The Nucleus, Chromosomes and Microbodies

The nucleus stores and uses hereditary information

In a typical eukaryotic cell, the nucleus contains most of the cell's genomic DNA in association with proteins and other molecules. It is more than a passive container: many processes related to gene regulation, DNA replication and RNA formation occur there. Robert Brown described the nucleus in plant cells in the nineteenth century, and later microscopy and staining techniques revealed chromatin. The shape, size and number of nuclei vary among cell types, but a prominent membrane-bound nucleus is a familiar feature of many eukaryotic cells.

Some cells naturally have more than one nucleus; other specialised cells may lose their nuclei during development. Mature erythrocytes in most mammals lack nuclei, and mature sieve-tube elements of flowering plants are also enucleate but remain integrated with supporting cells. These exceptions do not make them prokaryotes: they originated by differentiation of eukaryotic cells. The nucleus must be understood in the context of development rather than treated as a universal always-present structure in every mature eukaryotic cell.

The nuclear envelope is double layered and selective

The nuclear envelope contains an inner membrane and an outer membrane separated by a perinuclear space, typically on the scale of tens of nanometres. The outer membrane is often continuous with endoplasmic reticulum and may bear attached ribosomes. Both membranes are interrupted at nuclear pore complexes, large assemblies that regulate the movement of proteins and RNA-containing particles between nucleus and cytoplasm. Small molecules may diffuse through pores under suitable conditions, while larger macromolecules often require specific transport mechanisms.

Inside the envelope is nucleoplasm containing chromatin and one or more nucleoli. A nucleolus is not bounded by a membrane; it is a concentrated region where ribosomal RNA is produced and early ribosomal assembly occurs. Cells synthesising many proteins often have prominent nucleoli because ribosome production is in high demand. A chromosome and a nucleolus are therefore not alternative names for the same structure: chromosomes package DNA, while nucleoli are specialised sites in RNA and ribosome biogenesis.

Chromatin becomes conspicuous chromosomes during division

During interphase, much chromatin is distributed as less-condensed fibres rather than easily distinguished individual chromosomes. Before and during cell division, DNA is reorganised and condensed so individual chromosomes become visible by appropriate staining. Chromatin contains DNA, histone proteins, additional non-histone proteins and associated RNA. In a human diploid somatic cell there are typically 46 chromosomes arranged in 23 pairs; the long DNA molecules are packed into a nucleus only a few micrometres across.

A duplicated chromosome can contain two sister chromatids joined at a centromere region. The centromere is a chromosome region important for establishing the kinetochore, a protein assembly where spindle microtubules attach. Although textbook drawings may show a round point at the centre of a chromosome, the centromere is a specialised DNA/chromatin region rather than merely a hole in a drawing. A chromosome's centromere position determines its arm proportions.

Four chromosome shapes based on centromere placement

A metacentric chromosome has its centromere near the middle, giving roughly equal arms. In a submetacentric chromosome, the centromere is displaced from the midpoint, producing one shorter and one longer arm. An acrocentric chromosome has a centromere very close to one end, producing an extremely short arm and a long arm. A telocentric chromosome has a terminal centromere, with effectively one main arm. These are geometric labels independent of whether the chromosome contains a particular gene.

Some chromosomes contain a secondary constriction, separate from the centromere, and may show a small segment called a satellite. A satellite is identified by its position beyond the secondary constriction; it is not equivalent to a kinetochore or a microbody. In humans some acrocentric chromosomes have satellite regions, providing a practical example of why an additional constriction must not be misidentified as the primary centromere.

Microbodies are enzyme-containing membrane compartments

Microbodies are small membrane-enclosed organelles containing specialised enzymes in plant and animal cells. Peroxisomes are a familiar example and participate in oxidation reactions and detoxification of hydrogen peroxide through enzymes such as catalase. In plants, specialised microbodies also contribute to aspects of metabolism associated with photorespiration or conversion of stored reserves in germinating seeds. Although these processes may coordinate with mitochondria and chloroplasts, the organelles remain distinct compartments.

Microbodies should not be confused with lysosomes. Lysosomes are often characterised by acidic digestive hydrolases and roles in macromolecule turnover, while peroxisomes typically contain oxidative enzymes and catalase-related reactions. Both are bounded by a single membrane, but similar shape does not imply identical chemistry. The textbook introduces microbodies briefly, so a focused structural and functional distinction is more useful than inventing an extensive additional pathway here.

What to label in two important drawings

For the nucleus, draw two membranes, a nuclear pore, nucleoplasm, chromatin fibres and a nucleolus. For chromosomes, draw representative centromere positions and label the two arms where applicable. Add a kinetochore only at the proper centromere region, and add a satellite beyond a secondary constriction if including that special example. The organelle and chromosome drawings represent different scales and should not be superimposed as though they were a single microscopic view.

Quick Quiz

Model Answers

1. Show inner and outer nuclear membranes, perinuclear space, nuclear pores, nucleoplasm, chromatin and nucleolus; note outer membrane continuity with ER.

2. Metacentric has central centromere and equal arms; submetacentric is off-centre; acrocentric is near one end; telocentric is at the terminal end.

3. Chromatin is DNA with associated proteins; chromosomes are organisational units of chromatin; centromeres are specialised chromosome regions; kinetochores are protein complexes assembled on centromeres for microtubule attachment.

4. They develop from eukaryotic precursors and expel the nucleus during differentiation. Their developmental lineage and other biological features remain eukaryotic.

5. Peroxisomes, an example of microbodies, contain oxidative enzymes such as catalase. Lysosomes contain many acid hydrolases for controlled digestion.

6. A chromosome with a characteristic secondary constriction leaving a small distal fragment termed the satellite; this constriction is distinct from the main centromere.