Chromosome Structure and DNA Packaging

Chromosome structure and DNA packaging explains how the long DNA molecules of a genome are compacted, organized, and made accessible inside the nucleus. Packaging begins with the wrapping of DNA around histone proteins to form nucleosomes and continues through higher orders of folding into chromatin and, at cell division, condensed chromosomes.

Definition

Chromosome packaging is the hierarchical organization of genomic DNA with proteins, beginning with nucleosomes (DNA wrapped around histone octamers) and extending to compact chromatin fibers and condensed chromosomes, which together fit a genome into the nucleus while regulating access to the DNA.

Scope

The entry covers the nucleosome as the repeating unit of chromatin, the histone proteins and the histone octamer, linker histones and higher-order folding, the distinction between more and less compact chromatin states, and large-scale genome folding within the nucleus. It is a structural reference topic and does not give clinical guidance.

Key concepts

• Nucleosome (repeating unit of chromatin)
• Histone octamer (H2A, H2B, H3, H4)
• Linker histone H1
• Chromatin fiber and higher-order folding
• Euchromatin and heterochromatin
• Topologically associating domains
• Mitotic chromosome condensation

Mechanisms

The fundamental packaging unit is the nucleosome, in which about 147 base pairs of DNA wrap nearly twice around a core octamer of two copies each of histones H2A, H2B, H3, and H4; this repeating bead-on-a-string arrangement was identified by Kornberg and visualized by Olins and Olins. A linker histone (H1) binds where DNA enters and exits the nucleosome and promotes folding into a more compact chromatin fiber. Successive levels of folding and loop formation organize chromatin into domains, and chromosome-conformation methods such as Hi-C reveal that interphase genomes are partitioned into self-interacting topological domains and compartments. The degree of compaction varies between transcriptionally active, more open chromatin and condensed, generally repressed chromatin, and the entire genome is maximally condensed into discrete chromosomes during mitosis to allow accurate segregation.

Clinical relevance

How DNA is packaged influences which genes are accessible and how genomes are stably inherited, and chromatin organization is a major theme in molecular medicine and developmental biology. The entry describes structure for reference and is not a basis for individual clinical decisions.

History

In the early 1970s, biochemical digestion patterns and electron-microscopy images converged on the nucleosome model, articulated by Kornberg in 1974 and visualized as repeating spheroid units by Olins and Olins the same year. Later work characterized linker histones and higher-order folding, and from 2009 chromosome-conformation-capture methods began mapping how whole genomes fold in three dimensions.

Key figures

• Roger Kornberg
• Ada Olins
• Donald Olins
• Tom Misteli

Seminal works

• kornberg-1974
• olins-olins-1974
• lieberman-aiden-2009

Frequently asked questions

What is a nucleosome?

A nucleosome is the basic repeating unit of chromatin: a stretch of DNA wrapped around a core of eight histone proteins. Strings of nucleosomes fold into more compact chromatin fibers.

Why does DNA need to be packaged at all?

The DNA in a single human cell is about two meters long and must fit inside a microscopic nucleus; packaging compacts it, protects it, and organizes it so the right genes can be accessed when needed.