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Chapter 20

BIOL 110 Chapter Notes - Chapter 20: Start Codon, Ribosomal Rna, Transfer Rna

Course Code
BIOL 110
Denise Woodward

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Tutorial 20
Thursday, October 20, 2016
1:23 PM
From Gene to Protein
Gene to Protein: The Central Dogma
o DNA transcribed to mRNA, mRNA translated into proteins
o DNA codes for protein synthesis via transcription and translation
o Template strand is used to make mRNA
mRNA is complementary to the template strand of the DNA molecule and is identical
to the coding strand
o Transcriptions occurs in the nucleus
o mRNA leaves the nucleus and associates with a ribosome
o Because prokaryotes do not have a nucleus, proteins can be translated from mRNA
Gene to Protein: The Triplet Code
o Sequence of nucleotides determines the type of protein that is synthesized
Four possible bases, 20 amino acids
o Code was cracked by Nirenberg when he found the codon for phenylalanine
o mRNA codons code for amino acids or stop codons
o Every newly synthesized protein begins with methionine (start codon)
o Because most organisms use this code it indicates a common origin
o Codons are fixed and redundant
o Redundancy protects against the effects of mutation
Gene to Protein: Transcription
o DNA transcribed to mRNA
o Occurs in nucleus of eukaryotes
o Two DNA strands separate
o RNA polymerase II separates the DNA strands and joins the RNA nucleotides along the
exposed template strand
o Process is initiated when transcription factors bind to the gene's starting point
o Starting point is a promoter
o RNA polymerase II adds nucleotides to the 3 end of the elongating RNA molecule
o Enzyme moves down the strand, unwinding, and the DNA helix reforms after a sequence has
been transcribed
o Continues until a terminator sequence signals the end of the RNA synthesis
o Transcription: initiation, elongation, termination
Gene to Protein: Translation Ingredients
o mRNA, rRNA, and tRNA are all needed for protein synthesis
o rRNA combines with proteins to form ribosomes
o Ribosomes
Two subunits; large and small
o tRNA molecules transport amino acids to the growing polypeptide chain
Has an amino acid attachment site for a particular amino acid and an anticodon
Sequence of three nucleotides that is complementary to a sequence of bases in
the mRNA strand
o Enzyme aminoacyl-tRNA synthetase
Ensures that a given tRNA molecule picks up only its specific amino acid
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