1. Chapter 17 From Gene to Protein

2. 2 DNA contains the genes that make us who we are. The characteristics we have are the result of the proteins our cells produce during the process of transcription and translation. 2 The Bridge Between DNA and Protein

3. Main Questions: Somehow the information content in DNA-- - -the specific sequence of nucleotides along the DNA--strands needs to be turned into protein. How does this information determine the organism’s appearance? How is the information in the DNA sequence translated by a cell into a specific trait?

4. The Bridge Between DNA and Protein RNA is the single stranded compound that carries the message from the DNA to the ribosome for translation into protein. Recall, DNA = A,T,C,G; RNA= A,U,C,G The order of these bases carries the code for the protein which is constructed from any or all of the 20 amino acids.

5. RNA RNA is used because it is a way to protect the DNA from possible damage. Many copies of RNA can be made from one gene, thus, it allows many copies of a protein to be made simultaneously.

6. mRNA and RNA Polymerase mRNA is the “messenger” or vehicle that carries the genetic information from the DNA to the protein synthesizing machinery. RNA polymerase pries apart the DNA and joins RNA nucleotides together in the 5’-->3’ direction (adding, again, to the free 3’ end). RNA polymerase is just like DNA polymerase, but it doesn’t need a primer.

7. Transcription and Translation The process of going from gene to mRNA is called transcription. Translation is the process that occurs when the mRNA reaches the ribosome and protein synthesis occurs.

8. The mRNA produced during transcription is read by the ribosome and results in the production of a polypeptide. The polypeptide is comprised of amino acids. The specific sequence of amino acids is determined by the genetic code on the DNA. Transcription and Translation

9. Transcription The gene determines the sequence of bases along the length of the mRNA molecule. One of the two regions of the DNA serves as the template. The DNA is read 3’-->5’ so the mRNA can be synthesized 5’-->3’ Not all regions of DNA codes for protein.

10. There are numerous segments of DNA to which transcription factors bind. These govern the synthesis of mRNA and regulate gene expression. Promoter sequence Termination sequence Enhancers Transcription

11. 11 Other Functions of Non-Coding DNA Other regions of non-coding DNA are involved in regulating gene expression, coding for tRNA molecules, and ensuring that the DNA maintains its length (telomeres). 11

13. tRNA Structure and Function tRNA, like mRNA, is made in the nucleus and is used over and over again. tRNA binds an aa at one end and has an anticodon at the other end. The anticodon acts to base pair with the complementary code on the mRNA molecule, and delivers an aa to the ribosome.

14. 14 Additionally, in eukaryotes, once genes get transcribed, the RNA that is produced is often modified before getting translated. 14 Transcription and Translation

15. 15 Post Transcriptional Modification In eukaryotes, once the primary transcript is made, it is spliced and modified before getting translated into protein. 15

16. mRNA Modification The initial transcript (~8000 bp) is reduced (to ~1200 on average). The large, non-encoding regions of the DNA that get transcribed are spliced out. Introns--intervening regions are removed. Exons--expressed regions are kept.

17. mRNA Modification Some untranslated regions of the exons are saved because they have important functions such as ribosome binding.

20. Translation mRNA triplets are called codons. Codons are written 5’-->3’ Codons are read 5’-->3’ along the mRNA and the appropriate aa is incorporated into the protein according to the codon on the mRNA molecule. As this is done, the protein begins to take shape.

22. 22 Protein Synthesis Many copies of protein can be made simultaneously within a cell using a single mRNA molecule. This is an efficient way for the cell to make large amounts of protein in times of need. 22

23. Polyribosome Here you can see an mRNA transcript being translated into many copies of protein by multiple ribosomes in a eukaryote. This is a way in which the cell can efficiently make numerous copies of protein.

25. Polyribosome Here it is again in a prokaryote. The process essentially the same between prokaryotes and eukaryotes. The main exception is where it occurs.

26. One Main Difference Between prokaryotes and eukaryotes, there is one main difference between transcription and translation. The two processes can occur simultaneously in prokaryotes because they lack a nucleus. In eukaryotes, the two processes occur at different times. Transcription occurs in the nucleus, translation occurs in the cytoplasm.

27. 27 Translation So how, exactly, does the cell translate genetic code into protein? 27

28. The Genetic Code Scientists began wondering how the genetic information contained within DNA instructed the formation of proteins. How could 4 different base pairs code for 20 different amino acids? 1:1 obviously didn’t work; a 2 letter code didn’t work either; but a 3 letter code would give you more than enough needed.

29. The Genetic Code Codons are composed of triplets of bases. 61 of the 64 codons code for amino acids. 3 of the codons code for stop codons and signal an end to translation. AUG--start codon

30. Genetic Code The genetic code is said to be redundant. More than one triplet codes for the same amino acid. One triplet only codes for one amino acid. The reading frame is important because any error in the reading frame codes for gibberish.

31. Ribosomes rRNA genes are found on chromosomal DNA and are transcribed and processed in the nucleolus. They are assembled and transferred to the cytoplasm as individual subunits. The large and small subunits form one large subunit when they are attached to the mRNA.

32. Ribosomes The structure of ribosomes fit their function. They have an mRNA binding site, a P-site, an A-site and an E-site. A-site (aminnoacyl-tRNA) holds the tRNA carrying the next aa to be added to the chain. P-site (peptidyl-tRNA) holds the tRNA carrying the growing peptide chain. E-site is the exit site where the tRNAs leave the ribosome. Each of these are binding sites for the mRNA.

33. The 3 Stages of Protein Building 1. Initiation 2. Elongation 3. Termination All three stages require factors to help them “go” and GTP to power them.

34. 1. Initiation Initiation brings together mRNA, tRNA and the 2 ribosomal subunits. Initiation factors are required for these things to come together. GTP is the energy source that brings the initiation complex together.

35. 1. Initiation Initiation brings together mRNA, tRNA and the 2 ribosomal subunits. Initiation factors are required for these things to come together. GTP is the energy source that brings the initiation complex together.

36. 2. Elongation The elongation stage is where aa’s are added one by one to the growing polypeptide chain. Elongation factors are involved in the addition of the aa’s. GTP energy is also spent in this stage.

37. 3. Termination Termination occurs when a stop codon on the mRNA reaches the “A-site” within the ribosome. Release factor then binds to the stop codon in the “A-site” causing the addition of water to the peptide instead of an aa. This signals the end of translation.

38. Polypeptide Synthesis As the polypeptide is being synthesized, it usually folds and takes on its 3D structure. Post-translational modifications are often required to make the protein function. Adding fats, sugars, phosphate groups, etc. Removal of certain proteins to make the protein functional. Separately synthesized polypeptides may need to come together to form a functional protein.