Histones Organize DNA Into Compact & Manageable Structures
- If you tried packing a long rope into a small box without a proper system, it would tangle and overflow.
- DNA faces a similar challenge, as it fits nearly 2 meters of material into a microscopic nucleus.
- The solution is histones.
Definition
Histone
A protein that provides structural support for DNA in eukaryotic cells. Histones are positively charged, allowing them to interact with negatively charged DNA, helping to compact it into a condensed form called chromatin.
How Is DNA Wrapped Around Histones?
- DNA wraps around a core of eight histone proteins like a thread around a spool.
- These eight histones form an octamer that serves as the foundation for DNA wrapping.
- This is also referred to as a nucleosome.
Definition
Nucleosome
A nucleosome is the basic structural unit of chromatin, consisting of DNA wrapped around a core of histone proteins.
Components of a Nucleosome
- Histone Core
- Composed of eight histone proteins: two copies each of H2A, H2B, H3, and H4.
- This core forms the foundation for DNA wrapping.
- DNA Wrapping
- About 147 base pairs of DNA coil around the histone core, forming 1.65 turns.
- The negatively charged DNA interacts with the positively charged histones, ensuring stability.
- Linker DNA and Histone H1
- Linker DNA connects adjacent nucleosomes, with lengths ranging from 20 to 80 base pairs.
- H1 histone binds to the linker DNA, stabilizing the nucleosome and facilitating higher-order chromatin structures.
Definition
Linker DNA
Short stretches of DNA that connect adjacent nucleosomes in chromatin.
- Students are required to know that nucleosomes are DNA wrapped around a core of eight histones, with H1 binding to linker DNA.
- The names of the four core histones are not required by the IB, but understanding their arrangement helps with visualization.
Role of Histones in Nucleosomes
- DNA Compaction: Histones enable long DNA molecules to compact into chromatin, fitting into the nucleus.
- Gene Regulation: Modifications to histones (e.g., acetylation, methylation) regulate DNA accessibility for transcription, replication, and repair.
- Structural Integrity: The histone-DNA interaction maintains the nucleosome’s stability under cellular conditions.
- The nucleosome is like a spool of thread, with histones as the spool and DNA as the thread wrapped around it.
- This compact design allows DNA to be stored efficiently while remaining accessible when needed.
- What proteins make up the nucleosome core, and how many are there in total?
- What role does histone H1 play in nucleosome structure?
