Nucleus

The nucleus (Figure 1-1-1) is the site of deoxyribonucleic acid (DNA) replication and transcription of DNA into precursor ribonucleic acid (RNA) molecules. It contains all of the enzymes required for replication and repair of newly synthesized DNA, as well as for transcription and processing of precursor RNA molecules. It is enclosed by the nuclear envelope and contains the nuclear lamina, nucleolus, and chromatin.

Nuclear Envelope

The nuclear envelope is a double membrane containing pores that are approximately 90 nm in diameter. The outer nuclear membrane is continuous with the endoplasmic reticulum.

Nuclear Lamina

The nuclear lamina is a latticelike network of proteins that include lamins. Lamins attach chromatin to the inner membrane of the nuclear envelope and participate in the breakdown and reformation of the nuclear envelope during the cell cycle. Phosphorylation of the lamina (by lamin kinase) during prophase of mitosis initiates nuclear disassembly into small vesicles.

Nucleolus

The nucleolus is responsible for ribosomal RNA (rRNA) synthesis and ribosome assembly. It contains three morphologically distinct zones:

• Granular zone—found at the periphery; contains ribosomal precursor particles in various stages of assembly.

• Fibrillar zone—centrally located; contains ribonudear protein fibrils.

■ Fibrillar center—contains DNA that is not being transcribed.

Chromatin

Chromatin is a complex of DNA histone proteins, and nonhistone proteins.

• DNA—a double-stranded helical molecule that carries the genetic information of the cell. It exists in three conformations: B DNA, Z DNA, and A DNA

■ Histone proteins—positively charged proteins enriched with lysine and arginine residues. They are important in forming two types of structures in chromatin: nucleo-somes and solenoid fibers. The nucleosomes are the basic repeating units of the chromatin fiber, having a diameter of approximately 10 nm.

■ Nonhistone proteins—include enzymes involved in nuclear functions such as replication, transcription, DNA repair, and regulation of chromatin function. They are acidic or neutral proteins.

Figure 1-1-1

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