AP Biology DNA The Genetic Material Biology---Yippee!

Scientific History  Understanding that DNA is the genetic material  T.H. Morgan (1908)  Frederick Griffith (1928)  Avery, McCarty & MacLeod (1944)  Hershey & Chase (1952)  Watson & Crick (1953)  Meselson & Stahl (1958)

Genes are on chromosomes  T.H. Morgan  working with fruit flies  Determined: genes are on chromosomes  The question then was: is it the protein or the DNA of the chromosomes that are the genes?  through 1940 proteins were thought to be genetic material because their structure is more complex 1908 | 1933

Frederick Griffith 1928  was working to find cure for pneumonia (Streptococcus pneumonia bacteria)  harmless live bacteria mixed with heat-killed infectious bacteria causes disease in mice  substance passed from dead bacteria to live bacteria = “Transforming Factor”

The “Transforming Factor” Transformation something in heat-killed bacteria could still transmit disease-causing properties to the harmless bacteria live pathogenic strain of bacteria live non-pathogenic strain of bacteria mice diemice live heat-killed pathogenic bacteria mix heat-killed pathogenic & non-pathogenic bacteria mice livemice die A.B. C. D.

 Avery, McCarty & MacLeod  purified both DNA & proteins from Streptococcus pneumonia bacteria to determine which will transform non- pathogenic bacteria?  injected protein into bacteria  no effect  injected DNA into bacteria  transformed harmless bacteria into virulent bacteria  Concluded  DNA is the transforming factor 1944 What ’ s the conclusion?

Confirmation of DNA  Hershey & Chase  classic “blender” experiment  worked with bacteriophage  viruses that infect bacteria  grew phage viruses in 2 media, radioactively labeled with either  35 S in their proteins  32 P in their DNA  infected bacteria with labeled phages 1952 | 1969 Hershey

Blender experiment  Radioactive phage & bacteria in blender  35 S phage  radioactive proteins did NOT enter bacteria  32 P phage  radioactive DNA did enter bacteria  Confirmed DNA is “transforming factor” Taaa-Daaa!

Summary  Used radioactive labels to identify genetic material  Labeled DNA with radioactive phosphorus ( 32 P)  Labeled protein coats with radioactive sulfur ( 35 S)  Blender  Examined newly infected host cells (pellet) and found only radioactive phosphorus label (not sulfur)  ***DNA is the genetic material

Hershey & Chase Alfred HersheyMartha Chase 1952 | 1969 Hershey

Nucleic Acids (review) section 10.2  Examples:  RNA (ribonucleic acid)  single helix  DNA (deoxyribonucleic acid)  double helix  Structure:  monomers = nucleotides RNADNA

Nucleotides 3 parts 1. nitrogen base  Adenine, guanine, cytosine & thymine 2. pentose sugar (5 Carbon) ribose in RNA deoxyribose in DNA 3. phosphate (PO 4 ) group Nitrogen base I’m the A,T,C,G or U part!

Erwin Chargaff  DNA composition: “Chargaff’s rules”  varies from species to species  Amount of thymine = amount of adenine  Amount of cytosine = amount of guanine 1947

Structure of DNA  Watson & Crick  developed double helix model of DNA  other scientists working on question:  Rosalind Franklin  Maurice Wilkins  Linus Pauling 1953 | 1962 Franklin WilkinsPauling

Rosalind Franklin ( )

Rosalind Franklin & Maurice Wilkins (1950) 1. used X-ray crystallography to study the structure of DNA.  In this technique, X-rays are diffracted as they passed purified DNA.  The diffraction pattern can be used to deduce the three-dimensional shape of molecules.

Watson and Crick 1953 article in Nature CrickWatson

James Watson and Francis Crick 1. Concluded that the structure of DNA is a double helix. (After Watson viewed Franklin’s x-ray diffraction photo) 2. Backbone consists of alternating sugar & phosphate units. 3. Attached to the backbone are four kinds of bases. I. AdenineIII. Cytosine II. GuanineIV. Thymine

Twist

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 16.5

Copying DNA  Replication of DNA  base pairing allows each strand to serve as a template for a new strand

Franklin Stahl Matthew Meselson  Meselson & Stahl experiment used radioactive nitrogen to determine that DNA replication is semi-conservative (each of the 2 new DNA molecules is half original or parent DNA and half newly made) Semi-conservative replication 1958

AP Biology DNA Replication

 DNA used to make DNA  Making an exact copy of the DNA before the cell divides  original strand serves as a template for the new strand  Each resulting double-stranded DNA molecule is made of one original and one new strand ( ½ parent template and ½ new DNA)  semi-conservative replication

Anti-parallel strands  DNA molecule has “direction”  complementary strand runs in opposite direction  Replication only occurs in the 5’ to 3’ direction

Bonding in DNA weak bonds covalent phosphodiester bonds hydrogen bonds strong bonds

Base pairing in DNA  Pairing  A : T  2 bonds  C : G  3 bonds

Replication: 1st step  Unwind DNA  Helicase enzyme  unwinds part of DNA helix  stabilized by single-stranded binding proteins single-stranded binding proteins replication fork helicase

DNA Polymerase III Replication: 2nd step  Build daughter DNA strand  add new complementary bases to 3’’ end of growing DNA strand  Enzyme  DNA polymerase III  strand only grows 5  3

Loss of DNA  With each replication, small segments at the end of our chromosomes (called telomeres) are lost  This may be part of the aging process  When enough DNA is lost the cell can no longer divide  An enzyme called telomerase is able to add on to the end of chromosomes – but it is inactivated in most of our cells  Cancer cells keep the enzyme active and can divide forever