Nucleus

Definition

The membrane-bound organelle of a eukaryotic cell that houses the chromosomes and controls gene expression, cell division, and hereditary information. In arthropods, the nucleus varies in size and chromatin organization across cell types: polyploid nuclei in insect salivary glands and fat body, condensed nuclei in spermatozoa, and syncytial nuclei in early embryonic development. The nuclear envelope, perforated by nuclear pores, separates nucleoplasm from cytoplasm and regulates molecular traffic.

Etymology

Latin, diminutive of nux (nut), meaning kernel or seed inside a fruit.

Example

In Drosophila melanogaster, the giant polytene chromosomes of larval salivary gland nuclei allow cytogenetic mapping of gene loci; each nucleus contains about 1024 copies of the genome aligned in register, producing visible banding patterns used to identify chromosomal inversions and deletions.

Synonyms

• karyon

Related Terms

• Nucleolus
• Chromosome
• nuclear envelope
• Cytoplasm
• organelle
• polyteny
• syncytium
• karyotype

Usage Notes

Distinguished from the atomic nucleus in physics. In cell biology, 'nucleus' without qualification always refers to the cellular organelle. Arthropod nuclei show notable specializations: insect nurse cell nuclei become highly polyploid to supply mRNA to developing oocytes, while arachnid spermatozoa often exhibit compact, non-motile nuclei adapted for cryptic female sperm storage. The adjectival form is 'nuclear' (NOO-klee-ur), not to be confused with 'nucleolar' (pertaining to the nucleolus).