3.1Transcription and Translation of Genes


Central Dogma

Fig. 3-1. Central dogma

The genetic information of a protein specifically refers to the information that determines its primary structure (i.e., the amino acid sequence) and, at the substance level, to the nucleotide sequence (the base sequence) of DNA. The genetic information of DNA is copied to mRNA (messenger RNA; see 3.2) - molecules synthesized using DNA as a template - and is consequently converted to the amino acid sequence of a protein. The concept of genetic information flowing in one direction from DNA to mRNA to proteins is called the central dogma of molecular biology (Fig. 3-1). This concept is a basic principle common to all organisms - both prokaryotes and eukaryotes - including bacteria and humans. mRNA synthesis means the transcription of the genetic information in DNA (the base sequence) to the base sequence of RNA, while protein synthesis refers to the translation of information in one language (the sequence of mRNA) into that of another (the amino acid sequence) (Fig. 3-2).

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Genetic Codes

Specifically, the genetic information of DNA is its base sequence. On the other hand, the genetic code is defined as the base sequence of mRNA transcribed using DNA as a template, and a particular three-base sequence known as a codon corresponds to one amino acid. There are 43 = 64 codons, encoding 20 amino acids (Fig. 3-2). As an example, 5’-AUG-3’ - a code of mRNA - corresponds to the amino acid methionine (Fig. 3-3). A protein consisting of 400 amino acids linked together is derived from 1,200 DNA bases and 1,200 mRNA bases. Here, the 1,200-base section of the entire DNA is the gene for this protein.
AUG encodes methionine in addition to serving as the initiation codon for protein synthesis. Following the determination of the first amino acid, the next three-base sequence determines the next amino acid, and so on. Figure 3-2 includes three termination codons. When protein synthesis proceeds to the termination codon (which does not encode any amino acids), the protein synthesis is terminated. The region between the initiation codon and the termination codon is called the coding region.

Fig. 3-2. Genetic code table

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Sense Strand of DNA

Of the double strands of DNA, the one complementary to the template strand for RNA synthesis is called the sense strand (Fig. 3-3). The base sequence of mRNA can be obtained by replacing the Ts in the sense strand with Us. Codons on the sense strand are almost the same as those on mRNA, e.g., ATG on the sense strand corresponds to AUG on mRNA. The strand of the double-stranded genome DNA that serves as the sense strand depends on the gene.

Fig. 3-3. Genes and genetic information

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Gene Expression

Gene function refers to the process of mRNA synthesis based on genetic information and the resulting production of a protein - this is also described as gene expression. The situation of a gene not functioning is described as the suppression of gene expression.

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