Concepts Summary

• The discovery of introns in eukaryotic genes forced the redefinition of the gene at the molecular level. Today, a gene is often defined as a sequence of DNA nucleotides that is transcribed into a single RNA molecule.

• Introns are noncoding sequences that interrupt the coding sequences (exons) of genes. Common in eukaryotic cells but rare in bacterial cells, introns exist in all types of genes and vary in size and number. They comprise four major types: group I introns, group II introns, nuclear pre-mRNA introns, and tRNA introns.

• The results of experiments in the late 1950s and early 1960s suggested that genetic information is carried from DNA to ribosomes by short-lived RNA molecules called messenger RNA. An mRNA molecule has three primary parts: a 5' untranslated region, a protein-coding sequence, and a 3' untranslated region.

• Bacterial mRNA is translated immediately after transcription and undergoes little processing.

• The primary transcript (pre-mRNA) of a eukaryotic protein-encoding gene is extensively processed: a modified nucleotide and methyl groups, collectively termed the cap, are added to the 5' end of pre-mRNA; the 3' end is cleaved and a poly(A) tail is added; and introns are removed.

• The process of RNA splicing takes place within a structure called the spliceosome, which is composed of several small nuclear RNAs and proteins. RNA splicing takes place in a two-step process that entails RNA - RNA interactions among snRNAs of the spliceosome and the pre-mRNA.

• Some introns found in rRNA genes and mitochondrial genes are self-splicing.

• Some pre-mRNAs undergo alternative splicing, in which different combinations of exons are spliced together or different 3' cleavage sites are used.

• Messenger RNAs may also be altered by the addition, deletion, or modification of nucleotides in the coding sequence, a process called RNA editing.

• Transfer RNA serves as a bridge between amino acids and the genetic information carried in mRNA. Transfer RNAs are relatively short molecules that assume a common secondary structure and contain modified bases. Most organisms have multiple copies of tRNA genes; the tRNAs transcribed from these genes are extensively processed in bacterial and eukaryotic cells.

• Ribosomes are the sites of protein synthesis in the cell. Each ribosome is composed of several rRNA molecules and a number of proteins that form a large and a small subunit. Genes for rRNA exist in multiple copies; the primary transcripts from these genes are extensively modified after transcription in bacterial and eukaryotic cells. In eukaryotic cells, rRNA processing is carried out by small nucleolar RNAs.

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