DNA Structure and Function

DNA Structure and Function

Chapter 13

Miescher Discovered DNA

• 1868• Johann Miescher investigated the chemical

composition of the nucleus• Isolated an organic acid that was high in

phosphorus• He called it nuclein• We call it DNA (deoxyribonucleic acid)

Mystery of the Hereditary Material

• Originally believed to be an unknown class of proteins

• Thinking was– Heritable traits are diverse– Molecules encoding traits must be diverse– Proteins are made of 20 amino acids and

are structurally diverse

Structure of the Hereditary Material• Experiments in the 1950s

showed that DNA is the hereditary material

• Scientists raced to determine the structure of DNA

• 1953 - Watson and Crick proposed that DNA is a double helix

Figure 13.2Page 217

Griffith Discovers Transformation

• 1928• Attempting to develop a vaccine• Isolated two strains of Streptococcus

pneumoniae– Rough strain was harmless– Smooth strain was pathogenic

Griffith Discovers Transformation

1. Mice injected with live cells of harmless strain R.

2. Mice injected with live cells of killer strain S.

3. Mice injected with heat-killed S cells.

4. Mice injected with live R cells plus heat-killed S cells.

Mice die. Live S cells in their blood.

Mice live. No live R cells in their blood.

Mice die. Live S cells in their blood.

Mice live. No live S cells in their blood.

Figure 13.3Page 218

Transformation

• What happened in the fourth experiment?

• The harmless R cells had been transformed by material from the dead S cells

• Descendents of the transformed cells were also pathogenic

Oswald & Avery

• What is the transforming material?• Cell extracts treated with protein-digesting

enzymes could still transform bacteria• Cell extracts treated with DNA-digesting

enzymes lost their 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

cytoplasm

bacterial cell wall plasma

membrane

Figure 13.4bPage 219

Hershey & Chase’s Experiments

• Created labeled bacteriophages– Radioactive sulfur – Radioactive phosphorus

• Allowed labeled viruses to infect bacteria

• Asked: Where are the radioactive labels after infection?

virus particle labeled with 35S

virus particle labeled with 32P

bacterial cell (cutaway view)

label outside cell

label inside cell

Hershey and

Chase Results

Figure 13.5Page 219

Information• Mon., 28 November

– Chapter 14 and 16 highlights• Wed., 30 November

– Final exam review – BRING YOUR QUESTIONS!– Instructor evaluations

• Mon., 12 December, 2:15-4:15pm in C317– Final Exam

• [email protected]

• Exam 3 will be returned at the end of this class.

Structure of Nucleotides in DNA

• Each nucleotide consists of– Deoxyribose (5-carbon sugar)

– Phosphate group

– A nitrogen-containing base

• Four bases– Adenine, Guanine, Thymine, Cytosine

Nucleotide Bases

phosphate group

deoxyribose

ADENINE (A)

THYMINE (T)

CYTOSINE (C)

GUANINE (G)

Figure 13.6Page 220

Composition of DNA

• Chargaff showed:– Amount of A relative to G differs among

species

– Always: A=T and G=C

Rosalind Franklin’s Work

• Expert in X-ray crystallography

• Examined DNA fibers

• Concluded that DNA was some sort of helix

Watson-Crick Modelof DNA

• 2 nucleotide strands– Run in opposite directions

– Held together by H bonds between bases

• A binds with T and C with G

• Molecule is a double helix

DNA Structure Helps Explain How It Duplicates

• DNA is 2 nucleotide strands held together by H bonds

• H bonds between 2 strands are easily broken

• Each single strand then serves as template for new strand

DNA Replication

newnew old old

• Each parent strand remains intact

• Every DNA molecule is half “old” and half “new”

Figure 13.9Page 222

Base Pairing during

Replication

Each old strand serves as the template for complementary new strand

Figure 13.10Page 223

Enzymes in Replication

• Enzymes unwind the two strands

• DNA polymerase attaches complementary nucleotides

• DNA ligase fills in gaps

• Enzymes wind two strands together

A Closer Look at Strand Assembly

Energy for strand assembly is provided by removal of two phosphate groups from free nucleotides

newlyformingDNAstrand

one parent DNA strand

Figure 13.10Page 223

Continuous and Discontinuous Assembly

Strands can only be assembled in the 5’ to 3’ direction

Figure 13.10Page 223

DNA Repair

• Mistakes can occur during replication

• DNA polymerase can read correct sequence from complementary strand and, together with DNA ligase, can repair mistakes in incorrect strand

Information• Mon., 28 November

– Chapter 14 and 16 highlights• Wed., 30 November

– Final exam review – BRING YOUR QUESTIONS!– Instructor evaluation

• Mon., 12 December, 2:15-4:15pm in C317– Final Exam

• [email protected]

• Exam 3 will be returned ... Now!

Exams

Grades

• A = 100-79 • B = 78-70 • C = 69-61 • D = 60-53

• Average = 63• High = 99