Atlas de Genética

Atlas de Genética


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DNA strand is
called antisense RNA.
B. Translation
During translation the sequence of codons
made up of the nucleotide bases in mRNA is
converted into a corresponding sequence of
amino acids. Translation occurs in a reading
framewhich is defined at the start of translation
(start codon). Amino acids are joined in the
sequence determined by the mRNA nucleotide
bases by a further class of RNA, transfer RNA
(tRNA). Each amino acid has its own tRNA,
which has a region that is complementary to its
codon of the mRNA (anticodon). The codons 1,
2, 3, and 4 of the mRNA are translated into the
amino acid sequence methionine (Met), glycine
(Gly), serine (Ser), and isoleucine (Ile), etc.
Codon 1 is always AUG (start codon).
C. Stages of translation
Translation (protein synthesis) in eukaryotes
occurs outside of the cell nucleus in ribosomes
in the cytoplasm. Ribosomes consist of subunits
of numerous associated proteins and RNA
molecules (ribosomal RNA, rRNA; p. 204).
Translation begins with initiation (1): an initia-
tion complex comprising mRNA, a ribosome,
and tRNA is formed. This requires a number of
initiation factors (IF1, IF2, IF3, etc.). Then elon-
gation (2) follows: a further amino acid, deter-
mined by the next codon, is attached. A three-
phase elongation cycle develops, with codon
recognition, peptide binding to the next amino
acid residue, and movement (translocation) of
the ribosome three nucleotides further in the 3!
direction of the mRNA. Translation ends with
termination (3), when one of three mRNA stop
codons (UAA, UGA, or UAG) is reached. The
polypeptide chain formed leaves the ribosome,
which dissociates into its subunits. The bio-
chemical processes of the stages shown here
have been greatly simplified.
D. Structure of transfer RNA (tRNA)
Transfer RNA has a characteristic, cloverleaf-
like structure, illustrated here by yeast phenyl-
alanine tRNA (1). It has three single-stranded
loop regions and four double-stranded \u201cstem\u201d
regions. The three-dimensional structure (2) is
complex, but various functional areas can be
differentiated, such as the recognition site (an-
ticodon) for the mRNA codon and the binding
site for the respective amino acid (acceptor
stem) on the 3! end (acceptor end).
References
Brenner, S. , Jacob. F., Meselson, M.: An unstable
intermediate carrying information from
genes to ribosomes for protein synthesis.
Nature 190:576\u2013581, 1961.
Ibba, M., Söll, D.: Quality control mechanisms
during translation. Science 286:1893\u20131897,
1999.
Watson J.D. et al.: Molecular Biology of the
Gene. 3rd ed. Benjamin/Cummings Publish-
ing Co., Menlo Park, California, 1987.
Fundamentals
Passarge, Color Atlas of Genetics © 2001 Thieme
All rights reserved. Usage subject to terms and conditions of license.
45The Flow of Genetic Information: Transcription and Translation
A. Transcription
C. Stages of translation
D. Structure of transfer RNA (tRNA)
mRNA
DNA double helix Rewinding Unwinding
5'
3'
1. Initiation 2. Elongation 3. Termination
5' end
3' end
Modified
nucleotides
Loop 1
Loop 2
Loop 3
1. Cloverleaf
1. structure
2. Three-dimensional structure
T stem
T loop
Variable
loop
Anticodon
stem
D loop
Anticodon
loop
D stem
Acceptor stem
Acceptor
end
76
72
69
7
12
26
Anticodon
38
32
44
20
54 64
4
3'5'
CCA U G G G C U A U C GGG C C AAA GG CC
Codons
3'5'
mRNA
1 2 3 4 5 6 7 8
Polypeptide chain
Methionine Glycine Serine Isoleucine
Glycine
tR
N
A
C C G
Alanine
tR
N
A
C C U
B. Translation
A G G
Ser
C C G
GlyMet
A U G G G C U C C
Ribosome
U A C
Met
A U G G G C U C C
A G G
LeuPhe Ser
U A GU C CC U G
3'
Anticodon
A
U
G
C
C
G
U
ACUC
AG
G
G G A
G A G C
C
C
A
G
A
C
UG A A
A
C
U
G
C
G
C
C
A
A
U
U
A
A
C
U G
G
A G
G UU
AA C C
G
C U
G
CU
U
C
3'
5'
3'
Transcription
Passarge, Color Atlas of Genetics © 2001 Thieme
All rights reserved. Usage subject to terms and conditions of license.
46
Genes and Mutation
The double helix structure of DNA is the basis of
both replication and transcription as seen in the
preceding pages. The information transmitted
during replication and transcription is arranged
in units called genes. The term gene was intro-
duced in 1909 by the Danish biologist Wilhelm
Johannsen (along with the terms genotype and
phenotype). Until it was realized that a gene
consists of DNA, it was defined in somewhat ab-
stract terms as a factor (Mendel\u2019s term) that
confers certain heritable properties to a plant or
an animal. However, it was not apparent how
mutations could be related to the structure of a
gene. The discovery thatmutations also occur in
bacteria and other microorganisms paved the
way to understanding their nature (see p. 84).
The organization of genes differs in prokaryotes
and eukaryotes as shown below.
A. Transcription in prokaryotes and
eukaryotes
Transcription differs in unicellular organisms
without a nucleus, such as bacteria (prokary-
otes, 1), and in multicellular organisms
(eukaryotes, 2), which have a cell nucleus. In
prokaryotes, themRNA serves directly as a tem-
plate for translation. The sequences of DNA and
mRNA correspond in a strict 1:1 relationship,
i.e., they are colinear. Translation begins even
before transcription has completely ended.
In contrast, a primary transcript of RNA precur-
sor mRNA) is formed first in eukaryotic cells.
This is a preliminary form of the mature mRNA.
The mature mRNA is formed when the noncod-
ing sections are removed from the primary
transcript, before it leaves the nucleus to act as a
template for forming a polypeptide (RNA pro-
cessing).
The reason for these important differences is
that functionally related genes generally lie to-
gether in prokaryotes and that noncoding seg-
ments (introns) are present in the genes of
eukaryotes (see p. 50).
B. DNA and mutation
Coding DNA and its corresponding polypeptide
are colinear. An alteration (mutation) of the
DNA base sequence may lead to a different
codon. The position of the resulting change in
the sequence of amino acids corresponds to the
position of the mutation (1). Panel B shows the
gene for the protein tryptophan synthetase A of
E. coli bacteria and mutations at four positions.
At position 22, phenylalanine (Phe) has been re-
placed by leucine (Leu); at position 49, glutamic
acid (Glu) by glutamine (Gln); at position 177,
Leu by arginine (Arg). Every mutation has a de-
fined position. Whether it leads to incorpora-
tion of another amino acid depends on how the
corresponding codon has been altered. Differ-
ent mutations at one position (one codon) in
different DNA molecules are possible (2). Two
differentmutations have been observed at posi-
tion 211: glycine (Gly) to arginine (Arg) and Gly
to glutamic acid (Glu). Normally (in the wild-
type), codon 211 is GGA and codes for glycine
(3). A mutation of GGA to AGA leads to a codon
for arginine; amutation to GAA leads to a codon
for glutamic acid (4).
C. Types of mutation
Basically, there are three different types of mu-
tation involving single nucleotides (pointmuta-
tion): substitution (exchange), deletion (loss),
and insertion (addition). With substitution, the
consequences depend on how a codon has been
altered. Two types of substitution are distin-
guished: transition (exchange of one purine for
another purine or of one pyrimidine for
another) and transversion (exchange of a purine
for a pyrimidine, or vice versa). A substitution
may alter a codon so that a wrong amino acid is
present at this site but has no effect on the read-
ing frame (missensemutation), whereas a dele-
tion or insertion causes a shift of the reading
frame (frameshift mutation).