MB06 Intro to packing of chromosomes/DNA/chromatin

Eukaryotic DNA challenge

1. All DNA in a eukaryotic cell must fit inside a small, membrane-bound nucleus.

2. This requires significant compaction strategies.

Eukaryotic DNA challenge 1

All DNA in a eukaryotic cell must fit inside a small, membrane-bound nucleus.

Eukaryotic DNA challenge 2

This requires significant compaction strategies.

Histones (definition)

1. Histones are ~10 nm thick proteins that DNA wraps around to assist with compaction.

2. They serve as scaffolds for DNA organization in the nucleus.

Histones (definition) 1

Histones are ~10 nm thick proteins that DNA wraps around to assist with compaction.

Histones (definition) 2

serve as scaffolds for DNA organization in nucleus.

DNA wrapping around histones

1. DNA coils around histones, but this alone is insufficient for full nuclear compaction.

2. Higher-order organization is required.

DNA wrapping around histones 1

DNA coils around histones, but this alone is insufficient for full nuclear compaction.

DNA wrapping around histones 2

Higher-order organization is required.

Nucleosome (definition)

A nucleosome consists of an octamer of histones (8 histone proteins) with DNA wrapped around it.

Nucleosome size

Nucleosomes have a diameter of ~30 nm.

Nucleosome function

Repeated wrapping of DNA around nucleosomes allows a very large genome to be packaged into a small nuclear volume.

Higher-order chromatin structure

Nucleosomes can assemble into larger fibers, further compacting DNA beyond the level of single nucleosomes.

Importance of histones

Histones are essential for DNA packaging and nuclear organization but also create the challenge of needing to balance compaction with accessibility.

Accessibility problem

DNA must remain accessible for polymerases during replication and transcription. Over-condensation would prevent access to the genetic code.

Histone modifications

Chemical modifications to histones regulate whether DNA is tightly coiled (heterochromatin) or relaxed (euchromatin). These modifications control accessibility.

Euchromatin (definition)

Euchromatin is relaxed chromatin that is transcriptionally active and accessible for replication and gene expression.

Dynamic chromatin regulation

Cellular processes govern when DNA coils tightly for storage versus when it relaxes for transcription/replication, balancing compaction with function.