BIOL 114
Biology Dept
Kenyon College
Chromosome Structure
and Cell Division
Fall SectionSpring Section 1Spring Section 2

Bacterial Chromosome Structure
Prokaryotic cells (bacteria) contain their chromosome as circular  DNA.  Usually the entire genome is a single circle, but often there are extra circles called plasmids. The DNA is packaged by DNA-binding proteins.

The bacterial DNA is packaged in loops back and forth.  The bundled DNA is called the nucleoid.  It concentrates the DNA in part of the cell, but it is not separated by a nuclear membrane (as in eukaryotes.)  The DNA does form loops back and forth to a protein core, attached to the cell wall.

From Bacterial Diversity

The DNA is accessible to enzymes that make RNA and protein (see Chapter 4).  In the bacterial cell, the DNA gets transcribed to RNA, and the RNA gets translated to protein before it is even completed. How does this differ from a eukaryotic cell?

Eukaryotic Chromosome Structure
Eukaryotic cells contain their DNA within the nuclear membrane.
The DNA double helix is bound to proteins called histones.  The histones have  positively charged (basic) amino acids to bind the negatively charged (acidic) DNA.  Here is an SDS gel of histone proteins, separated by size (those migrating down farthest are smaller).
From Virtual Fly Lab

The DNA is wrapped around the histone core of eight protein subunits, forming the nucleosome.   The nucleosome is clamped by histone H1.  About 200 base pairs (bp) of DNA coil around one histone.  The coil "untwists" so as to generate one negative superturn per nucleosome.

Life, the Science of Biology, by Purves, Orians, & Heller, 5th ed., 1997
Click on image to see molecular structure
from Protein Data Bank (pdb 1aoi)

This form of DNA is active chromatin; it can be "expressed" (transcribed and translated) to make RNA and proteins (Chapter 4). 

How is the chromatin "read"?  How does the cell know which genes to use when?  Part of the answer comes from the histone code of histone modifications (as well as DNA modifications).  For example:
  • Methylation of histone or of DNA usually turns a gene off.
  • Acetylation of histone usually turns a gene on.
  • Phosphorylation -- we're not sure what that does.

Eukaryotic Chromosome condensation
After DNA has been replicated for mitosis (cell division), the chromatin condenses.The nucleosomes zig-zag back and forth to form a flat ribbon:

Life, the Science of Biology, by Purves, Orians, & Heller, 5th ed., 1997

The ribbon forms a coil, which then loops back and forth attached to a nuclear matrix -- similar to the protein core of bacteria, but greatly extended.  During mitosis, several more layers of coiling result in fully condensed chromatin (see textbook Ch. 9).

In mitosis, the chromosomes appear as the thick rod-shaped bodies which can be stained and visualized under light microscopy.

The modern way to visualize condensed chromosomes is by FISH -- fluorescence in situ hybridization.  In this method, fluorescent antibody-tagged DNA probes hybridize to their complementary sequences in the chromosomes.  By using FISH probes with different colored fluorophores, one can color each human chromosome independently, and thus identify all 23 chromosomes.  This is called chromosome painting.

 Chapter 2: Bacterial Cell Division
Eukaryotic Cell Division by Mitosis and Meiosis