DNA STRUCTURE Unit 2: Molecular Genetics
DNA is the Hereditary Substance!! DNA is the winner!
Next Step: Learn about it’s physical & chemical structure • structure & function are intimately connected.
The Chemical Composition of DNA • DNA is made of 3 different components: • A phosphate group • A deoxyribose sugar • A nitrogenous base.
The Phosphate Group • A group of 4 oxygen atoms surrounding a central phosphorus atom found in the backbone of DNA
Deoxyribose Sugar • Sugar molecule containing 5 carbons... • The 2’ carbon has lost an hydroxyl group (-OH).
Nitrogenous Bases • Nitrogenous Bases – an alkaline, cyclic molecule containing nitrogen. • In DNA, there are 4 types: • Adenine • Thymine • Guanine • Cytosine
2 Types of Nitrogenous Bases: • Pyrimidines: • Composed of one ring of nitrogen atoms. • Thymine • Cytosine • Purines • Composed of two ring of nitrogen atoms. • Adenine • Guanine
Structure of Nucleotides • Each of DNA’s four nucleotides comprise of a deoxyribose sugar attached to a phosphate group and a nitrogenous base.
The nitrogenous base is attached to the 1’C of the sugar by a glycosyl bond. A glycosyl bond is a bond between a sugar and another organic molecule. The phosphate group is attached to a 5’ C of the sugar by an ester bond...phosphodiester bond.
A Model of DNA Linus Pauling (California) Rosalind Franklin & Maurice Wilkins (London) Watson & Crick (Cambridge)
X-Ray Diffraction Analysis • Franklin & Wilkins were using X-Ray Diffraction Analysis of DNA to determine it’s structure. • How does it work? • The X-rays are deflected by atoms in the molecules, producing a pattern of lighter and darker lines on a photographic film. The pattern that is produced is analyzed and the 3-D structure can be determined mathematically.
X-Ray Diffraction Patterns • The pattern revealed that DNA has the shape of a helix, is about 2nm in diameter and has a complete helical turn every 3.4 nm. • 1nm = 10 -9 m
DNA consists of 2 antiparallel strands of nucleotides. Antiparallel – parallel but in opposite directions. The 5’ end of one strand aligns with the 3’ end of the other. 5’ (Phosphate) 3’ (Sugar)
DEMO – Antiparallel! Each student is a nucleotide. Find a Partner (he or she is your conjugate base). Shake each other’s right hands (H-Bonds) Line up in two lines (each one is one strand of DNA) Put your left hand on right shoulder of the person in front of you (phosphodiester bonds). At the end of each line there will be one free left hand. It is your 3’ END!!
Complementary Base Pairing • Pairing of the nitrogenous base of one strand of DNA with the nitrogenous base of another strand • Erwin Chargaff (1949) • A+T, • G +C
Purines are nitrogenous bases that have 2 rings: adenine and guanine • Pyrimidines are nitrogenous bases that have only 1 ring: thymine and cytosine. • One purine + one pyrimidine otherwise DNA structure would look “wonky”.
The bases of the two DNA strands are bound by hydrogen bonds. Hydrogen bonds on their own are relatively weak, but multiple hydrogen bonds are stronger. A & T – 2 H-Bonds C& G – 3 H-Bonds