1. Cell Division, Genetics, Molecular Biology
20.2 Gene Expression
Transcription/Translation
Protein Synthesis

2. 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)

3. 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

4. Ex) A U G C U U A A A G C C U G A

5. Gene Expression
Way information in a gene is converted into a specific trait through 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

6. Gene Expression
Central Dogma: 2 step process of transferring genetic information from DNA to RNA, then from RNA to protein

7. 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

8. 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

9. 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

10. 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

11. Review
Transcription

12. 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

13. Translation - Initiation
Occurs when ribosome recognizes specific sequence on mRNA – 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

14. tRNA Structure

15. Translation - Elongation
Start codon recognized by ribosome - 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

17. Translation - Termination
Ribosome reaches one of THREE STOP CODONS - 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!