Structure of DNA• Deoxyribonucleic Acid is a type of nucleic acid.• Nucleic acids are polymers of nucleotides.• DNA was discovered in 1868, but the

significance of what it was used for was unknown.

• Even when chromosomes were discovered, it was controversial whether it was the DNA or the proteins that transmitted hereditary traits.

Oswald Avery• 1877-1955

In 1944, Avery discoveredthat it was the DNA, andnot the proteins in chromosomes that wereinvolved in transmittingtraits.

*is this completely true???

Nucleotides:

There are four nitrogen bases:Pyrimidines have one ring. Purines have two rings

Erwin Chargaff

(1905-2002)

Studied the chemistry ofDNA structure. Discovered:% Adenine = % Thymine% Guanine = % Cytosine(1:1 purines:pyrimidines)At the time, the structureof DNA was unknown, so the significance of thisdiscovery was minimal at the time.

Maurice Wilkens & Rosalind FranklinIn the early 1950’s, Wilkens hiredFranklin to work in his lab atKing’s College in London. Herwork was to do X-ray crystallography on DNA in orderto figure out it’s structure. She made many images, themost famous being “Photo 51”

Francis Crick & James WatsonWere both working atCambridge University in 1951. Using information from Avery, Chargaff, and Franklin, they eventually built a model of DNA thatcombined the ideas of all the other scientists working on the structure of DNA.

Crick, Watson, and Wilkens shared the Nobel Prize in Medicine in 1962.

Their model is still on display in London

Here’s what the structure is like:• Sugar – Phosphate “backbone”• Nitrogen bases pairing in the middle• Entire molecule is twisted into a double helix

The nitrogen bases follow “Chargaff’s Rules”Adenine bonds to ThymineCytosine bonds to Guanine

Because of this, if we know the sequence of bases on one strand of DNA, we can predict the sequence of bases on the complementary strand.

A C G G C T A C T A C T G C C G A T G A T G

Now…. about the strands: They are running “anti-parallel”…Each deoxyribose looks like this:

The carbons are numbered 1-5The 1’ carbon bonds to a Nitrogen baseThe 5’ and 3’ end attach to phosphate groups

eukaryotic DNA has “ends”

The 5’ end will attach to aphosphate group and then end.

The 3’ end will attach to an-OH group (no phosphate) and then end.

On one end there will be a 3’ and a 5’ endThe other end will be a 5’ and a 3’ end

A picture to make it make sense:

These are called “Anti-parallel”strands

DNA REPLICATION

• Begins at “origins of replication” – numerous places all at the same time.• Helicase (an enzyme) untwists the DNA at each

origin of replication.(there’s a lot of other steps in here I’m leaving out for the sake of time and complexity…)

• “free” nitrogen bases form new hydrogen bonds• DNA polymerase (an enzyme) will finish new

nucleotidesbut only to the 3’ end of both strands.• Since nucleotides are only added to the 3’ end,

what happens at the 5’ end???

End result of replication:

How do we know both new strands are identical?

Genes• A gene is a segment of DNA that codes for the

production of a polypeptide.• Genes are not always continuous (introns and

exons)• Genes may overlap• You may have multiple copies (multigene

families)• Some genes are silent, some are turned on or off• Some genes can “jump” from one chromosome

to another (transposons)• Some genes cause apoptosis (self-destruction) of

cells that are damaged or mutated