6.1 - DNA and RNA Structure

Functions of DNA

1. Primary Source of Heritable Information:

   • DNA stores genetic information and passes it from one generation to the next.

2. Protein Synthesis:

   • DNA contains instructions for making proteins through the central dogma of molecular biology: DNA -> RNA -> Protein.

Prokaryotic vs. Eukaryotic DNA

• Eukaryotic Cells:

  • DNA is found in the nucleus.

  • DNA is organized into linear chromosomes.

• Prokaryotic Cells:

  • DNA is located in the nucleoid region.

  • DNA is typically circular.

  • Plasmids: Prokaryotes can also contain plasmids, which are small, circular DNA molecules. Plasmids replicate independently of chromosomal DNA and often carry genes beneficial for survival under specific conditions.

The discovery of DNA's structure was a collaborative effort that involved key contributions from several scientists. Chargaff’s ratios, Franklin’s X-ray diffraction images, and Watson and Crick’s modeling all played crucial roles. Understanding the double-helix structure of DNA has been fundamental to the study of genetics and molecular biology, as it explains how genetic information is stored, replicated, and used to make proteins.

Plasmids and Genetic Exchange in Bacteria

• Gene Exchange: Bacteria can exchange genes found on plasmids with neighboring bacteria through a process called horizontal gene transfer. This exchange allows bacteria to acquire new traits, such as antibiotic resistance, enhancing their survival.

• Expression of Acquired Genes: Once the DNA is exchanged, bacteria can express the newly acquired genes, producing the corresponding proteins.

• Genetic Engineering: Scientists can insert genes of interest (e.g., the human insulin gene) into plasmids to create recombinant plasmid DNA. Restriction enzymes are used to cut and insert specific DNA fragments, enabling the production of proteins like insulin in bacterial cells.

Differences Between DNA and RNA

• DNA (Deoxyribonucleic Acid):

  • Double-stranded

  • Base pairs: Adenine (A) pairs with Thymine (T), and Cytosine (C) pairs with Guanine (G).

  • Sugar: Deoxyribose

• RNA (Ribonucleic Acid):

  • Single-stranded

  • Base pairs: Adenine (A) pairs with Uracil (U), and Cytosine (C) pairs with Guanine (G).

  • Sugar: Ribose

Key Discoveries and Principles

• Chargaff’s Rule: In any species, the amount of adenine (A) equals the amount of thymine (T), and the amount of cytosine (C) equals the amount of guanine (G). This pairing is consistent across all species.

  • Purines: Adenine (A) and Guanine (G)

  • Pyrimidines: Cytosine (C), Thymine (T) in DNA, and Uracil (U) in RNA

• Bonding in DNA:

  • Hydrogen Bonds: Hold together the base pairs in the DNA double helix.

  • Phosphodiester Bonds: Connect the sugar-phosphate backbone.

Contributions of Key Scientists

• Rosalind Franklin: Used X-ray diffraction to show the helical shape of DNA, known as Photo 51.

• James Watson and Francis Crick: Built the first accurate 3D model of the DNA double helix, using Franklin's data and Chargaff’s rules.