Gene Expression: From DNA to Protein
Background
Watson & Crick
Beedle & Tatum
How Genes are Expressed
Processes: Transcription (DNA to mRNA) Translation (mRNA to protein)
Importance of location Eukaryotes
Prokaryotes
RNA
Review
Types of RNA in Gene Expression mRNA (messenger)-provides code for
protein tRNA (transfer)-carries amino acids rRNA (ribosomal)-directs translation
Transcription
Requires DNA-dependent RNA Polymerase ▪ Core enzyme + sigma factor = holoenzyme
Three forms in Eukaryotic Nuclei▪ RNA Pol I –Pre-rRNA▪ RNA Pol II**--Pre-mRNA▪ RNA pol III –Pre-tRNA
Stage One: Intiation
RNA recognizes region to be transcribed Promotor regions▪ TATA box▪ CAAT box
Stage Two: Elongation
Complementary mRNA produced from DNA
Stage Three: Termination
Often zones with high GC levels Loop and release of mRNA Protection
5’ capped with 7-methyl G 3’ tail polyadenylation
Translation
Prokaryotic cells translate immediately into protein
Eukaryotic cells process mRNA before translation occurs Cutting out Introns Protecting transcript
Processing mRNA
Splicing occurs by sequence snRNA and snRNP (spliceosomes),
and/or ribozymes (self-splicing enzymes)
Many different proteins can result from one transcript
Requirements for Translation
mRNA Transcript
tRNA Carrier for amino acids
Ribosomes Two sub-units of protein with three
pieces of RNA
Nucleotide Sequence to Protein
Codons- 3 nucleotide codes for amino acids 20 amino acides 64 codons 3 stop codons (UGA, UAA, UAG) 1 Start codon (AUG-methionine)
Each codon brings in a tRNA anti-codon that contributes one amino acid to the protein
Stage One: Initiation
Small sub-unit of ribosome binds to mRNA
Methionine “start” Large sub-unit of ribosome enters Initiation factors involved
Stage Two: Elongation
Connection Next tRNA enters 1st tRNA “hands” amino acid to 2nd tRNA,
then leaves Ribosome slides to free the next space Continuation for length of protein Elongation factors involved
Stage Three: Termination
Stop Codon is reached RF’s bind to stop codon Protein is released Ribosomal units disassociate
For our next session:
Exploring factors influencing gene expression
Expression and repression of genes
References
Brooker, R. J. (2011). Concepts of Genetics + Connect Plus Access Card. McGraw-Hill Science Engineering, New York, NY.
Brooker, R. J., (2012). Genetics: analysis and principles. 4th Ed. McGraw-Hill Higher Education, New York, NY.
King, M. D. (2013). The Medical Biochemistry Page. Retrieved from: http://themedicalbiochemistrypage.org/home.html.
McClean, P. E. (2013). Transcription. Retrieved from: http://www.ndsu.edu/pubweb/~mcclean/plsc731/transcript/transcript1.htm
Unknown (2013). Three types of RNA polymerase in eukaryotic nuclei. Retrieved from: http://mcb.berkeley.edu/courses/mcb110/ZHOU/lec.5-7.euk_trxn_apparatus.pdf
Image Sources
Cap & Tail http://classconnection.s3.amazonaws.com/681/flashcards/894681/jpg/coding1330783434942.jpg
RNA Elongation http://limbiclab.files.wordpress.com/2012/12/limbic_lab_dna_transcription_diagram1.png
RNA Initiation http://9e.devbio.com/images/ch05/0503fig1.gif
Processing http://www.genomebc.ca/glossary/Alternative%20Splicing%20(colourful)_Image.gif
Translation Initiation http://kvhs.nbed.nb.ca/gallant/biology/translation_initiation.jpg
Translation Elongation http://kvhs.nbed.nb.ca/gallant/biology/translation_elongation.jpg
Translation Termination http://utminers.utep.edu/rwebb/assets/images/17.17_Termination_of_transla.jpg