Summary
DNA wraps around histone proteins to form nucleosomes that coil into compact chromatin.
In depth
About 147 base pairs wrap around a histone octamer (two each of H2A, H2B, H3 and H4) to make a nucleosome, like beads on a string. H1 helps fold the fibre into a 30 nm fibre and loops. Heterochromatin is tightly packed and silent; euchromatin is open and active.
Two metres of DNA fit into a nucleus about 6 micrometres wide.
The 30 nm fibre seen in test tubes is not clearly present in living cells, where chromatin appears as irregular clutches and loops. Cohesin and condensin, protein rings that extrude loops, structure interphase and mitotic chromosomes. Linker histone H1 binds the DNA entering and leaving each nucleosome.
Histone tails project out and carry modifications that act as a code. Histones are among the most conserved proteins; histone H4 from peas and cows differs by only 2 amino acids. Archaea have histone-like proteins, and bacteria use proteins such as HU and H-NS plus supercoiling to compact DNA.
Example: in sperm, histones are replaced by protamines, which pack DNA even more tightly.
Frequently asked questions
What is DNA Packaging?
DNA wraps around histone proteins to form nucleosomes that coil into compact chromatin.
Which area of biology covers DNA Packaging, and how advanced is it?
DNA Packaging is one of the 52 topics in Molecular Biology. It is rated Beginner, so it suits learners who are starting out in biology. Related topics in this area include Origin of Replication, Histone Modification, Gene Expression.
Can you give an example of DNA Packaging?
Chromosomes are most condensed in metaphase.
What should I learn before DNA Packaging?
DNA Packaging is a Beginner topic, so no earlier topic is required. Other Beginner topics in Molecular Biology that pair well with it include Central Dogma, DNA Replication, Base Pairing.
What topics are related to DNA Packaging?
The closest topics to DNA Packaging are Origin of Replication, Histone Modification, Gene Expression, Riboswitches. Comparing them side by side is a good way to see how the ideas connect.
How can I check that I understand DNA Packaging?
Read the summary and the in-depth section, then use the quick check on the interactive lesson for DNA Packaging. To practise the whole of Molecular Biology, use the Molecular Biology quiz.
How is DNA Packaging different from Origin of Replication?
DNA Packaging: DNA wraps around histone proteins to form nucleosomes that coil into compact chromatin. Origin of Replication: Specific sites where replication begins, singular in bacteria and multiple in eukaryotic chromosomes. Read both side by side and note what each one explains.
How should I study DNA Packaging step by step?
First, read the summary and write the definition in your own words. Second, read the in-depth section and note any example. Third, compare it with Origin of Replication and explain the difference aloud. Finally, take the Molecular Biology quiz and come back to this page for any question you miss.