Lecture 7. The Structure Of DNA Flashcards
Who showed that each building block of DNA is a nucleotide?
Phoebus Aaron Theodor Levene
What is the difference between ribose and deoxyribose?
Ribose has a hydroxyl group on the C-2’ whilst deoxyribose has a hydrogen atom where the hydroxyl group was
Which nitrogenous bases are purines?
Adenine and Guanine
Which nitrogenous bases are pyrimidine?
Cytosine, Thymine and Uracil
Where do glycosidic bonds form between sugars and purines?
Between sugar C-1’ and N-9
Where do glycosidic bonds form between sugars and pyrimidines?
Between sugar C-1’ and N-1
Nucleoside
A sugar and nitrogenous base joined together by a glycosidic bond
Nucleotides
Phosphorylated nucleosides
Where do ester bonds form between nucleosides and phosphates?
Between C-5’ and the negatively charged end of the phosphate
How do you recognise the start and the end of the DNA sequence?
Base sequence is read from 5’ (phosphate group) to 3’ (Hydroxyl group on sugar)
What was the Tetranucleotide Model that Levene proposed in 1930?
The four nucleotides occurred in tetranucleotide blocks with the bases pointing outwards and thus DNA was simple and repetitive and couldn’t be the genetic material
Who disproved the tetranucleotide model and when was it disproven?
Erwin Chargaff
How was the tetranucleotide sequence disproven?
The percentages of all nitrogenous bases were not the same
%T = %A and %G = %C
What is the alpha helix?
The basic structure preset in many proteins
How was the alpha helix discovered?
X-ray crystallography - cross formed which is typical of a helical structure
How was DNA stretched into long fibres that could be mounted in front of an X ray source?
X-ray fibre diffraction
Who discovered A-DNA and what did they do to the stretched DNA?
Maurice Wilkins, stretched DNA and air dried it
Who discovered B-DNA and what did they do to the stretched DNA?
Raymond Gosling and Rosalind Franklin, stretched DNA and left it hydrated
What was ‘Photo 51’
The clear cross from the x-ray fibre diffraction that showed DNA was helical. Allowed the dimensions of B-DNA to be deduced
B-DNA dimensions
Height of one helical turn = 3.6nm
Separation of the bases = ~0.34nm
Diameter of the helix = 2nm
Which way are the polynucleotide chain wound in the double helix?
Right-handed (clockwise)
A-DNA
Occurs in low hydration conditions: whether this occurs naturally is uncertain
B-DNA
The most structurally stable form under physiological conditions
A-DNA
Taken up physiologically by stretches of alternating pyrimidines and purines
