DNA Structure and Function Deoxyribonucleic acid DNA Stores

DNA Structure and Function

Deoxyribonucleic acid (DNA) • Stores and transmits genetic information from one generation to the next • Present in all organisms, but different (unique) in each individual – Except identical twins!

DNA History • Important experiments – Griffith – Avery – Hershey and Chase • Discovering its structure – Chargaff – Franklin and Wilkins – Watson and Crick model

Griffith: a “transforming principle” • 1928 • Investigated two forms of bacteria by injecting them into mice – “S form” have a smooth coating – “R form” have a rough coating • Only the live S type is virulent (kills mice) • Mice injected with a combination of heatkilled S bacteria and live R bacteria also died

Griffith’s Experiment • “Transforming principle”– Griffith found live S bacteria in blood samples of the dead mice • Some material must have transferred from dead S bacteria to live R bacteria • Avery (1944) suggested that DNA was probably the genetic material that was “transformed”

Hershey and Chase • 1952 • Conclusive evidence for DNA as the genetic material • Studied bacteriophages (viruses that take over bacteria cells) • Phage structure: – DNA molecule – Protein coat • This could help answer the DNA vs. protein debate!

Hershey and Chase • Protein has sulfur but not much phosphorous • DNA has phosphorous but not sulfur • Hershey and Chase grew phages in cultures with radioactive sulfur or phosphorous isotopes • Experiment 1 – Bacteria infected with phages that had radioactive sulfur in their protein – Used a blender to separate bacteria from parts of the phages that remained outside the bacteria – No radioactivity found in the bacteria

Hershey and Chase • Experiment 2 – Bacteria infected with phages grown in radioactive phosphorous – Used a blender to separate bacteria from parts of phage that remained outside bacteria – Radioactivity present inside bacteria • These results showed that bacteriophages’ DNA had entered bacteria but protein had not! • Finally convinced scientists that DNA and not protein is the genetic material • Also called the “blender experiment”

Hershey and Chase

Discovering DNA • Griffith- discovered a “transforming principle” (hereditary molecule) • Avery- suggested DNA as the transforming principle • Hershey and Chase- proved that it was DNA, not protein, that was the hereditary molecule (“blender experiment”)

Discovering DNA Structure • Erwin Chargaff (1950) showed the amounts of the four DNA bases • In ALL organisms, the amount of adenine (A) always equaled the amount of thymine (T) • The amount of cytosine (C) always equaled the amount of guanine (G) – If A = 15%, then T = ? ? ? , G = ? ? ? , and C = ? ? ?

Chargaff’s Rule • Adenine pairs with thymine • Guanine pairs with cytosine • The bases form weak hydrogen bonds

DNA Structure • Franklin and Wilkins (1952) – X-ray crystallography photographs of DNA • Watson and Crick (1953) – Built the first model of DNA using Franklin’s x-rays – Double helix model- two strands twisted together, antiparallel

DNA Structure • DNA molecule is a polymer made of many nucleic acid monomers, called nucleotides • Nucleotide has three parts: – Phosphate group – Deoxyribose (sugar) – Nitrogen-containing base • • Adenine Guanine Cytosine Thymine

One DNA Strand • The backbone of the molecule is alternating phosphates and deoxyribose sugar • The rungs (across) are nitrogenous bases.

Four Nitrogenous Bases • Purines (double ring) – Adenine (A) – Guanine (G) • Pyrimidines (single ring) – Thymine (T) – Cytosine (C)

Base Pairing • A always pairs with T (2 hydrogen bonds) • C always pairs with G (3 hydrogen bonds) • Hydrogen bonds are weak but there are millions of them in a single DNA molecule!

Antiparallel strands • DNA strands are complementary – Fit together (one purine and one pyrimidine) – Opposite of each other • One strand goes from 5’ to 3’ • The other strand is in the opposite direction going 5’ to 3’ – For example, if one strand’s bases are ATCTT, what will the other strand be? – TAGAA

Helix