DNA Structure and Function

Learning Outcomes

Explain the main results of the Hershey-Chase experiment

Describe the structure of DNA and the evidence used to deduce the structure Describe how DNA replicates and explain the role of each component of the replication machinery

Johannes Friedrich Miescher

Isolated nuclei from the rest of the cell. Treated the nuclei with various chemicals and was able to recover thin filaments of material Showed that the substance contained C, N, O, H, and P (no sulfur) Proposed that it was the genetic material.

1st to isolate DNA, in 1868

(he called it “nuclein”)

Controversy --

Is the genetic material protein or nucleic acid?

Protein – much more complex Nucleic acids – simple -- only 4 monomers

Hershey Chase Experiment (1952)

They knew that DNA contained phosphorus (P) But not sulfur (S) And proteins contained sulfur (in the amino acids Met, Cys) But not phosphorus (P)

Question – Is the genetic material protein or DNA?

Bacteriophages are viruses that infect bacteria. Simple composition of protein coat surrounding DNA

Hershey Chase Experiment

DNA being injected into bacterium

Virus particle coat proteins labeled with 35S

35S remains outside cells

Labeled DNA being injected into bacterium

Virus DNA labeled with 32P

32P remains inside cells

DNA is the genetic material

Questions remaining --

1) How is it replicated?

2) How does it code for proteins?

In order to answer these questions – need to know the structure

DNA Structure (1953)

James Watson and Francis Crick

Nucleotide Structure

Sugar (deoxyribose)

Nitrogenous base (can be A, G, C, or T)

GUANINE (G) deoxyguanosine triphosphate

Phosphate groups

Nucleotide Structure

Thymine (T)

Adenine (A) Guanine (G)

Pyrimidines

Purines

Cytosine (C)

Chargaff’s Rule

In DNA –

A = T

G = C Erwin Chargaff 1905 –2002

Franklin’s X-ray Diffraction of DNA

This image shows that -- 1)The structure is helical. 2) The diameter of the helix is uniform (~ 20 nm) 3) The length of the twist is ~ 34 nm

Rosalind Franklin 1920-1958

DNA Structure

Complementary Base Pairing in DNA

Watson and Crick suggest a mechanism the cell can use to copy DNA.

one base pair

DNA Strands are Antiparallel The sugar-phosphate backbones run in opposite directions.

DNA Replication

semi-conservative each parent strand is the template for a new daughter strand

DNA polymerase is responsible for making the new polynucleotide strand

DNA Replication

primer parent strand

DNA polymerase requires a primer

DNA Replication

DNA polymerase can only add nucleotides to the 3’ end of a polynucleotide

DNA Replication

DNA polymerase reads the parent strand and adds the complementary nucleotide to the new strand

DNA Polymerase

Some limitations --

DNA Pol requires a primer

DNA Pol can only add nucleotides to a 3’ OH

DNA Pol can make mistakes

errors in replication – 1/1000 – 1/billion

~ 1/1000 mutations will NOT be repaired

Chromosomes

In eukaryotes, DNA molecules are packaged into chromosomes.

Chromosomes

Prokaryotes usually have one circular DNA molecule that is their chromosome.