Essential Cell Biology 5th edition

Essential Concepts
263
• Eukaryotic pre-mRNAs go through several additional RNA processing
steps before they leave the nucleus as mRNAs, including 5ʹ RNA
capping and 3ʹ polyadenylation. These reactions, along with splicing,
take place as the pre-mRNA is being transcribed.
• Translation of the nucleotide sequence of an mRNA into a protein
takes place in the cytoplasm on large ribonucleoprotein assemblies
called ribosomes. As the mRNA moves through the ribosome, its
message is translated into protein.
• The nucleotide sequence in mRNA is read in consecutive sets of three
nucleotides called codons; each codon corresponds to one amino
acid.
• The correspondence between amino acids and codons is specified
by the genetic code. The possible combinations of the 4 different
nucleotides in RNA give 64 different codons in the genetic code. Most
amino acids are specified by more than one codon.
• tRNAs act as adaptor molecules in protein synthesis. Enzymes called
aminoacyl-tRNA synthetases covalently link amino acids to their
appropriate tRNAs. Each tRNA contains a sequence of three nucleotides,
the anticodon, which recognizes a codon in an mRNA through
complementary base-pairing.
• Protein synthesis begins when a ribosome assembles at an initiation
codon (AUG) in an mRNA molecule, a process that depends on
proteins called translation initiation factors. The completed protein
chain is released from the ribosome when a stop codon (UAA, UAG,
or UGA) in the mRNA is reached.
• The stepwise linking of amino acids into a polypeptide chain is catalyzed
by an rRNA molecule in the large ribosomal subunit, which thus
acts as a ribozyme.
• The concentration of a protein in a cell depends on the rates at
which the mRNA and protein are synthesized and degraded. Protein
degradation in the cytosol and nucleus occurs inside large protein
complexes called proteasomes.
• From our knowledge of present-day organisms and the molecules
they contain, it seems likely that life on Earth began with the evolution
of RNA molecules that could catalyze their own replication.
• It has been proposed that RNA served as both the genome and the
catalysts in the first cells, before DNA replaced RNA as a more stable
molecule for storing genetic information, and proteins replaced RNAs
as the major catalytic and structural components. RNA catalysts in
modern cells are thought to provide a glimpse into an ancient, RNAbased
world.
KEY TERMS
alternative splicing messenger RNA (mRNA) RNA polymerase
aminoacyl-tRNA synthetase polyadenylation RNA processing
anticodon promoter RNA splicing
codon protease RNA transcript
exon proteasome RNA world
gene reading frame small nuclear RNA (snRNA)
gene expression ribosomal RNA (rRNA) spliceosome
general transcription factors ribosome transcription
genetic code ribozyme transfer RNA (tRNA)
initiator tRNA RNA translation
intron RNA capping translation initiation factor