1. Concept 16.3 A Chromosome consists of a DNA molecule packed together with proteins
The bacterial chromosome is a double-stranded, circular DNA molecule associated with a small amount of protein
Eukaryotic chromosomes have linear DNA molecules associated with a large amount of protein

2. In a bacterium, the DNA is “supercoiled” and found in a region of the cell called the nucleoid

3. Chromatin, a complex of DNA and protein, is found in the nucleus of eukaryotic cells
Chromosomes fit into the nucleus through an elaborate, multilevel system of packing
For the Cell Biology Video Cartoon and Stick Model of a Nucleosomal Particle, go to Animation and Video Files.

5. DNA packing How do you fit all that DNA into nucleus?
DNA coiling & folding
double helix
nucleosomes
chromatin fiber
looped domains
chromosome
nucleosomes
“beads on a string”
1st level of DNA packing
histone proteins have high proportion of positively charged amino acids (arginine & lysine)
bind tightly to negatively charged DNA

6. Nucleosomes “Beads on a string” 1st level of DNA packing
8 histone molecules
“Beads on a string”
1st level of DNA packing
histone proteins
protein molecules
Made up of positively charged amino acids
bind tightly to negatively charged DNA

7. Nucleosomes are coiled into 30 nm chromatin fiber
30 nm chromatin fibers are folded into looped domains
Looped domains are packed into chromosome

8. Nucleosome (10 nm in diameter)
Figure 16.22a
Nucleosome (10 nm in diameter)
DNA double helix (2 nm in diameter) H1
Histone tail
Figure Exploring: Chromatin Packing in a Eukaryotic Chromosome
Histones
Nucleosomes, or “beads on a string” (10-nm fiber)
DNA, the double helix
Histones

9. Replicated chromosome (1,400 nm)
Figure 16.22b
Chromatid (700 nm)
30-nm fiber
Loops
Scaffold
300-nm fiber
30-nm fiber
Figure Exploring: Chromatin Packing in a Eukaryotic Chromosome
Replicated chromosome (1,400 nm)
Looped domains (300-nm fiber)
Metaphase chromosome

10. DNA double helix (2 nm in diameter)
Figure 16.22c
DNA double helix (2 nm in diameter)
Figure Exploring: Chromatin Packing in a Eukaryotic Chromosome

11. Nucleosome (10 nm in diameter)
Figure 16.22d
Nucleosome (10 nm in diameter)
Figure Exploring: Chromatin Packing in a Eukaryotic Chromosome

12. Figure 16.22e
30-nm fiber
Figure Exploring: Chromatin Packing in a Eukaryotic Chromosome

13. Loops Scaffold Figure 16.22f
Figure Exploring: Chromatin Packing in a Eukaryotic Chromosome
Loops
Scaffold

14. Chromatid (700 nm) Figure 16.22g
Figure Exploring: Chromatin Packing in a Eukaryotic Chromosome

15. Chromatin undergoes changes in degree of packing during the cell cycle
During interphase, some chromatin is organized into a 10-nm fiber, but much is compacted into a 30-nm fiber, through folding and looping
chromosomes are not highly condensed during interphase

16. Figure 16.23
Figure Impact: Painting Chromosomes
5 m

17. Most chromatin is loosely packed in the nucleus during interphase and condenses prior to mitosis
Loosely packed chromatin is called euchromatin
During interphase a few regions of chromatin (centromeres and telomeres) are highly condensed into heterochromatin