idea of the transformation principle and sought to find out what it was made of. He found it was DNA by using inactivating enzyme and exposing them to the transformation principle until a combination resulted in the loss of the transformation principle.
When Avery and his colleagues had obtained what was concluded to be the transforming factor from the IIIS virulent cells, they treated the fraction with proteases, ribonuclease, and
deoxyribonuclease, followed by the assay for retention or loss of transforming ability. What were the purpose and results of these
experiments? What conclusions were drawn?
Avery used proteases, deoxyribonucleases and ribonucleases because they are inactivating enzymes, therefore once one of them inactivated the transformation principle it could be determined what the principle was made of. It was found that the deoxyribonuclease inactivated the transforming principle, the conclusion was the transforming principle is made of DNA
Why were 32P and 35S chosen in the Hershey Chase experiment? Discuss the rationale and conclusions of this experiment
32P and 35S were used as tags because the radioactivity made them visible. 32P was meant to appear on DNA, because P is in DNA. 35S was used because S is in proteins. radioactivity was found in bacteria that had been infected with bacteriophages with the 32P, this was not the case for the bacteria infected with the 35S. The conclusion was that the transforming principle is made of DNA and not proteins.
Does the design of the Hershey–Chase experiment distinguish between DNA and RNA as the molecule serving as the genetic material? Why or why not?
No, because the experiment is designed to distinguish DNA and proteins. DNA is tagged with P which is not present in proteins and proteins are tagged with S which is not present in proteins. However P is present in both DNA and RNA so the results do not distinguish DNA from RNA.
. Describe the various characteristics of the Watson–Crick double
helix model for DNA.
2 antiparallel polynucleotide chains, coiled around a central axis twisted into a right handed helix
each chain has covalently (phosphodiester bond) joined nucleotides, the bases are on the inside and the chains are held together by H bonds
What might Watson and Crick have concluded, had Chargaff’s data from a single source indicated the following base composition?
A T G C
% 29 19 21 31
The data given show that A = C and T = G, which do not normally pair, making it unlikely that a tight helical structure
would form. This observation would likely lead to the interpretation of DNA as a single-stranded or other nonhydrogenbonded structure. Such structures would not produce the
regular X-ray diffraction pattern observed by Wilkins and
Franklin, which indicated a double-helical structure.
How do covalent bonds differ from hydrogen bonds? Define base complementarity.
h bonds are weak electrostatic attractions, they are strong when in numbers like in the helix.
phosphodiester bonds/ covalent bonds are stronger bonds because of similar electronegativity between atoms
base complementarity refers to the affinity between bases where A-T and G-C
One of the most common spontaneous lesions that occurs in DNA under physiological conditions is the hydrolysis of the amino group of cytosine, converting it to uracil. What would be the
effect on DNA structure if a uracil group replaced cytosine?
Assuming the lesion is not repaired, the result would be a base substitution of G-C to A-T after two rounds of replication, because cytosine pairs with guanine and uracil pairs with adenine.
What are the molecular composition and arrangement of the
components in the nucleosome?
nucleosomes are made of DNA 147 bps wrapped around histone proteins. arranges in packaged structures that are coiled and stacked, looped and folded.
Describe the transitions that occur as nucleosomes are coiled and
folded, ultimately forming a chromatid.
As chromosome condensation occurs, five or six nucleosomes coil together to form a 30-nm fiber called a solenoid. These fibers form a series of loops that further condense into the chromatin fiber, which is then coiled into chromosome arms
making up each chromatid
Provide a comprehensive definition of heterochromatin, and list as many examples as you can.
telomere, centromere, inactivated x chromosomes
chromatin that never uncoils it remains condensed even during interphase, repetitive DNA like in satellite DNA
What is the experimental basis for concluding that DNA replicates semi conservatively in bacteria?
Mathew Meselson and Franklin Stahl
by tagging the DNA with 15N in E.coli, they found that after many generations N15 was in the DNA. They separated the old and new DNA by placing it in a medium where the new ones were lighter and the old ones were heavier and sank to the bottom. The results were that the DNA has intermediate density, proving that each DNA mol had both old and new. They ruled out dispersive by isolating the hybrid molecule and denaturing it with heat which separated the two strands, they observed that each strand was either old or new and not both like the dispersive theory suggests. Also, with each generation the DNA got lighter and lighter i.e newer and newer.
Describe the role of 15N in the Meselson–Stahl experiment.
By uniformly labeling the nitrogenous bases of the DNA of E. coli with the heavy isotope 15N, and allowing synthesis of “new” DNA in the presence of the light isotope 14N, it was possible to “follow” the “old” and “new” DNA
