Unit 1: DNA Structure Flashcards
What two experiments confirmed that DNA was the genetic material of bacteria, viruses, and eukaryotes ?
1) Griffith experiment
2) Hershey chase experiment
Describe the Griffith Experiment
Griffith used two bacterial strains of streptococcus pneumonia to infect mice.
-one of the strains, the S strain was the virulent one and would kill the infected mice, while the other was the R strain that was harmless.
The reason the S strain was virulent was b/c this particular bacterial could produce a polysaccharide capsule that would protect it from the host immune system. The R strain on the other hand did not have this capsule and hence could not evade the host immune system and thus would be destroyed.
one of the most surprising experiments was when he injected a heat killed S bacteria and a live R bacteria into a healthy mouse. He expected the mouse to live, as the harmful bacteria was already killed prior to injection. Instead he found was that the S type had the same smooth coat as the S strain. This meant that somehow the property of the dead bacteria was transferred over to the R bacteria transforming it such that it could now make the smooth polysaccharide and thus make it very violent DNA.
What is the transforming principle?
genetic properties can be transferred from one bacterial strain to another by extracting DNA from the first strain and adding it to the second strain
The DNA that is taken up by a bacterium
How did phage infection show that DNA is the genetic material of viruses?
When the DNA and protein components of bacteriophages are labeled w/different radioactive isotopes, only the DNA is transmitted to the progeny phages produced by infecting bacteria
Describe the Hershey Chase experiment:
This experiment was trying to distinguish between protein and DNA being the genetic material of viruses. This experiment uses T2 bacteriophages that had either their DNA radioactively labeled with P32 or protein labeled with S35. Following infection, the bacteria were subjected to centrifugation with two fractions separated. One fraction contained the virus capsules that were on the outer surface of the bacteria and the other fraction was the bacteria itself. The first fraction contained all the S radioactive label while the second fraction which went into the bacteria carried the P radioactive label. This demonstrated that the viral DNA entered the bacteria was where replication and incorporation into the progeny phage occur thus confirming the DNA is the genetic material of virus
Through what process do eukaryotic cells take up foreign DNA and incorporate it into its own genome and expression of the DNA results in a new trait
through transfection.
Eukaryotic cells can acquire a new phenotype as the result of transfection by added DNA
A nucleoside consists of a..?
purine or pyrimidine base linked to the 1’ carbon of a pentose sugar
A nucleotide consists of a..?
of a nitrogenous base, pentose sugar, and a phosphate group. The pyrimidine or purine linked to the sugar is called a nucleoside.
What are two differences between DNA and RNA ?
DNA: A-T, C-G
RNA: U-T, C-G
Another difference between DNA and RNA is in the group at the 2’ position of the sugar
-DNA has a deoxyribose sugar (2’H); RNA has a ribose sugar (2’-OH)
Where is the phosphodiester bond between nucleotides formed? How does this affect how the DNA chain is extended?
between the hydroxyl group of the 3’ carbons of the growing chain and the phosphate group on the 5’ carbon of the incoming nucleotide. Thus the chain is always extended in the 5’-3’ direction with the top end of the chain having the free 5’ end and the growing end of the chain having a 3’ end
What is the Watson-Crick DNA form?
B form of DNA which consists of two anti-parallel strands that maintain the same width throughout the entire length of the molecule b/c purine will always pair with pyrimidine and vice versa in the complementary AT and GC base pair
How many hydrogen bonds form between AT and CG pairs?
A-T: 2 hydrogen bonds
C-G: 3 hydrogen bonds
What is meant by complementary base pairing?
base pairs match up in the pairing reactions in double helical nucleic acids
A-T in DNA
A-U in RNA
C-G
explain major and minor groove
The double helix has a major groove and a minor grove. When the helix is twisted, DNA will have areas where the backbones are further apart (major groove) alternating w/areas where the backbones are closer together (minor).
Proteins that regulate transcription or replication bind to which groove, why?
the major groove b/c there would be less steric hindrance from the backbones
The degree of DNA winding can be affected by?
confirmation of the DNA helix and proteins bound to specific sites on the DNA
What is meant when DNA is overwound?
B-form DNA that has fewer than 10.4 base pairs per turn of the helix
What is meant when DNA is underwound?
B-form DNA that has more than 10.4 base pairs per turn of the helix
When does supercoiling occur in DNA?
when DNA bends over on itself (crosses over its own axis) causing more tension, twisted and condensed form, and hence more energy than the stable relaxed form of DNA
-supercoiling occurs only in ‘closed’ DNA w/no free ends.
Why is supercoiling needed?
1) to compact the large amount of DNA found w/in a cell
2) in prokaryotes supercoiling plays a role in replication initiation
True or False: closed DNA is either circular DNA or linear DNA in which the ends are anchored so that they are not free to rotate
TRUE
The linking number (L) is the sum of what?
The sum of twist (T) and writhe (W)
L= T + W
where twist (T) represents the number of helical turns the two strands make and writhe (W) represents the supercoiling of the helix
In a relaxed circular molecule where there is no supercoiling, the linking number would be equal to what?
it would be equal to the twist (T) since W would equal 0.
TRUE OR FALSE: A linking number can have different combinations of T and W provided their sum is unchanged
True, the linking number remains constant as long as the closed DNA is intact and there have been no breaking and reforming of bonds
