Lecture+1_handout

Discovery of DNA

• 1860s: Miescher discovers DNA in nuclei, initially termed nuclein, which is now known as nucleic acid.

• 1920s: Levene determines that DNA is a long chain made up of subunits called nucleotides.

  • Nucleotide Composition: Each nucleotide consists of a sugar, a base (which can be either a purine—A or G—or a pyrimidine—C or T).

Chargaff's Rules and DNA Structure

• 1950s: Chargaff demonstrates that the amounts of Adenine (A) equals Thymine (T) and Guanine (G) equals Cytosine (C).

• 1950s: Franklin and Wilkins utilize X-ray diffraction to show that DNA is a helix.

Watson and Crick Model

• 1953: Watson and Crick propose the Double Helix structure of DNA.

  • Nucleotide Structure: Nucleotides consist of a nucleoside (sugar + base) and a phosphate group.

Nucleotide Structure

• Deoxyribonucleotides: Comprised of multiple carbon atoms, denoted as:

  • 5’ to 3’ polarity that arises from the arrangement of phosphodiester bonds.

• Nucleotides are linked together by phosphodiester bonds, creating a phosphodiester backbone that has a directionality from 5’ to 3’.

DNA Double Helix Characteristics

• The double helix structure is formed by two strands wrapping around each other:

  • Backbone: Phosphodiester backbone located outside.

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

• The two strands are antiparallel, with orientation of 5’ to 3’ on one strand and 3’ to 5’ on the complementary strand.

Stability of DNA Structure

• The stability of double-stranded DNA is maintained by:

  • Base stacking forces between adjacent bases.

  • Hydrogen bonds between complementary base pairs (A-T and G-C), providing specificity in pairing.

Hydrogen Bond Formation

• A hydrogen bond forms when a hydrogen atom covalently bonded to a highly electronegative atom (e.g., oxygen or nitrogen) is attracted to another electronegative atom.

DNA Hybridization and Packaging

• Hybridization: The process of combining complementary strands of nucleic acids, forming hybrid molecules like those seen in Fluorescence In Situ Hybridization (FISH).

• DNA Packaging in Cells:

  • Bacteria: Contain a single, circular chromosome.

  • Eukaryotes: Have multiple linear chromosomes.

Chromatin Structure

• The fundamental unit of chromatin is the nucleosome, comprising both DNA and histone proteins, facilitating the organization of DNA.

• Chromatin can be visualized through electron microscopy, showing the compact structure of DNA.

Chromosomes

• Chromosomes are condensed structures formed from chromatin, characterized by:

  • Looped domains and the 30-nm fiber structure.

Key Terms Summary

• Nucleotide: Subunit of DNA.

• Nucleoside: Sugar + Base.

• Antiparallel: Orientation of DNA strands.

• Phosphodiester bond: Link between nucleotides.

• Hydrogen bonds: Bonds between base pairs.

• Base stacking force: Stabilizing interaction between bases.

• Base pairing rules: A pairs with T; G pairs with C.

• Complementary: Matching nucleotides on opposite strands.

• Hybridization: Combining nucleic acid strands.

• Chromatin and chromosome: DNA packaging forms.

• Plasmid: Circular DNA in bacteria.

• Nucleosome: Structure of DNA and histone complex.

• Histones: Proteins that package DNA.

• DNA Structure Summary: Nucleoside/nucleotide, polarity, and double helix.

• DNA Packaging: Includes descriptions of bacterial/Eukaryotic packaging mechanisms, emphasizing nucleosome formation and 30-nm fiber organization.