DNA Structure and Function

Chapter 9

Miescher Discovered DNA

• 1868

• Johann Miescher investigated chemical composition of nucleus

• Isolated organic acid high in phosphorus

• He called it nuclein

• We call it DNA (deoxyribonucleic acid)

Griffith Discovers Transformation

• 1928

• Attempting to develop a vaccine

• Isolated two strains of Streptococcus pneumoniae– Rough strain was harmless

– Smooth strain was pathogenic

Griffith's experiment

Griffith Discovers Transformation

Transformation

• Harmless R cells were transformed by material from dead S cells

• Descendents of transformed cells were also pathogenic

What Is the Transforming Material?

• Avery found protein-digesting enzymes did not change results– extracts still transformed bacteria

• But treated with DNA-digesting enzymes– extracts lost transforming ability

• Concluded that DNA, not protein, transforms bacteria

Bacteriophages

• Viruses that infect bacteria

• Consist of protein and DNA

• Inject their hereditary material into bacteria

Bacteriophages

Hershey-Chase experiments

Subunits of DNA

Watson-Crick Model

Hershey and Chase’s Experiments

• Created labeled bacteriophages– Radioactive sulfur

– Radioactive phosphorus

• Allowed labeled viruses to infect bacteria

• Where were the radioactive labels after infection?

virus particlelabeled with 35S

DNA (blue)being injected into bacterium

35S remainsoutside cells

virus particlelabeled with 32P

DNA (blue)being injected into bacterium

35P remainsinside cells

Fig. 9-2, p.139

Hershey and Chase Results

35S remains outside cells

32P remains inside cells

Structure of DNA

2nm diameter overall

0.34 nm between each pair of bases

3.4 nm length of each full twist of helix

In 1953, Watson and Crick showed that DNA is a double helix

DNA close up

Watson-Crick Model

Watson and Crick

Watson-Crick Model

• DNA molecule is a double helix

• Consists of two nucleotide strands that

run in opposite directions

• Strands are held together by hydrogen

bonds between bases

• A binds with T, C binds with G

Structure of Nucleotides in DNA

• Each nucleotide consists of

– Deoxyribose (5-carbon sugar)

– Phosphate group

– A nitrogen-containing base

• There are four bases:

– Adenine, Guanine, Thymine, Cytosine

Nucleotide Bases

phosphate group

deoxyribose

ADENINE (A)

THYMINE (T)

CYTOSINE (C)

GUANINE (G)

Composition of DNA

• Amount of adenine relative to guanine

differs among species

• Amount of adenine always equals amount of thymine, and amount of guanine always equals amount of cytosine

A=T and G=C

DNA Structure Allows It to Duplicate

• Two nucleotide strands held together by

hydrogen bonds

• Hydrogen bonds between two strands

are easily broken

• Each single strand serves as template

for new strand

Rosalind Franklin’s Work

• Expert in x-ray crystallography

• Used technique to examine DNA fibers

• Concluded that DNA was some sort of helix

2-nanometer diameter overall

0.34-nanometer distance between each pair of bases

3.4-nanometer length of each full twist of the double helix

In all respects shown here, the Watson–Crick model for DNA structure is consistent with the known biochemical and x-ray diffraction data.

The pattern of base pairing (A only with T, and G only with C) is consistent with the known composition of DNA (A = T, and G = C). Fig. 9-6, p.141

DNA Models

DNA Replication

newnew old old

• Each parent strand

remains intact

• Every DNA

molecule is half

“old” and half “new”

Base Pairing during

Replication

Each old strand is template for new complementary strand

DNA replication details

Base Pairing during Replication

Enzymes in Replication

• Enzymes unwind the two strands and

complementary base pairs unzip

• DNA polymerase attaches new

complementary nucleotides

• DNA ligase fills in gaps

• Enzymes wind two strands together

DNA Repair

• Mistakes can occur during replication

• DNA polymerase reads correct

sequence from complementary strand

and, together with DNA ligase, repairs

mistakes in incorrect strand

Clones

• Nuclear transfer from adult cell

• Structural and functional problems– Most adult DNA inactive

• Potential benefits– Replacement organs– Endangered animals

Cloning

• Making a genetically identical copy of

an individual

• Researchers have been creating clones

for decades

• Clones can be created by embryo

splitting (artificial twinning)

How Dolly was created

Cloning

Impacts, Issues Video

Goodbye Dolly

More Clones

• Numerous species been clonedMice, pigs, cattle, cats, etc.

• Most cloning attempts are still unsuccessful

• Many clones have defects

• Clones may vary in their phenotype

More Clones