Lecture 22: Chromosome Packing

  • Exam Preparation Advice

    • Study in parts (half of lecture content a day) to manage material better; use quizzes and assignments to track your progress.

  • Genetic Material Overview

    • Focus on how gene products are formed from genetic material.

    • Reviewed previous topics: transmission genetics, gene inheritance, linkage, and epistasis.

  • DNA Structure Recap

    • DNA looks like a helix, consists of chromosomes containing genes.

    • Discussed how chromosomes condense during cell division (metaphase).

    • Chromatin vs. Chromosome: Chromatin is less condensed, while chromosomes are highly condensed.

  • Chromosome Composition

    • Each body cell has 23 pairs of chromosomes (total 46).

    • Approximately 6 billion base pairs, equating to about 2 meters of DNA per cell.

    • Overall, a human body contains about 30 trillion cells, with DNA length allowing extensive measurements (can circle the Earth multiple times).

  • Transition from DNA to Chromatin

    • Chromatin: Complex of DNA and proteins (specifically histones) that allows DNA to fit inside the nucleus.

    • Two types of chromatin:

      • Euchromatin: Loosely packed, accessible for transcription.

      • Heterochromatin: Tightly packed, generally inaccessible.

  • Types of Heterochromatin

    • Constitutive Heterochromatin: Always tightly packed, found in regions like centromeres.

    • Facultative Heterochromatin: Can switch between eu- and heterochromatin, plays a role in gene regulation.

  • Histones and Nucleosomes

    • Core Histones: H2A, H2B, H3, H4 (8 total in a nucleosome). Histone H1 acts as a linker.

    • Nucleosome: DNA wrapped around core histones, forming the basic packing unit of chromatin.

  • Levels of Compaction

    • 10 nm Fiber: DNA wrapped around histones (beads on a string).

    • 30 nm Fiber (Solenoid Model): Higher compaction, involves six nucleosomes, forming the beginning of supercoiling.

    • Histone H1 increases compaction significantly (40 times tighter).

    • 300 nm Fiber: Chromatin loops formed; allows for accessibility to genetic material.

    • 700 nm Fiber (Heterochromatin): Further condensation, less accessible for transcription.

  • Regulation of Chromatin Structure

    • Changes from euchromatin to heterochromatin depend on cellular signals which are typically gene products.

    • Compaction needed for mitosis and meiosis to occur efficiently; influences accessibility of DNA during these processes.

  • Cell Cycle Overview

    • Interphase consists of G1, S, and G2 phases where most gene expression occurs before chromatin condenses for mitosis.

    • During mitosis, chromosomes are highly condensed, resulting in minimal gene transcription.

    • Review different stages of mitosis and their relation to chromatin structure.

  • Key Concepts to Remember

    • Understand the structural aspects of nucleosomes and overall chromatin organization.

    • Familiarity with the terminology and differences between types of chromatin is crucial.

    • Be able to explain processes involved in DNA condensation and compaction during cell division.