Unit 4: Nucleic Acids

1. Salient Features and Chemical Composition of DNA and RNA

Nucleic acids are macromolecules that store and transmit genetic information. They are composed of repeating units called nucleotides.

Every nucleotide consists of three essential components:

Pentose Sugar: Deoxyribose in DNA and Ribose in RNA.
Phosphate Group: Provides the acidic nature and negative charge to the nucleic acid.
Nitrogenous Bases:
- Purines: Adenine (A) and Guanine (G).
- Pyrimidines: Cytosine (C), Thymine (T) in DNA, and Uracil (U) in RNA.

In 1953, James Watson and Francis Crick proposed the double-helix model of DNA. This discovery revolutionized molecular biology.

Double Stranded: DNA consists of two polynucleotide chains wound around each other in a right-handed spiral.
Antiparallel Polarity: One strand runs in the 5' to 3' direction, while the other runs in the 3' to 5' direction.
Complementary Base Pairing: Adenine (A) always pairs with Thymine (T) via two hydrogen bonds, and Guanine (G) always pairs with Cytosine (C) via three hydrogen bonds.
Sugar-Phosphate Backbone: The "rails" of the DNA ladder are made of alternating sugar and phosphate groups.
Dimensions: The diameter of the helix is 20Å (2 nm), and one complete turn covers 34Å (3.4 nm), containing roughly 10 base pairs.

DNA can exist in different structural forms depending on hydration and salt concentration.

B-DNA: The most common form found under physiological conditions (right-handed).
A-DNA: A wider and shorter right-handed helix, often found in dehydrated conditions.
Z-DNA: A left-handed helix with a "zigzag" backbone, often involved in gene regulation.

Genetic Reservoir: Stores the complete set of instructions for building and operating an organism.
Replication: Ensures that genetic information is accurately passed from parents to offspring during cell division.
Protein Synthesis: Provides the template for mRNA synthesis (transcription).

RNA is typically single-stranded and shorter than DNA. It contains the sugar ribose and the base uracil instead of thymine.

Type	Structure	Function
mRNA (Messenger)	Linear, single-stranded	Carries genetic code from DNA to ribosomes.
tRNA (Transfer)	Clover-leaf shape (2D)	Transports specific amino acids to ribosomes during translation.
rRNA (Ribosomal)	Complex, folded	Forms the structural framework of ribosomes.

[Image of mRNA, tRNA, and rRNA structures]

Hydrogen Bonds: Always remember that G-C pairs are stronger than A-T pairs because they have 3 hydrogen bonds instead of 2.
Thymine vs. Uracil: In exam questions, if you see Uracil, the molecule is RNA. If you see Thymine, it is DNA.
Chargaff's Rule: For DNA, [A] = [T] and [G] = [C]. The sum of purines equals the sum of pyrimidines.

Confusing the sugar: Ribose is for RNA, Deoxyribose is for DNA.
Writing that RNA is always linear. tRNA has a highly specialized clover-leaf folding.