Molecular Biology: DNA Structure

• Molecular biology is the study of the formation, structure, and function of macromolecules essential to life.
• Focus is given to nucleic acids and proteins, especially their roles in cell replication and transmitting genetic information.

Discovery of DNA Structure

• James Watson and Francis Crick discovered the double helical structure of DNA in 1953.
• Rosalind Franklin (X-ray diffraction) and Erwin Chargaff (chemical composition) provided the scientific framework for the discovery of DNA structure.
• Rosalind Franklin used X-ray diffraction to study wet fibers of DNA.
• The diffraction pattern suggested structural features of DNA such as a being helical, having more than one strand, and containing 10 base pairs per complete turn.
• Erwin Chargaff pioneered biochemical techniques for isolating, purifying, and measuring nucleic acids from living cells.
• Chargaff's rule says the percent of adenine (A) equals the percent of thymine (T), and the percent of cytosine (C) equals the percent of guanine (G).
• As a result A+G = T+C, meaning purines equal pyrimidines.

Terminology

• Genome: all the genetic material in a cell.
• Gene: a segment of DNA that encodes a functional product, usually a protein.
• Genotype: the genes of an organism.
• Phenotype: the expression of the genes.

The Genome

• The genome contains 23 chapters or chromosomes.
• Each chapter contains 48 to 250 million letters (A, C, G, T).
• The genome contains over 3.2 billion letters in total.
• Most cells contain at least one copy of all 23 chapters (chromosomes).
• Mature red blood cells in humans are an exception, they lack a genome because they become enucleated during development.

Chromosomes

• Chromosomes are large subcellular structures found in the nuclei of most eukaryotic cells.
• Each chromosome consists of a single very long DNA molecule.
• Humans have 22 pairs of numbered chromosomes (autosomes) and one pair of sex chromosomes (XX or XY), for a total of 46.
• Each pair of chromosomes contains one chromosome from each parent, and children therefore inherit half of their chromosomes from their mother and half from their father.

Nucleotide

• A nucleotide consists of a nitrogen-containing base (adenine, guanine, thymine, or cytosine in DNA; adenine, guanine, uracil, or cytosine in RNA), a phosphate group, and a sugar (deoxyribose in DNA; ribose in RNA).
• Chromosomes have proteins called histones that bind to DNA.
• DNA has two strands that twist into the shape of a spiral ladder called a helix.
• DNA contains four building blocks called nucleotides: adenine (A), thymine (T), guanine (G), and cytosine (C).
• The nucleotides attach to each other (A with T, and G with C) to form chemical bonds called base pairs, which connect the two DNA strands.
• Genes are short pieces of DNA that carry specific genetic information.