Genetic Material and Chromatin Structure

Overview of Upcoming Exam

Five lectures leading up to the next exam.

Urging students to start preparing today.

Suggestion: split study material into manageable portions (e.g., half today, half tomorrow).

Quiz and Packback Assignments

A quiz is associated with the current module.

Packback curiosity scores are important, ensure minimum scores on all three assignments.

Due date: Assignments due on the 30th.

Exam Grading Timeline

Goals to have previous exam graded by next Friday.

Introduction to Molecular Genetics

Starting with understanding genetic material, specifically DNA.

Previous coverage of DNA structure: double helix, base pairing rules (A to T, C to G), concepts of inheritance.

Focus on chromosomes, gene location, and linkage during meiotic processes (meiosis).

Chromosome Structure

Chromosomes exist in various forms: single chromosomes, sister chromatids, homologous chromosomes.

Eukaryotic cell nucleus contains condensed genetic material during metaphase.

Chromatin: DNA in its loose form, packaged within the nucleus when not condensed into chromosomes.

Nucleolus: Site of ribosome synthesis within the nucleus.

Condensation and Packaging of DNA

Question raised: How do we condense two meters of DNA into eukaryotic cells?

Roughly 6,000,000,000 base pairs of DNA in each cell, translating to about two meters of DNA in length.

Average eukaryotic cell size: 50 micrometers; DNA must fit into a much smaller nucleus (~10 nanometers).

Chromatin Structure

Chromatin: Composition of DNA and proteins (histones).

Histones: Proteins crucial for DNA condensation; required during cell division.

• Four core histones (H2A, H2B, H3, H4) exist as dimers in nucleosomes.
• Histone H1: linker histone, assists in compacting DNA further.

Types of Chromatin

Euchromatin: Loosely packed, accessible regions of genes for transcription.

Heterochromatin: Tightly packed, inaccessible portions.

• Divided into Constitutive (always heterochromatin) and Facultative (can alternate states between euchromatin and heterochromatin).

Model of Chromatin Compaction

Historical models of DNA folding:

• Raw Fiber Model: Proposed by E.J. Dupraw, inherently flawed, lacked a clear mechanism for folding.
• Nucleosome Model: Refined in the 1970s, where DNA wraps around histone proteins creating nucleosomes.

Hierarchical Organization of Chromatin

10 nanometer fiber: Simplest level, beginning of nucleosome formation.

30 nanometer fiber: Structure called solenoid or zigzag configurations, compacting further.

300 nanometer fiber: Chromatin loops, still euchromatic, accessible for transcription.

700 nanometer fiber: Heterochromatic, DNA tightly packed, not accessible for transcription.

Proteins involved in condensing chromatin signal from cellular processes, preparing for replication or mitosis.

Cell Cycle and Chromatin Structure

Interface: Most time spent in cell cycle where chromatin is less condensed, accessible.

Mitosis: Densely packed chromosomes during metaphase.

• Protein production mainly occurs in G1, S, and G2 phases, minimal during mitosis due to tightly packed chromosomes.

Key Takeaways:

Understand different forms of chromatin, histone functions, chromatin condensation levels, and the process of DNA packaging within the cell cycle.

Questions to Consider:

When does crossing over occur during meiosis?

Differences between euchromatin and heterochromatin under specific conditions.