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The Nucleosome

The Nucleosome

DNA in a human cell is almost 2m long and it is packed inside a tiny nucleus that is only a few microns in diameter. The level of compaction of DNA inside the nucleus is astonishing. It is organized into several sequentially higher levels of compaction to fit into such a tiny space. The most compact form of DNA is a chromosome that can be seen under a microscope in a dividing cell.
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The Nucleosome Core Particle

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Nucleosomes are the DNA-histone complex, where the DNA strand is wound around the histone core. The histone core is an octamer containing two copies of H2A, H2B, H3, and H4 histone proteins.
The paradox
Nucleosomes, paradoxically, perform two opposite functions simultaneously. On the one hand, their main responsibility is to protect the delicate DNA strands from physical damage and help achieve a higher compaction ratio. While on the other hand, they must allow polymerase enzymes to access DNA...

Cooperative Binding of Transcription Regulators

Cooperative Binding of Transcription Regulators

Transcriptional regulators bind to specific cis-regulatory sequences in the DNA to regulate gene transcription. These cis-regulatory sequences are very short, usually less than ten nucleotide pairs in length. The short length means that there is a high probability of the exact same sequence randomly occurring throughout the genome. Since regulators can also bind to groups of similar sequences, this further increases the chances of random binding. Transcriptional regulators form dimers that...

Single-Strand DNA Binding Proteins

Single-Strand DNA Binding Proteins

For successful DNA replication, the unwinding of double-stranded DNA must be accompanied by stabilization and protection of the separated single strands of the DNA. This crucial task is performed by single-strand DNA-binding (SSB) proteins. They bind to the DNA in a sequence-independent manner, which means that the nitrogenous bases of the DNA need not be present in a specific order for binding of SSB proteins to it. The binding of SSB proteins straightens single-stranded DNA (ssDNA) and makes...

The Nucleosome

The Nucleosome

Human DNA is almost two meters long. However, it is compressed inside a tiny nucleus measuring only a few microns in diameter. To make this degree of compaction possible, DNA is organized into several sequential levels so that it can fit into such a tiny space. The most compact form of DNA is a chromosome that can be seen under a microscope in a dividing cell.
In a chromosome, DNA is wound twice around a protein complex called a histone octamer core, which consists of 8 histone proteins. This...

The Nucleosome Core Particle

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Nucleosomes are the DNA-histone complex, where the DNA strand is wound around the histone core. The histone core is an octamer containing two copies of H2A, H2B, H3, and H4 histone proteins.
Nucleosomes, paradoxically, perform two opposite functions simultaneously. On the one hand, their primary aim is to protect the delicate DNA strands from physical damage and help achieve a higher compaction ratio. On the other hand, they must allow polymerase enzymes to access histone-bound DNA during...

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Related Experiment Video

Updated: Jul 18, 2026

DamID-seq: Genome-wide Mapping of Protein-DNA Interactions by High Throughput Sequencing of Adenine-methylated DNA Fragments

DamID-seq: Genome-wide Mapping of Protein-DNA Interactions by High Throughput Sequencing of Adenine-methylated DNA Fragments

Published on: January 27, 2016

Human Dmc1 protein binds DNA as an octameric ring.

S I Passy¹, X Yu, Z Li

¹Department of Biology, Rensselaer Polytechnic Institute, 425 Jordan Road, Troy, NY 12180, USA.

Proceedings of the National Academy of Sciences of the United States of America

|September 15, 1999

Summary

Human Dmc1 (hDmc1) forms octameric rings, unlike bacterial RecA. This meiosis-specific recombinase binds DNA within its ring structure, suggesting a conserved functional mechanism in genetic recombination.

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Area of Science:

Molecular Biology
Genetics
Biochemistry

Background:

The bacterial RecA protein is central to homologous genetic recombination and shares structural similarities with F1-ATPase and helicases.
RecA forms a hexameric ring structure, a motif also observed in F1-ATPases and ring helicases.

Purpose of the Study:

To investigate the structural and functional characteristics of the human meiosis-specific recombinase, hDmc1.
To compare the oligomeric state and DNA-binding properties of hDmc1 with its bacterial homolog, RecA.

Main Methods:

Structural analysis of human Dmc1 (hDmc1) protein.
DNA-binding assays to determine the active conformation of hDmc1.

Main Results:

Human Dmc1 (hDmc1) forms octameric rings, distinct from the hexameric rings of RecA.
Unlike RecA and Rad51, hDmc1 does not form helical filaments.
The hDmc1 ring structure binds DNA in its central channel, indicating this is likely the active conformation.

Conclusions:

The RecA-like ring structure is conserved from bacteria to humans.
Some RecA homologs, like hDmc1, may function exclusively as rings, while others can form both rings and filaments.