3.1 The Nucelus

The Nucleus

The nucleus contains the master plan for all cell structures and activities encoded in the DNA of the chromosomes. It also houses the molecular machinery necessary for DNA replication and the synthesis and processing of various types of RNA. Macromolecular transfer between the nuclear and cytoplasmic compartments is tightly regulated, and the proteins required for proper nuclear function are imported from the cytoplasm.

Key Components of the Nucleus:

1. Nuclear Envelope:
   • Composed of two membrane layers surrounding the nucleus, separated by a narrow (30-50 nm) perinuclear space.
   • The outer portion of the envelope is a direct continuation of the endoplasmic reticulum (ER).
   • The inner portion, called the nuclear lamina, consists of fibrous proteins (mainly intermediate filament proteins called lamins) that help stabilize the nuclear envelope.
   • Nuclear pore complexes (nucleoporins) are present at sites where the inner and outer membranes of the nuclear envelope fuse. These complexes regulate most bidirectional transport between the nucleus and the cytoplasm.
2. Chromatin:
   • Chromatin is the chromosomal material in a largely uncoiled state.
   • Two types of chromatin can be distinguished:
     • Heterochromatin: Tightly packed DNA associated with several functions, including gene regulation and chromosome integrity protection. Its dense packing contributes to these roles.
     • Euchromatin: Lightly packed chromatin (composed of DNA, RNA, and proteins) that is rich in gene concentration. Euchromatin is often (but not always) under active transcription. Approximately 92% of the human genome is euchromatic, while the remainder is heterochromatin.
   • Chromatin mainly consists of coiled DNA strands bound to basic proteins called histones and various non-histone proteins.
   • The nucleosome is the basic structural unit of chromatin, consisting of a core of eight small histones (two copies each of H2A, H2B, H3, and H4) around which DNA wraps (approximately 150 base pairs). Each nucleosome also includes a larger linker histone (H1) that binds both the wrapped DNA and the core surface.
   • Further folding of DNA bound to nucleosomes results in the next level of chromatin organization (30 nm fiber). Higher orders of chromatin coiling lead to the formation of microscopically visible stained structures known as chromosomes. These structures play a crucial role during mitosis and meiosis when chromatin condenses.
3. Chromatin Pattern and Cell Activity:
   • The chromatin pattern within a nucleus provides insights into the cell’s activity.
   • Cells with lightly stained nuclei are generally more active in protein synthesis than those with condensed, dark nuclei.
   • In nuclei with abundant euchromatin (loose and less dense chromatin, ready for transcription and subsequent protein synthesis), more DNA surface is available for RNA transcription.
4. Sex Chromatin:
   • In humans, males have one X chromosome and one Y chromosome, while females have two X chromosomes.
   • The X and Y chromosomes contain genes that determine an individual’s sex.
   • Most somatic cells (body cells) contain 22 pairs of autosomes in addition to one pair of sex chromosomes (XX in females and XY in males).
   • Each chromosomal pair consists of homologous chromosomes, which carry different alleles of the same genes from different parents.
   • Somatic cells are considered diploid (2N chromosomes, totaling 46), while sperm and mature oocytes are haploid (N chromosomes).