Nucleus (biology), membrane-bound structure of a
cell that plays two crucial roles. The nucleus carries
the cell's genetic information that determines if the
organism will develop, for instance, into a tree or a
human; and it directs most cell activities including
growth, metabolism, and reproduction by regulating
protein synthesis (the manufacture of long chains of
amino acids). The presence of a nucleus distinguishes
the more complex eukaryotic cells of plants and
animals from the simpler prokaryotic cells of bacteria
and cyanobacteria that lack a nucleus.
The nucleus is the most prominent structure in the
cell. It is typically round and occupies about 10
percent of the cell's total volume. The nucleus is
wrapped in a double-layered membrane called the
nuclear envelope. The space between the nuclear
envelope layers is called perinuclear space. The
nuclear envelope is attached to a network of
membrane-enclosed tubules that extends throughout
the cell called the endoplasmic reticulum. The nuclear
envelope is perforated by many holes, called nuclear
pores, that permit the movement of selected molecules
between the nucleus and the rest of the cell, while
blocking the passage of other molecules.
The nucleus contains the nucleolus, which
manufactures protein-producing structures called
ribosomes. Genetic information in the form of
deoxyribonucleic acid (DNA) is stored in threadlike,
tangled structures called chromatin within the nucleus.
During the process of cell division known as mitosis, in
which the nucleus divides, the chromatin condense
into several distinct structures called chromosomes.
Each time the cell divides, the heredity information
carried in the chromosomes is passed to the two newly
formed cells.
The DNA in the nucleus also contains the instructions
for regulating the amount and types of proteins made
by the cell. These instructions are copied, or
transcribed, into a type of ribonucleic acid (RNA)
called messenger RNA (mRNA). The mRNA is
transported from the nucleus to ribosomes, where
proteins are assembled.
Contributed By: John B. Ferguson, Sc.B., M.Phil., Ph.D.
Professor of Biology, Division of Natural Sciences and
Mathematics, Bard College. Director, Distinguished
Scientist Lecture Series, Bard Center.
Bio
|
