Unit 3: Lipids and Nucleic Acids
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.
[Image of saturated vs unsaturated fatty acid structure]
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
6. Exam Focus: Comparison Tables & Tips
Exam Tip: Remember Chargaff's Rule: In any double-stranded DNA, the amount of Adenine equals Thymine (A=T) and Guanine equals Cytosine (G=C). This is a frequent numerical question topic!
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.