Nucleus and Cell Cycle Phases

Study Notes

The nucleus contains DNA blueprint for all cell structures, and the molecular machinery to replicate DNA and synthesize three types of RNA (rRNA, mRNA, tRNA).
The nucleus does not produce proteins; it imports needed proteins from the cytoplasm.
The nucleus is usually a rounded or elongated structure in the center of the cell, visible with light microscopy (LM) as a dark blue or black structure.
The nucleus is surrounded by a nuclear envelope composed of two parallel membranes, the perinuclear cisterna, and nuclear pores.
These nuclear pores allow for controlled exchange of substances between the nucleus and the cytoplasm.
Chromatin is mainly coiled DNA and basic proteins (histones), arranged as nucleosomes.
Two types of chromatin exist: heterochromatin (electron dense, inactive) and euchromatin (less coiled, active).
The nucleolus, a spherical structure rich in rRNA and protein, is typically basophilic; it synthesizes ribosome subunits that migrate to the cytoplasm through nuclear pores.

The cell cycle is a series of events preparing cells for division into two daughter cells.
Interphase is the longest period of the cell cycle, where the cell grows, and replicates its genetic material.
Interphase has three stages:
- G1 (gap 1) phase: Cell growth, synthesis of macromolecules essential for DNA duplication begins.
- S (synthesis) phase: DNA is duplicated.
- G2 (gap 2) phase: Cell prepares for mitosis, including centriole duplication, and energy storage.
Mitosis is the shorter phase where the nucleus and cytoplasm divide, creating two daughter cells.
G0 phase (outside cycle): A resting phase, cells may exit the cell cycle either permanently or temporarily.