Chapter 14_post

Genetics: A Conceptual Approach

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Unformatted text preview: Chapter 14 RNA Molecules and RNA Processing Dr. Ed Otto George Mason University Key objectives Examine the structures of the major classes of RNA molecules Understand how these RNAs are processed/modified before they assume their biological roles in the cell Gene Organization Early Definition of the Gene 1902 Archibald Garrod correctly suggested that genes encode proteins For many years, the working definition of a gene was a set of nucleotides that specify the amino acid sequence of a protein Colinearity In 1958, Francis Crick proposed that genes and proteins are colinear that there is a direct correspondence between the nucleotide sequence of DNA and the amino acid sequence of a protein Bacterial vs Eukaryotic Genes While the concept of colinearity is generally true for genes found in bacteria, it is not true for many eukaryotic genes Early hints that eukaryotic gene structure was different included observations that:- eukaryotic cells contain more DNA than is required to encode proteins - large RNA molecules in the nucleus are absent from the cytoplasm, suggesting that nuclear RNAs undergo some sort of change before they are exported to the cytoplasm Eukaryotic Gene Organization Early 1970s: it was discovered that coding sequences in eukaryotes are interrupted by nucleotides that do not specify amino acids This discovery was made when eukaryotic viral DNA was hybridized to mRNA transcribed from it and examined using electron microscopy: the DNA was much longer than the mRNA, and regions of the DNA looped out from the hybridized molecules Subsequent to this discovery, it was found that the coding sequences of many eukaryotic genes are interrupted by nucleotides that do not encode amino acids- Noncoding regions = _______ (or intervening sequences) For these genes, all of the DNA nucleotides are initially transcribed into RNA (pre-mRNA), but after transcription, the introns are removed by splicing and the remaining segments (exons) are joined to form the mature mRNA Eukaryotic Gene Organization Transcription and Splicing Transcription and Splicing Introns are common in eukaryotic genes but rare in bacterial genes Introns are present in nuclear genes and in mitochondrial and chloroplast genes Size and number of introns:- vary from gene to gene- are directly related to increasing organismal complexity (e.g. yeast genes only have a few short introns; introns in Drosophila are longer and more numerous) Introns tend to be longer than exons, so most eukaryotic genes contain more noncoding that coding nucleotides Introns All classes of eukaryotic genes contain introns Four major types of introns Introns Updated Definition of the Gene Defining a gene as a sequence of nucleotides that encodes amino acids is now considered an oversimplification - this definition excludes introns and other nucleotides that are transcribed, but do not encode amino acids- also excludes sequences that encode rRNA, tRNA, and other...
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Chapter 14_post - Chapter 14 RNA Molecules and RNA...

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