Chapter 10 - Molecular Biology of the Gene

Bio 100 - Molecular Genetics 4Transcription (DNA ----> RNA)During transcription, the DNA code is passed to mRNA1. A segment of the DNA helix unwinds and unzips, and complimentary RNAnucleotides pair with the DNA nucleotides of one strand2. Initiation - RNA nucleotides are joined by the enzyme RNA polymerase, and themRNA molecule is formedRNA polymerase must be must be instructed where to start and where to stop thetranscribing processThe “start transcribing” signal is a nucleotide sequence called a promoter – located inthe DNA next to the beginning of the gene3. After initiation, the RNA (mRNA) strand is elongatesFinally, there is termination – where the RNA polymerase reaches a special sequence ofbases in the DNA template called the terminatorRNA ProcessingIn eukaryotes, after the mRNA strand is formed, it passes from the nucleus to thecytoplasm where it becomes associated with ribosomesBut, before it leaves the nucleus, the mRNA is modified or processedOne kind of processing is the addition of extra nucleotides (The “cap” and “tail” ) to theends of the RNA transcriptAnother type of processing results from the fact that DNA is comprised of long stretchesof noncoding regions (=introns) that interrupt nucleotide sequences that code for aminoacidsThe coding regions - the parts of the gene that are expressed - are called exonsBoth exons and introns are transcribed from DNA into RNARNA splicing - before the RNA leaves the nucleus, the introns are removed and theexons are joined to produce the mRNA with a continuous coding sequence