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Transcription To carry out its information-bearing function DNA must do more than copy itself It must also express its information The first step a cell takes in reading out a needed part of its genetic instructions is to copy a portion of its DNA nucleotide sequence-a gene-into an RNA nucleotide sequence All RNA is manufactured from a DNA template in a process called transcription Since DNA cannot leave the nucleus or the mitochondrial matrix eukaryotic transcription must take place2 During transcription a DNA sequence is read by anRNA polymerase which produces a complementaryantiparallelRNA strandTranscription begins with the opening and unwinding of a small portion of the DNA double helix to expose the bases on each DNA strand1 One of the two strands of the DNA double helix then acts as a template for the synthesis of an RNA molecule The beginning of transcription is called initiation1 In initiation a group of proteins called initiation factors help to position eukaryotic RNA polymerase correctly at the promoter aid in pulling apart the two strands of DNA to allow transcription to begin and release RNA polymerase from the promoter into the elongation mode once transcription has begun1 A promoter is a sequence of DNA nucleotides that designates a beginning point for transcription1 Like DNA polymerase RNA polymerases catalyze the formation of the phosphodiester bonds that link the nucleotides together to form a linear chain1 The transcription start point is part of the promoter The first base-pair located at the transcription start point is designated 1 base-pairs located before the start point such as those in the promoter are designated by negative numbers1 The most commonly found nucleotide sequence of a promoter recognized by the RNA polymerase of a given species is called the consensus sequence1 Variation from the consensus sequence causes RNA polymerase to bond less tightly and less often to a given promoter which leads to those genes being transcribed less frequently Fig1
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