Page 21 - Color Atlas Physiology

The Cell inal. In cell division, this process is the means
by which duplication of genetic information
The cell is the smallest functional unit of a (replication) is achieved.
living organism. In other words, a cell (and no Messenger RNA (mRNA) is responsible for
smaller unit) is able to perform essential vital code transmission, that is, passage of coding
functions such as metabolism, growth, move- sequences from DNA in the nucleus (base
ment, reproduction, and hereditary transmis- sequence) for protein synthesis in the cytosol
sion (W. Roux) (! p. 4). Growth, reproduction, (amino acid sequence) (! C1). mRNA is
and hereditary transmission can be achieved
formed in the nucleus and differs from DNA in
Fundamentals and Cell Physiology membrane, cytosol or cytoplasm (ca. 50 vol.%), uracil (U) instead of thymine. In DNA, each
that it consists of only a single strand and that
by cell division.
it contains ribose instead of deoxyribose, and
Cell components: All cells consist of a cell
and membrane-bound subcellular structures
amino acid (e.g., glutamate, ! E) needed for
known as organelles (! A, B). The organelles of
synthesis of a given protein is coded by a set of
three adjacent bases called a codon or triplet
eukaryotic cells are highly specialized. For in-
stance, the genetic material of the cell is con-
(C–T–C in the case of glutamate). In order to
transcribe the DNA triplet, mRNA must form a
centrated in the cell nucleus, whereas “diges-
tive” enzymes are located in the lysosomes.
complementary codon (e.g., G–A–G for gluta-
mate). The relatively small transfer RNA
Oxidative ATP production takes place in the
codon in the ribosomes (! C2). tRNA contains
The cell nucleus contains a liquid known
as karyolymph, a nucleolus, and chromatin.
a complementary codon called the anticodon
Chromatin contains deoxyribonucleic acids
1 mitochondria. (tRNA) molecule is responsible for reading the
for this purpose. The anticodon for glutamate
(DNA), the carriers of genetic information. Two is C–U–C (! E).
strands of DNA forming a double helix (up to RNA synthesis in the nucleus is controlled
7 cm in length) are twisted and folded to form by RNA polymerases (types I–III). Their effect
chromosomes 10µm in length. Humans nor- on DNA is normally blocked by a repressor pro-
mally have 46 chromosomes, consisting of 22 tein. Phosphorylation of the polymerase oc-
autosomal pairs and the chromosomes that curs if the repressor is eliminated (de-repres-
determine the sex (XX in females, XY in males). sion) and the general transcription factors at-
DNA is made up of a strand of three-part tach to the so-called promoter sequence of the
molecules called nucleotides, each of which DNA molecule (T–A–T–A in the case of poly-
consists of a pentose (deoxyribose) molecule, a merase II). Once activated, it separates the two
phosphate group, and a base. Each sugar strands of DNA at a particular site so that the
molecule of the monotonic sugar–phosphate code on one of the strands can be read and
backbone of the strands (...deoxyribose – transcribed to form mRNA (transcription,
phosphate–deoxyribose...) is attached to one ! C1a, D). The heterogeneous nuclear RNA
of four different bases. The sequence of bases (hnRNA) molecules synthesized by the poly-
represents the genetic code for each of the merase have a characteristic “cap” at their 5!
roughly 100 000 different proteins that a cell end and a polyadenine “tail” (A–A–A–...) at the
produces during its lifetime (gene expression). 3! end (! D). Once synthesized, they are im-
In a DNA double helix, each base in one strand mediately “enveloped” in a protein coat, yield-
of DNA is bonded to its complementary base in ing heterogeneous nuclear ribonucleoprotein
the other strand according to the rule: adenine (hnRNP) particles. The primary RNA or pre-
(A) with thymine (T) and guanine (G) with cy- mRNA of hnRNA contains both coding
tosine (C). The base sequence of one strand of sequences (exons) and non-coding sequences
the double helix (! E) is always a “mirror (introns). The exons code for amino acid
image” of the opposite strand. Therefore, one sequences of the proteins to be synthesized,
strand can be used as a template for making a whereas the introns are not involved in the
new complementary strand, the information coding process. Introns may contain 100 to
8 content of which is identical to that of the orig- 10 000 nucleotides; they are removed from the
!
Despopoulos, Color Atlas of Physiology © 2003 Thieme
All rights reserved. Usage subject to terms and conditions of license.