Protein Synthesis

1. Protein Synthesis Transcription and Translation

2. The Central Dogma • The information encoded with the DNA nucleotide sequence of a double helix is transferred to a mRNA molecule. • The mRNA molecule travels out of the nucleus and attaches to a ribosome • Using the RNA nucleotide sequence and the genetic code, the ribosome assembles a protein

3. The Central Dogma (brief) • DNA is copied to mRNA • mRNA is used as blueprint to make protein

4. DNA  Protein: in 3 easy steps! • Transcription • RNA modification • Translation

5. Genes and DNA • A gene is a specific sequence of DNA nucleotides • For each specific protein used by a cell, there is a specific DNA sequence (gene) located on a chromosome • 1 gene  1 polypeptide

6. RNA Structure • RNA (ribonucleic acid) are nucleotides very similar to DNA • Nitrogenous bases include Cytosine, Guanine, Adenine, and Uracil (instead of Thymine) • Form three basic structures • mRNA – messenger RNA • rRNA – ribosomal RNA • tRNA – transfer RNA

7. RNA vs DNA • RNA has an oxygen on the 2’ carbon of the ribose sugar

8. RNA vs DNA • RNA is single stranded, DNA is double stranded • GCAT vs CUGA

9. Types of RNA molecules • mRNA (messenger): • Relays DNA sequence information to ribosome • rRNA (ribosomal): • Combines with proteins to form ribosomes • tRNA (transfer): • Acts as bridge between nucleotide sequence and growing polypeptide chain

10. Transcription • The process by which the nucleotide base sequence of a DNA molecule is copied into a mRNA molecule • 3 steps: • Initiation • Elongation • Termination • Proteins required: • RNA polymerase • Transcription factors

11. RNA Polymerase • Creates a mRNA molecule complimentary to template strand of DNA • Works in the 5’ 3’ direction • Requires transcription factors to begin its work

12. Initiation • Proteins called transcription factors bind to DNA region upstream from gene • Proteins bind to region called promoter • RNA polymerase attaches to double helix at beginning of gene

14. Elongation • RNA polymerase creates a mRNA molecule with bases complimentary to the template strand • Template strand = Anti-sense strand

15. Termination • RNA polymerase reaches end of gene and detaches from double helix • mRNA transcript is released

17. Animations • Transcription showing full complex • Transcription – cool sounds

18. Sense or Anti-sense? • The sense strand of a gene has the same base sequence as the mRNA transcript • The anti-sense strand is used as the template

19. Transcript Modification • Before a mRNA transcript exits the nucleus it is modified in 3 three (tres) ways… • Addition of 5’ cap • Addition of poly-A tail • Removal of introns

20. 5’ cap and poly-A tail • Protective cap is placed on 5’ end • A long repetitive sequence of adenine nucleotides are added to 3’ end, also for protection

21. mRNA splicing • Not all of a transcribed DNA sequence will be translated • Genes are composed of introns and exons • Introns are removed from mRNA transcripts by splicosomes

22. Transcription Review • How is RNA polymerase similar to DNA polymerase III? How are they different? • Will the mRNA transcript have the same nucleotide sequence as the sense or anti-sense strand of DNA? • How are RNA and DNA different? • Name 3 things that happen during mRNA modification.

23. Translation • messenger RNA (mRNA) is decoded at a ribosome to produce a specific polypeptide according to the rules specified by the genetic code. • 4 steps: • Activation • Initiation • Elongation • Termination • Requires: • Ribosomes (rRNA + proteins), mRNA, tRNA, and amino acids

24. Activation • Amino acid is joined with the correct tRNA • Reaction catalyzed by aminoacyl-tRNA-synthetase • Occurs continuously

25. tRNA - transfer • Specified amino acids are attached to tRNA • each anti-codon corresponds to the amino acid specified by the genetic code • Each tRNA has an anti-codon (3 nucleotides) • Anti-codon region base pairs with mRNA trascript

26. Initiation • Small ribosome subunit recognizes start sequence on mRNA and binds to it • Start codon, AUG, is recognized by tRNA carrying a Methionine amino acid • Large subunit completes the complex

27. Elongation • Ribosome moves down the mRNA in a 5’ 3’ direction • Every three mRNA nucleotides another amino acid is added to the growing polypeptide • 3 steps: • Codon recognition • Peptide bond formation • Translocation

28. Elongation: Codon Recognition • When the appropriate tRNA anticodon H-bonds to a mRNA codon at the ribosomal complex

29. Elongation: Peptide Bond Formation • A peptide bond is created between polypeptide chain and new amino acid • polypeptide is transferred to incoming tRNA

30. Elongation: Translocation • Ribosome shifts 3 nucleotides (reading frame) down mRNA transcript • tRNA unattached to polypeptide is released

31. Elongation Codon Recognition Peptide Bond Formation Translocation

32. Termination • The end of the mRNA coding sequence is reached • Stop codon is recognized by a release factor • Ribosome complex dissociates, protein is released

33. The Genetic Code • Each codon corresponds to a specific amino acid • Degenerate • 64 possible codons • only 20 amino acids • Several codons can code for the same amino acid • Ex. CCU, CCA, CCG, CCC = Proline • Universal • The same genetic code is used by all living organisms

34. The Genetic Code

35. Animations • Translation • Translation – no sound, basic

36. Summary Genetic information is encoded in the sequence of the DNA double helix. To access this information, the DNA sequence must be copied, or "transcribed", by enzymes known as RNA polymerases. The resulting messenger RNA (mRNA) molecules carry the genetic information to the protein-synthesizing machinery, where it is used to define the amino-acid sequence, and therefore the structure and function, of proteins.