Lecture 7 - DNA Structure Flashcards
Who are Freidrich, Flemming, and Zacharias and what did they do?
Freidrich - collected pus on bandages to obtain nuclei from WBC’s, “nuclein”
Flemming - first to observe chromosomes in dividing cells
Zacharias - extracted DNA from cells causes chromosomes to disappear
What properties of proteins make them seem likely to be the carriers of genetic information?
Proteins are the building blocks
Proteins are way more complex (20 amino acids) than DNA (4 nucleotide bases), and they thought DNA was super complex and had to be the more complex structure
What are the two strains of Streptococcus Pneumoniae?
R strain (rough colonies) - its benign, these would not make mice sick/die S strain (smooth colonies) - extremely severe, would kill the mice
Explain Griffith’s experiments + his results
Gave mice living/dead/heat shocked versions of the 2 strains of bacteria
Found that bacteria are capable of transferring genetic information through a process known as transformation - the heat killed S bacteria mixed with living R bacteria then produced living S bacteria in dead mice
Explain Avery’s experiments + his results
He used enzymes to destroy certain components (such as lipids/proteins)
Found that DNA is responsible for transformation, could not go from the R to S strain without it, mice would survive
Explain Hershey and Chase experiments + give their results
Mixed bacteria with radioactive phages (either DNA or proteins) to infect the bacteria, put in blender to knock the viruses off the bacteria cells, centrifuge and measure the radioactivity of the pellet
Given that DNA cells are heavy and sink to the bottom, when the pellet with DNA was found to be radioactive, it was known to be the carrier of genetic info. The protein did not sink with the bacteria, showing it was not getting into the cells
What are the differences in composition of DNA and RNA?
DNA - deoxyribose sugar (missing an O), ATGC, double stranded
RNA - ribose sugar, AUGC, single stranded
What is Chargraff’s rules?
DNA from any cell of any organisms should have a 1:1 ratio of pyrimidine and purine bases (the amount of guanine should be equal to cytosine and the amount of adenine should be equal to thymine)
What did Watson and Crick discover?
That DNA is a double helix - DNA had a sugar phosphate backbone with bases, these bases could form hydrogen bonds, + x-ray diffraction data (Rosalind Franklin) showed it was a long, thin helical molecule with a repeating structure
What are the different components of DNA structure?
Major groove, minor groove, antiparallel orientation, right handed helix, nucleotide pair - 0.34nm, 10 nucleotide pairs per turn, 2nm diameter, hydrogen bonds, and complementary pairing
What are the three forms of DNA and explain each
B-DNA - Main form of DNA in the cell (right handed)
A-DNA - Dehydrated B-DNA, the helix compresses by removing water
Z-DNA - longer and thinner than B-DNA (left handed)
Where does Z-DNA occur?
In regions where there are tracts of alternating purine/pyrimidines, also in areas of methylation
Possible role in regulating gene expression
Can exist in vivo
What are DNA quadruplexes?
G-quadruplexes - 4 stranded DNA, bases in the middle are interacting
What is supercoiled DNA? Can linear DNA have this feature?
The twisting of the double helical structure into a more compact form (takes up less space)
Originally seen in circular plasmid/viral DNA, linear DNA can be supercoiled in regions where the DNA is anchored
What is positive and negative supercoiling?
Positive - twisted in the same direction that the DNA helix is wound (right)
Negative - twisted in the opposite direction that the helix is wound (left)
What do topoisomerases (TI’s) in general do? Type I? Type II?
Convert DNA between relaxed and supercoiled states
Type I - makes nicks in one strand, causes relaxation of the supercoiled form, only one strand is cut
Type II - uses ATP to cut both strands of the DNA helix simultaneously in order to manage DNA tangles and supercoils. DNA gyrase can induce and relax supercoiling in bacteria
Do the strands of the helix need to separate and reanneal regularly? If so, why?
Yes, for replication, transcription, and repair
Explain DNA melting temperatures
Its the temperature in which half the absorbance change in reached. Can be different depending on each specific sequence of DNA
Does absorbance increase or decrease when strands are separate?
Increases
Can GC content change Tm? If so, why?
Yes, because they are held together by three hydrogen bonds, more bonds = greater strength, which requires more energy to break
**2nd step of PCR depends on the Tm, creating the wide range
