AP Bio Ch 17 part 2 translation

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AP Bio Ch 17 part 2 translation

  1. 1. From Genes to Proteins Translation Ch. 17 Sections 17.4, 17.5, 17.6, & 17.7
  2. 2. To assist you in your note taking… Key vocabulary terms are in green, bold, underlined font
  3. 3. Overview of Concepts 1. The genetic code is a triplet code 2. Translation is directed by RNA molecules 3. RNA plays many different roles in protein synthesis 4. Point mutations may affect protein formation
  4. 4. The triplet code There are 20 amino acids (the monomers of proteins) but only 4 nucleotides (the monomers of nucleic acids) How can just 4 bases code for 20 different amino acids?
  5. 5. The triplet code The genetic code is based on triplets of bases: a series of nonoverlapping, three nucleotide “words” We call these base triplets in the mRNA codons How did scientists figure out it was 3 bases for each codon?
  6. 6. The triplet code 4 nucleotides (A,C,T,G) x 1 in a sequence = 4 different combinations 4 nucleotides x 2 in a sequence = 16 different combinations 4 different nucleotides x 3 in a sequence = 64 different combinations (for 20 AA’s)
  7. 7. Only UGG codes for tryptophan The triplet code The code is redundant but unambiguous Each codon codes for only 1 amino acid unambiguous Some amino acids are coded for by more than one codon - redundant AGU & AGC both code for serine
  8. 8. How did scientists figure out what amino acid each codon codes for? 1960s - Nierenberg & Mathaei Used artificial RNA triplets in tubes with the components for building proteins Made chains of uracil first UUUUUUUUU Got all phenylalanines in a chain, so UUU must code for phenylalanine. Within a few years, they had decoded all 64 codons
  9. 9. What is translation? Translation is the process by which a cell interprets the codons along an mRNA molecule and builds a polypeptide
  10. 10. Who translates the code? Transfer RNA (tRNA) is the interpreter of the genetic code tRNA is the molecule responsible for converting the genetic code of nucleotides to the protein code of amino acids
  11. 11. How does tRNA work? The cell already has all 20 amino acids in its cytoplasm (either makes them itself or they are taken in through the organism’s diet) Each tRNA is a strand about 80 bases long Some bases are complementary to each other so it can hydrogen bond to itself Takes on a clover-leaf shape
  12. 12. tRNA On one end of the tRNA is an amino acid On the other end is an anticodon The anticodon is complementary to the codon in the mRNA
  13. 13. So codon by codon, the tRNAs deposit amino acids in the prescribed order, and the ribosome joins them into a polypeptide chain
  14. 14. Some practice  DNA template strand: ACCGGTCAGTAC 1. Make the mRNA from this template 2. What will be the tRNA anticodons?
  15. 15. Ribosomes  Ribosomes are the sites of protein synthesis  They are made up of ribosomal RNA (rRNA) & protein  Composed of 2 subunits: large & small  Subunits are made in the nucleolus  They join together at the mRNA to make a functional ribosome
  16. 16. Ribosomes Ribosomes bring together the mRNA and the tRNAs bearing the correct amino acids and bond those amino acids in the correct order There are 3 sites on the ribosome that function in this capacity: the E site, the P site, and the A site
  17. 17. A site - holds the tRNA with the next amino acid to be added to the chain P site - holds the tRNA carrying the growing polypeptide chain E site - releases tRNAs from the ribosome here P A
  18. 18. Translation has 3 stages Initiation Elongation Termination
  19. 19. Initiation Brings together mRNA, the first tRNA with the first amino acid, and the large & small subunits of the ribosome The first amino acid is methionine (codon AUG, the start codon) This establishes the reading frame The whole thing is called a “translation initiation complex” and GTP energy is required to build it
  20. 20. Elongation  More amino acids are added to the growing chain  There are 3 steps catalyzed by protein elongation factors
  21. 21. STEP 1 - Codon Recognition  the anticodon on the tRNA H-bonds with the codon in the A site 1. 2 GTPs for energy are used up here 2. An elongation factor protein catalyzes this step
  22. 22. STEP 2 - Peptide Bond Formation The large subunit catalyzes the formation of a peptide bond between the amino acid in the A site and the amino acid in the P site
  23. 23. STEP 3 Translocation The ribosome moves the tRNA in the A site to the P site The empty tRNA in the P site is moved to the E site and released GTP energy is required here
  24. 24. Termination Happens when one of the 3 stop codons reaches the A site on the ribosome A release factor protein binds to the stop codon & hydrolysis occurs to free the polypeptide chain
  25. 25. Polyribosomes Several ribosomes can be working at the same mRNA strand at the same time Strings of these ribosomes are called polyribosomes This helps the cell make more proteins more quickly
  26. 26. Proteins  As the polypeptide chain is being formed, it will begin to coil & fold in to its 3-D shape  The gene determines the order of the amino acids - the primary structure  The primary structure determines the secondary and tertiary structure
  27. 27. Proteins  Proteins may be further modified by the addition of sugars, lipids, or phosphate groups  Enzymes may cleave the polypeptide chain into smaller chains  2 or more polypeptide chains may join to make the quaternary structure of a functional protein
  28. 28. Proteins All translation begins in the cytosol on free ribosomes If the protein is destined to become part of an organelle or is to be shipped outside the cell, the ribosome will move to the ER and become an attached ribosome
  29. 29. Proteins There will be a signal peptide (a sequence of amino acids) that is recognized by a protein-RNA complex called a signal recognition particle (SRP) This particle brings the ribosome to the ER and translation continues there
  30. 30. Types of RNA  mRNA - messenger RNA (the code)  tRNA - transfer RNA (brings amino acids)  rRNA - ribosomal RNA (the ribosome)  Pre-mRNA - the primary transcript before editing  snRNA - part of sliceosomes  SRP RNA - part of the signal recognition particle  & others
  31. 31. What makes RNA so versatile? 1. It can H-bond to itself & to other nucleic acids 2. It has functional groups that allow it to act as an enzyme
  32. 32. Point Mutations A point mutation is a change in a single base pair in a gene They can have catastrophic consequence, or none at all There are 3 main types: Substitution Insertion Deletion
  33. 33. Substitution mutations A base pair is replaced with a different base pair Because there is redundancy in the genetic code, this may cause no problem at all It could also lead to a malformed protein and be the difference between life and death
  34. 34. Substitution  Think of it like a sentence:  Normal sentence would read  THE DOG BIT THE CAT  A point mutation might make the sentence read:  THE DOG BIT THE CAR This changes the meaning of the sentence, but not dramatically.
  35. 35. Changing a single base can cause a dramatic change: The base change codes for a different amino acid, making a different protein Example: sickle cell anemia
  36. 36.  Changing a single base may not cause any change at all:  The changed base may still code for the same amino acid  Proline is coded for by CCC, CCA, CCG, and CCU, So a change in the last base won’t make any difference to the amino acid that is added to the protein chain.
  37. 37. Insertions & Deletions •These mutations add an extra letter or two or delete letters •These mutations disrupt the reading frame and are usually more severe •Because of this they are called frameshift mutations
  38. 38. Frameshift Mutations  Think of it as a sentence again:  THE DOG BIT THE CAT  Adding an extra letter makes it:  THH EDO GBI TTH ECA T  It changes the entire sentence to nonsense. This kind of mutation has a more dramatic effect on the DNA sequence and is usually lethal

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