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Gene Expression

Gene Expression. How proteins are made. W O R K T O G E T H E R. what monomers make up proteins? what monomers make up nucleic acids (DNA and RNA)?. Nucleic Acids. Proteins. Primarily DNA , RNA. Thousands of different proteins. Made up of four different bases.

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Gene Expression

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  1. Gene Expression • How proteins are made.

  2. W O R K T O G E T H E R • what monomers make up proteins? • what monomers make up nucleic acids (DNA and RNA)?

  3. Nucleic Acids Proteins Primarily DNA, RNA Thousands of different proteins Made up of four different bases Made up of more than than 20 amino acids So, how does relatively simple-sounding DNA contain the information for building thousands of different proteins?

  4. Codons A “codon” is a sequence of three bases in DNA and RNA. Each codon codes for a different amino acid. This mRNA strand: codes for these amino acids: met cys glu leu trp

  5. The Genetic Code All 20 amino acids are coded for. Redundancy of codes is one protection against mutations.

  6. The Gene Concept • A “gene” is a segment of DNA that codes for a specific protein. • Only one side of the DNA double-helix (the “sense” or “coding” strand) contains the actual gene. • Genes are defined by promotor and terminator sequences in the DNA.

  7. Eukaryotic gene structure exons DNA promoter introns A typical eukaryotic gene consists of sequences of DNA called exons, which code for the amino acids of a protein (medium blue), and intervening sequences called introns (dark blue), which do not. The promoter (light blue) determines where RNA polymerase will begin transcription.

  8. A small protein is 30 amino acids long. How many nucleotides are needed to code for it? • 30 • 60 • 90 • Depends on which amino acids.

  9. The same protein that is 30 amino acids long needs how many codons to code for it? • 30 • 60 • 90 • Depends on the amino acids it is made of.

  10. Transcription • DNA stays in the nucleus. • To get information out of one gene on a strand of DNA, the gene must be transcribed. • An mRNA copy of a gene leaves the nucleus, so the original information (DNA) remains intact in the nucleus.

  11. RNA • RNA is a single-stranded nucleic acid. • RNA contains the bases adenine, uracil, guanine, and cytosine. • RNA contains the sugar ribose in its sugar-phosphate backbone.

  12. W O R K T O G E T H E R • Which of these is TRUE about RNA: • RNA has uracil instead of thymine. • RNA is a protein. • RNA is a single strand instead of a double-helix. • RNA never leaves the nucleus.

  13. gene DNA Transcription of the gene produces an mRNA with a nucleotide sequence complementary to one of the DNA strands. (nucleus) (cytoplasm) (a) Transcription messenger RNA Notice that transcription takes place in the nucleus. ribosome protein

  14. (a) Initiation DNA gene 1 gene 2 gene 3 RNA polymerase DNA promoter RNA polymerase binds to the promoter region of DNA near the beginning of a gene, separating the double helix near the promoter.

  15. (b) Elongation RNA DNA template strand RNA polymerase travels along the DNA template strand (blue), catalyzing the addition of ribose nucleotides into an RNA molecule (pink). The nucleotides in the RNA are complementary to the template strand of the DNA.

  16. (c) Termination termination signal At the end of a gene, RNA polymerase encounters a DNA sequence called a termination signal. RNA polymerase detaches from the DNA and releases the mRNA molecule.

  17. (d) Conclusion of transcription mRNA After termination, the DNA completely rewinds into a double helix. The RNA molecule is free to move from the nucleus to the cytoplasm for translation, and RNA polymerase may move to another gene and begin transcription once again.

  18. RNA synthesis and processing in eukaryotes DNA transcription initial RNA transcript add RNA cap and tail cap tail introns cut out and broken down RNA splicing completed mRNA to cytoplasm for translation RNA polymerase transcribes both the exons and introns, producing a long RNA molecule. Enzymes in the nucleus then add further nucleotides at the beginning (cap) and end (tail) of the RNA transcript. Other enzymes cut out the RNA introns and splice together the exons to form the true mRNA, which moves out of the nucleus and is translated on the ribosomes.

  19. gene RNA molecules DNA direction of transcription

  20. Transcription begins when: • RNA polymerase finds a start codon • RNA polymerase finds a promoter sequence • RNA polymerase finds a ribosome

  21. Base-pair matching, DNA mRNA T A C G A U T A G C C G A U

  22. W O R K T O G E T H E R • The enzyme that assembles RNA bases to make mRNA is: _________________ • This enzyme begins reading DNA at the ____________ sequence of a gene and ends at the ___________ sequence. • True or False: The entire DNA strand must be “unzipped” for transcription to take place. RNA Polymerase promoter terminator

  23. Translation • Once the gene has been transcribed into mRNA, the message must be translated to build a protein. • Ribosomes (made of rRNA) “read” the mRNA message and use the information to assemble aminoacids.

  24. gene Notice that translation takes place outside the nucleus, at the ribosomes. DNA (nucleus) (cytoplasm) messenger RNA Translation of the mRNA produces a protein molecule with an amino acid sequence determined by the nucleotide sequence in the mRNA. (b) Translation ribosome protein

  25. The players: • mRNA: Carries the encoded instructions for building a protein. • Ribosome (rRNA & protein structures): these act like enzymes to catalyze protein assembly. • tRNA: Transport RNA molecules that carry amino acids from the cytoplasm to the ribosome.

  26. To what class of molecules does tRNA belong? • Proteins • Carbohydrates • Lipids • Nucleic acids • Depends on which amino acid it carries.

  27. W O R K T O G E T H E R • What other molecule have we encountered has active sites and acts as a catalyst? How is a ribosome like this molecule? How is it different?

  28. Initiation: amino acid met methionine tRNA initiation complex small ribosomal subunit A tRNA with an attached methionine amino acid binds to a small ribosomal subunit, forming an initiation complex.

  29. Initiation: met tRNA mRNA The initiation complex binds to an mRNA molecule. The methionine (met) tRNA anticodon (UAC) base-pairs with the start codon (AUG) of the mRNA.

  30. Initiation: second tRNA binding site catalytic site met first tRNA binding site large ribosomal subunit The large ribosomal subunit binds to the small subunit. The methionine tRNA binds to the first tRNA site on the large subunit.

  31. Elongation: catalytic site met val The second codon of mRNA (GUU) base-pairs with the anticodon (CAA) of a second tRNA carrying the amino acid valine (val). This tRNA binds to the second tRNA site on the large subunit.

  32. Elongation: met peptide bond val The catalytic site on the large subunit catalyzes the formation of a peptide bond linking the amino acids methionine and valine. The two amino acids are now attached to the tRNA in the second binding position. Is this hydrolysis or dehydration synthesis?

  33. Elongation: catalytic site initiator tRNA detaches met val ribosome moves one codon to right The “empty” tRNA is released and the ribosome moves down the mRNA, one codon to the right. The tRNA that is attached to the two amino acids is now in the first tRNA binding site and the second tRNA binding site is empty.

  34. Elongation: catalytic site met his val The third codon of mRNA (CAU) base-pairs with the anticodon (GUA) of a tRNA carrying the amino acid histidine (his). This tRNA enters the second tRNA binding site on the large subunit.

  35. Elongation: met val his The catalytic site forms a new peptide bond between valine and histidine. A three-amino-acid chain is now attached to the tRNA in the second binding site. The tRNA in the first site leaves, and the ribosome moves one codon over on the mRNA.

  36. Termination: met val his arg completed peptide arg ile stop codon This process repeats until a stop codon is reached; the mRNA and the completed peptide are released from the ribosome, and the subunits separate.

  37. The ribosome has just bonded a series of amino acids into a chain. What has it built? • An amino acid. • A protein. • A nucleic acid. • Impossible to tell at this point.

  38. W O R K T O G E T H E R • When a tRNA leaves the ribosome, it goes off and finds another amino acid in the cell. Where do amino acids in human cells originally come from? Where do they come from in plant cells?

  39. gene (a) DNA etc. complementary DNA strand template DNA strand etc. codons (b) mRNA etc. anticodons (c) tRNA etc. amino acids (d) protein etc. methionine glycine valine

  40. direction of transcription RNA polymerase DNA mRNA protein ribosome

  41. DNA to mRNA to Protein A methionine (start) U G C C proline A U C serine U U

  42. DNA to mRNA to Protein G valine U U A threonine C U U stop codon G A

  43. Practice Transcription and Translation: http://learn.genetics.utah.edu/content/begin/dna/transcribe/

  44. The ribosome finds a promoter sequence. The ribosome finds a start codon. The ribosome breaks apart. Translation begins when:

  45. Interpret mRNA and build proteins. Construct mRNA. Replicate DNA. Facilitate cell division. The role of the ribosome is:

  46. Transcribe DNA and move mRNA out of the nucleus. Bind to the ribosome and mRNA chain together. Carry amino acids to the ribosome. Replace T with U when transcribing mRNA. The role of tRNA is:

  47. Genes Non-coding DNA Other proteins Nothing. They’re manufactured in the smooth ER. If genes code for proteins, what codes for enzymes?

  48. W O R K T O G E T H E R • Write out the mRNA strand that would be formed from this DNA segment:C A T A T G G G C T T A T A C • If the segment doesn’t include the start or stop codon, how many amino acids does it code for?

  49. W O R K T O G E T H E R • Suppose a segment of DNA contains the triplet ACG, and a mutation changes it to ACT. Would that cause a change in the resulting amino acid chain? Use your knowledge of transcription and translation to find the answer.

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