Honors Biology Chapter 13 DNA Structure and Function Modified By: R Honors Biology Chapter 13 DNA Structure and Function Modified By: R. LeBlanc 10/’11

Early scientific discoveries. . . Johann Friedrich Miescher (1868) discovered “nuclein” later to be named DNA. Biologists ignored his discovery for 75 years. P.A. Levene (1920) discovered that there were 4 nitrogen base molecules found in DNA. Fred Griffith (1928) while trying to find a vaccine for pneumonia, he discovered the process of transformation. He used two strains of pneumonia bacteria: A harmless strain (R) with a “rough” surface and a deadly strain (S) with a “smooth” surface.

Griffith’s Experiment What was the conclusion from this experiment?

How was this possible? Could it be TRANSFORMATION?

A Summary of Griffith’s Experiment This experiment clearly demonstrated the presence of a hereditary material & its ability to be transferred from one organism to another!

Which Substance (DNA or Protein) Carried the Hereditary Information? Oswald Avery (1944) added protein-digesting enzymes to cells, but transformation occurred anyway. DNA-destroying enzymes stopped transformation. So, what did Avery’s experiment prove? Most biochemists ignored Avery’s work, claiming that his results probably only applied to bacteria. Proteins were still believed to be the carriers of hereditary information.

What runs the cell, DNA or proteins? Researchers like Delbruck, Hershey, & Luria in the 1950’s began using viruses called bacteriophages to study the transfer of genetic information. These are made of only two things: DNA Protein.

Bacteriophages: Valuable Tools in Finding the Identity of the Hereditary Substance. genetic material viral coat sheath base plate tail fiber bacterial cell wall plasma membrane In the electron microscope image above, bacteriophages are infecting an E. coli bacterium. cytoplasm

Confirmation of DNA Function Alfred Hershey and Martha Chase (1952) used radioisotope tracers on viruses. What did this prove? (see the next slide) virus particle labeled with 35S virus particle labeled with 32P bacterial cell (cutaway view) label outside cell label inside cell

Hershey and Chase’s Experiments Chapter 10

The Search for DNA’s Structure Linus Pauling (1951) discovered the 3-D structure of proteins, & the presence of 20 essential amino acids. What were the 3D structures of proteins? Following his discovery, scientists began to believe that the three-dimensional structure of DNA could also be discovered. 3D Model of hemoglobin

The sequence of bases is different from species to species. The Base Pairing Rule In 1949, Erwin Chargaff disclosed 2 important clues to DNA’s structure: (1) The amount of adenine relative to guanine differs from one species to the next, (2) the amount of adenine in a DNA molecule is always equal to the amount of thymine & the amount of guanine is always equal to the amount of cytosine! Base pairing between the two nucleotide strands in DNA is constant for all species (A with T and C with G). The sequence of bases is different from species to species. one base pair

DNA - a spiral double helix Rosalind Franklin (left) - A crystallographer who identified the helical shape of DNA by using x-ray defraction. A picture of a DNA refraction-----------> Her work provided the evidence needed to solve the mystery of DNA structure.

Rosalind Franklin’s X-ray of DNA CAN YOU SEE THE X IN THE MODEL TO THE RIGHT?? Can you see the steps of the twisted ladder (helix) in the photograph? Why is the 4th step smeared?

Finally, in 1953, James Watson & Francis Crick (left) solved the mystery of the structure of the DNA molecule. For this achievement they were awarded the Nobel Prize. James

The Structure of DNA The DNA molecule is made up of smaller units known as nucleotides (shown below). A nucleotide is composed of three parts: A five-carbon deoxyribose sugar… A phosphate group… One of four nitrogen-containing bases: Adenine Guanine Cytosine Thymine

A “twisted ladder” shape known as a “double helix” is formed. DNA - chains (2 strands) of nucleotides joined by hydrogen bonds between bases. A “twisted ladder” shape known as a “double helix” is formed. A. DNA Replication: Each organism has its own unique nucleotide sequence in its DNA. DNA must be copied for cell division ( replication). The sides of the DNA ladder are made of sugars & phosphate groups. The “rungs” of the ladder are made of nitrogenous bases.

DNA Replication

Chapter 10 Comparing DNA and RNA

DNA Replication DNA polymerases attach free nucleotides to the unzipped strands. DNA ligases seal new short stretches of nucleotides into one continuous strand. In which direction is a DNA molecule built? 5’ to 3’

DNA Organization in Chromosomes Each chromosome consists of one DNA molecule. Proteins keep all the DNA from becoming a tangled mess. Histones are like spools for winding up small stretches of DNA. A nucleosome is a DNA-protein spool.