Page 245 - Color Atlas of Biochemistry

P. 245

236 Molecular genetics

Molecular genetics: overview mine for uracil—it is identical to that of the
sense strand. In this way, the DNA triplet TTC
Nucleic acids (DNA and various RNAs)are of gives rise in hnRNA to the RNA codon UUC.
central importance in the storage, transmis- RNA maturation. In eukaryotes, the hnRNA
sion, and expression of genetic information. initially formed is modified several times be-
The decisive factor involved is their ability to fore it can leave the nucleus as messenger
enter into specific base pairings with each RNA (mRNA, 4). During RNA maturation,
other (see p. 84). The individual processes in- superfluous (“intervening”) sequences (in-
volved,which aresummedup inanoverview trons) are removed from the molecule, and
here, are discussed in more detail on the fol- both ends of the transcript are protected by
lowing pages. the addition of further nucleotides (see
p. 246).
Translation. Mature mRNA enters the cyto-
A. Expression and transmission of genetic
plasm, where it binds to ribosomes,which
information
convert the RNA information into a peptide
Storage. The genetic information of all cells is sequence. The ribosomes (see p. 250) consist
stored in the base sequence of their DNA (RNA of more than 100 proteins and several RNA
only occurs as a genetic material in viruses; molecules (rRNA;see p. 82). rRNAplays a role
see p. 404). Functional sections of DNA that as a ribosomal structural element and is also
code for inheritable structures or functions involved in the binding of mRNA to the ribo-
are referred to as genes. The 30 000–40 000 some and the formation of the peptide bond.
human genes represent only a few percent of The actual information transfer is based on
the genome, which consists of approximately the interaction between the mRNA codons
9
5 10 base pairs (bp). Most genes code for and another type of RNA, transfer RNA
proteins—i. e., they contain the information (tRNA; see p. 82). tRNAs, of which there are
for the sequence of amino acid residues of a numerous types, always provide the correct
protein (its sequence). Every amino acid res- amino acid to the ribosome according to the
idue is representedin DNA by a codeword(a sequence information in the mRNA. tRNAs are
codon) consisting of a sequence of three base loaded with an amino acid residue at the 3
pairs (a triplet). At the level of DNA, codons end. Approximately in the middle, they
are defined as sequences of the sense strand presentthe tripletthatiscomplementary to
read in the 5 3 direction (see p. 84). A DNA each mRNA codon, known as the anticodon
codon for the amino acid phenylalanine, for (GAA in the example shown). If the codon UUC
example, is thus TTC (2). appears on the mRNA, the anticodon binds a
Replication. During cell division, all of the molecule of Phe-t-RNA Phe to the mRNA (5)
genetic information has to be passed on to the and thus brings the phenylalanine residue at
daughter cells. To achieve this, the whole of the other end of the molecule into a position
the DNA is copied during the S phase of the in which it can take over the growing poly-
cell cycle (see p. 394). In this process, each peptide chain from the neighboring tRNA (6).
strandservesasamatrix forthe synthesis Amino acid activation. Before binding to
of a complementary second strand (1;see the ribosomes, tRNAs are loaded with the
p. 240). correct amino acids by specific ligases (7;
Transcription. For expression of a gene—i. e., see p. 248). It is the amino acid tRNA ligases
synthesis of the coded protein—the DNA se- that carry out the transfer (translation) of the
quence information has to be converted into a genetic information from the nucleic acid
protein sequence. As DNA itself is not in- level to the protein level.
volved in protein synthesis, the information
is transferred from the nucleus to the site of
synthesis in the cytoplasm. To achieve this,
the template strand in the relevant part of
the gene is transcribed into an RNA (hnRNA).
Thesequenceofthis RNA is thus complemen-
tary to that of the template strand (3), but—
with the exception of the exchange of thy-

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