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Rna protein-synthesis
- 1. RNA, Ribosomes and Protein Synthesis
- 2. Learning Objectives Contrast RNA and DNA. Explain the process of transcription. Identify the genetic code and explain how it is read. Summarize the process of translation. Describe the central dogma of molecular biology.
- 3. Comparing RNA and DNA The sugar in RNA is ribose instead of deoxyribose. RNA is generally single stranded, not double stranded. RNA contains uracil in place of thymine.
- 4. The Role of RNA The roles played by DNA and RNA molecules in directing protein production are like the two types of plans builders use: A master plan A blueprint
- 5. Types of RNA The three main types of RNA are: Messenger RNA Ribosomal RNA Transfer RNA
- 6. Messenger RNA An mRNA molecule is a copy of the portion of DNA that will be used to make a protein. After being made in the nucleus, mRNA travels to the cytoplasm, the site of protein synthesis.
- 7. Ribosomal RNA Protein synthesis occurs on ribosomes, which are made up of two subunits. Both subunits consist of several molecules of ribosomal RNA (rRNA).
- 8. Transfer RNA During protein synthesis, transfer RNA molecules (tRNA) carry amino acids from the cytoplasm to the mRNA.
- 9. RNA Synthesis: Transcription In transcription, segments of DNA serve as templates to produce complementary mRNA molecules.
- 10. RNA Synthesis: Promoters RNA polymerase binds only to regions of DNA that have specific base sequences. These regions are called promoters. C A G U TA
- 11. RNA Synthesis: RNA Editing New RNA molecules sometimes require a bit of editing before they are ready to be read. Exons Introns Cap Tail
- 12. The Genetic Code RNA has four bases: adenine, cytosine, guanine, and uracil. These bases form a “language”: A, C, G, and U.
- 13. The Genetic Code: Codons The genetic code is read in three-letter groupings called codons. A codon is a group of three nucleotide bases in messenger RNA that specifies a particular amino acid. AUG AAC UCU
- 14. Genetic Code Table There are 64 possible three-base codons in the genetic code.
- 15. Reading Codons Start at the middle of the circle with the first letter of the codon and move outward. CAC = Histidine
- 16. Start and Stop Codons The methionine codon AUG serves as the “start” codon for protein synthesis. There are three “stop” codons. AUG = methionine = “start” codon UAA, UAG, and UGA are “stop” codons
- 17. Translation Transcribed mRNA directs the translation process. Translation is the process that makes proteins using the copy of the DNA code on the mRNA.
- 18. Translation: Transfer RNA To start translation, tRNA molecules bind to mRNA codons, carrying amino acids with them. anticodon
- 19. Translation: The Polypeptide Assembly The ribosome helps form a peptide bond. It breaks the bond holding the first tRNA molecule to its amino acid.
- 20. Translation: Completing the Polypeptide The ribosome reaches a stop codon, releasing the newly synthesized polypeptide and the mRNA molecule, completing the process of translation.
- 21. Roles of RNA in Translation All three major forms of RNA—mRNA, tRNA, and rRNA—are involved in the process of translation.
- 22. The Molecular Basis of Heredity The central dogma of molecular biology is that information is transferred from DNA to RNA to protein.
- 23. Gene Expression When a gene (segment) of DNA code is used to build a protein, scientists say that gene has been expressed.
Notas do Editor
- Read the lesson title aloud to students.
- Click to show each of the learning objectives. Students may already have some basic knowledge of RNA. Give students a few minutes to complete a quick write of whatever they know, or think they know, about RNA. Then, ask volunteers to come forward and write their responses on the board. Tell students that at the end of this lesson, they should be able to compare and contrast RNA and DNA and explain the process of transcription. Distribute the worksheet to students. Have students create their main idea circle in the middle of the page and write “RNA” in it. Then have them draw lines branching off the main idea, connected to circles that contain concepts related to RNA. Encourage students to use the worksheet to record significant information regarding RNA as you move through the lesson.
- Remind students that each nucleotide in DNA is made up of a 5-carbon sugar, a phosphate group, and a nitrogenous base. Explain that this is true for RNA as well, but that there are some important differences between RNA and DNA. Click to reveal the first difference. Tell students: The difference in the sugar is the distinction in the abbreviations RNA and DNA. Click to reveal the second difference. Tell students: This lesson will explain why RNA is single stranded. Click to reveal the third difference. Explain that for every “T” in the DNA code, RNA has a “U.” This means if forming a complimentary code, where there is an “A” in DNA, RNA would pair it with a “U”. Explain that these chemical differences make it easy for enzymes in the cell to tell DNA and RNA apart.
- Explain to students that the first step in decoding these genetic instructions is to copy part of the base sequence from DNA into RNA. RNA then uses these instructions to direct the production of proteins, which help determine an organisms’ characteristics. Make the analogy that the roles DNA and RNA molecules play in directing the production of proteins are like the two types of plans builders use. Click to reveal the first bullet point. Ask: What is a builder’s master plan used for? Answer: A master plan is the original information needed to construct a building. Tell students: Builders never bring a valuable master plan to the job site because it might be damaged or lost. Instead, they use a blueprint. Click to reveal the second bullet point. Ask: Why do you think builders work from blueprints? Answer: because they are inexpensive, disposable copies of the master plan and multiple copies can be made Tell students: An RNA molecule is like a disposable copy of a segment of DNA.
- Explain to students that there are several types of RNA and most of them are involved in protein synthesis. Tell students: RNA controls the assembly of amino acids into proteins. Each type of RNA molecule specializes in a different aspect of this job. Tell students: There are three main types of RNA. Click to reveal each type. Read each name. Tell students: You will learn more about each one in the upcoming slides. Address misconceptions: Emphasize the importance of RNA. Explain to students that DNA is the inherited genetic material, but RNA is the genetic material that carries out the instructions encoded in DNA. Without RNA, the instructions in DNA could not be used by cells.
- Tell students: Many DNA genes contain instructions for assembling amino acids into proteins. The RNA molecules that form as copies of these instructions are called messenger RNA. Ask: Where would the instructions for polypeptide synthesis be carried from and to? Answer: For polypeptide synthesis, messenger RNA moves from the nucleus to ribosomes in the cytoplasm. Click to reveal this answer.
- Explain to students that proteins are assembled on ribosomes, small organelles composed of two subunits. Click to reveal the image and information. Tell students: These subunits are made up of several ribosomal RNA (rRNA) molecules and as many as 80 different proteins. Click to reveal this point.
- Explain that when a protein is built, a third type of RNA molecule transfers each amino acid to the ribosome as it is specified by the coded messages in mRNA. These molecules are known as transfer RNA. Click to reveal the image and information. Ask students to come up with analogies for transfer RNA and have a class discussion in which they share their ideas.
- Ask: How does the cell make mRNA? Answer: Most of the work of making RNA takes place during the process of transcription. Tell students that, in prokaryotes, RNA synthesis and protein synthesis take place in the cytoplasm. In eukaryotes, RNA is produced in the nucleus, as shown in this illustration, and then moves to the cytoplasm for the purpose of protein synthesis. Explain that transcription requires an enzyme, known as RNA polymerase, that is similar to DNA polymerase. Click to highlight RNA polymerase. RNA polymerase binds to DNA during transcription and separates the DNA strands. Click to highlight the DNA strands. Tell students: This is like unzipping the two strands of DNA. RNA polymerase then uses one strand of DNA as a template from which to assemble nucleotides into a complementary strand of mRNA. Click to highlight the strand of RNA. The ability to copy a single DNA sequence into RNA makes it possible for a single gene to produce hundreds or even thousands of RNA molecules.
- Ask: How does RNA polymerase know where to start and stop making a strand of RNA? Answer: RNA polymerase doesn’t bind to DNA just anywhere. The enzyme binds only to promoters. Explain that promoters, which have specific base sequences, tell the enzyme where to start transcribing DNA. Point out how bases in the DNA strand are bound to complementary bases that will form the RNA strand. Ask for volunteers to label the bases in the base pairs with the letters A, C, G, T, or U, using the legend as a guide. Remind them that uracil in RNA is complementary to adenine in DNA. Once students have written in their answers, click to reveal the answers and verify that they are correct. Tell students: Transcription is just the first stage of RNA synthesis.
- Ask: What happens to the newly made mRNA before it leaves the nucleus? Answer: RNA editing Make the analogy that new mRNA molecules are like a writer’s first draft―they sometimes require editing before they are ready to be read. Tell students: The pieces of pre-mRNA molecules that are cut out, or “edited out,” and discarded are called introns. Ask for a volunteer to point out the introns in the pre-mRNA. Click to highlight the introns. Tell students: The remaining pieces are known as exons. Ask for a volunteer to point out the exons in the pre-mRNA. Click to highlight the exons. Point out that the remaining exons are spliced together. Click to highlight the exons splicied together. Tell students: Then, an RNA cap and tail are added to form the final mRNA molecule. Click to show labels and highlight the cap and tail. Ask: What is the purpose of making a large RNA molecule and then throwing parts of that molecule away? Answer: That’s a good question, and biologists still don’t have a complete answer. Some pre-mRNA molecules may be cut and spliced in different ways in different tissues, so a single gene can actually produce several different mRNA molecules. Introns and exons may also play a role in evolution, making it possible for very small changes in DNA sequences to have dramatic effects on how genes affect cellular function.
- Ask: What is the genetic code and how is it read? Answer: The four bases of RNA form the “language” called the genetic code. The genetic code is read three “letters” at a time, so that each “word” is three bases long and corresponds to a single amino acid. Remind students that the four bases of RNA are adenine, cytosine, guanine, and uracil. Click to highlight these in the legend.
- Tell students: A codon consists of three consecutive bases that specify a single amino acid to be added to the polypeptide chain. All living organisms read the genetic code in this way, three bases at a time. Ask: What are the three-letter groups of codons shown here? Answer: AUG, AAC, and UCU Click to highlight each group as students provide the answers.
- Explain to students that because there are four different bases in RNA, there are 64 possible three-base codons (4 × 4 × 4 = 64) in the genetic code. Reading them is simplified by the use of a genetic code table like that shown. Tell students: Most amino acids can be specified by more than one codon. For example, six different codons—UUA, UUG, CUU, CUC, CUA, and CUG—specify leucine. Click to highlight the codons that specify leucine. Tell students: Only one codon—UGG—specifies the amino acid tryptophan. Click to highlight the UGG codon. Point out that although the code table shown here applies to most multicellular organisms, including animals and plants, there are slight differences in the code as used by some microorganisms, as well as mitochondria and chloroplasts.
- Step students through reading, or decoding, a codon. Use CAC as your example. Ask volunteers to decode CAC. Have a student find the first letter in the set of bases at the center of the circle. Click to highlight the correct letter. Then, have a volunteer find the second letter of the codon, A, in the “C” quarter of the next ring. Click to highlight the correct letter. Have a volunteer find the third letter, C, in the next ring, in the “C-A” grouping. Click to highlight the correct letter. Then, have the class read the name of the amino acid in that sector—in this case, histidine. Click to highlight the amino acid name. Practice using the genetic code “decoder” by having volunteers provide codons for other volunteers to identify the associated amino acid. Then, reverse the process: Select amino acids and ask for other volunteers to provide the codon or codons that represent them. Ask: How many amino acids does each codon represent? Answer: one Ask: How many codons can code for a single amino acid? Answer: from one to six
- Continue the analogy of the genetic code being a language and point out that languages need punctuation marks. In the genetic code, punctuation marks are “start” and “stop” codons. The methionine codon AUG serves as the initiation, or start, codon for protein synthesis. Click to highlight the start codon and summary label. Explain that following the start codon, mRNA is read, three bases at a time, until it reaches one of three different stop codons, which end translation. Ask for volunteers to point out the three different stop codons. Click to highlight the first stop codon (UAA). Click again to highlight each base for the second codon (UAG). Click to show third base option (UGA) and summary label.
- Explain to students that translation occurs after transcription. Transcribed mRNA directs the translation process. Click to highlight mRNA. Remind students that in a eukaryotic cell, transcription goes on in the cell’s nucleus. Translation is carried out by ribosomes after the transcribed mRNA enters the cell’s cytoplasm.
- Tell students: Translation begins when a ribosome attaches to an mRNA molecule in the cytoplasm. Explain that translation initiates at AUG, the start codon. Click to highlight AUG. Tell students: tRNA molecules carry in the amino acids coded for by the codon. Since AUG always codes for methionine, the first amino acid brought in for every round of translation is methionine. Click to highlight methionine. As each codon passes through the ribosome, more tRNAs bring the proper amino acids into the ribosome. Click to highlight amino acid being brought in. Each transfer RNA has an anticodon whose bases are complementary to the bases of a codon on the mRNA strand. The tRNA attaches its anticodon to the appropriate codon of the mRNA. Click to highlight the anticodon. Ask: What is the anticodon for methionine codon, the start codon? Answer: UAC Ask: What amino acid does the mRNA codon UUC bring to the polypeptide chain? Answer: phenylalanine Ask: If an mRNA codon has the bases CUA, what bases will the corresponding transfer RNA anticodon have? Answer: GAU
- Explain to students that the ribosome helps form a peptide bond between the first and second amino acids—methionine and phenylalanine. Click to highlight. Point out that, at the same time, the bond holding the first tRNA molecule to its amino acid is broken. That tRNA then moves into a third binding site, from which it exits the ribosome. Click to highlight. Tell students that the ribosome then moves to the third codon, where tRNA brings in the amino acid specified by the third codon. Click to highlight. Explain that the ribosome moves along the mRNA from right to left, binding new tRNA molecules and amino acids. Address misconceptions: Tell students: Keep in mind the overall picture and relationship between transcription and translation. Ask: What is the product of transcription? Answer: mRNA Ask: What is the product of translation? Answer: a protein Ask: Where do the amino acids come from that make up the protein? Answer: They are available in the cell and are picked up by the tRNA molecules. The tRNA molecules then bring them into ribosome to be used to build the protein.
- Tell students: The translation process continues until the ribosome reaches one of the three stop codons. Ask a volunteer to identify the stop codon in this illustration. Click to highlight the answer (UGA). Explain that when the ribosome reaches the stop codon, it releases both the newly synthesized polypeptide and the mRNA molecule, completing the process of translation. Click to highlight the polypeptide and mRNA molecule. Ask: What would happen if the stop codon mistakenly had been made into a regular codon? Answer: Translation would not stop; another amino acid would be attached and the protein product would have an error in its structure.
- Remind students that all three major forms of RNA are involved in the process of translation. Click to reveal messenger RNA. Tell students: The mRNA molecule carries the coded message that directs the process of translation. Click to reveal transfer RNA. Tell students: The tRNA molecules deliver the amino acids, enabling the ribosome to translate the mRNA’s message into protein form. Click to reveal ribosomal RNA. Tell students: The ribosomal RNA molecules hold ribosomal proteins in place and may even carry out the chemical reaction that joins amino acids together. Point out to students that these components, which translate the genetic code for the purpose of synthesizing new protein molecules, are common to all organisms.
- Ask: What is the central dogma of molecular biology? Answer: The central dogma of molecular biology is that information is transferred from DNA to RNA to protein. Click to reveal this answer. Write the following on the board: DNA → RNA → Protein Tell students this represents the central dogma of molecular biology. Call on several volunteers to express the central dogma in their own words. Then, lead a discussion about its implications and limitations. Ask: What does the central dogma imply about the role of RNA? Answer: It’s the step between DNA and proteins. Point out that the central dogma does have limitations. For example, it doesn’t represent the other roles of RNA. There are also exceptions to this dogma, including viruses that transfer information in the opposite direction, from RNA to DNA.
- Have students consider the illustration of gene expression in this slide―the way in which DNA, RNA, and proteins are involved in putting genetic information into action in living cells. Point out to students that this illustration is a simplification and have them review the other figures used in this presentation if they are confused on that point. Tell students: The protein made at the end of transcription and translation is the ultimate use or expression of the code in the DNA segment used in transcription. Scientists will say that region of DNA has been expressed. This means its code was used to build a protein. Summarize gene expression in the following manner: DNA carries information for specifying the traits of an organism. The cell uses the sequence of bases in DNA as a template for making mRNA. The codons of mRNA specify the sequence of amino acids in a protein. Proteins, in turn, play a key role in producing an organism’s traits. Explain that many RNA molecules are not translated into proteins but still play important roles in gene expression. Relate the diagram to the central dogma of molecular biology. Ask a volunteer to point out the DNA in the illustration. Click to highlight the DNA. Ask a volunteer to point out the RNA in the illustration. Click to highlight the RNA. Then, ask a volunteer to point out the protein in the illustration. Click to highlight the protein. Share with students that one of the most interesting discoveries of molecular biology is the near-universal nature of the genetic code. Although some organisms show slight variations in the amino acids assigned to particular codons, the code is always read three bases at a time and in the same direction. Despite their enormous diversity in form and function, living organisms display remarkable unity at life’s most basic level, the molecular biology of the gene. Ask students to write a paragraph stating and explaining the central dogma of molecular biology.