BIO181

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Arizona State University *

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181

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Biology

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Apr 3, 2024

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docx

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BIO181 Transcription Translation Worksheet If any questions are unanswered, you will receive a score of 0. The extension questions are required . Name: Class Time: Part 1: Transcription ______________________________________________________________________________________ READ THIS : DNA is often referred to as a genetic blueprint. In the same way that blueprints contain the instructions for construction of a building, the DNA found inside the nuclei of cells contains the instructions for assembling a living organism. The DNA blueprint carries its instructions in the form of genes. In most cases the genes direct the production of a polypeptide, from which other more complex proteins, such as enzymes or hormones, may be constructed. These polypeptides and other molecules run the organism’s metabolism and, in multicellular organisms, dictate what each cell’s job is. So, what is the language of these instructions and how are they read and decoded by the cellular organelles? This activity will focus on the decoding of genes in eukaryotes. ______________________________________________________________________________________ Note - These are the same images. The image on the left is provided as a Google drawing. To edit it, draw on it, add circles or labels to it, double click the image while in Google Docs. Add in what you need to, then hit “Save and Close”. The image on the right is clearer but cannot be edited. 1. Refer to Model 1. a. What is the base-pair rule for a DNA strand matching an RNA strand? For DNA, Adenine pairs with Thymine and Guanine pairs with Cytosine. For RNA, Adenine pairs with Uracil and Guanine pairs with Cytosine. b. Compare this base-pair rule with that of two DNA strands. C ompared to two DNA strands, the base-pair rule states that for DNA, DNA also pairs T with A because DNA does not have Uracil like RNA. In DNA, Thymine bonds to Adenine and Adenine bonds to Thymine. In RNA, Thymine bonds to Adenine, but Adenine bonds to Uracil. 1

2. Which strand of the DNA contains the “blueprint” for the pre-mRNA? The template strands of the DNA contains the “blueprint” for the pre-mRNA. 3. Consider Model 1 a. In which direction is the DNA template strand read? The DNA template strand is read from 3’ to 5’. b. The DNA template strand and pre-mRNA strand are anti-parallel. With this in mind label the 3ʹ and 5ʹ ends of the pre-mRNA strand in Model 1. The left end is the 3’ end, and the right end is the 5’ end. c. In which direction is the pre-mRNA molecule constructed? On which end are new bases added? The pre-mRNA molecule is constructed from 5’ to 3’. _______________________________________________________________________________________ READ THIS: In eukaryotes the enzyme RNA polymerase joins with several transcription factor proteins at the promoter, which is a special sequence of base pairs on the DNA template strand that signals the beginning of a gene. The transcription factor proteins, along with the RNA polymerase, is called the transcription initiation complex . This moves along the DNA template strand at about 40 base pairs per second producing pre-mRNA. When the RNA polymerase reaches the terminator sequence of base pairs on the DNA template strand, it completes the production of pre-mRNA and releases it into the nucleoplasm. _______________________________________________________________________________________ 4. Where on the DNA strand does the transcription initiation complex form? The transcription initiation complex forms on the promotor of the DNA strand. 5. Imagine the diversity of functions in which your cells participate. a. Do you think that the cells in your big toe contain the same DNA as the neurons in your brain? Yes, I think that the cells in my big toe contain the same DNA as the neurons in my brain because every cell in the human body has the same DNA. 2

b. Will an individual cell transcribe every, or almost every, gene in its genome? Explain. No, an individual cell cannot transcribe every, or almost every, gene in its genome because some of the products are not needed for that specific action. Note - These are the same images. The image on the left is provided as a Google drawing. To edit it, draw on it, add circles or labels to it, double click the image while in Google Docs. Add in what you need to, then hit “Save and Close”. The image on the right is clearer but cannot be edited. 6. Compare the pre-mRNA to the mRNA leaving the nucleus in Model 1. a. What has been removed from the pre-mRNA to make it into mRNA? An intron was removed from the pre-mRNA to make it into mRNA. b. What has been added to the mRNA that was not present in the pre-mRNA, and where on the mRNA strand are the additional items located? A methyl cap was added to the mRNA that was not present in the pre-mRNA. The additional items are located on the poly (A) tail that was added to the 3’ end. The poly A tail and a 5’ cap are added. 7. Can mRNA diffuse through a membrane? Why or why not? mRNA cannot diffuse through a membrane because it has to have a transport protein. 3

______________________________________________________________________________________ READ THIS: Introns are sections of pre-mRNA that are noncoding. That is, they don’t provide useful information for the production of the polypeptide being synthesized. There is evidence that suggests these introns allow certain sections of DNA to code for different polypeptides when different sections are removed. The removal of specific sections is triggered by a signal response in the cell. The portions of the pre-mRNA that remain are called exons . The methyl cap (sometimes called the GTP cap or 5ʹ cap) helps the mRNA molecule move through the nuclear pore and attach to a ribosome, its final destination. mRNA is a short lived molecule. Once in the cytoplasm the mRNA will be subject to exonucleases that immediately start removing individual nucleotides from the 3ʹ end of a nucleic acid. The individual mRNA nucleotides will then be free to be used again during the process of transcription. ______________________________________________________________________________________ 8. The human genome contains about 25,000 genes and yet produces about 100,000 different polypeptides. Propose an explanation of how this is possible. Introns allow certain sections of the DNA to code for different polypeptides when different sections are removed from the DNA strand. Different mRNA combinations make up different genes. 9. Using the information in Read This section , develop a hypothesis to explain the advantage of the poly-A tail added to the 3’ end of the mRNA. The poly A tail prevents the information carrying part of the mRNA from being removed by the exonucleases before the formation of a polypeptide can occur. It also protects the coding on the mRNA. 10.Different mRNA molecules can have poly-A tails of different lengths. Considering the purpose of adding the poly-A tail (from the previous question), why are some tails longer than others? Justify your answer. Some tails are longer than others because proteins that are needed for long periods of time will come from mRNA with long poly A tails. Proteins that are needed for brief periods of time will come from mRNA with shorter poly A tails. Some tails are longer so that they can last longer in the cytoplasm. 11.Summarize the steps of transcription. 1. Initiation: recognition sites, transcription factors, RNA polymerase unzips DNA and reads the transcription region and produces RNA strands. 2. Elongation: elongating DNA. 5

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