BSC-2010 Lecture Notes - Lecture 12: Genetic Code, Intron, Gene Expression

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11 Nov 2016
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CH 17
Gene expression, the process by which DNA directs protein synthesis,
includes two stages: transcription and translation
Transcription is the synthesis of RNA using information in DNA
Transcription produces messenger RNA (mRNA)
Translation is the synthesis of a polypeptide, using information in the mRNA
Ribosomes are the sites of translation
Prokaryotes are much simpler than eukaryotes
The flow of information from gene to protein is based on a triplet code: a
series of nonoverlapping, three-nucleotide words
The DNA sequence where RNA polymerase attaches is called the promoter;
in bacteria, the sequence signaling the end of transcription is called the
terminator
The stretch of DNA that is transcribed is called a transcription unit
These noncoding regions are called intervening sequences, or introns
The other regions are called exons because they are eventually expressed,
usually translated into amino acid sequences
RNA splicing removes introns and joins exons, creating an mRNA molecule
with a continuous coding sequence
Spliceosomes consist of a variety of proteins and several small nuclear
ribonucleoproteins (snRNPs) that recognize the splice sites
Ribozymes are catalytic RNA molecules that function as enzymes and can
splice RNA
Some genes can encode more than one kind of polypeptide, depending on
which segments are treated as exons during splicing, This is called
alternative RNA splicing
A cell translates an mRNA message into protein with the help of transfer
RNA (tRNA)
Grabs amino acids out of the cytosol and adds them to a polypeptide
A tRNA molecule consists of a single RNA strand that is only about 80
nucleotides long
Accurate translation requires two steps
o First: a correct match between a tRNA and an amino acid, done by the
enzyme aminoacyl-tRNA synthetase
o Second: a correct match between the tRNA anticodon and an mRNA
codon
A ribosome has three binding sites for tRNA
o The P site holds the tRNA that carries the growing polypeptide chain
o The A site holds the tRNA that carries the next amino acid to be added
to the chain
o The E site is the exit site, where discharged tRNAs leave the ribosome
During its synthesis, a polypeptide chain begins to coil and fold
spontaneously to form a protein with a specific shapea three-dimensional
molecule with secondary and tertiary structure
Mutations are changes in the genetic material of a cell or virus
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