Chapter 5: The Structure of Large Biological Molecules

Overview of Biological Macromolecules

Biological macromolecules are large, complex molecules essential for life. They are typically polymers, made by joining smaller units called monomers through dehydration reactions. The four major classes are carbohydrates, lipids, proteins, and nucleic acids.

• Carbohydrates: Serve as energy sources and structural materials.

• Lipids: Function in energy storage, membrane structure, and signaling.

• Proteins: Perform a vast array of functions including catalysis, structure, transport, and regulation.

• Nucleic Acids: Store and transmit genetic information.

Table: Macromolecules, Monomers, Polymers, and Bonds

Nucleic Acids

Role of Nucleic Acids and the Central Dogma

Nucleic acids (DNA and RNA) are polymers of nucleotides that function in the storage, transmission, and expression of genetic information. The Central Dogma of molecular biology describes the flow of genetic information: DNA is transcribed into RNA, which is then translated into protein.

• DNA (Deoxyribonucleic acid): Stores genetic information in cells.

• RNA (Ribonucleic acid): Acts as a messenger and functional molecule in protein synthesis.

Central Dogma:

• DNA → RNA → Protein

Nucleotide Structure

A nucleotide is the monomer of nucleic acids, composed of three parts:

• Phosphate group

• Pentose sugar (deoxyribose in DNA, ribose in RNA)

• Nitrogenous base (Adenine, Thymine, Cytosine, Guanine in DNA; Uracil replaces Thymine in RNA)

Nucleotides are joined by phosphodiester bonds between the phosphate of one nucleotide and the sugar of the next, forming a sugar-phosphate backbone.

Differences Between DNA and RNA

• DNA: Double-stranded, contains deoxyribose, bases are A, T, C, G.

• RNA: Single-stranded, contains ribose, bases are A, U, C, G.

Base Pairing and Chargaff's Rules

DNA strands are held together by hydrogen bonds between complementary bases:

• Adenine (A) pairs with Thymine (T)

• Guanine (G) pairs with Cytosine (C)

Chargaff's Rules: In any DNA sample, the amount of A equals T, and the amount of G equals C.

DNA Structure and Discovery

• DNA is a double helix with antiparallel strands.

• Each turn of the helix contains about 10 base pairs (3.4 nm per turn).

• Watson and Crick, using Rosalind Franklin's X-ray diffraction data, proposed the double helix model in 1953.

Transcription and Translation

The process of gene expression involves two main steps:

1. Transcription: DNA is used as a template to synthesize messenger RNA (mRNA).

2. Translation: mRNA is decoded by ribosomes to assemble amino acids into a polypeptide (protein).

Example:

• DNA sequence: 5'-ACCGGTAATGGA-3'

• Complementary DNA strand: 3'-TGGCCATTACCT-5'

• mRNA transcript: 5'-ACCGGUAAUGGA-3' (U replaces T)

Chapter 6: A Tour of the Cell

Cell Size, Shape, and Common Features

Cells are the fundamental units of life. Despite their diversity, all cells share certain basic features:

• Plasma membrane: Encloses the cell, controlling the movement of substances in and out.

• Cytosol: Semifluid substance within the membrane.

• Chromosomes: Carry genetic information.

• Ribosomes: Synthesize proteins.

Prokaryotic vs. Eukaryotic Cells

• Prokaryotic cells: Lack a nucleus; DNA is located in a region called the nucleoid. They have a plasma membrane, cytoplasm, ribosomes, and often a cell wall.

• Eukaryotic cells: Have a true nucleus enclosed by a nuclear envelope and various membrane-bound organelles.

Additional info: Eukaryotic cells are typically larger and more complex than prokaryotic cells.