Rucete ✏ Campbell Biology In a Nutshell
Unit 3 GENETICS — Concept 16.3 A Chromosome Consists of a DNA Molecule Packed Together with Proteins
A single eukaryotic chromosome contains one very long DNA molecule. To fit inside the nucleus, this DNA is highly organized and tightly packed with proteins, forming a complex known as chromatin. This packing is dynamic, changing during the cell cycle to support replication, gene expression, and cell division.
DNA Packing in Bacteria vs. Eukaryotes
- Bacterial chromosomes are circular DNA molecules associated with proteins, but not enclosed in a nucleus
- DNA is supercoiled and located in a region called the nucleoid
- In contrast, eukaryotic chromosomes are linear and form chromatin, a DNA-protein complex that is tightly regulated and structured
- Example: The E. coli genome is 4.6 million base pairs—100× more than a virus, but 1/1,000 of the DNA in a human somatic cell
- If stretched out, the DNA from one E. coli cell would be ~1 mm long—about 1,000× longer than the bacterial cell itself
Chromatin Structure and Histones
- Histones are positively charged proteins that bind tightly to negatively charged DNA
- The four main types are highly conserved across eukaryotes
- DNA wraps around a histone core (2 copies each of 4 types) to form a nucleosome—the basic unit of chromatin
- Nucleosomes resemble "beads on a string" in 10-nm fibers seen under electron microscopy
- Each nucleosome has DNA wrapped twice around the histone core; linker DNA connects nucleosomes
- Histone tails extend outward and can be modified to affect gene expression
Euchromatin vs. Heterochromatin
- Euchromatin: loosely packed chromatin; accessible to transcription machinery
- Heterochromatin: tightly packed chromatin; generally not transcribed
- During interphase, euchromatin allows active gene expression, while heterochromatin remains transcriptionally silent
- Chromatin state is regulated by other proteins that help organize chromatin into looped domains, sometimes attaching to the nuclear lamina or matrix
Chromosome Condensation in Mitosis
- During prophase, chromatin condenses into visible chromosomes
- Proteins condensin I and II help form DNA loops and compact them around a central scaffold
- Condensin II forms large loops early in prophase
- Condensin I forms smaller loops later, increasing density
- This loop-based compaction results in sister chromatids being highly condensed at metaphase
- By metaphase, each chromatid is ~700 nm wide and ready for segregation
Spatial Organization in the Nucleus
- Even during interphase, chromatin is not tangled randomly
- Each chromosome occupies a distinct territory within the nucleus
- Homologous chromosomes are generally not located together
- This compartmentalization may help regulate gene activity and prevent entanglement during division
Dynamic Nature of Chromatin
- Chromatin structure changes dynamically depending on cellular needs
- It is loosened for replication or transcription and condensed for mitosis
- Chemical modifications to histones and DNA affect chromatin structure and gene expression (explored more in Chapter 18)
- The ability to compact, modify, and reorganize chromatin is crucial for cell function, replication, and development
In a Nutshell
Eukaryotic chromosomes consist of long DNA molecules tightly packed with proteins into chromatin. This structure allows massive DNA sequences to fit in the nucleus while still being accessible when needed. Chromatin can shift between open (euchromatin) and closed (heterochromatin) states, and is further condensed into chromosomes during mitosis.