Inside the Cell Nucleus: From Chromatin to Chromosomes

The Eukaryotic Cell Nucleus

General Characteristics

The cell nucleus is a membranous organelle that allows for the compartmentalization of DNA. It contains proteins called histones and acts as the cell's control center.

Number, Shape, Location, and Size

• Number: Most cells have a single nucleus. Some, like liver cells, can exceptionally have two. Multinucleated cells formed by cell fusion are called syncytia, while those formed by mitosis without cell division are called plasmodia.
• Shape, Location, and Size: The nucleus is typically located in the center of an animal cell. In plant cells, it is often pushed to one side by the large central vacuole.

Structure and Composition of the Nucleus

The nucleus can be observed in two main states: the interphase nucleus and the mitotic nucleus.

The Interphase Nucleus

Its components include the nuclear envelope, chromatin, nucleolus, and nucleoplasm.

Nuclear Envelope

This is a double membrane consisting of an inner and an outer nuclear membrane, separated by the perinuclear space. The outer membrane is continuous with the Endoplasmic Reticulum (ER). The membranes communicate through nuclear pores, which regulate the transport of molecules like proteins and RNA. Each pore is part of a complex structure integrated by eight protein blocks. Between the nucleoplasm and the inner nuclear membrane lies the nuclear lamina, a fibrous network made of intermediate filaments.

Chromatin

Chromatin is a complex of DNA and proteins (primarily histones) that condenses to form chromosomes. Its fundamental structure is the nucleosome.

• Nucleosome: A discoidal protein complex (an octamer of histones) around which a segment of DNA is wound.
• H1 Histone: A linker histone that helps to further compact the DNA.

This chain forms the basic 10nm chromatin fiber, which coils into a thicker 30nm fiber.

Types of Chromatin

• Euchromatin: Less condensed and transcriptionally active.
• Heterochromatin: Highly condensed and transcriptionally inactive. There are two types:
  • Constitutive Heterochromatin: Always inactive.
  • Facultative Heterochromatin: Can be active or inactive depending on the cell type.

Nucleolus

A prominent structure within the interphase nucleus, its primary function is the synthesis and assembly of ribosomal subunits. The nucleolus is composed of RNA, DNA (the nucleolar organizer region), and proteins. Cytoplasmic ribosomal proteins are imported and assembled with ribosomal RNA (rRNA).

Nucleoplasm

Also called karyoplasm, this is the aqueous matrix of the nucleus, composed of water, solutes, and various proteins.

The Mitotic Nucleus

During mitosis, chromatin undergoes extreme structuring and condensation to form visible chromosomes. The size and number of chromosomes vary and are species-specific. Cells can be:

• Haploid (n): One set of chromosomes.
• Diploid (2n): Two sets of chromosomes.
• Polyploid (more than 2n): More than two sets of chromosomes.

Metaphase Chromosome Structure

A metaphase chromosome consists of two identical sister chromatids joined at the centromere.

• Kinetochores: Protein plates at the centromere that connect the chromosome to the microtubules of the mitotic spindle.
• Telomeres: The protective caps at the ends of the chromatids that prevent the loss of genetic information during replication.
• Satellites: Segments of the chromosome separated from the main body by a secondary constriction.

Chromosome Organization

The chromatin fiber is further organized into loops anchored to a protein scaffold, leading to the highly compacted structure of a metaphase chromosome.

Karyotype

The karyotype refers to the specific characteristics (number, size, shape) of the chromosomes of a species. Its graphical representation is called a karyogram. The primary function of the chromosome is to ensure the accurate division of DNA between daughter cells.