Notes on Chromosome Structure and DNA Sequence Organization

Chapter 11: Chromosome Structure and DNA Sequence Organization

11.1 Viral and Bacterial Chromosomes Are Relatively Simple DNA Molecules

• Viral and Bacterial Chromosomes

  • Comprise single nucleic acid molecules.
  • Largely lack associated proteins, resulting in smaller sizes compared to eukaryotic chromosomes.
  • Contain less genetic information than eukaryotic forms.

• Viral Chromosomes

  • Can be comprised of either DNA or RNA and can be single-stranded or double-stranded.
  • Can be circular (e.g., bacteriophage lambda) or linear molecules; e.g., $ext{bacteriophage } ext{ is ds DNA}$.
  • Circularity is not mandatory for replication, as seen in some viruses.

• Table 11.1: Genetic Material of Representative Viruses and Bacteria

  • Lists various organisms, types of nucleic acid, whether it is single-stranded (SS) or double-stranded (DS), and lengths in micrometers.

11.2 Mitochondria and Chloroplasts Contain DNA Similar to Bacteria and Viruses

• Mitochondria and Chloroplasts

  • Contain their own DNA, inherited maternally in most organisms.
  • Their DNA closely resembles the DNA found in viruses and bacteria, suggesting evolutionary origins.

• Mitochondrial DNA (mtDNA)

  • Typically exists as double-stranded, closed circles, lacking chromosomal proteins and mostly devoid of introns.
  • Replication is enzymatically dependent on proteins coded by nuclear DNA.

• Chloroplast DNA (cpDNA)

  • Larger than mtDNA, contains more genes, has both introns and duplications, and aids in photosynthesis.

11.3 Specialized Chromosomes Reveal Variations in the Organization of DNA

• Polytene Chromosomes

  • Visible in certain tissues and represent paired homologs; appear as individualized bands (chromomeres) under light microscopy.
  • Characterized by DNA undergoing many rounds of replication without strand separation.

• Lampbrush Chromosomes

  • Found in meiotic chromosomes of oocytes in various vertebrates.
  • Notable for extensive DNA looping and large numbers of condensed areas (chromomeres).

11.4 DNA Is Organized into Chromatin in Eukaryotes

• Chromatin

  • Chromatin fiber composes eukaryotic chromosomes that change structure throughout the cell cycle.
  • During interphase, chromatin is uncoiled and replicated; during cell division, it condenses into visible chromosomes.

• Histones and Nucleosomes

  • DNA in eukaryotes is characterized by being complexed with positively charged histones, playing essential roles in structure and function.
  • Nucleosomes form the fundamental units of chromatin, appearing as "beads on a string" along DNA strands.

• Chromatin Remodeling

  • Essential for gene expression, requiring relaxation of compact structures.
  • Histone tails undergo various chemical modifications (acetylation, methylation, phosphorylation) impacting genetic activity.

• Heterochromatin vs. Euchromatin

  • Euchromatin: Uncoiled and actively transcribed genes.
  • Heterochromatin: More condensed and typically genetically inactive areas; plays a role in structural integrity and gene expression.

11.5 Eukaryotic Genomes Demonstrate Complex Sequence Organization Characterized by Repetitive DNA

• Repetitive DNA

  • Comprises sequences repeated multiple times within chromosomes; often does not encode proteins.
  • Includes categories like satellite DNA (highly repetitive, found in centromeric regions) and middle repetitive sequences.

• Middle Repetitive Sequences

  • Include Variable Number Tandem Repeats (VNTRs) and Short Tandem Repeats (STRs), important for genetic fingerprinting.
  • Transposable elements like SINEs and LINEs can relocate within the genome.

11.6 The Vast Majority of a Eukaryotic Genome Does Not Encode Functional Genes

• Pseudogenes
  • Comprise a minor portion of the eukaryotic genome, typically representing evolutionary remnants.
  • These noncoding sequences have undergone mutations and are not transcriptionally active.