Genetics Chapter 12 DNA Organization in Chromosomes Summary

12.1 I Viral and Bacterial Chromosomes

• DNA as genetic material organized into genes and chromosomes.

• -Bacterial and viral chromosomes:

  • Single nucleic acid molecule, small, less genetic info than eukaryotes.

• Viral chromosomes:

  • DNA/RNA, ss/ds, linear/circular, inert until in host cell.

• Bacterial chromosomes:

  • Circular, double-stranded DNA, compacted into nucleoid, associated with proteins (HU and H-NS).

12.2 I Supercoiling

• Supercoiling aids DNA compaction, particularly in closed-circular molecules.

• Linkage Number (L) determines supercoiling state:

  • Linear DNA: 20 turns, Relaxed Circular: L = (+20), Underwound Circular: L = (+18).

12.3 I Polytene Chromosomes

• Present in various tissues, large enough for microscopic viewing.

• DNA undergoes replication without strand separation or cytoplasmic division, leading to 1000-5000 DNA strands.

• Puff regions indicate high-level gene activity (transcription).

12.4 I Chromatin

• Chromatin is uncoiled eukaryotic DNA during interphase; condenses during cell division.

• Histones: positively charged proteins aiding in DNA packaging; five types exist (H1, H2A, H2B, H3, H4).

• Chromatin remodeling is essential for DNA-protein interactions, facilitating replication and gene expression.

Chemical Modifications

• Key modifications include:

  • Acetylation (neutralizes charge, facilitates transcription),

  • Methylation (positive correlation with gene activity),

  • Phosphorylation (adds phosphates to serine/histidine).

Types of Chromatin

• Euchromatin: active, uncoiled, unstained during interphase.

• Heterochromatin: condensed, inactive, stained, replicates later in S phase. Ex: Telomeres (chromosome integrity), Centromeres (chromosome movement).