Protein-coding DNA sequences

Complementary (coding) DNA (cDNA) DNA that is generated from a messenger RNA (mRNA) and contains only protein-coding DNA sequences. 

Transcription. For expression of a gene—i. e., synthesis of the coded protein—the DNA sequence information has to be converted into a protein sequence. As DNA itself is not involved 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). The sequence of this RNA is thus complementary to that of the template strand (3), but— with the exception of the exchange of thy-... 

Fig. 20.2. Simplified scheme describing the central dogma in molecular biology. DNA is replicated and passed from one generation to the next. For protein biosynthesis, DNA sequence is first transcribed into complementary messenger RNA (mRNA) sequence which, by means of the adapter molecule transfer RNA (tRNA), is translated into protein sequence. The translation follows the genetic code where a nucleotide triplet (e.g., AGC) codes for an amino acid (e.g., serine) [522]...

In previous sections many of the eukaryotic proteins and DNA sequences that participate in transcription and its control have been introduced. In this section, we focus on assembly of transcription preinitiation complexes involving RNA polymerase II (Pol II). Recall that this eukaryotic RNA polymerase catalyzes synthesis of mRNAs and a few small nuclear RNAs (snRNAs). Mechanisms that control the assembly of Pol II transcription preinitiation complexes, and hence the rate of transcription of protein-coding genes, are considered in the next section. 

A transcription unit represents the combination of regulatory and coding DNA sequences that together make up an expressible unit, whose expression leads to synthesis of a gene product that often is a protein but also may be an RNA molecule. In prokaryotes, proteins in a specific metabolic pathway are often encoded by genes that are clustered and transcribed into one polycistronic mRNA. A polycistronic mRNA encodes for multiple proteins. In such mRNAs, ribosomes are recruited to internal translational initiation sites through an interaction between the 16S ribosomal RNA and the so-called Shine-Delgamo sequence located immediately upstream of the translational start codon that is used to initiate protein synthesis. 

The amount of sample required is quite small as little as 10 mole is typical So many peptides and proteins have been sequenced now that it is impossible to give an accurate count What was Nobel Prize winning work m 1958 is routine today Nor has the story ended Sequencing of nucleic acids has advanced so dramatically that it is possible to clone the gene that codes for a particular protein sequence its DNA and deduce the structure of the protein from the nucleotide sequence of the DNA We 11 have more to say about DNA sequencing m the next chapter... 

A potentially general method of identifying a probe is, first, to purify a protein of interest by chromatography (qv) or electrophoresis. Then a partial amino acid sequence of the protein is deterrnined chemically (see Amino acids). The amino acid sequence is used to predict likely short DNA sequences which direct the synthesis of the protein sequence. Because the genetic code uses redundant codons to direct the synthesis of some amino acids, the predicted probe is unlikely to be unique. The least redundant sequence of 25—30 nucleotides is synthesized chemically as a mixture. The mixed probe is used to screen the Hbrary and the identified clones further screened, either with another probe reverse-translated from the known amino acid sequence or by directly sequencing the clones. Whereas not all recombinant clones encode the protein of interest, reiterative screening allows identification of the correct DNA recombinant. 

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