The Cell Nucleus

Every cell with the exception of red blood cells has a nucleus. However, there are cells with two nuclei (some liver cells) or greater numbers of nuclei, e. g., osteoclasts in bony tissue (5−20 nuclei) or skeletal muscle cells (more than 1000 nuclei). Cells without nuclei can no longer divide. The nuclei are separated from the surrounding cytoplasm by two elementary membranes (nuclear membranes, nuclear envelope) but are connected to the endoplasmic reticulum by so-called nuclear pores. The nucleus usually contains a clearly defined round structure, the nucleolus. Its task is the production of ribosomal RNA (rRNA). It is therefore inconspicuous in inactive cells, but is welldefined in metabolically active cells with increased protein synthesis. Multiple nucleoli may occur in such cells. The size and shape of the nucleus vary from cell to cell: its form may be round, lobulated, or extended. Its shape and structure also depend at any one time on the current phase of the cell’s cycle. For instance, in the phase of cell division, filiform structures—the chromosomes—become apparent, while these are invisible in the phase between divisions, the so-called interphase.
Chromosomes and Genes
Chromosomes are the carriers of hereditary characteristics called genes. The human cell nucleus contains 46 chromosomes (diploidy) in the form of 23 chromosome pairs (23 male, 23 female chromosomes). The individual chromosomes can be distinguished by their total length, the lengths of their arms, and the position of their segmentations. By these means, individual chromosomal pairs can be assigned to specific groups (karyotyping)and numbered in decreasing size from 1 to 22, with the 23rd pair determining sex. With the exception of the sex chromosomes (heterochromosomes = allosomes), male and female chromosomes (homologous chromosomes = autosomes) correspond to each other in their hereditary characteristics. Whereas the female human has two sex chromosomes of equal size, the male human has one large and one small sex chromosome.
In humans the 23 chromosome pairs contain about twice 30000−40000 hereditary markers or genes. Each of their genes occurs twice in each cell of the body, namely one male and one female (diploidy). In contrast, the germ cells (egg and sperm cells) each have only a single set of chromosomes (haploidy). With 23 chromosomes and a total complement of 30000−40000, each chromosome therefore contains about 1300−1700 genes.

a, b Chromosome set of a normal human cell. (After Langman)

a The chromosomes are prepared and viewed by cultivating the cells in an artificial medium. This is followed by treatment with a colchicine solution, which blocks the mitoses in metaphase. The cells are then fixed, spread on a slide, and stained

b The chromosomes shown in a are arranged in a karyotype by total length and position of the centrosome. The two sex chromosomes (XY) determine the sex (male in this case)