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Cell Division, Genetics, Molecular
Biology
20.2 Gene Expression
Transcription/Translation
Protein Synthesis
Amino Acids
 Frederick Sanger 1952:
- proteins consist of sequence of molecules called amino acids
- specific sequence of amino acids determines chemical properties
of each protein
- proteins produced by cell determine structure, function &
development of cell
 Proteins responsible for inherited traits
 Sequence of base pairs in DNA makes up genetic code of organism
 Genetic code determines how amino acids strung together (how
proteins are made)
Genetic Code
 In a gene: each set of 3 bases is a CODON
 Genetic code always interpreted in terms of mRNA codon rather
than original DNA sequence
 Each mRNA codon codes for a specific amino acid
 Only 20 amino acids found in proteins, depend on combination of
bases in codon
 Start and stop codons initiate or terminate protein synthesis
Ex)
A U G C U UAAA G C C U GA
Gene Expression
 Way information in a gene is converted into a specific trait through
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production of a polypeptide (protein)
- products of all genes = polypeptides
RNA is utilized to convert genes into proteins
- messenger RNA (mRNA)
- transfer RNA (tRNA)
- ribosomal RNA (rRNA)
2 stages of gene expression: transcription and translation
Transcription: genetic information converted from DNA sequence
into mRNA, carries information from nucleus to cytoplasm
Translation: genetic information from mRNA used to synthesize
polypeptide chains of specific amino acids
Gene Expression
 Central Dogma: 2 step process of transferring genetic
information from DNA to RNA, then from RNA to protein
Transcription Overview
 DNA sequence copied (transcribed) into the sequence of a single
stranded mRNA molecule
 THREE PROCESSES:
1. Initiation: RNA polymerase binds to DNA at specific site near
beginning of gene
2. Elongation: RNA polymerase uses DNA as template to build
mRNA molecule
3. Termination: RNA polymerase passes the end of gene and
stops
 mRNA then released from template strand
 Carried through nuclear pores, into cytoplasm of cell
Transcription – Initiation
 Only one strand of the double-stranded DNA is transcribed for each
gene
- template strand: one that is transcribed
- either strand can serve as the template strand for different genes
 RNA polymerase binds to segment of DNA)
- opens double helix
- binds in front of gene to be transcribed
Transcription - Elongation
 DNA strand to be transcribed =
template
 RNA polymerase moves along
template strand of DNA and
begins building mRNA in 5’ to 3’
direction
 Promoter not transcribed
 mRNA strand complementary to
DNA template strand EXCEPT
thymine is replaced with URACIL
Transcription - Termination
 RNA polymerase reaches end of gene
 Termination sequence: RNA polymerase recognizes it as end
 Transcription stops, mRNA disconnects from DNA template
strand
 RNA polymerase free to bind to another promoter region
- transcribe another gene
 mRNA free and diffuses out of nucleus into cytoplasm
Review
Transcription
Translation Overview
 mRNA now in the cytoplasm needs to be
translated
- puts amino acids together according to
genetic information and creates proteins
 Ribosomes: site of translation
- composed of 2 subunits: clamp together
around mRNA, moves along mRNA
 THREE PROCESSES:
1. Initiation
2. Elongation
3. Termination
Translation - Initiation
 Occurs when ribosome recognizes specific sequence on mRNA –
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binds to it
Ribosome moves along mRNA in 5’ to 3’ direction
- adds amino acids to polypeptide chain once it reads a codon
Must start reading in correct spot on mRNA
- START codon (AUG)
- ensures ribosome translates code using reading frame of mRNA
molecule
- results in correct sequence of amino acids
Transfer RNA (tRNA) carries amino acids back to ribosome
structure of tRNA contains an ANTICODON
- complementary to codon of mRNA
- tells tRNA which amino acid to bind to
- amino acid binds to opposite end
- tRNA delivers amino acid to ribosome
tRNA Structure
Translation - Elongation
 Start codon recognized by ribosome
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- codes for methionine
2 sites for tRNA to attach
- A (aminoacyl)
- P (peptidyl)
tRNA with anticodon complementary to start codon enters P site
Next tRNA carrying required amino acid enters the A site
- peptide bond formed between both amino acids
Ribosome shifts over one codon so that SECOND tRNA is now in P
site
Released first tRNA from P site and allowed THIRD tRNA to enter
empty A site
- like a tickertape!
Released tRNA’s recycled back into cytoplasm
Translation - Termination
 Ribosome reaches one of THREE STOP CODONS
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- UGA, UAG, UAA
Do not code for an amino acid, no corresponding tRNA’s
Protein release factor recognizes that ribosome has stalled and
helps release polypeptide chain from ribosome
TERMINATION.
Translation
Transcribe & Translate!