Is it true that the nucleus consists of a nuclear envelope?

Nucleus

The nucleus consists of a nuclear envelope that is continuous with the ER, chromatin, matrix, and a nucleolus the site of ribosomal ribonucleic acid (rRNA) synthesis and initial ribosomal assembly. The nuclear envelope contains pores for bidirectional transport and is supported by intermediate filament proteins, the lamins. Chromatin consists of euchromatin (eu = true), which is an open form of DNA that is actively transcribed, and heterochromatin that is quiescent. There is a sequential packing of chromatin beginning with the DNA double helix, which is combined with histones to 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. H1 is 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.
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 separation of cytoplasm (cytokinesis) occurs through the action of an actin contractile ring.

The cell cycle consists of interphase (G1, S, and G2), and the stages of mitosis (M): prophase, prometaphase, metaphase, anaphase, and telophase. The cell cycle is regulated at the G1/S and G2/M boundaries (checkpoints) by phosphorylation of complexes of a protein kinase [cyclin-dependent kinase (Cdk) protein] and a cyclin (cytoplasmic oscillator). For example, the G2/M interface is regulated by M-Cdk complex (formerly called Mitosis Promoting Factor, MPF), which is responsible for the phosphorylation of spindle proteins, histones, and lamins.

Phosphorylation of lamins results in their breakdown as well as the dissolution of the nuclear envelope. There are different cyclins and Cdks for each of the cell cycle checkpoints. Overarching the Cdks are the Cdk inhibitors that 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 genes that have been well studied are retinoblastoma gene (Rb) and p53. Rb is active (suppressing growth) in the hypophosphorylated state and inactive in the hyperphosphorylated form. In its nonphosphorylated form Rb serves as a brake on the cell cycle at the G1/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 transcription of cell cycle genes, allowing cells to traverse the G1/S interface.

Mutations in Rb occur in tumors; a mutation has the same effect as inactivating Rb leading to uncontrolled cell proliferation as E2F trancribes cell cycle genes. p53 is a protective gene or molecular policeman, which prevents the replication of damaged DNA and stimulates repair. p53 acts as a transcription factor and also works through the Cdk inhibitors to arrest the cell cycle at the G1/S interface. p53 mutations are found in many human tumors.

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