Gene Expression - Translation

Gene Expression—Translation How do cells synthesize polypeptides and convert them to functional proteins? Why? The message in your DNA of who you are and how your body works is carried out by cells through gene expression. In most cases this means synthesizing a specific protein to do a specific job. First, mRNA is transcribed from the DNA code. Then, the mRNA sequence is translated into a polypeptide sequence. Model 1 – Codons mRNA nucleotides Amino acids U C A G U UUU Phe UUC Phe UUA Leu UUG Leu UCU Ser UCC Ser UCA Ser UCG Ser UAU Tyr UAC Tyr UAA stop UAG stop UGU Cys UGC Cys UGA stop UGG Trp U C A G C CUU Leu CUC Leu CUA Leu CUG Leu CCU Pro CCC Pro CCA Pro CCG Pro CAU His CAC His CAA Gln CAG Gln CGU Arg CGC Arg CGA Arg CGG Arg U C A G A AUU Ile AUC Ile AUA Ile AUG Met (start) ACU Thr ACC Thr ACA Thr ACG Thr AAU Asn AAC Asn AAA Lys AAG Lys AGU Ser AGC Ser AGA Arg AGG Arg U C A G G GUU Val GUC Val GUA Val GUG Val GCU Ala GCC Ala GCA Ala GCG Ala GAU Asp GAC Asp GAA Glu GAG Glu GGU Gly GGC Gly GGA Gly GGG Gly U C A G Third Base First Base Second Base 1. Model 1 defines the code scientists have discovered that relates the nucleotide sequence of mRNA to the amino acid sequence of polypeptides. a. What do the letters U, C, A, and G in Model 1 represent? mRNA nucleotides b. What do the abbreviations such as Phe, Ile, Ala, and Gly in Model 1 represent? amino acids c. The language of mRNA is often described as a “triplet code.” Explain the significance of this reference. Three nucleotides are needed to code one amino acid Gene Expression—Translation 1 2. If an mRNA molecule had 300 nucleotides in the coding region of the strand, how many amino acids would be in the polypeptide that was synthesized? Show mathematical work to support your answer. 300/3 = 100 3. Consider the information in Model 1. a. How many different codons (triplets) code for the amino acid Proline (Pro)? 4 b. Compare all of the codons for Proline. What are the similarities and differences? The codons for proline all start with CC but the third base is different c. Considering that mistakes can occur during transcription and DNA replication, what advantage is there for an organism to have multiple mRNA sequences code for the same amino acid? If one codon was to make a mistake or fail, the mRNA would still translate properly 4. Using the mRNA codon chart in Model 1, complete the following: DNA → mRNA → TAC CTT AUG GAA CGG GCC ATG UAC GTC ACT CAG UGA polypeptide sequence → met Glu Ala Tyr Gln 5. According to the table in Model 1, what amino acid is at the beginning of every polypeptide? AUG 6. The codons shown in Model 1 are used in all species on Earth with very little variation. What might scientists conclude from this? All species might have a common ancestor or the theory of evolution 2 Model 2 – Translation Amino acid Uncharged tRNA His Ala His A UAC Charged tRNA Ser Met Leu G UA Leu Ala Met CGG A C Ser G U A Anticodon Ribosome A AC U CG U AC A U G U U G G C C A G C C A U U A A 5´ CGG U C G A U G U U G G C C A G C C A U U A A 3´ mRNA Initiation Elongation U A C A C G AC U C G G U A G Ser Met Ala His Leu H2O Release factor A U G U U G G C C A G C C A U U A A Termination 7. Refer to Model 2. a. What are the three stages of translation? initiation, elongation, termination b. Define each of the terms used in your answer to part a as they are used in everyday language. initiation means to start, elongation means to make longer, and termination means to end Gene Expression—Translation 3 8. According to Model 2, when the mRNA leaves the nucleus, to which cellular organelle does it attach? the ribosome 9. The mRNA attaches to the organelle at the sequence AUG. What is the significance of this sequence of nucleotides? its the start codon 10. Describe the movement of the ribosome as translation occurs. the ribosome moves along the mRNA chain from 5 to 3 Read This! The ribosome is a large complex of ribosomal RNA (rRNA) and proteins. It consists of two subunits. The smaller subunit binds to the mRNA strand and the larger subunit holds the tRNA molecules in place while the covalent peptide bond is formed between the amino acids. Several ribosomes can attach to an mRNA molecule simultaneously. This allows for many polypeptide chains to be synthesized at once. 11. The tRNA molecules in a cell are short sequences of nucleotides (about 80 bases) that contain an anticodon and carry a specific amino acid. a. Find the tRNA in Model 2 that is carrying the Histidine (His). What sequence of nucleotides makes the anticodon on this tRNA molecule? GUA b. What codon on mRNA would match this anticodon? CAU c. Verify that the codon you wrote in part b codes to Histidine by looking at the table in Model 1. d. What anticodon would be found on a tRNA molecule carrying Glycine (Gly)? (Note: There are several correct answers here.) CCG, CCA, CCU, CCC 12. The “t” in tRNA is short for transfer. In a complete sentence, explain why this molecule is called transfer RNA. It transfers the correct amino acid in the correct sequence 4 13. During elongation, how many tRNA molecules are held in the ribosome at the same time? 2 14. What will happen to the unattached tRNA once it has delivered its amino acid? It is free to pick up and carry another amino acid 15. Describe two things that occur during termination as illustrated in Model 2. A release factor binds to the last codon and stops the process of adding more amino acids to the polypeptide. There is a water molecule that is added to the end of the amino acid chain 16. Explain how the term “translation” applies to the synthesis of proteins from DNA instructions. the form of nitrogen bases read in a set of three called codons that is being translated into the language of proteins Gene Expression—Translation 5 Extension Questions 17. The codons of mRNA are a set of three nucleotides with four possible bases in combination. a. Show mathematically that there are 64 permutations possible when three bases are used. 4 x 4 x 4 = 64 b. Show mathematically that two bases as a codon would not be sufficient to code for all 20 known amino acids. 2x2x2=8 18. A silent mutation is one that does not affect protein structure. Write a code for an original DNA strand containing at least 12 bases, and then mutate the original DNA so that the final protein is unaffected. original: TAC AAA CCC GGA mutated: TAC AAG CCC GGA 19. In prokaryotic cells, translation begins before transcription is finished. Give two reasons why this would not be possible in eukaryotic cells. Prokaryotic cells don't have a nucleus whereas eukaryotic cells do. Transcription takes place in the nucleus and translation takes place on the ribosomes in the cytoplasm. 6