which is an open form of DNA that is actively transcribed, and hete-
rochromatinthat is quiescent. There is a sequential packing of chromatin
beginning with the DNA double helix, which is combined with histonesto
form the nucleosomes,the smallest unit of chromatin structure. This is the
“beads on a string”structure with the histones forming the octamer
arrangment of paired H2A, H2B, H3, and H4. H1is the linker histone.
The nucleosomes are connected by strands of protein free DNA, so called
linker DNA.Nucleases degrade the linker DNA, but nucleosome particles
are protected against micrococcal nuclease activity because of the close
interaction of DNA with histone proteins. The next orders of packing are
the 30 nm chromatin fibril, the chromatin fiber with loops of chromatin
fibrils, and chromatin fibers loosely or tightly packed in euchromatin and
heterochromatin respectively.
During cell division, DNA is accurately replicated and divided
equally between two daughter nuclei. Equal distribution of chromosomes
is accomplished by the microtubules of the mitotic spindle.The separa-
tion of cytoplasm (cytokinesis)occurs through the action of an actin con-
tractile ring. The cell cycle consists of interphase (G 1 , S, and G 2 ), and the
stages of mitosis (M): prophase, prometaphase, metaphase, anaphase,
and telophase.The cell cycle is regulated at the G 1 /S and G 2 /M boundaries
(checkpoints) by phosphorylation of complexes of a protein kinase
[cyclin-dependent kinase (Cdk) protein] and a cyclin (cytoplasmic
oscillator).For example, the G 2 /M interface is regulated by M-Cdk com-
plex(formerly called Mitosis Promoting Factor, MPF), which is respon-
sible for the phosphorylation of spindle proteins, histones, and lamins.
Phosphorylation of laminsresults in their breakdown as well as the disso-
lution of the nuclear envelope. There are different cyclins and Cdks for
each of the cell cycle checkpoints. Overarching the Cdks are the Cdk
inhibitorsthat form an additional regulatory layer at each of the cell cycle
checkpoints. Study of the cell cycle is critical to an understanding of the
regulation of abnormal proliferation as occurs in cancer cells. Two tumor
suppressor genesthat have been well studied are retinoblastoma gene
(Rb)andp53.Rb is active (suppressing growth) in the hypophosphory-
lated state and inactive in the hyperphosphorylated form. In its nonphos-
phorylated form Rb serves as a brake on the cell cycle at the G 1 /S interface
by binding to the transcription factor, E2F.Stimulation by growth factors
results in phosphorylation and release of the brake; E2F is free to turn on
High-Yield Facts 17