DNA and Chromosome Structure: From Nucleotides to Genomes

While several forms of DNA exist, they differ in their twist and geometry:

• B-DNA: The most common form in living cells. It is a right-handed spiral with a diameter of approximately 2 nm and about 10.4 base pairs (bp) per rotation.
• A-DNA: A wider, right-handed form that is common in bacteriophages.
• Z-DNA: A left-handed, zig-zagging structure often found near transcription start sites.
• H-DNA: A triple-stranded structure that may play a role in controlling gene expression.

Genomic Complexity and Diversity

A genome is the complete set of genetic material in an organism. The complexity and size of genomes vary drastically across the domains of life:

• Viruses: The smallest genomes (5–200 Kbp). They can consist of single or double-stranded DNA or RNA, and can be circular or linear.
• Bacteria: Typically possess a single circular chromosome of double-stranded DNA (1–10 Mbp). They often contain smaller, autonomously replicating loops called plasmids.
• Archaea: Possess circular genomes (500 Kbp–6 Mbp). Interestingly, while they are prokaryotic, their DNA is coiled using histones, similar to eukaryotes.
• Eukaryotes: The most complex (5 Mbp to >100 Gbp). Their DNA is organized into multiple linear chromosomes.

It is important to note that genome size does not always correlate with organismal complexity—a phenomenon known as the C-value paradox. For example, an Amoeba can have nearly 100 times more DNA than a human. This is largely because eukaryotic genomes contain vast amounts of repetitive DNA and non-coding (intergenic) regions. In humans, only about 30-40% of the genome is unique sequence, while the remaining ~58% is repetitive.

DNA Packaging in Eukaryotes

Because eukaryotic DNA is so massive, it must be highly compacted to fit within the nucleus. This packaging is organized into chromatin. The process occurs in several hierarchical steps:

1. Nucleosomes (10 nm fiber): The first level of compaction involves DNA wrapping around an octamer of histone proteins (two each of H2A, H2B, H3, and H4). This creates a "beads on a string" appearance, resulting in a 7-fold shortening of the DNA. The histone H1 helps stabilize the linker DNA between nucleosomes.
2. 30 nm fiber: The nucleosomes coil into a solenoid structure, further shortening the DNA by about 7-fold.
3. 300 nm fiber (Radial Loops): The 30-nm fibers attach to a non-histone protein scaffold via specific sequences called matrix attachment regions (MARs).
4. Metaphase Chromosome: During cell division, the chromatin reaches its maximum condensation (up to 1400 nm) to form the visible chromosomes seen in mitosis.

Note on Chromatin States: Not all chromatin is packed equally. Euchromatin is less condensed and contains actively expressed genes, whereas heterochromatin is highly condensed and contains fewer expressed genes. Heterochromatin can be constitutive (permanently condensed, like at centromeres and telomeres) or facultative (variable condensation based on transcription needs).

In bacteria, compaction is achieved through supercoiling (often controlled by enzymes like DNA gyrase) and the formation of a nucleoid. In eukaryotes, specialized proteins called SMC proteins (such as condensin and cohesin) help manage the structural maintenance and condensation of these massive chromosomes.