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Gene Transcription and RNA ModificationLec1617OLD

Gene Transcription and RNA ModificationLec1617OLD - Gene...

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1 Gene Transcription and RNA Modification Introduction At the molecular level, a gene is a segment of DNA used to make a functional product – either an RNA or a polypeptide; transcription is the first step in gene expression. Transcription refers to the copying of a DNA sequence into an RNA sequence (see fig. 12.1). Structural genes encode the amino acid sequence of a polypeptide; transcription of a structural gene produces messenger RNA (mRNA). The mRNA sequence determines the amino acid sequence in the polypeptide, which determines the function of the polypeptide; the function of the protein determines traits. The path from gene to trait is called the central dogma of genetics (see fig. 12.1). Overview of Transcription Gene expression requires base sequences that perform different functional roles. Gene expression is the overall process by which the information within a gene is used to produce a functional product that can determine a trait in play with the environment. Figure 12.2 shows common organization of a gene. A key concept is that DNA base sequences define the beginning and end of a gene and regulate the level of RNA synthesis. Gene expression requires base sequences that perform different functional roles; for example, the promoter provides a site to begin transcription, and the terminator specifies the end of transcription; these two sequences cause RNA synthesis to occur within a defined location. The strand that is actually transcribed (used as the template) is termed the template strand. The opposite strand is called the coding strand or the sense strand; the base sequence is identical to the RNA transcript except for the substitution of uracil in RNA for thymine in DNA. Transcription factors recognize the promoter and regulatory sequences to control transcription. mRNA sequences such as the ribosomal-binding site and codons direct translation. The three stages of transcription are initiation, elongation, and termination (see fig. 12.3). Initiation – the promoter functions as a recognition site for transcription factors, which enable RNA polymerase to bind to the promoter; following binding, the DNA is denatured into a bubble known as the open complex.
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2 Elongation – RNA polymerase slides along the DNA in an open complex to synthesize RNA.
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