DNA: Deoxyribonucleic Acid The Carrier of Genetic Information ESSENTIAL QUESTIONS: 1. Which experiments led to the discovery of DNA as the genetic material? 2. What is the basic structure of DNA? 3. What is the basic structure of a chromosome?

p. 303 So Which Came First? DNA or Proteins/Enzymes Gene Expression/ Protein Synthesis

What material contains the inheritable information? Proteins? Carbohydrates? Lipids (fats)? or Nucleic Acids? TIMELINE: 1900s – Proteins were believed to carry inheritable information First major experiment performed on mice using bacteria to identify the transforming substance – DNA, not protein, is demonstrated to be involved in viral reproduction. Research supports the hypothesis that DNA carries inheritable traits 1944 – DNA is definitely identified as the transforming principle Structure of DNA was proposed

Frederick Griffith Experiment Griffith experimented with mice and 2 strains of the bacteria Streptococcus pneumonia to see which strain would be transformed. Mouse lives Mouse dies Mouse lives Mouse dies

The Avery and Hershey-Chase Experiments We knew: Chromosomes carried genetic information But Which Part? Protein or DNA? Hershey and Chase used radioactive isotopes to “label” or tag the DNA and the protein of the viruses some viruses were grown so that their DNA contained radioactive phosphorous ( 32 P) other viruses were grown so that their protein coats contained radioactive sulfur ( 35 S)

The Avery and Hershey-Chase Experiment After the labeled viruses were allowed to infect bacteria, only bacteria infected with the 32 P viruses had the 32 P label in their interior The conclusion was that the genes that viruses use to specify new viruses are made of DNA and not protein Figure 12.2 The Hershey-Chase experiment.

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Discovering the Structure of DNA DNA is comprised of subunits called nucleotides each DNA nucleotide has three parts a central deoxyribose sugar a phosphate group an organic nitrogen base

Nucleotides differ with regards to their bases large bases (purines) with double-ring structure either adenine (A) or guanine (G) small bases (pyrimidines) with single rings either cytosine (C) or thymine (T) Erwin Chargaff noted that DNA molecules always had equal amounts of purines and pyrimidines Chargaff’s rule suggested that DNA had a regular structure the amount of A always equaled the amount of T the amount of C always equaled the amount of G A or G T or C

Complementary base pairing in DNA 1)Hydrogen bonding between the nitrogenous bases holds the two chains in a helix 2) Base pairing follows specific rules Base-pairing rules for DNA Adenine (A) forms two hydrogen bonds with thymine (T) Guanine (G) forms three hydrogen bonds with cytosine (C) Vid

Rosalind Franklin’s work in 1953 using X-ray diffraction revealed that DNA had a regular structure that was shaped like a corkscrew, or helix Francis Crick and James Watson elaborated on the discoveries of Franklin and Chargaff and deduced that the structure of DNA was a double helix (built in 1953) two strands of DNA bound together by hydrogen bonds between the bases because a purine of one strand binds to a pyrimidine on the other strand to form a base pair, the molecule keeps a constant thickness

DNA & Replication

Why does DNA replicate? Make new cells for: Growth Repair Reproduction

Cell division and DNA replication Cells divide in a process called Mitosis  Growth, Repair, Replacement Before cells divide they have to double cell structures, organelles and their genetic information

How the DNA Molecule Copies Itself The two strands of DNA that form the double helix DNA molecule are complementary to each other each chain is essentially a mirror image of the other

DNA Replication is called Semiconservative Replication the DNA unzips and new complementary strands are assembled using each parent strand as a template leaving one original strand preserved in each duplex created.

How the DNA Molecule Copies Itself DNA replicates through the use of several enzymes: RNA polymerase Adds an RNA primer to jump start replication DNA polymerase adds the correct complementary nucleotide to the growing daughter strand but can only add nucleotides to the 3´ end of an existing strand or primer Helicase unwinds the DNA to expose the templates creating a replication fork DNA Ligase seals fragments of DNA together

How nucleotides are added in DNA replication

How the DNA Molecule Copies Itself At the replication fork, a primer must first be added to give a place for DNA polymerase to start DNA replicates by adding to the 3’ end (From 5’ to 3’) using one template, DNA polymerase adds nucleotides in a continuous fashion: leading strand DNA polymerase can build a new strand in one direction only Second strand is assembled is segments called okazaki fragments, each one beginning with a primer the segments are joined together by DNA ligase to form the lagging strand

Building the leading and lagging strands

DNA Replication Steps 1. DNA unwinding 2. Unzipping (hydrogen bonds break) 3. New strands made– free floating nucleotides form to build new strand Note: DNA replication is said to be Semi-conservative because: One strand is the original (conserved) One strand is freshly assembled (semi-half)

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Because so much DNA is being replicated in the many cells of the body, there is a potential for errors to occur DNA repair involves comparing the daughter strand to the parent DNA template to check for mistakes. The proofreading is not perfect because mutations are still possible, although rare; however, genetic variation is the raw material of evolution

DNA replication DNA Double helix game medicine/dna_double_helix/