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).