Genetics- Chapter 12 DNA Organization in Chromosomes

Chapter 12: DNA Organization in Chromosomes

12.4 DNA Is Organized into Chromatin in Eukaryotes

• Chromatin: Form in which eukaryotic chromosomes exist during interphase; chromatin uncoils and decondenses.

  • Dispersed throughout the nucleus during interphase.

  • Condenses into visible chromosomes during cell division.

Histones

• Histones: Positively charged proteins associated with chromosomal DNA, composed largely of lysine and arginine.

  • Facilitate electrostatic bonding to DNA's negatively charged phosphate groups.

  • Five main types: H1, H2A, H2B, H3, and H4.

Nucleosomes

• Nucleosomes: Structural units of chromatin, resembling "beads on a string."

  • Each nucleosome consists of DNA wrapped around a core of histone proteins.

  • Functionally condensed to form intact chromatids during cell division.

Chromatin Remodeling

• Chromatin Remodeling: Necessary for DNA-protein interactions and gene expression.

  • Involves relaxing the chromatin structure to expose DNA regions.

  • Allows mechanisms for reversing inactive states for regulatory processes.

Superhelix

• Superhelix: DNA in eukaryotic nuclei forms a packaging structure that tightly coils around histones.

  • Histone tails (unstructured) protrude through the minor groove of DNA.

Chemical Modifications

• Chemical Modifications: Modifications to histone tails that affect gene activity.

  • Acetylation: Enzyme histone acetyltransferase (HAT) attaches acetyl groups to lysine, neutralizing positive charges.

  • Methylation: Enzyme methyltransferase adds methyl groups to histones, positively correlating with gene activity.

  • Phosphorylation: Enzyme kinase adds phosphate groups to serine and histidine, playing a role in regulating gene expression.

Methylation of DNA

• Methylation: Involves the addition of methyl groups to cytosine bases, forming 5-methylcytosine, typically correlating negatively with gene activity.

  • CpG Islands: Areas rich in cytosine and guanine dinucleotides often involved in gene regulation.

Euchromatin and Heterochromatin

• Euchromatin: Loose structure, active in gene expression, appears unstained during interphase.

• Heterochromatin: Condensed structure, primarily inactive genes, appears stained in interphase.

  • Telomeres: Protect chromosome ends, maintaining integrity.

  • Centromeres: Essential for proper separation during cell division.

12.5 Chromosome Banding

• Chromosome Banding Techniques: Applied to identify regions along mitotic chromosomes, resembling polytene chromosome bands.

  • C-banding: Particularly stains centromeres (heterochromatin).

  • G-banding: Involves differential staining and enzymatic digestion of chromosomes for detailed visual patterns.

Human Chromosome Banding

• Nomenclature in Chromosome Banding: Based on G-banding, where regions of chromosomes are labeled following the banding patterns observed.

12.6 Centromeres and Kinetochores

• Centromeres: Serve as primary constrictions on chromosomes, critical for the separation of homologs during mitosis and meiosis.

• Kinetochore Proteins: Attach to spindle fibers, essential for correct chromosome movement.

12.7 Eukaryotic Genome Characteristics

• Pseudogenes: Represent a small portion of the genome (2-10%) constituting protein-encoding genes. Often evolved from ancestral genes, now nonfunctional due to significant mutations.