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Gene expression
- the process by which DNA directs protein synthesis,
- includes two stages:
- –Transcription
- (mRNA synthesis)
- –Translation (polypeptide, or protein,
- synthesis)
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Transcription
- copying
- think of medical transcriptionist
- The synthesis of RNA under the direction of DNA
- Produces messenger RNA (mRNA)
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Translation
- think languages: language of nucleotides to language of amino acids
- The actual synthesis of a polypeptide
- Occurs under the direction of mRNA
- Occurs on ribosomes
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Codons
nonoverlapping base triplets of genetic information
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RNA synthesis
- Catalyzed by RNA polymerase (pries the DNA strands apart and hooks together the RNA nucleotides)
- works 5’-3’, but doesn’t need a primer like DNA polymerase does
- Follows the same base-pairing rules as DNA, except that in RNA, uracil substitutes for thymine
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Transcription (stages)
- Initiation
- Elongation
- Termination
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Promoters
- signal the initiation of RNA synthesis
- RNA polymerase binds to promoter, DNA strands unzip, transcription commences
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Post-Transcriptional Modification of RNA
- The 5' end receives a modified nucleotide cap
- The 3' end gets a poly-A tail
- RNA splicing removes introns and joins exons, forming the final mRNA molecule which now exits the nucleus, entering the cytoplasm
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Exons
the sequences that will be translated into protein
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tRNA
translates an mRNA message into protein in a ribosome
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Ribosome 
- 2 Subunits
- E site (Exit site)
- A site (Aminoacyl-tRNA binding site)
- P site (Peptidyl-tRNA binding site)
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Mutations
Changes in the genetic material of a cell
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Point mutations
Changes in just one base pair of a gene
- –Base-pair substitutions
- –Base-pair insertions or deletions
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Mutagens
Physical or chemical agents that can cause mutations
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carcinogens
cancer-causing chemicals
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histones
Proteins responsible for the first level of DNA packing in chromatin and binds tightly to DNA
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