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Unformatted text preview: Gene Transcription and RNA Modification A gene is a DNA segment used to make a RNA or a polypeptide; transcription is the 1 st step in gene expression . Transcription is the copying of a DNA sequence into an RNA sequence. Structural genes encode the amino acid sequence of a polypeptide; transcription of a structural gene produces 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. Overview of T ranscription Gene expression is the process by which the information in a gene is used to produce a functional product that can determine a trait with the environment. Key concept=DNA base sequences define a genes beginning & end and regulate RNA synthesis level. Gene expression requires base sequences that perform different functions; for example, the promoter and the terminator cause RNA synthesis to occur within a defined location. The strand that is 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. 3 stages of transcription are initiation, elongation, and termination: I nitiation 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. Elongation RNA polymerase slides along the DNA in an open complex to synthesize RNA. Termination a terminator causes RNA polymerase and the RNA transcript to dislocate from the DNA. RNA transcripts have different functions: Well over 90% of all genes are structural genes producing mRNA; the other RNA molecules are never translated. Nonstructural genes are not translated; but have important cellular functions and can still confer traits. In some cases, the RNA transcript becomes part of a complex that contains protein subunits; for example, ribosomes, spliceosomes, signal recognition particles. T ranscription in Bacteria Understanding of gene transcription came from studies involving bacteria and bacteriophages; much from E . coli. A promoter is a short sequence of DNA that initiates transcription at an exact location; usually located just upstream of the site where transcription of a gene actually begins; the bases in a promoter sequence are numbered in relation to the transcription start site. The start site has the first base used as a template for RNA transcription, denoted +1....
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