The structure of the DNA double helix was described by Watson and Crick in 1953. Explain the structure of the DNA double helix, including its subunits and the way in which they are bonded together.(Total 8 marks)

Please have that paperwork out & your journal… HW check!

• subunits are nucleotides;• one base, one deoxyribose and one phosphate in each

nucleotide;• description / diagram showing base linked to deoxyribose C1

and phosphate to C5;• four different bases – adenine, cytosine, guanine and thymine;• nucleotides linked up with sugar-phosphate bonds;• covalent / phosphodiester bonds;• two strands (of nucleotides) linked together;• base to base;• A to T and G to C;• hydrogen bonds between bases;• antiparallel strands;• double helix drawn or described;

Accept any of the points above if clearly explained in a diagram.• [8]

11DNA and Its Role in Heredity

11 DNA and Its Role in Heredity

• 11.1 What Is the Evidence that the Gene Is DNA?

• 11.2 What Is the Structure of DNA?

• 11.3 How Is DNA Replicated?

• 11.4 How Are Errors in DNA Repaired?

• 11.5 What Are Some Applications of Our Knowledge of DNA Structure and Replication?

Homework Check—Journal 7.1

Quiz Thursday!

11-1:• Not in IB, but ...Not in IB, but ...

• Experiments on bacteria and Experiments on bacteria and viruses demonstrated that DNA IS viruses demonstrated that DNA IS the GENETIC MATERIAL!the GENETIC MATERIAL!

11.1 What Is the Evidence that the Gene Is DNA?

By the 1920s: chromosomes consisted of DNA & proteins.

A new dye stained DNA, provided circumstantial evidence—DNA’s the genetic material:

It was in the right place

It varied among species

It was present in the right amount

11.1 What Is the Evidence that the Gene Is DNA?

Frederick Griffith, working w/2 strains of

Streptococcus pneumoniae

“transforming principle” from dead cells of 1 strain produced a heritable change in the other strain.

Figure 11.1 Genetic Transformation of Nonvirulent Pneumococci

11.1 What Is the Evidence that the Gene Is DNA?

To identify the transforming principle, Oswald Avery:

Treated samples to destroy different molecules (RNA, DNA, Protein)

If DNA was destroyed, the transforming principle was lost.

Figure 11.2 Genetic Transformation by DNA (Part 1)

Figure 11.2 Genetic Transformation by DNA (Part 2)

11.1 What Is the Evidence that the Gene Is DNA?

Hershey-Chase experiment:

• Is DNA or protein the genetic material?using bacteriophage T2 virus

• Bacteriophage proteins labeled with 35SDNA labeled with 32P

Figure 11.3 Bacteriophage T2: Reproduction Cycle

Bacteriophage T2

Protein coat

DNA

Figure 11.3 Bacteriophage T2: Reproduction Cycle

Bacteriophage T2

Protein coat

DNA

DNA

Bacteriophage T2 attaches to the surface of a bacterium and injects its DNA.

Viral genes take over the host’s machinery and synthesizes new viruses.

The bacterium bursts, releasing about 200 viruses.

Figure 11.4 The Hershey–Chase Experiment (Part 1)

Figure 11.4 The Hershey–Chase Experiment (Part 2)

Figure 11.5 Transfection in Eukaryotic Cells

DNA Origami!• http://www.dnai.org/teacherguide/pdf/

origami_inst.pdf

• Don’t forget...

www.Msleejichs.wikispaces.com

11-2 SUMMARY:• DNA: double helix of 2 DNA: double helix of 2

ANTIPARALLEL polynucleotide ANTIPARALLEL polynucleotide chainschains

• 2 chains joined by H bonds 2 chains joined by H bonds between nucleotide bases—pair A-between nucleotide bases—pair A-T , G-CT , G-C

11-2 Recap ?s:

• What’s the evidence that Watson & What’s the evidence that Watson & Crick used to come up with the Crick used to come up with the double helix model for DNA?double helix model for DNA?

• How does the double helical How does the double helical STRUCTURE of DNA relate to its STRUCTURE of DNA relate to its FUNCTION?FUNCTION?

11.2 What Is the Structure of DNA?

Structure of DNA was determined using…

Figure 11.6 X-Ray Crystallography Helped Reveal the Structure of DNA

Photo 11.1 X-ray diffraction pattern of DNA.

11.2 What Is the Structure of DNA?

Chemical composition also provided clues:

Bases:

Figure 3.23 Nucleotides Have Three Components

repeat fig 3.23 here

11.2 What Is the Structure of DNA?

1950: Erwin Chargaff

Figure 11.7 Chargaff’s Rule

11.2 What Is the Structure of DNA?

Model building

Linus Pauling—

Figure 11.8 DNA Is a Double Helix (A)

11.2 What Is the Structure of DNA?

X-ray crystallography

Figure 11.8 DNA Is a Double Helix (B)

11.2 What Is the Structure of DNA?

Key features of DNA:

11.2 What Is the Structure of DNA?

Complementary base pairing:

Figure 11.9 Base Pairing in DNA Is Complementary (Part 1)

Figure 11.9 Base Pairing in DNA Is Complementary

5′ end

3′ end

TA pairs have twohydrogen bonds.

CG pairs have three hydrogen bonds.

The strands both run in a 5′-to-3′ direction—they are antiparallel.3′ end

Each phosphate group links the 3′ carbon of one sugar to the 5′ carbon of the next sugar along the backbone.

Pairs of complementary bases form hydrogen bonds that hold the two strands of the DNA double helix together.

Photo 11.2 Computer-simulated space-filling model of DNA.

11.2 What Is the Structure of DNA?

Antiparallel strands:

11.2 What Is the Structure of DNA?

Functions of DNA:Functions of DNA:

11.2 What Is the Structure of DNA?

• Genetic material is precisely replicated

Replication Model

Remember this?!?

Then 7.1

11-3 Recap:• Meselson and Stahl showed that DNA Meselson and Stahl showed that DNA

replication is replication is semiconservativesemiconservative: each : each parent strand serves as a template for a parent strand serves as a template for a new strandnew strand

• A complex of proteins, most notably A complex of proteins, most notably DNA polymerasesDNA polymerases, is involved, is involved

• New DNA is polymerized in New DNA is polymerized in one one direction onlydirection only

• Since the 2 strands are antiparallel, 1 Since the 2 strands are antiparallel, 1 strand is made continuously and the strand is made continuously and the other is made in other is made in Okazaki fragmentsOkazaki fragments that that are eventually joinedare eventually joined

11-3 Recap ?s:

• How did the Meselson-Stahl expt How did the Meselson-Stahl expt work?work?

• What are 5 enzymes needed for What are 5 enzymes needed for DNA replication? Role of each?DNA replication? Role of each?

• How does the leading strand of How does the leading strand of DNA differ from the lagging DNA differ from the lagging strand?strand?

11.3 How Is DNA Replicated?

The DNA is a template for

11.3 How Is DNA Replicated?

Three possible replication patterns:

Figure 11.10 Three Models for DNA Replication

11.3 How Is DNA Replicated?

Meselson and Stahl

Figure 11.11 The Meselson–Stahl Experiment (Part 1)

Figure 11.11 The Meselson–Stahl Experiment (Part 2)

11.3 How Is DNA Replicated?

Results of their experiment

AnimationsAnimations!

• Meselson & Stahl Expt

• Replication Part 1

• Part deux

• Leading & Lagging

11.3 How Is DNA Replicated?

Two steps in DNA replication:Two steps in DNA replication:

Figure 11.12 Each New DNA Strand Grows from Its 5′ End to Its 3′ End (Part 1)

Figure 11.12 Each New DNA Strand Grows from Its 5′ End to Its 3′ End (Part 2)

11.3 How Is DNA Replicated?

DNA replicates in both directions, forming

Figure 11.13 Two Views of DNA Replication

11.3 How Is DNA Replicated?

DNA helicase

Figure 11.14 Replication in Small Circular and Large Linear Chromosomes (A)

11.3 How Is DNA Replicated?

Large linear chromosomes

Figure 11.14 Replication in Small Circular and Large Linear Chromosomes (B)

11.3 How Is DNA Replicated?

DNA polymerasesDNA polymerases:

Figure 11.15 DNA Polymerase Binds to the Template Strand (Part 1)

Figure 11.15 DNA Polymerase Binds to the Template Strand (Part 2)

11.3 How Is DNA Replicated?

primer required to start DNA replication—

Figure 11.16 No DNA Forms without a Primer

11.3 How Is DNA Replicated?

Cells have several DNA polymerases!

Figure 11.17 Many Proteins Collaborate in the Replication Complex

11.3 How Is DNA Replicated?

At replication fork:

Figure 11.18 The Two New Strands Form in Different Ways

11.3 How Is DNA Replicated?

Okazaki fragments

Figure 11.19 The Lagging Strand Story (Part 1)

Figure 11.19 The Lagging Strand Story (Part 2)

11.3 How Is DNA Replicated?

Telomeres

Figure 11.21 Telomeres and Telomerase

11.3 How Is DNA Replicated?

Human chromosome telomeres (TTAGGG) are repeated about 2500 times.

Chromosomes can lose 50–200 base pairs with each replication. After 20–30 divisions, the cell dies.

11.3 How Is DNA Replicated?

Some cells—bone marrow stem cells, gamete-producing cells—have telomerase that catalyzes the addition of telomeres.

90% of human cancer cells have telomerase; normal cells do not. Some anticancer drugs target telomerase.

IB Review

Quiz FRI!

11-4 Recap:• DNA replication ain’t perfect!DNA replication ain’t perfect!

• DNA can also be damaged or DNA can also be damaged or naturally alterednaturally altered

• Repair mechanisms exist that Repair mechanisms exist that detect and repair mismatched or detect and repair mismatched or damaged DNAdamaged DNA

11.4 How Are Errors in DNA Repaired?

DNA polymerases make mistakes

11.4 How Are Errors in DNA Repaired?

DNA polymerase

Figure 11.22 DNA Repair Mechanisms (A)

11.4 How Are Errors in DNA Repaired?

The newly replicated DNA is scanned for mistakes by other proteins.

Mismatch repair mechanism detects mismatched bases—the new strand has not yet been modified (e.g., methylated in prokaryotes) so it can be recognized.

If mismatch repair fails, the DNA is altered.

Figure 11.22 DNA Repair Mechanisms (B)

11.4 How Are Errors in DNA Repaired?

DNA can be damaged by radiation, toxic chemicals, and random spontaneous chemical reactions.

Excision repair: enzymes constantly scan DNA for mispaired bases, chemically modified bases, and extra bases—unpaired loops.

Figure 11.22 DNA Repair Mechanisms (C)

• IB Finished with Chapter 11!Quiz tomorrow (3.3, 3.4) & 7.1, 7.2

11-5 Recap:• Knowledge of the mechanisms of Knowledge of the mechanisms of

DNA replication led to DNA replication led to development of techniques for development of techniques for making multiple copies of DNA making multiple copies of DNA sequences of DNA molecules!sequences of DNA molecules!

11-5 Recap ?s:

• What do primers do in PCR?What do primers do in PCR?

• Why are dideoxyribonucleosides Why are dideoxyribonucleosides used in DNA sequencing?used in DNA sequencing?

11.5 What Are Some Applications of Our Knowledge of DNA Structure and Replication?

Copies of DNA sequences can be made by the polymerase chain reaction (PCR) technique.

PCR is a cyclical process:• DNA fragments are denatured by

heating.• A primer, plus nucleosides and DNA

polymerase are added.• New DNA strands are synthesized.

Figure 11.23 The Polymerase Chain Reaction

11.5 What Are Some Applications of Our Knowledge of DNA Structure and Replication?

PCR results in many copies of the DNA fragment—referred to as amplifying the sequence.

Primers are 15–20 bases, made in the laboratory. The base sequence at the 3′ end of the DNA fragment must be known.

11.5 What Are Some Applications of Our Knowledge of DNA Structure and Replication?

DNA polymerase that does not denature at high temperatures (90°C) was taken from a hot springs bacterium, Thermus aquaticus.

11.5 What Are Some Applications of Our Knowledge of DNA Structure and Replication?

DNA sequencing determines the base sequence of DNA molecules.

Relies on altered nucleosides with fluorescent tags that emit different colors of light.

DNA fragments are then denatured and separated by electrophoresis.

Figure 11.24 Sequencing DNA (Part 1)

Figure 11.24 Sequencing DNA (Part 2)

• Replication Activity

• Sequencing Activity