32. Biochemistry: Lipids, Nucleic Acids & DNA

OCW Scholar

Session Overview

Modules	Organic Materials
Concepts	denaturing proteins, lipids and their self assembly into bilayers, nucleic acids, DNA, and encoding information for protein synthesis, history of the discovery of DNA double helix structure
Keywords	denaturing, disulfide bond, triglyceride, zwitterion, aliphatic compound, cell wall, nucleotide, amphipathic, amine, base pairs, codon, Laue pattern, alpha helix, Oswald Avery, Erwin Chargaff, Rosalind Franklin, Francis Crick, James Watson, Maurice Wilkins, x-ray crystallography, biological replication
Chemical Substances	fats, oils, cholesterols, hormones, phospholipid, phosphatide, phosphatidylethanolamine, ribose, 2-deoxyribose, DNA, RNA, guanine, cytosine, adenine, thymine
Applications	cooking food, pickling food, hair styling, laundry detergent

Prerequisites

Before starting this session, you should be familiar with:

Prior sessions in this Organic Materials module (starting with Session 27), and particularly the fundamentals of protein structure as described in Session 31:
- Composition (primary structure of amino acid chains)
- Packing (secondary structures of alpha helix, beta pleated sheet, and random coil),
- Conformation (tertiary structure or folding caused by R-group interactions)
X-ray crystallography, as introduced in Session 18: X-Ray Diffraction Techniques

Learning Objectives

After completing this session, you should be able to:

Describe several methods of denaturing proteins and provide practical examples of each method.
List the characteristics and properties of lipids.
List the characteristics and properties of nucleic acids.
Draw the chemical structure of DNA, and indicate the role of hydrogen bonding in forming the structure.
Describe how the chemical structure of DNA carries information and supports replication.

Reading

Book Chapters	Topics
[Saylor] 24.6, "The Molecules of Life."	Overview of lipids and nucleic acids
[PB-EOC] 20-1 through 20-5 in Chapter 20, "Lipids."	Fatty acids; waxes; fats and oils; soaps, detergents, and micelles; phospholipids
[PB-EOC] 21-2, 21-5, and 21-8 in Chapter 21, "Nucleosides, Nucleotides, and Nucleic Acids."	Overview of nucleic acids; DNA and heredity; biosynthesis of proteins via translation

Lecture Video

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Resources

This resource may not render correctly in a screen reader. Lecture Slides (PDF - 4.1MB)

Lecture Summary

After a quick review of the previous session, Prof. Sadoway begins with a discussion of denaturing proteins – disrupting protein secondary and tertiary structures. Examples of the denaturing process include:

Changing temperature (frying an egg)
Changing pH (pickling food)
Introducing oxidizing/reducing agents to rearrange disulfide bonds (hair styling)
Introducing detergents, which destabilize hydrophobic pockets

Next, the lecture introduces lipids.

Lipids are defined by their properties, not by their chemical composition; they are soluble in solvents of low polarity.
Examples include fats, oils, cholesterols, and hormones. Some lipids have a hydrophilic head and hydrophobic tail; arranged in two layers, tail to tail, they produce a lipid bilayer capable of self-assembly.
The lipid bilayer is the basis of cell walls. It includes integral proteins that change conformation in response to certain conditions, creating dynamic physical responses in the cell wall – once again, chemistry as the animating force of life.

After lipids, the lecture proceeds with the structure of nucleic acids.

Nucleic acids carry information that directs metabolic activity, including replication.
They are macromolecules whose structural unit is the nucleotide, consisting of three building blocks: a sugar (ribose or 2-deoxyribose), a phosphate acting as a bridge and a 'spacer,' and an amine group (one from the set AGCU for RNA, and AGCT for DNA).
The amine groups combine in pairs (A-T or C-G), and intermolecular spacing and hydrogen bonds lead to the double helix structure.
The order of A-T and C-G pairs ("letters") in chains creates "word" sequences of the genetic code.

This lecture ends with a history of the discovery of DNA's double helix structure: early insights by Oswald, Chargaff's rule, Franklin's x-ray crystallography experiment and its subsequent controversial use by Watson and Crick.

Homework

Problems (PDF)

Solutions (PDF)

For Further Study

Supplemental Readings

Watson, J. D., and F. H. C. Crick. "Molecular Structure of Nucleic Acids." Nature 171 (1953): 737-738. ( This resource may not render correctly in a screen reader. PDF)

Franklin, R., and R. G. Gosling. "Molecular Configuration in Sodium Thymonucleate." Nature 171 (1953): 740-741. ( This resource may not render correctly in a screen reader. PDF)

Judson, Horace Freeland. The Eighth Day of Creation: Makers of the Revolution in Biology. 25th anniversary edition. Woodbury, NY: Cold Spring Harbor Laboratory Press, 1996. ISBN: 9780879694784.

Maddox, Brenda. Rosalind Franklin: The Dark Lady of DNA. New York, NY: HarperCollins, 2002. ISBN: 9780060184070.

Watson, James D. The Double Helix: A Personal Account of the Discovery of the Structure of DNA. New York, NY: Touchstone/Simon & Schuster, 2001 (original publication 1968). ISBN: 9780743216302.

People

Oswald Avery

Erwin Chargaff

Francis Crick, James Watson, Maurice Wilkins — 1962 Nobel Prize in Physiology or Medicine

Rosalind Franklin

Culture

"It's a Chemical Reaction, That's All." Music and lyrics by Cole Porter. From Silk Stockings. Fred Astaire, Cyd Charisse; directed by Rouben Mamoulian and Roy Mack. Original film release MGM, 1957. DVD, Warner Home Video, 2003.

Hank Ballard and the Midnighters. "The Twist." (B-side with "Teardrops on Your Letter.") Original release: King Records, 1959. Available on many reissue compilations, such as Their Very Best: Hank Ballard & The Midnighters. K-Tel, 2007.