Page 217 - Color Atlas of Biochemistry

208 Organelles

Nucleus proteins can enter the nucleus without dif -
culty. By contrast, larger proteins (over
40 kDa) can only pass through the nuclear
A. Nucleus
pores if they carry a nuclear localization se-
The nucleus is the largest organelle in the eu- quence consisting of four successive basic
karyotic cell. With a diameter of about 10 µm, amino acids inside their peptide chains (see
it is easily recognizable with the light micro- p. 228). mRNAs and rRNAs formed in the nu-
scope. This is the location for storage, replica- cleus cross the pores into the cytoplasm as
tion, and expression of genetic information. complexes with proteins (see below).
The nucleus is separated from the cyto-
plasm by the nuclear envelope, which consists C. Relationships between the nucleus and
of the outer and inner nuclear membranes.
cytoplasm
Each of the two nuclear membranes has two
layers, and the membranes are separated Almost all of the RNA in the cell is synthesized
from each other by the perinuclear space. in the nucleus. In this process, known as
The outer nuclear membrane is continuous transcription, the information stored in DNA
with the rough endoplasmic reticulum and is transcribed into RNA (see p. 242). As men-
is covered with ribosomes. The inner side of tioned above, ribosomal RNA (rRNA) is mainly
the membrane is covered with a protein layer produced in the nucleolus, while messenger
(the nuclear lamina), in which the nuclear and transfer RNA (mRNA and tRNA) are
structures are anchored. formed in the region of the euchromatin. En-
The nucleus contains almost all of the cell’s zymatic duplication of DNA—replication—also
DNA (around 1% of which is mitochondrial only takes place in the nucleus (see p. 240).
DNA). Together with histones and structural Thenucleotidecomponents requiredfor
proteins, the nuclear DNA forms the chroma- transcription and replication have to be im-
tin (see p. 238). It is only during cell division ported into the nucleus from the cytoplasm.
that chromatin condenses into chromosomes, Incorporation of these components into RNA
which are also visible with the light micro- leads to primary products, which are then
scope. During this phase, the nuclear mem- altered by cleavage, excision of introns, and
brane temporarily disintegrates. the addition of extra nucleotides (RNA matu-
During the phase between cell divisions, ration; see p. 242). It is only once these pro-
the interphase, it is possible to distinguish cess have been completed that the RNA mol-
between the more densely packed hetero- ecules formed in the nucleus can be exported
chromatin and loose euchromatin using an into the cytoplasm for protein synthesis
electron microscope. Active transcription of (translation;see p. 250).
DNA into mRNA takes place in the region of The nucleus is not capable of synthesizing
the euchromatin. A particularly electron- proteins. All of the nuclear proteins therefore
dense region is noticeable in many have to be imported—the histones with which
nuclei—the nucleolus (several nucleoli are DNA is associated in chromatin, and also the
sometimes present). The DNA in the nucleolus so–called non–histone proteins (DNA poly-
contains numerous copies of the genes for merases and RNA polymerases, auxiliary and
rRNAs (see p. 242). They are constantly under- structural proteins, transcription factors, and
going transcription, leading to a high local ribosomal proteins). Ribosomal RNA (rRNA)
concentration of RNA. already associates with proteins in the nucle-
olus to form ribosome precursors.
A special metabolic task carried out by the
B. Nuclear pores +
nucleus is biosynthesis of NAD . The immedi-
The exchange of substances between the nu- ate precursor of this coenzyme, nicotinamide
+
cleus and the cytoplasm is mediated by pore mononucleotide (NMN ), arises in the cyto-
complexes with complicated structures, plasm and is then transported into the nucle-
which traverse the nuclear membrane. The olus, where it is enzymatically converted into
+
+
nuclear pores consist of numerous proteins the dinucleotide NAD . Finally, NAD then re-
that form several connected rings of varying turns to the cytoplasm.
diameter. Low-molecular structures and small

Koolman, Color Atlas of Biochemistry, 2nd edition © 2005 Thieme
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