Chromosomes and Chromatin Organization

Bacterial vs eukaryotic chromosomes

Bacteria: one circular chromosome (± plasmids), housed in the nucleoid (no membrane), smaller genome. Eukaryotes: multiple linear chromosomes plus non-nuclear (mitochondrial) DNA, more tightly packaged.

Supercoiling

Topoisomerase and gyrase relieve positive supercoiling / introduce negative supercoiling to manage torsional stress during replication/transcription.

Chromatin

DNA + histone proteins. A nucleosome = ~146 bp of DNA wrapped around a histone octamer (2 each of H2A, H2B, H3, H4) + ~44 bp linker DNA, clamped by histone H1.

Euchromatin vs. heterochromatin

Euchromatin: loosely packed, active, being transcribed/replicated. Heterochromatin: densely packed, inactive, not being replicated.

Karyotype

: An image/map of all the chromosomes in a cell. Humans: 22 pairs of autosomes + 1 pair of sex chromosomes (23 pairs total).

Sister chromatids

Two identical copies of a chromosome (post-S-phase), joined at the centromere; fully condensed chromosome structure is only seen during mitosis.