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Lecture slides for Bio I, Hon. on 4/17/07.
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Information Transfer and Protein Synthesis
“Central Dogma”DNA
mRNA
Protein(polypeptide)
• 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)
Transcription
Transcription is the process by which one strand of the DNA is copied to form a
complimentary strand of messenger RNA (mRNA)
Transcription
Transcription
initiation
elongation
termination
initiation
• RNA polymerase binds to the DNA at the promoter region
• DNA unwinds exposing the coding strand
elongation
• DNA acts as a template (pattern) for the mRNA
• RNA polymerase moves along DNA elongating (making longer) the mRNA strand
termination
• RNA polymerase reaches the terminator region on the DNA
• RNA is release RNA polymerase falls off
A transmission electron micrographof an unidentified operon of thebacterium Escherichia coli, x72,600.Ribosomes attach to mRNA, andprotein synthesis begins even beforetranscription is complete.
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
Enzymes attach a cap of chemically modified guanine
nucleotides (methyl-guanine, or mG) to the starting end of
the mRNA molecule
Other enzymes then replace part of the
opposite end with a tail of 100–200 adenine
nucleotides called a poly-A tail
The final step in mRNA processing involves
removal of some internal segments of the RNA that do not code for protein
called introns
The parts of the transcript that remain (and code for protein) are called exons. They are joined together in a process called splicing.