Lecture 15 Flashcards
What are the structures of DNA and RNA? How do they differ?
DNA and RNA have similar structures, both consist of a phosphate-sugar backbone with nitrogenous bases joined to the sugars. However they differ in the nitrogenous base and the sugar used. DNA uses deoxyribose and uses the nitrogenous bases of A,C,T and G. RNA uses ribose and uses the nitrogenous bases A, C, U and G.
How did Chargaff disprove the tetranucleotide hypothesis?
Chargaff found that the ratio of nitrogenous bases in organisms was not 1:1:1:1 and hence showed that the base sequence could not simply repeat over and over.
What were chargaff’s rules? How did he work this out?
His first rule was that the conc of Adenine = the conc of Thymine and that the conc of Guanine = the conc of Cytosine. His second rule was that the composition of DNA varies based on species.
He determined these rules because he found from examining the DNA of many organisms that the concentration of A and T were always roughly equal and the concentration of C and G were always roughly equal and that the proportion between species was different.
What was Griffith’s experiment? What did it show?
Griffith’s experiment took dead smooth strain cells and added them to a mix of rough strain cells, the rough strain were non virulent while the smooth strain was virulent. He found that when this was done and the mix tested on mice that the mice would die with S cells in them. This showed that the information for genes must be a non living chemical.
What was Avery’s experiment? What did it show?
Avery’s experiment was to take heat killed smooth cells and removed RNA from one batch, protein from another and DNA from another. He then mixed each batch with live rough cells and found that only the one with the removed DNA did not lead to new smooth cell bacteria. This showed that DNA was the genetic material.
What is the DNA structure?
A double stranded helix of constant diameter with bases perpendicular to length.
Which three bases are pyrimidines? Which are purines? Whats the difference?
Uracil, cytosine and thymine are pyrimadines and consist only of a pyrimadine ring. Adenine and guanine are purines and consist of both a pyrimadine ring and an imidazole ring.
How was it determined that pyrimidines must always bind with purines?
If a purine were to bind with a purine the diameter would be larger than if a pyrimadine binded with a pyrimadine and hence the diameter of DNA would not be constant.
What difference seperates ribose and deoxyribose?
Ribose has an OH on its 2nd carbon from the base, deoxyribose has only an H group on its 2nd carbon from the base.
How are phosphodiester bonds formed? What does this mean for the growth of DNA?
The hydroxyl group on the third carbon of the sugar on one nucleotide reacts with the phosphate group attached to the 5th carbon on another nucleotide, as such DNA can only grow in the 5’ to 3’ direction.
What does it mean for the DNA strands to be antiparallel?
The two strands are flipped, growing in opposite directions to each other (though both are still 5’ to 3’).
How are the DNA strands held together? How many for each?
Hydrogen bonding, A and T are bonded by two hydrogen bonds while C and G are bonded by three hydrogen bonds.
