Molecular Structure of Chromosomes and Transposable Elements

Definition: Chromosomes are structures that contain an organism's genetic material, comprising complexes of DNA and proteins.
Genome: The complete set of genetic material possessed by an organism.
- In prokaryotes: typically a single circular chromosome.
- In eukaryotes: a complete set of linear nuclear chromosomes.
- Eukaryotes also possess mitochondrial and, in plants, chloroplast genomes.

The main role of genetic material is to store information necessary for developing organism traits, primarily through protein-coding genes.
Critical DNA functions include:
- Synthesis of RNA and cellular proteins.
- Replication of chromosomes.
- Proper segregation and compaction of chromosomes to fit within cells.

Examination of three features of chromosomes:
- General organization of functional sites on a chromosome.
- Transposition of transposable elements (TEs).
- Molecular mechanisms of chromosome compaction.

Prokaryotic chromosomal DNA is usually a circular molecule, millions of nucleotides long.
Examples: E. coli has ~4.6 million base pairs; Haemophilus influenzae has ~1.8 million base pairs.
A bacterial chromosome typically includes thousands of genes, with intergenic regions between them.
Must have at least one origin of replication to initiate DNA replication.
Compaction of chromosomal DNA (1000-fold) is achieved through:
- Formation of loop domains (microdomains).
- Use of nucleoid-associated proteins (NAPs) that stabilize the structure.

Supercoiling: Compacts bacterial chromosomes via additional twists, forming negative or positive supercoils.
- Negative supercoiling enhances strand separation, aiding in replication and transcription.
- Controlled by enzymes such as DNA gyrase (introduces negative supercoils) and DNA topoisomerase I (relaxes negative supercoils).

Eukaryotic chromosomes are usually linear and composed of multiple sets of chromosomes.
Contain numerous origins of replication, centromeres, and telomeres essential for stability and replication.
Classification of DNA sequences:
- Unique: Found once in the genome.
- Moderately repetitive: Found several hundred to thousands of times.
- Highly repetitive: Found tens of thousands to millions of times.

Definition: Small segments of DNA that can integrate into new locations within the genome.
- First identified by Barbara McClintock in corn.
- Classified into simple transposons (move directly to new sites) and retrotransposons (use RNA intermediate for movement).
Transposition can cause various genetic changes, such as mutations or chromosomal rearrangements.

Nucleosomes are the repeating unit of eukaryotic chromatin, comprised of DNA wrapped around histone proteins.
Composed of core histones (H2A, H2B, H3, H4) and linker histone (H1), facilitating further compaction of DNA into higher-order structures.

DNA in eukaryotic cells is organized into chromatin, which can exist in forms of heterochromatin (tightly packed, transcriptionally inactive) and euchromatin (loosely packed, active).
Interactions among nucleosomes and specialized proteins lead to various topological configurations within the nucleus.

During mitosis, cohesin ensures sister chromatid cohesion while condensin facilitates chromosome compaction.
These complexes are essential for proper alignment and separation of chromosomes during cell division.

Four levels of organization:
- Level 1: Distinct territories of chromosomes in the nucleus.
- Level 2: Compartment organization within territories.
- Level 3: Structural domains (TADs).
- Level 4: Nucleosome interactions forming zigzag structures.

The structure and organization of chromosomes are crucial for the proper functioning and stability of genetic material. Transposable elements play significant roles in genetic diversity and evolution, while the specific mechanisms of compaction affect gene expression and regulation throughout the cell cycle.