The Chemical Composition of DNA DNA STRUCTURE

1920s- It was determined that DNA comprises three main components: Deoxyribose sugar Phosphate group Nitrogenous base The Chemical Composition of DNA

Nitrogeneous Bases PurinesAdenine (A)Guanine (G)PyrimidinesThymine (T)Cytosine (C)

Solving the DNA mystery Erwin Chargaff discovered two rules that helped lead to the discovery of the double helix structure of DNA. This strongly hinted towards the base pair makeup of the DNA, although Chargaff was not able to make this connection himself. The first and best known achievement was to show that in natural DNA the number of guanine units equals the number of cytosine units and the number of adenine units equals the number of thymine units. He also discovered that the total amount of purines equals the total amount of pyrimidines in an organism’s genome. G = C A = T Purines = Pyrimidines

Nucleotide Structure in detail Sugar molecule containing 5 carbons that has lost the –OH (hydroxyl group) on its 2 carbon Deoxyribose sugar Group of four oxygen atoms surrounding a central phosphorous atom found in the backbone of DNA Phosphate group An alkaline, cyclic molecule containing nitrogen Held together by hydrogen bonds The source of variation is found in the nitrogenous bases. Nitrogeneous base

Nucleotides making up DNA DNA is composed of many nucleotides Nucleotides are held together by phosphodiester bonds Therefore, DNA is a polymer

Deoxyribose Sugar Five-carbon cyclic ring structure Attached to a phosphate group and nitrogeneous base The first four of the five carbon atoms, together with an oxygen atom, form a five membered ring The carbon atoms are numbered clockwise, starting with the carbon atom to the immediate right of the oxygen atom This first carbon atom is designated as 1 ' A deoxyribose sugar has a hydroxyl group (OH) on the 3 ' carbon, and a hydrogen atom (H) on the 2 ' carbon

Deoxyribose sugar The nitrogeneous base is attached to the 1 ' carbon of the sugar by a glycosyl bond The phosphate group is attached to the 5 ' carbon by an ester bond

Know your bonds: The nitrogeneous base is attached to sugar by a glycosyl bond The phosphate group is attached to sugar by an ester bond Nucleotides are held together by phosphodiester bonds Base pairs are held together by hydrogen bonds

Building a model for DNA structure California: Linus Pauling London: Rosalind Franklin & Maurice Wilkins Cambridge University: James Watson & Francis Crick Franklin’s X-ray crystallography (X-ray diffraction analysis) results helped Watson and Crick to determine the molecular structure of DNA X-ray results revealed that DNA has the shape of a helix DNA has a diameter of 2 nm A complete helical turn  every 3.4 nm 1 nm=10 -9

Rosalind Franklin James Watson and Francis Crick (1953)

DNA Structure: The Double Helix DNA consists of two anti-parallel strands of nucleotides Anti-parallel: parallel running in opposite directions The bases of one strand are paired with the bases of the other strand The 5 ' end of one strand of DNA aligns with the 3 ' end of the other strand in a double helix.

Complementary Base Pairing The nitrogeneous base pairs are arranged above each other, perpendicular to the axis of the DNA melecule A purine is always bonded to a pyrimidine Adenine with Thymine Guanine with Cytosine This type of pairing is termed complementary base pairing

A purine is always bonded to a pyrimidine Adenine with ThymineGuanine with Cytosine Diameter of DNA is 2 nm If 2 purines are bonded together (A-A or A-G or G-G), the DNA molecule would be wider If 2 pyrimidines are bonded together (T-T or T-C or C-C), the DNA molecule would be less What about Thymine with Guanine? Or Adenine with Cytosine?

DNA What about Thymine with Guanine? Or Adenine with Cytosine? Even though, they would have the desired diameter, within the DNA, the molecules would be unstable because of the lack of hydrogen bonding. Only Adenine can hydrogen bond with Thymine and Guanine can hydrogen bond with Cytosine. Double helix turns in a clockwise direction DNA is a right-handed helix making one complete turn every 10 molecules A full helical twist is 3.4 nm, therefore, the distance between adjacent base pairs is 0.34 nm.

DNA The two strands of DNA run anti-parallelOne strand runs in 5 ' – 3 ' directionThe other strand runs in 3 ' – 5 ' direction The 3 ' end terminates with the hydroxyl group of the deoxyribose sugar The 5 ' end terminates with a phosphate group Therefore, every DNA molecule has an intrinsic directionality

5- ' ATGCCGTTA-33'-3'-TACGGCAAT-5 ' DNA