DNA Structure

• What is the role of DNA?

  Duplication and Controlled Development of the cell
• What are the structures of DNA?

  Primary,Secondary,and Tertiary
• Primary Structure

  Sequence of nucleotides
• Secondary Structure

  Double Helix
• Tertiary structure


  Chromosomes
• Parts of a nucleotide

  Sugar,Phosphate, and Nitrogen Base
• Ribose


  -OH group at C'
• Deoxyribose 


  Has -H group at C'
• Adenine and Thymine

  2 hydrogen bonds
• Guanine and Cytosine


  3 hydrogen bonds
• Which nucleotide pair is stronger?

  Guanine and Cytosine
• DNA methylation

  The addition of methyl groups to bases of DNA after DNA synthesis; may serve as a long-term control of gene expression
• How is the double helix held together?

  Two antiparallel polynucleotide strands held together by H bases between complementary bases
• B-DNA

  Alpha Helix (Right handed spiral)

  Exists in the presence of water and normal base pairing
• A-DNA

  Forms when little water is present
• Z-DNA

  Forms when there are particular base sequence
• Hairpin

  A structure of RNA when it folds back on itself and forms intramolecular hydrogen bonds between complementary nucleotides.
• Stem

  Nucleotides fold on itself
• Palindrome

  Inverted repeat that is complementary to itself
• DNA supercoiling


  The formation of additional coils in DNA due to twisting forces. This occurs in the tertiary structure.
• Underwound

  Negative supercoiling
• Overwound

  Positive supercoiling
• Topoisomerases

  Induce supercoiling by adding or removing rotations
• Type 1


  Break 1 strand to remove rotations
• Type 2

  Break both strands to add or remove rotations
• Euchromatin

  Condenses and decondenses where transcription occurs
• Heterochromatin

  Stays condensed, not much transcription
• Histones

  Postively charged and there are 5 major types
• Chromosomal Scaffolding


  Help fold and pack DNA
• High mobility group proteins

  Replication and transcription machinery
• Chromatin Structure

  Nucleosome, Chromatosome, and Linker DNA
• Nucleosome: a structural unit of a eukaryotic chromosome, consisting of a length of DNA coiled around a core of histones.


  DNA wrapped 2x around histone
• Chromatosome

  Histone H1 locks DNA onto nucleosome octamer
• Linker DNA

  Chromatosomes are seperated by 30bp. Looks like beads on a string
• 30 nm fiber

  Nucleosomes fold on themselves

  Dense

  Tightly packed structure
• 300 nm fiber

  Contain 20-100kb

  Packed and folded to produce 250nm fiber
• 250 nm fiber


  Helically coiled to produce chromatids
• Can chromosomes form without nucleosomes?

  Yes, but condensins are responsible for structure. Chromosome interactions during interphase are not random.
• Topologically Associated Domains

  Large regions of chromatin that interact with each other

  May be on same chromosome or different chromosomes
• Chromosomal Puffs

  Relaxed chromatin and sites of transcription