Lec 1- Molecular Biology 1- 1st Year PT

Course Introduction

Dr. Suheila Mohamed introduces the first lecture of Biochemistry 1, which covers molecular biology and nucleotide chemistry. She encourages students to study biochemistry regularly rather than letting lectures accumulate, and points them to Med Dose Academy’s Telegram groups for free course explanations.

1. Nucleotides and Nucleosides

• Nucleotides are the basic building blocks of DNA and RNA. Each nucleotide contains:
  1. A nitrogenous base
  2. A five-carbon sugar (pentose)
  3. One or more phosphate groups
• A nucleoside contains only a nitrogenous base and a sugar. Adding phosphate group(s) to a nucleoside makes a nucleotide.
• Hydrolysis can split a nucleotide into a nucleoside and phosphate. Further breakdown of the nucleoside separates the base and sugar.

2. Sugars and Nitrogenous Bases

The pentose sugar is:

• Ribose in RNA
• Deoxyribose in DNA. Compared with ribose, deoxyribose lacks an oxygen-containing hydroxyl group at carbon 2′. This difference contributes to DNA’s greater stability.

Nitrogenous bases are grouped by ring structure:

• Purines: adenine (A) and guanine (G), each with two fused rings
• Pyrimidines: cytosine (C), uracil (U), and thymine (T), each with one ring

Adenine, guanine, and cytosine occur in both DNA and RNA. Thymine is characteristic of DNA, while uracil is characteristic of RNA. The lecture notes that thymine may occur as a minor modified base in some tRNA.

3. Naming Nucleosides and Nucleotides

Nucleoside names are derived from the base:

• Purines generally take the -osine ending: adenine → adenosine; guanine → guanosine
• Pyrimidines generally take the -idine ending: cytosine → cytidine; uracil → uridine; thymine → thymidine

Phosphate groups are indicated as mono-, di-, or triphosphate, depending on whether there is one, two, or three. A d- prefix indicates a deoxyribose-containing nucleotide, as in dAMP. The lecturer stresses learning common abbreviations because they may appear in exam questions.

4. Other Roles of Nucleotides

Nucleotides and related molecules have important cellular functions beyond forming DNA and RNA:

• Energy transfer: ATP (adenosine triphosphate) is a major cellular energy source. It supports processes such as muscle activity, nerve signaling, membrane transport, and biosynthesis. Breaking high-energy bonds between ATP’s phosphate groups releases energy, converting ATP to ADP or AMP.
• Cell signaling: Cyclic AMP (cAMP) and cyclic GMP (cGMP) act as second messengers, relaying signals inside cells, including responses to hormones.
• Chemical-group transfer:
  • SAM (S-adenosylmethionine) is an important methyl-group donor.
  • PAPS is an activated sulfate donor.
• Coenzyme components: Nucleotide-containing coenzymes include NAD, NADP, FMN, and FAD. They participate in metabolic reactions, including oxidation–reduction and biosynthetic processes.

5. Nucleic-Acid Structure and DNA

DNA and RNA are polymers: long chains made from repeating nucleotide units. DNA stores genetic instructions, while RNA helps carry and interpret those instructions in protein production.

DNA is usually described as a double helix made of two strands:

• Each strand has a sugar–phosphate backbone on the outside, with the bases directed inward.
• Adjacent nucleotides in a strand are linked by phosphodiester bonds, joining the 3′ hydroxyl of one sugar to the 5′ phosphate of the next nucleotide.
• The two strands run in opposite directions, or antiparallel: one runs 5′→3′ and the other 3′→5′. New DNA is synthesized in the 5′→3′ direction.
• Complementary base pairing holds the strands together through hydrogen bonds: A pairs with T using two hydrogen bonds, and G pairs with C using three. The sequence of bases encodes genetic information.

6. Mitochondrial DNA

Mitochondrial DNA is described as a small, circular, double-stranded molecule, distinct from the predominantly linear DNA in the nucleus. The lecture gives it as roughly 0.3–1% of a cell’s total DNA.

Human mitochondrial DNA encodes 13 proteins, 22 tRNAs, and 2 rRNAs. Many of its proteins are involved in oxidative phosphorylation, the process used to produce cellular energy.

Mitochondrial DNA is associated with maternal inheritance. Mutations can contribute to muscle and neurological disorders; the lecture highlights myopathies and connects them to the high energy demands of muscle and nerve tissue.

Speakers and Sources

• Dr. Suheila Mohamed — lecturer; identified as being from Med Dose Academy
• Med Dose Academy — educational source and course provider mentioned in the lecture