Genetics Ch. 11

• Eukaryotic Chromosome Structure

  Organelles with DNA
• Levels of DNA structure

  1. Primary structure: The nucleotide sequence

  2. Secondary structure: DNA double helix

  3. Tertiary structure: Higher order folding (packing)
• Supercoiled DNA

  • Overwound or underwound, causing it to twist on itself
  • Makes the structure more compact
  • Stabilized by proteins
  • Occurs in both prokaryotic and eukaryotic cells
  • Topoisomerases (enzymes) that add or remove rotations of DNA
  • Opens for transcription
  • Closes for packing
• Bacterial DNA

  • A circular chromosome
  • In the nucleoid region
  • highly folded into a series of loops with proteins attached
• Interphase DNA

  • Still packed (interphase)
  • DNA + (packing proteins) = chromatin
• Tertiary Structure of DNA (chromatin)

  • Nucleosome: Basic Unit of Chromatin
  • Nucleosome are linked with histones (core: H2A, H2B, H3, H4)- H1 locks everything in
• Histones

  • High in Lys and Arg (positively charged Amnio Acids)
  • The positively charged amino Acids attract the negatively charged sugar-phosphate backbone of DNA
  • Tails (acetylation or phosphorylation) can change chromatin structure to allow replication and transcription
• Positive Supercoiling (overrotated)

  helix twist on itself
• Negative Supercoiling (underrotated)

  the helix twists on itself in the opposite direction
• Types of Eukaryotic DNA

  Euchromatin and Heterochromatin
• Euchromatin

  • is the structure of the DNA where transcription occurs
  • Decondensed (30nm fiber); transparent
  • Located: chromosome arms- where the genes are that encode
  • Types of Sequences: Unique sequence
  • Presence of Genes: Many genes
  • When replicated: S Phase
  • Transcription: Often
  • Crossing Over: Common
• Heterochromatin

  • is more condensed
  • darkly stained
  • Location: at centromeres, telomers and other places where genetic erosion is prevented
  • Presence of Genes: Few genes
  • When Replicated: Late S phase
  • Transcription: Infrequent
  • Crossing Over- Uncommon
• Types of Heterochromatin

  Constitutive and Facultative
• Constitutive 


  centromeres and telomeres; always heterochromatin
• Facultative

  X inactivation; only heterochromatic at certain stages of development
• Epigenetic


  changes effect of chromatin structure; turning on and off the expression of a gene
• Compaction

  prevents transcription of DNA into RNA
• Active Euchromatin

  has nucleosome that can bock DNA transcription
• Chromatin Remodeling

  required for DNA transcription
• Telomeres

  • ends of chromosomes
  • prevents genetic erosion
  • usually consist of repeating A & T followed by several G (G's don't encode for product)
• Shelterin

  multiprotein complex; bind to telomers and protects the ends from repair
• T-Loops


  may form after the G rich 3' overhang to protect the end from degradation
• DNA Polymerase

  can't replicate the ends of chromosomes, so telomeres get shorter with every cell division
• Germs cells, steam cells have telomerase to length telomeres and prevent shortening
• Senescence

  What too short telomers lead to; is the cell will not do anything, having no function, so it will no longer replication; this can be good in some cases (ex. stopping cancer cells)