A&P Lecture 1.3: Nucleus

This lecture note on Anatomy and Physiology is linked to A&P Lecture 1: The Cell to further explain the physiology of the Nucleus.

ANATOMY AND PHYSIOLOGY LECTURE 1.3

NUCLEUS

The nucleus is a large, membrane-bound structure usually located near the center of the cell. It may be spherical, elongated, or lobed, depending on the cell type. All cells of the body have a nucleus at some point in their life cycle, although some cells, such as red blood cells, lose their nuclei as they develop. Other cells, such as skeletal muscle cells and certain bone cells, called osteoclasts, contain more than one nucleus.

The nucleus consists of nucleoplasm surrounded by a nuclear envelope composed of two membranes separated by a space. At many points on the surface of the nuclear envelope, the inner and outer membranes fuse to form porelike structures called nuclear pores. Molecules move between the nucleus and the cytoplasm through these nuclear pores.

Deoxyribonucleic acid (DNA) is mostly found within the nucleus , although small amounts of DNA are also found within mitochondria. Nuclear DNA and associated proteins are organized into discrete structures called chromosomes. The proteins include histones, which are important for the structural organization of DNA, and other proteins that regulate DNA function. During most of the life cycle of a cell, the chromosomes are dispersed throughout the nucleus as delicate filaments referred to as chromatin. During cell division, the dispersed chromatin becomes densely coiled, forming compact chromosomes. At the beginning of cell division, each chromosome consists of two chromatids , which are attached at a single point called the centromere. The kinetochore, a protein structure within the centromere, provides a point of attachment for microtubules during cell division.

DNA determines the structural and functional characteristics of the cell by specifying the structure of proteins. Proteins form many structural components of the cell and all the enzymes, which regulate most chemical reactions in the cell. DNA establishes the structure of proteins by specifying the sequence of their amino acids. DNA is a large molecule that does not leave the nucleus but functions by means of an intermediate, ribonucleic acid (RNA) , which can leave the nucleus through nuclear pores. DNA determines the structure of messenger RNA (mRNA), ribosomal RNA (rRNA), and transfer RNA (tRNA) (all described in more detail on p. 86). A sequence of nucleotides in a DNA molecule that specifies the structure of a protein or RNA molecule is called a gene.

Because mRNA synthesis occurs within the nucleus, cells without nuclei accomplish protein synthesis only as long as the mRNA produced before the nucleus degenerates remains functional. The nuclei of developing red blood cells are expelled from the cells before the red blood cells enter the blood, where they survive without a nucleus for about 120 days. In comparison, many cells with nuclei, such as nerve and skeletal muscle cells,

survive as long as the person survives. 

A nucleolus is a somewhat rounded, dense region within the nucleus that lacks a surrounding membrane . Usually, one nucleolus exists per nucleus, but several nucleoli may also be seen in the nuclei of rapidly dividing cells. The nucleolus incorporates portions of 10 chromosomes (5 pairs), called nucleolar organizer regions. These regions containDNA from which rRNA is produced. Within the nucleolus, the subunits of ribosomes are manufactured