1 DNA: The Genetic Material Chapter 14. 2 Outline Genetic Material Experiments Chemical Nature of Nucleic Acids Three-Dimensional Structure of DNA – Watson.

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DNA: The Genetic Material

Chapter 14

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Outline

• Genetic Material Experiments• Chemical Nature of Nucleic Acids• Three-Dimensional Structure of DNA

– Watson and Crick• Replication

– Semi Conservative– Replication Process

• Eukaryotic DNA Replication• One-Gene/One-Polypeptide Hypothesis

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Hammerling Experiment

• Hammerling Experiment– Cells of green alga (Acetabularia) were cut

into pieces and observed to see which were able to express hereditary information.

Discovered hereditary information is stored in the cell’s nucleus.

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Transplantation Experiments

• Briggs and King (1952), and Steward (1958) conducted several experiments that conclusively determined each nucleus in a eukaryotic cell contains a full set of genetic instructions.

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Transplantation Experiments

• Several experiments were required to conclusively determine which substance made up genes.

– Griffith experiment documented movement of genes from

one organism to another (transformation)movement of material can alter the

genetic makeup of the recipient cell

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Avery and Hershey-Chase Experiments

• Avery experiment– removed almost all protein from bacteria,

and found no reduction in transforming activity

• Hershey-Chase– labeled DNA and protein with radioactive

isotope tracer determined hereditary information was

DNA, not protein

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Hershey - Chase

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Chemical Nature of Nucleic Acids

• DNA made up of nucleic acids– Each nucleotide is composed of a five

carbon sugar, a phosphate group, and an organic base.

nucleotides distinguished by the bases reaction between phosphate group of

one nucleotide and hydroxyl group of another is dehydration synthesis

phosphodiester bond

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Chemical Nature of Nucleic Acids

• Purines - large bases– adenine and guanine

• Pyrimidines - small bases– cytosine and thymine

Chargaff’s ruleA = T and G = C

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Nucleotides

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Three-Dimensional Structure of DNA

• X-ray diffraction suggested DNA had helical shape with a 2 nanometer diameter.

– Watson and Crick deduced DNA is an inter-twined double helix.

complementary base-pairingpurines pairing with pyrimidines

constant 2 nanometer diameter antiparallel configuration

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DNA Double Helix

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Semi-Conservative Replication

• Each chain in the helix is a complimentary mirror image of the other.

– double helix unzips and undergoes semi-conservative replication

each strand original duplex becomes one strand of another duplex

confirmed by Meselson-Stahl experiment

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Meselson - Stahl

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Replication Process

• Replication of DNA begins at one or more sites (replication origin).

– DNA polymerase III and other enzymes add nucleotides to the growing complementary DNA strands.

require a primer can only synthesize in one direction

endonucleasesexonucleases

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DNA Replication

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Replication Process

• DNA polymerase cannot link the first nucleotides in a newly synthesized strand.

– RNA polymerase (primase) constructs an RNA primer.

• DNA polymerase adds nucleotides to 3’ end.– Leading strand replicates toward replication

fork.– Lagging strand elongates from replication

fork. Okazaki fragments

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DNA Synthesis

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Replication Process

• DNA ligase attaches fragment to lagging strand.

– Because synthesis of the leading strand is continuous and the lagging strand is discontinuous, the overall replication of DNA is referred to as semi-discontinuous.

• DNA gyrase removes torsional strain introduced by opening double helix.

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Replication Process

• Opening DNA double helix– initiating replication– unwinding duplex– stabilizing single strands– relieving torque

• Building a primer• Assembling complementary strands• Removing the primer• Joining Okazaki fragments

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DNA Replication Fork

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Replisome

• Replisome is a macromolecular protein machine (replication organelle).

– fast, accurate replication of DNA during cell division

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Stages of Replication

• Initiation– always occurs at the same site

• Elongation– majority of replication spent in elongation

• Termination– exact details unclear

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Eukaryotic DNA Replication

• Eukaryotes usually have multiple, large chromosomes.

– multiple origins of replication

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One-Gene/One-Polypeptide Hypothesis

• Genes produce their effects by specifying the structure of enzymes.

– Each gene encodes the structure of one enzyme (Beadle and Tatum).

Many enzymes contain multiple polypeptide subunits, each encoded by a separate gene.

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One-Gene / One-Polypeptide

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Summary

• Genetic Material Experiments• Chemical Nature of Nucleic Acids• Three-Dimensional Structure of DNA

– Watson and Crick• Replication

– Semi Conservative– Replication Process

• Eukaryotic DNA Replication• One-Gene/One-Polypeptide Hypothesis

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