From DNA to Proteins

1. From DNA to Proteins Chapter 13

2. Life’s Book of Instructions • DNA is like a book of instructions • The instructions are written in the alphabet of A,C,G, and T • All species use the same nucleotide subunits but the sequence differs from species to species. • The instructions for each protein is written in a segment of DNA called a gene. • Genes are linear stretches of DNA that codes for the assemble of amino acids into a polypeptide chain

3. The Path from Genes to Proteins • Two steps • Transcription: Molecules of RNA are produced on the DNA template in the nucleus • Translation: RNA molecules are shipped from the nucleus to the cytoplasmand are used as a template for polypeptide assembly.

4. Central Dogma • The central dogma is a description of how information flows in a cell to make proteins DNA RNA Protein Transcription Translation

5. RNA • Three types of RNA are needed for protein synthesis • Messenger RNA (mRNA) carries the blueprint for protein assembly to the ribosome. • Ribosomal RNA (rRNA) combines with proteins to form ribosomes upon which polypeptides are assembled. • Transfer RNA (tRNA) brings the correct amino acid to the ribosome.

6. RNA polymerase How RNA is assembled • Transcription begins when RNA Polymerase sits down on the promoter region just upstream of the gene. • The RNA Polymerase will unwind the DNA strand and begin adding ribonucleotides to the template in the order specified by the DNA template. Promoter region

7. transcribed DNA winds up again DNA to be transcribed unwinds Newly forming RNA transcript The DNA template at the assembly site RNA Polymerase • RNA Polymerase unwinds the DNA • Ribonucleotides (brown) are brought in and temporally bound to the DNA template inside the polymerase

8. Transcription of RNA inside the RNA polymerase is determined by the DNA template. growing RNA transcript 3’ 5’ 3’ 5’ direction of transcription

9. transcribed DNA winds up again DNA to be transcribed unwinds Newly forming RNA transcript The DNA template at the assembly site • As the RNA polymerase moves down the DNA strand it opens up the DNA to allow the RNA to be built. • As the DNA exits the RNA polymerase the two strands of DNA are joined back together and the RNA is disjoined from the DNA. • The new RNA strand is pushed out a different opening from the double stranded DNA. • As the RNA polymerase comes to the end of the gene, the polymerase falls off and the RNA detaches and floats away.

10. Maturation of mRNA • Introns must be cut out and exons must spliced together. • Introns are nonsense pieces of DNA that interrupt the message. • Exons are the informative portions of the message • A cap is added to the 5’ end • Helps direct the mRNA to the Ribosome • A poly-A tail is added to the 3’ end • Protects the 3’ end from endonucleases that will chewing up the message

11. (snipped out) (snipped out) unit of transcription in a DNA strand exon intron exon intron exon 3’ 5’ transcription into pre-mRNA poly-A tail cap 5’ 3’ 5’ 3’ mature mRNA transcript Fig. 14.9, p. 229

12. The Genetic Code • The message is read in triplets called Codons • Cells have 64 kinds of codons • Only 20 amino acids

13. Fig. 14.11, p. 230

14. Translation Ribosome • The mRNA in the cytoplasm joins with a Ribosome which is made out of rRNA • The mRNA runs through the ribosome

15. tRNA inTranslation codon in mRNA • tRNA shuttle the appropriate amino acids to the ribosome and mRNA • The tRNA contains an anticodon that matches up to the codons on mRNA anticodon amino acid

16. Translation • In the Ribosome the mRNA runs through the top • There are two sites that allow tRNA’s with their accompanying amino acids to bind Binding site for mRNA P (first binding site for tRNA) A (second binding site for tRNA)

17. Translation • Only tRNA’s that have the appropriate anticodons which match to the mRNA codons can bind to the mRNA in the ribosome

18. Translation • As the two tRNA’s bind to the codons their amino acids become in close enough proximity to form a peptide bond. Peptide bond forms between adjacent Amino acids

19. Once the amino acids have joined together the tRNA in the first position breaks loose both from the condon and its amino acid. It then floats away to find another amino acid.

20. Translation • Once the first tRNA is gone, the Ribosome moves the mRNA one codon further through. • The tRNA that was in the second postion is now dragged into the first position. • This leaves room for another tRNA with its amino acid to come into the ribosome

21. Binding site for mRNA P (first binding site for tRNA) A (second binding site for tRNA) Fig. 14.14a, p. 232

22. Fig. 14.14b, p. 233

23. Mutations • Two types of mutations • Base pair substitution mutations • One base is copied incorrectly during DNA replication • A protein may incorporate the wrong amino acid • Frame-shift mutations • One or more base pairs become inserted into DNA or are deleted from it. • This almost always makes the protein nonfunctional and is often lethal.

24. VALINE original base triplet in a DNA strand a base substitution within the triplet (red) Base pair substitution As DNA is replicated, proofreading enzymes detect the mistake and make a substitution for it: POSSIBLE OUTCOMES: OR One DNA molecule carries the original, unmutated sequence The other DNA molecule carries a gene mutation VALINE PROLINE THREONINE Fig. 14.15, p. 234 LEUCINE GLUTAMATE HISTIDINE

25. mRNA transcribed from the DNA PART OF PARENTAL DNA TEMPLATE resulting amino acid sequence ARGININE GLYCINE TYROSINE TRYPTOPHAN ASPARAGINE altered message in mRNA A BASE INSERTION (RED) IN DNA the altered amino acid sequence ARGININE GLYCINE LEUCINE LEUCINE GLUTAMATE Fig. 14.16, p. 234

26. Unwinding of gene regions of a DNA molecule TRANSCRIPTION Pre mRNA Transcript Processing mRNA rRNA tRNA protein subunits Mature mRNA transcripts ribosomal subunits mature tRNA Convergence of RNAs TRANSLATION Cytoplasmic pools of amino acids, tRNAs, and ribosomal subunits Synthesis of a polypetide chain at binding sites for mRNA and tRNA on the surface of an intact ribosome FINAL PROTEIN Fig. 14.18, p. 237 Destined for use in cell or for trasport

27. Click to view animation. animation