Packaging of DNA Helix

Nucleoid is the region in prokaryotic cells where DNA is organized and associated with proteins, despite the absence of a true nucleus.

The process by which a very long DNA molecule is compactly organised inside the cell nucleus so that it fits within the limited nuclear space and remains functional is called DNA packaging.

Positively charged basic proteins rich in lysine and arginine that associate with DNA to help in its packing in eukaryotic cells are called histones.

A structural unit composed of eight histone protein molecules around which DNA is wrapped is called histone octamer.

The basic repeating unit of chromatin formed by DNA wrapped around a histone octamer is called nucleosome.

The thread-like complex of DNA and proteins present in the nucleus of eukaryotic cells is called chromatin.

Proteins other than histones that are associated with chromatin and help in higher-order DNA packaging and regulation are called non-histone chromosomal (NHC) proteins.

The human genome contains approximately 6.6 × 10⁹ base pairs of DNA per diploid cell. If fully stretched out, the DNA from a single human cell would measure approximately 2.2 metres in length. Yet this DNA must fit inside a nucleus that is only about 10⁻⁶ m (1 micrometre) in diameter.

This extreme compaction is not just about saving space. DNA packaging also:

• Regulates gene expression - tightly packed regions prevent transcription; loosely packed regions allow it.​
• Protects DNA from tangling and mechanical damage during cell division.​
• Silences genes that should not be expressed in a specific cell type.

The Nucleoid:

In prokaryotes (e.g., E. coli), there is no true nucleus. Instead, the genetic material is confined to the nucleoid, an irregularly shaped region within the cytoplasm.

Key facts about prokaryotic DNA:

• E. coli DNA length: approximately 1.1–1.36 mm (1100 µm) in perimeter.​
• Contains about 4.6 million base pairs.​
• The cell itself is only 2–3 µm in length, so DNA must be compacted ~400–700 times.​
• Prokaryotic DNA is negatively charged and circular (not linear).​

DNA Packaging in Prokaryotes

Steps of Prokaryotic DNA Compaction:

​Proteins involved in prokaryotic packaging:

• HU proteins - positively charged histone-like DNA-binding proteins; assist supercoiling.​
• DNA Gyrase - an enzyme that introduces negative supercoils.​​
• Topoisomerase I - maintains the supercoiled state.​
• RNA connectors - assist loop domain formation.

Eukaryotic cells have far more DNA (~1000 times more than prokaryotes) and require a multi-level, hierarchical packaging system involving histone proteins and non-histone chromosomal (NHC) proteins.

Histones: The Packaging Proteins:

Small, positively charged (basic) proteins rich in the amino acids lysine and arginine, which interact with the negatively charged DNA phosphate backbone to facilitate compaction.

Five types of histones:

Histone Octamer: 

A structural core made of 8 histone molecules - two copies each of H2A, H2B, H3, and H4 - around which DNA wraps to form a nucleosome.

Nucleosome: The Basic Unit:

The fundamental, repeating structural unit of chromatin, consisting of a segment of DNA wrapped around a histone octamer.​

Nucleosome

Numerical facts:

• Total DNA per nucleosome: ~200 base pairs (bp)​
• DNA wrapped around the octamer: ~146 bp (in 1.65 turns)
• Linker DNA (between nucleosomes): ~20–90 bp (varies by species and tissue)​
• Diameter of nucleosome fibre ("beads-on-string"): ~10 nm

Chromatosome:

A chromatosome forms when histone H1 attaches to the linker DNA at the entry and exit sites of a nucleosome.

Levels of Eukaryotic DNA Compaction:

Non-Histone Chromosomal (NHC) Proteins:

A heterogeneous collection of proteins (other than histones) found in eukaryotic chromatin. They form the scaffold to which DNA loops are anchored during higher-order compaction and also regulate DNA replication, RNA synthesis, and nuclear processes.

Euchromatin vs Heterochromatin

After the nucleosome-to-solenoid-to-loops hierarchy, the resulting chromatin fibre in interphase nuclei exists in two distinct forms:​

Types of heterochromatin:

• Constitutive heterochromatin: Permanently condensed (e.g., centromeres, telomeres); contains repetitive, non-coding DNA.​
• Facultative heterochromatin: Can switch between active and inactive states depending on developmental stage (e.g., inactivated X chromosome = Barr body).

Prokaryote vs Eukaryote Packaging