DNA Structure & Replication

Ch Griffith’s Transformation Experiment 1928 – Frederick Griffith is studying how certain strains of bacteria cause pneumonia and inadvertently makes a discovery about how genetic information is passed from organism to organism His Experiment :  Grow two slightly different strains (types) of bacteria  One strain proven harmless and other deadly  Laboratory mice are injected with these strains

Griffith’s Results

What caused Griffith’s results? The heat-killed strain passed on its disease- causing ability to the live harmless strain. In Griffith’s words, one strain of bacteria was TRANSFORMED into another. Was the “Transforming” Factor: Protein or DNA?

Avery, MacLeod, McCarty Isolate macromolecules present in S-strain (virulent) bacteria to see which is capable of transforming R-strain (non-virulent) bacteria. Only DNA can transform!

The Hershey-Chase Experiment Alfred Hershey & Martha Chase studied viruses, which are non-living particles smaller than a cell that can infect living organisms. Bacteriophages: specific group of viruses that infect bacteria. OBJECTIVE: To determine which part of the virus (protein or DNA) enters a bacteria it is infecting.

What did Hershey & Chase do? If Hershey and Chase could determine which part of the virus entered an infected cell, they would learn whether genes were made of protein or DNA. To accomplish this, they grew viruses in cultures containing radioactive isotopes of phosphorus-32 ( 32 P) and sulfur-35 ( 35 S). Some viruses had P-32 in their DNA, and others had S- 25 in their protein coat. If S-35 is found in the bacteria, it would mean that viruses release their protein and if P-32 is found in the bacteria it would mean that viruses release their DNA.

Recall: Method of Bacteriophage Infection oWhen a bacteriophage enters a bacterium, the virus attaches to the surface of the cell and injects its genetic information into it. oThe viral genes replicate to produce many new bacteriophages, which eventually destroy the bacterium. oWhen the cell splits open, from viral overload, hundreds of new viruses burst out and can infect surrounding cells

Hershey-Chase Results DNA, So… The genetic material in bacteriophages was the DNA, (not the protein)!!!

Ch : DNA Structure nucleotides Made of monomers called nucleotides Nucleotide structure:

A nucleotide can have one of four bases: Types of bases:  Adenine  Guanine  Cytosine  Thymine A & G are bigger and are called purines C & T are smaller and are called pyrimidines

Chargaff’s Rule & Rosalind Franklin Chargaff (A=T; C=G) % G % C% A% T Discovered that in almost any DNA sample, the % G nearly equals the % C and the % A nearly equals the % T Franklin: X-ray picture helps figure out structure used x-ray diffraction to get information about the structure of DNA. She aimed an X-ray beam at concentrated DNA samples and recorded the scattering pattern of the X- rays on film.

Watson & Crick Using clues from Franklin’s X-ray pattern, shown to them by Maurice Wilkins, James Watson and Francis Crick built a 3-D model that explained how DNA carried information and could be copied. Watson, Crick & Wilkins were awarded the 1962 Nobel Prize in Physiology or Medicine for their work.

Base-Pairing Watson & Crick discovered that bonds can only form between certain base pairs, Adenine & Thymine and Cytosine & Guanine. purines pyrimidines The base-pairing rule means that purines only pair with pyrimidines, making the rungs equally spaced like a ladder. The nitrogenous bases are held together by hydrogen bonds. – A & T are held together by TWO hydrogen bonds – C & G are held together by THREE hydrogen bonds

DNA is a “double-helix” or twisted ladder: o o The “backbone” or sides of the DNA molecule are made up of alternating sugars and phosphates and the “rungs” are made up of interlocking nitrogen bases. o o The sugars and the phosphates are held together by covalent bonds and the nitrogen bases are held together by hydrogen bonds.

Molecular Structure of DNA

DNA Structure

Chromosomes & DNA PROKARYOTES : In prokaryotic cells ( bacteria ), DNA is located in the cytoplasm. Most prokaryotic cells have a single DNA molecule that holds all of the genetic information EUKARYOTES : 1000x the DNA as prokaryotes DNA is held in chromosomes located in the nucleus Chromosome # varies widely from one species to the next

DO NOW: What category of macromolecules does DNA fit into? What is the monomer of a DNA molecule? What are the three parts of this molecule? How do the nucleotides fit together in a molecule of DNA? How is the 3D structure of DNA described?

3 end 5 end 3 end 5 end  P P P P P P P P

DNA Replication Tutorials i_replication_vo2.html Excellent narrated animation; pause, highlight, take notes; use Styrofoam model i_replication_vo2.html Good additional animation of lagging strand ns/replication1.html ns/replication1.html

DNA Replication Before a cell can divide, it’s DNA must be replicated or copied in the S- phase of the cell cycle. In most prokaryotes, replication begins at a single point and continues in two directions. In eukaryotes, replication occurs in hundreds of places simultaneously and proceeds until complete. Sites of replication are called replication forks.

– DNA replication begins at the origins of replication –DNA unwinds at the origin to produce a “bubble” –Replication proceeds in both directions from the origin –Replication ends when products from the bubbles merge with each other – DNA replication occurs in the 5’ 3’ direction –Replication is continuous on the 3’ 5’ template –Replication is discontinuous on the 5’ 3’ template, forming short segments 10.5 DNA replication proceeds in two directions at many sites simultaneously Copyright © 2009 Pearson Education, Inc.

Animation: Leading Strand Animation: Leading Strand 10.5 DNA replication proceeds in two directions at many sites simultaneously – Proteins involved in DNA replication –DNA helicase unwinds the double helix –DNA polymerase adds nucleotides to a growing chain –DNA ligase joins small fragments into a continuous chain Copyright © 2009 Pearson Education, Inc. Animation: Lagging Strand Animation: Lagging Strand Animation: DNA Replication Review Animation: DNA Replication Review Animation: Origins of Replication Animation: Origins of Replication

Origin of replication Parental strand Daughter strand Bubble Two daughter DNA molecules

Parental DNA 3 5 DNA polymerase molecule DNA ligase 3 5 Overall direction of replication Daughter strand synthesizedcontinuously Daughterstrandsynthesized in pieces

Visual Summary of DNA replication Animation

Replication Bubbles

Parentalmolecule of DNA Nucleotides Both parental strands serve as templates Two identical daughter molecules of DNA  Each “new” strand of DNA consists of one original template strand and one newly made strand.  This allows for proofreading, using the template strand as the “master”. DNA REPLICATION is semi-conservative

Chromosome Structure Eukaryotic chromosomes contain DNA and protein tightly packed together to from chromatin. Chromatin consists of DNA tightly coiled around proteins called histones. DNA & histone molecules form nucleosomes, which pack together to form thick fibers of chromosomes.

Do Now: Why would DNA need to be photocopied? What is the name of this process? Where does it occur? Replication is described as semi- conservative. Explain. Distinguish between the leading and lagging strand during DNA replication.