Unit 3: Lipids and Nucleic Acids
Table of Contents
1. Classification and Properties of Fatty Acids
Lipids are a diverse group of organic compounds that are insoluble in water but soluble in organic solvents. Fatty acids are the simplest lipids, consisting of a hydrocarbon chain with a terminal carboxyl group.
Saturated vs. Unsaturated Fatty Acids
- Saturated Fatty Acids: Contain no double bonds between carbon atoms in the chain. They are generally solid at room temperature (e.g., Palmitic acid).
- Unsaturated Fatty Acids: Contain one or more double bonds. These double bonds create "kinks" in the chain, usually making them liquid at room temperature (e.g., Oleic acid).
- Essential Fatty Acids: These are fatty acids that the body cannot synthesize and must be obtained from the diet, such as Linoleic acid.
2. Phospholipids, Glycolipids, and Steroids
Complex lipids serve as structural components of cell membranes and signaling molecules.
- Phospholipids: Consist of two fatty acids, a glycerol backbone, and a phosphate group. They are amphipathic, meaning they have a hydrophilic head and hydrophobic tails, forming the basis of the lipid bilayer.
- Glycolipids: Lipids with a carbohydrate attached by a glycosidic bond. They are crucial for cell recognition and stability of the cell membrane.
- Steroids: Characterized by a carbon skeleton with four fused rings. Cholesterol is a key steroid in animal membranes, while plants contain phytosterols.
3. Nucleic Acids: Nucleosides and Nucleotides
Nucleic acids (DNA and RNA) are polymers of nucleotides that store and transmit genetic information.
Components
- Pentose Sugar: Ribose in RNA and Deoxyribose in DNA.
- Nitrogenous Bases:
- Purines: Adenine (A) and Guanine (G).
- Pyrimidines: Cytosine (C), Thymine (T - in DNA only), and Uracil (U - in RNA only).
- Phosphate Group: Attached to the 5' carbon of the sugar.
Nucleoside = Nitrogenous Base + Sugar
Nucleotide = Nucleoside + Phosphate group
4. Double Helical Model of DNA
Proposed by Watson and Crick in 1953, the DNA structure is described as a double helix.
Salient Features
- Two Polynucleotide Chains: The chains run anti-parallel (one 5' to 3', the other 3' to 5').
- Sugar-Phosphate Backbone: Located on the outside, with bases projecting inward.
- Base Pairing: Adenine pairs with Thymine (2 hydrogen bonds), and Guanine pairs with Cytosine (3 hydrogen bonds).
- Helical Dimensions: One full turn is 3.4 nm long and contains 10 base pairs.
5. Physical Properties and Types of DNA
Physical and Chemical Properties
- Denaturation (Melting): Separation of the two strands due to high temperature or extreme pH.
- Renaturation (Annealing): Re-association of separated strands when conditions return to normal.
- UV Absorption: DNA absorbs UV light strongly at 260 nm.
Types of DNA
| DNA Form | Helix Direction | Base Pairs per Turn | Occurrence/Notes |
|---|---|---|---|
| B-DNA | Right-handed | 10 | The standard form found in living cells. |
| A-DNA | Right-handed | 11 | Found in dehydrated conditions. |
| Z-DNA | Left-handed | 12 | Has a "zig-zag" backbone; often found in GC-rich regions. |
6. Exam Focus: Comparison Tables & Tips
Common Mistakes
- Mistake: Forgetting that Uracil is only in RNA and Thymine is only in DNA.
- Mistake: Thinking that steroids are composed of fatty acids. Correction: Steroids are non-saponifiable lipids and do not contain fatty acids.
Frequently Asked Questions
Q: What is the biological significance of the 3 hydrogen bonds between G and C?
A: GC pairs are more stable and harder to separate than AT pairs (which have only 2 bonds). DNA with high GC content has a higher melting temperature (Tm).
Q: Why is DNA the preferred genetic material over RNA?
A: The absence of the 2'-OH group in deoxyribose makes DNA chemically more stable and less prone to hydrolysis than RNA.