DNA Structure and Organization in the Cell Notes

DNA Structure and Organization in the Cell

  • DNA Double Helix

    • Comprised of two polynucleotide strands.

    • Each nucleotide consists of:

    • Deoxyribose sugar

    • Phosphate group

    • Nitrogenous base (A, T, G, C)

  • Nitrogenous Bases

    • Purines: Double-ring compounds (Adenine, Guanine)

    • Pyrimidines: Single-ring compounds (Cytosine, Thymine (DNA only), Uracil (RNA only))

  • Structure Representation

    • The DNA helix resembles a ladder:

    • “Rungs” = nitrogenous bases

    • “Sides” = phosphate and sugar

    • Types of bonds present in DNA:

    • Glycosyl bonds

    • Phosphodiester bonds

    • Hydrogen bonds

  • Complementary Base Pairing

    • A purine bonds with a pyrimidine:

    • A -- T

    • G -- C

    • Maintains dimensions of the helix.

    • Chargaff’s Rule:

    • % Adenine = % Thymine

    • % Cytosine = % Guanine

    • Antiparallel Structure: Strands run in opposite directions; one 5' to 3' and the other 3' to 5'.

  • From DNA to Chromosomes

    • Chromosome formation and structure:

    • Chromatid, telomere, centromere

    • Involves proteins like histones

  • DNA Organization

    • Genome: Complete genetic make-up of an organism.

    • Gene: Basic unit of heredity, coding sequences for proteins.

    • Nucleoid: Structure in bacteria housing chromosomal DNA

    • Regulatory Sequences: DNA sequences that regulate gene activity.

  • Prokaryotic DNA

    • Found in circular chromosomes, no nucleus, contained within the nucleoid.

    • DNA is supercoiled with protein assistance, regulated by topoisomerase I and II (gyrase).

    • Antibiotics may target topoisomerase II.

  • Eukaryotic DNA

    • Found in much greater quantities than in prokaryotes and located in the nucleus.

    • Humans have about 3 million base pairs of DNA coiled tightly within cell nuclei (~4 μm diameter).

  • DNA as Genetic Material

    • Must replicate accurately and code for protein synthesis.

    • Essential for reproduction and cellular function.

  • The Cell Cycle and Mitosis

    • Cells undergo mitosis to divide into two identical daughter cells, each containing the same DNA as the parent cell.

  • DNA Replication:

    • Research by Matthew Meselson and Franklin Stahl identified potential replication methods:

    1. Conservative replication

    2. Semi-conservative replication

    3. Dispersive replication

    • The Semiconservative model claims each replicated DNA consists of one original and one new strand.