Honors BiologyHonors Biology

Chapter 13Chapter 13DNA Structure and DNA Structure and

FunctionFunction

Modified By: R. LeBlancModified By: R. LeBlanc10/’1110/’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)(R) with a “rough” surface and a deadly strain (S)(S) with a “smooth” surface.

Griffith’s ExperimentGriffith’s Experiment

What was the conclusion from this experiment?

How was this possible? Could it be TRANSFORMATION?

A Summary of Griffith’s A Summary of Griffith’s ExperimentExperiment

This experiment clearly This experiment clearly demonstrated the presence of a demonstrated the presence of a

hereditary material & its hereditary material & its ability to be transferred from ability to be transferred from

one organism to another!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 Bacteriophages: Valuable Tools in Finding the Identity of the Hereditary Substance.Identity of the Hereditary Substance.

–genetic material

–viral coat

–sheath

–base plate

–tail fiber

bacterial cell wall

plasma membrane

cytoplasm

In the electron microscope image

above, bacteriophages are infecting an E. coli

bacterium.

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)bacterial cell (cutaway view)

label outside celllabel outside cell label inside celllabel inside cell

Chapter 10Hershey and Chase’s Experiments

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 Base Pairing RuleThe Base Pairing Rule

Base pairing between the two nucleotide strands in DNA is

constant for all species (A with T and C with GA with T and C with G).

The sequence of bases is different from

species to species.one base pairone base pair

–In 1949, Erwin ChargaffErwin Chargaff disclosed 2 important clues to DNA’s structure: (1) The amount of adenine relative (1) The amount of adenine relative to guanine differs from one species to the next,to guanine differs from one species to the next, (2) the (2) the amount of amount of adenineadenine in a DNA molecule is always equal in a DNA molecule is always equal to the amount of to the amount of thyminethymine & the amount of & the amount of guanineguanine is is always equal to the amount of always equal to the amount of cytosinecytosine!!

DNA - a spiral double helix

Rosalind FranklinRosalind 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 DNACAN 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?

James

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.

The Structure of DNA The DNA molecule is made up of smaller

units known as nucleotides nucleotides (shown below). A nucleotide is composed of threethree parts:

• A five-carbon deoxyribose sugardeoxyribose sugar…• A phosphate groupphosphate group…• One of four nitrogen-containing basesnitrogen-containing bases:

• AdenineAdenine• GuanineGuanine• CytosineCytosine• ThymineThymine

DNADNA - chains (2 strands) of nucleotides joined by hydrogen bondshydrogen bonds between bases.

A “twisted laddertwisted ladder” shape known as a “double helixdouble helix” is formed.

A. DNA Replication: Each organism has its own unique nucleotide unique nucleotide sequencesequence in its DNA. DNA must be copied for cell division ( replicationreplication).

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 10Comparing 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.