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BIOLOGY SEMESTER ONE UNIT 17 CHECKLIST UNIT 17: FROM GENE TO PROTEIN In this module you will investigate protein synthesis, a process in which cells build proteins. Translation is the term used to describe this process, as the sequences of DNA nucleotides are transcribed and translated by a various forms of RNA into the specific protein coded for by that gene sequence. By copying the DNA and using the copy to make proteins, it reduces the risk of the original DNA being altered or damaged, thus reducing the risk of mutations. Three forms of RNA are involved in transferring the DNA information from the nucleus into the cytoplasm to waiting ribosomes, where protein synthesis occurs. Messenger RNA (mRNA) are the “transcribed,” coded copies of the DNA sequence, and move from the nucleus to the cytoplasm. Ribosomal RNA (rRNA) is the major component of the ribosomes that decodes mRNA; transfer RNA (tRNA) assist in polypeptide (protein) construction by bringing in the specific amino acids that string together to create the protein. Protein synthesis begins with the “unzipping” of DNA by the enzyme helicase in the nucleus. As the DNA nucleotides unbind from their partner pairs, mRNA nucleotides attach to the DNA strand along the gene (the section of DNA that has the information for a particular protein). This process (making a complementary RNA copy of a DNA strand) is called transcription. The DNA strand that mRNA complements is called the template strand. The process of transcription can be divided into three stages: Initiation, Elongation and Termination, each regulated by a large number of proteins ,such as transcription factors and co-activators, that ensure the correct gene is properly transcribed. The DNA strand is read in the 3' to 5' direction, and the mRNA is transcribed in the opposite (5' to 3') direction by the RNA polymerase. The structure of RNA is very similar to the DNA structure, with the exception that, in RNA, the nucleotide uracil takes the place that thymine occupies in DNA. Once the gene has been transcribed, the single strand of mRNA releases from the DNA and leaves the nucleus through nuclear pores, migrating into the cytoplasm. The second part of protein synthesis is known as translation. Translation occurs in the cytoplasm, where the ribosomes are located. Ribosomes are mainly rRNA, and a single ribosomal complex consists of both a small and a large subunit that surround the incoming mRNA. In translation, the mRNA sequence is used as a template to guide the synthesis of a chain of amino acids that together form a specific protein. A third type of RNA, called transfer RNA, sequentially pairs with the mRNA to link the correct amino acid sequences together to make the protein. The grouping and order of bases on an RNA strand have significance: The bases can be thought of as individual letters of three letter words, each word having its own meaning; these “words” are referred to as codons. Most codons are translated into specific Creative Commons Attribution 3.0 Unported License 1 BIOLOGY SEMESTER ONE UNIT 17 amino acids (i.e. CUU is Leucine, ACU is Threonine). However, some of the codons are specialized for a different purpose: for example, UAA, UAG, and UGA are “stop codons,” signalling the end of the gene sequence and instructing the ribosome to stop translating. For synthesis of protein, a succession of tRNA molecules charged with appropriate amino acids are brought together with an mRNA molecule and matched up by base-pairing through their anti-codons. The amino acids are linked together to extend the growing protein chain, and the tRNAs are released. This whole complex of processes is carried out by the ribosomal complex, formed from two main chains of RNA, called ribosomal RNA (rRNA), and more than 50 different proteins. Translation proceeds in four phases: Activation, Initiation, Elongation, and Termination (all describing the growth of the amino acid chain, or polypeptide, that is the product of translation). 1. In activation, the first step in translation, the correct amino acid is joined to the correct transfer RNA (each tRNA molecule can only carry the specific amino acid that correlates with its “anticodon”—anticodons are tRNA sequences that may complement mRNA codons). When the tRNA has an amino acid linked to it, it is referred to as "charged." 2. Initiation involves the binding of the small ribosomal subunit to the 5' end of mRNA. “ Initiation factors” are a group of proteins that assist in the binding process. 3. Elongation occurs when the next charged tRNA in line binds to the ribosome to pair with the next mRNA codon. 4. Termination of the polypeptide chain occurs when the ribosome reaches a stop codon (UAA, UAG, or UGA). When this happens, no tRNA can recognize it and bind, but a “releasing factor” can recognize one of these three codons and cause the release of the polypeptide chain from the ribosome. Once the amino acid chain has been established, the resulting protein undergoes a series of primary, secondary and tertiary folding. This is a result of the numerous molecular interactions between amino acid groups within the protein chain that create bonds between groups along the chain. Once the protein is in its final form, it often requires further activation by enzymes. Only then is it ready to be released to undertake its biological function. Creative Commons Attribution 3.0 Unported License 2 BIOLOGY SEMESTER ONE UNIT 17 LEARNING OBJECTIVES At the end of this module you should be able to do the following: 1. Distinguish between the “one gene-one enzyme” hypothesis and the “one gene-one polypeptide” hypothesis and explain why the original hypothesis was changed. 2. Explain how RNA differs from DNA. 3. Briefly explain how information flows from gene to protein. 4. Describe the events and locations associated with transcription and translation, and compare between eukaryotes and bacteria. 5. Using a chart, be able to identify what amino acids are specified by various codons. 6. Define and discuss redundancy and ambiguity in the genetic code. 7. Explain the significance of the reading frame during translation. 8. Explain the evolutionary significance of a nearly universal genetic code. 9. Explain how RNA polymerase recognizes where transcription should begin. Describe the role of the promoter, the terminator, and the transcription unit. 10. Describe the functional and evolutionary significance of introns. 11. Explain why, due to alternative RNA splicing, the number of different protein products an organism can produce is much greater than its number of genes. 12. Describe the significance of polyribosomes. 13. Explain what determines the primary structure of a protein and describe how a polypeptide must be modified before it becomes fully functional. 14. Define “point mutations”. Distinguish between base-pair substitutions and base-pair insertions. Give an example of each and note the significance of such changes. 15. Distinguish between a mis-sense and a nonsense mutation. CHECK LIST Read Chapter 17: From Gene to Protein, of Campbell and Reece’s Biology, 9th Ed. As you are reading, address each of the learning objectives listed above. M flash cards for the terminology list provided. This will be beneficial for studying for the midterm and final exams later in the semester. You may be able to review the PowerPoint Lecture and other resources for this unit. Refer to your instructor’s notes for more details. Review the video clip “Protein Synthesis,” and answer the questions in the “Protein Synthesis Video Worksheet.” Creative Commons Attribution 3.0 Unported License 3 BIOLOGY SEMESTER ONE UNIT 17 Discussion Post: o Using the diagram here as reference, describe the events of transcription and translation, in your own words, as it occurs in the eukaryotic cell. Be sure to include all of the names of the structures and molecules involved, as well as the location of each in the cell. These descriptions will be posted on the discussion forum. You will be responsible for reading and commenting on two of your classmates’ posts. Include any corrections you deem needed, and also point out anything they have summarized in a way that you find helpful in understanding protein synthesis. For extra practice, try the Self Quiz or Practice Test on the Mastering Biology Website. To log onto the website, use the access code provided in your textbook. You will also find other resources, such as downloadable MP3 tutorials for each chapter, a glossary, and an electronic copy of your text—you can catch up on your reading anywhere! KEY TERMS 5' cap amino acid anticodon base-pair substitution chaperonin codon endomembrane system exon frameshift mutation gene expression insertion intron messenger RNA (mRNA) missense mutation mutagen mutation nonsense mutation P site point mutation poly (A) tail Creative Commons Attribution 3.0 Unported License polyribosome (polysome) primary transcript promoter reading frame ribosomal RNA (rRNA) ribosome ribozyme RNA polymerase RNA splicing spliceosome 4 BIOLOGY SEMESTER ONE UNIT 17 template strand terminator transcription transcription factor transcription initiation complex transcription unit transfer RNA (tRNA) translation triplet code ROOT WORDS TO KNOW 1 anti- = opposite (anticodon: a specialized base triplet on one end of a tRNA molecule that recognizes a particular complementary codon on an mRNA molecule) exo- = out, outside, without (exon: a coding region of a eukaryotic gene that is expressed) intro- = within (intron: a noncoding, intervening sequence within a eukaryotic gene) muta- = change; -gen = producing (mutagen: a physical or chemical agent that causes mutations) poly- = many (poly-A tail: the modified end of the 3[H11032] end of an mRNA molecule consisting of the addition of some 50 to 250 adenine nucleotides) trans- = across; -script = write (transcription: the synthesis of RNA on a DNA template) SOURCES Campbell, N. A. (2008). Biology, Eighth Edition. San Francisco: Pearson, Benjamin Cummings. Pearson Education. (2010). Retrieved 2010, from Mastering Biology : http://session.masteringbiology.com University of Leicester. (2009, Feb 11). Gene Regulation and Expression. Retrieved May 2010, from Virtual Genetics Education Centre: http://www.le.ac.uk/genetics/genie/vgec/he/expression.html 1 (Pearson Education, 2010) Creative Commons Attribution 3.0 Unported License 5 BIOLOGY SEMESTER ONE UNIT 17 NANSLO Biology Core Units and Laboratory Experiments by the North American Network of Science Labs Online, a collaboration between WICHE, CCCS, and BCcampus is licensed under a Creative Commons Attribution 3.0 Unported License; based on a work at rwsl.nic.bc.ca. Funded by a grant from EDUCAUSE through the Next Generation Learning Challenges. Creative Commons Attribution 3.0 Unported License 6