information transfer part 1
DESCRIPTION
Lecture slides for Bio I, Hon. on 4/17/07.TRANSCRIPT
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Information Transfer and Protein Synthesis
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“Central Dogma”DNA
mRNA
Protein(polypeptide)
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• DNA stores/transmits a “code”
• 4 nitrogen bases A, T, C, G
• 20 essential amino acids
Molecular Code
A colored scanning electron micrograph ofa group of human chromosomes (x6,875)
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Transcription
Transcription is the process by which one strand of the DNA is copied to form a
complimentary strand of messenger RNA (mRNA)
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Transcription
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Transcription
initiation
elongation
termination
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initiation
• RNA polymerase binds to the DNA at the promoter region
• DNA unwinds exposing the coding strand
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elongation
• DNA acts as a template (pattern) for the mRNA
• RNA polymerase moves along DNA elongating (making longer) the mRNA strand
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termination
• RNA polymerase reaches the terminator region on the DNA
• RNA is release RNA polymerase falls off
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A transmission electron micrographof an unidentified operon of thebacterium Escherichia coli, x72,600.Ribosomes attach to mRNA, andprotein synthesis begins even beforetranscription is complete.
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RNA Processing
• the primary mRNA transcript may be 200,000 nucleotides in length
• mRNA in the cytoplasm is only a few 1000 nucleotides in length
• mRNA is modified (processed) before leaving the nucleus
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Enzymes attach a cap of chemically modified guanine
nucleotides (methyl-guanine, or mG) to the starting end of
the mRNA molecule
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Other enzymes then replace part of the
opposite end with a tail of 100–200 adenine
nucleotides called a poly-A tail
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The final step in mRNA processing involves
removal of some internal segments of the RNA that do not code for protein
called introns
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The parts of the transcript that remain (and code for protein) are called exons. They are joined together in a process called splicing.