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

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

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.

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

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

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

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

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

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

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

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

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

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

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

Figure 16.23 Figure 16.23 Impact: Painting Chromosomes 5 m

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