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