The Structure of DNA

and how we figured it out Why Should We Care About the Shape of DNA? ● The shape of DNA has to allow it to… ○ carry genetic information from generation to generation ○ be accessible to control the cell ○ be copied easily for mitosis Monomer of DNA

● Made up of monomers called nucleotides ● 3 parts to a nucleotide ○ phosphate group ○ 5-carbon sugar, deoxyribose ○ nitrogenous base A Little More About the Nitrogenous Base ● There are 4 bases ○ adenine (A) ○ thymine (T) ○ cytosine (C) ○ guanine (G) A Little More About the Nitrogenous Base ● Bases are organized by their shape ○ Pyrimidines- contain one ring, C & T ○ Purines- contain 2 rings, A & G Erwin Chargaff

● 1950 ● Through chemical analysis found that… ○ the # of adenine = # of thymine ○ the # of cytosine = # of guanine ● Known as “Chargaff’s Rules” Rosalind Franklin

● 1952 ● Extracted DNA and took pictures of it with X-rays ● Saw an X-shaped pattern, suggesting a helix shape Rosalind Franklin James Watson & Francis Crick

● 1953 ● Saw Franklin’s data ● Created a 3-D double helix model of DNA ● Won the Nobel Prize in 1962 DNA Structure

● Double helix shape (twisted ladder) ● Two strands of nucleotides ○ phosphate group and deoxyribose form the “backbone” ○ nucleotides form “rungs” of the ladder DNA Structure DNA Structure

● Two strands of nucleotides ○ sugar of one nucleotide bonds covalently with the phosphate of another ○ nitrogenous bases join by hydrogen bonding ■ G&C have 3 hydrogen bonds ■ A&T have 2 hydrogen bonds DNA Structure DNA Structure

● Each strand of nucleotides is complementary to the other ● Base pairing explains Chargaff’s rules ○ adenine only bonds with thymine ○ cytosine only bonds with guanine Base Pairing

● If one strand of DNA has a nucleotide sequence of CTGCTA, the complementary strand will have a sequence of GACGAT