DNA STRUCTURE4.2

1920S It was determined that DNA comprises of three main

components: deoxyribose sugar, phosphate group (-), nitrigenous base.

These three components make up a ________________. These are held together by _______________ bonds (between the ____________ and ___________ groups.

Erwin Chargaff (1949) Proportion of adenine = proportion of _______________ Proportion of guanine= proportion of _______________ Proportion of pyrimidines = proportion of __________

Structure of Nucleotides Nitrogenous base attached to 1’ carbon of the sugar

by a glycosyl bond. Phosphate group attached to the 5’ carbon by an ester bond.

DNA... THAT COMPLICATED MOLECULE Carries hereditary information Controls cellular processes Accommodates for individual variation Replicates itself

1953: THE YEAR OF DNA What was known:

Chemical composition of DNA (not structure!) Relative proportions of nitrogenous bases to each other.

What was the structure? Linus Pauling: California Rosalind Franklin & Maurice Wilkins: London

Using X-ray diffraction analysis/ X-ray crystallography Molecule bombarded with X-rays X-rays deflected by atoms pattern of

lighter and darker lines on photographic film. Math used to decipher the 3-D shape.

James Watson & Francis Crick: Cambridge U. Wilkins presented them with informal look at Franklin’s diffraction

pattern before it was published (boo!) Revealed that DNA has the shape of a helix about 2 nm in diameter,

with a complete helical turn every 3.4 nm. Used Franklin’s info + Chargoff’s results to build the famous model of

the double-helix structure of DNA in 1953. Wilkins, Watson, and Crick awarded the Nobel Prize in 1962.

ROSALIND FRANKLIN Born to liberal-minded parents she was

raised in the same manner as her brothers. At 15, she decided to pursue a career in

science, despite the lack of female role models in the scientific community in the 1930s.

Did extensive research with X-ray chrystallography.

Died of cancer at the age of 37 in 1958, working until the very end in her wheelchair.

She may have been awarded the Nobel Prize had she been alive.

DNA STRUCTURE: THE DOUBLE HELIX Consists of two antiparallel strands of

_______. Bases of one strand paired with bases

in the other strand facing inward towards each other.

Purine is always paired with a pyrimidine: complimentary base-pairing. Which scientist led to this idea? If sequence of one strand is known, the

sequence of the complimentary strand is known.

THE HELIX HAS A CONSISTENT DIAMETER OF 2NM.

Yeah, so? This is consistent with the base-pairing

rules If two purines bond together: wider at

some points. If two pyrimidines bond together, less than

2nm. Complimentary base pairs held

together by hydrogen bonds Individual H-bond weak ‘strength in numbers’: DNA is stable.

ANTIPARALLEL... One strand runs in the 5’ to 3’ direction. One strand runs in the 3’ to 5’ direction. 3’ end:

Terminates with the hydroxyl group of the deoxyribose sugar 5’ end:

Terminates with a phosphate group.

By convention, only the 5’ – 3’ strand written when indicating the sequence of nucleotides. Why? _______________________

SEATWORK/HOMEWORK Page 216, #1-3, 5-10.