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1Lesson 4: Transcription and Translation
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3>>> Gene Expression
4A gene is a section of deoxyribonucleic acid (DNA) located on a chromosome. Specific sequences of nucleotide bases in the gene result in codes that the cell must interpret. Some codes direct the cell to produce proteins; other codes produce structures necessary for life to continue. When gene codes have been interpreted and their product has been produced, scientists say the gene has been expressed.
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6Every living organism on Earth uses the process of gene expression. The genetic code stored in DNA is interpreted during gene expression resulting in the organism’s appearance or phenotype. Scientists can measure gene expression to learn at what level a specific gene influences the appearance and the functioning of an organism. For example, a diagnostic test exists that can determine the level of expression of a specific gene in breast cancer cells. The results of the test can predict whether an individual will or will not respond favorably to a particular drug. Testing of this type offers the patient an opportunity for individualized medicine. Let’s take a closer look at how the cell is able to read the genetic code found in DNA.
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8After DNA replication, the cell uses ribonucleic acid (RNA), enzymes, and other molecules to read the genetic code. An RNA molecule is similar to a DNA molecule in that it is also a long chain of nucleotides. 
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10However, RNA and DNA differ in three significant ways:
11RNA is generally a single strand; DNA is double- stranded
12RNA uses the sugar ribose; DNA uses deoxyribose
13RNA uses the nucleotide uracil; DNA uses thymine
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15There are three types of cellular RNA that play important roles in producing proteins:
16Messenger RNA (mRNA) carries the instructions for combining amino acids to form proteins
17Ribosomal RNA (rRNA) combines with proteins to form ribosomes
18Transfer RNA (tRNA) carries amino acids to the ribosome
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20Using the processes of transcription and translation, RNA uses the coding strand of a DNA molecule to create a working template for a single gene. From this template, RNA helps build proteins. This process is called protein synthesis. Protein synthesis influences gene expression by regulating, increasing or decreasing, its rate of expression.
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22The nucleus of almost every cell in your body contains all your 20,000 genes. However, not every cell in your body needs to use every gene. As a result, genes are regulated to control the amount and timing of their product’s appearance. Prokaryotes regulate gene expression using DNA-binding proteins to control transcription. Eukaryotes control most genes individually using sophisticated systems to control transcription.
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24Gene regulation provides the cell a way to control its structure and functioning. Controlling the amount, timing, and location of gene expression plays a significant role in the cell’s biological processes including cell differentiation, morphogenesis, and adaptability. If the genes related to growth, for example, are released in the wrong amounts, at the wrong time, or in the wrong location, they can result in health issues such as cancer and birth defects.
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27>>> Transcription
28The nucleotide base codes that make up our DNA hold the instructions for creating the proteins and functional molecules, like RNA, that make life possible. In order for these codes to be interpreted by the cell, DNA must first go through the process of transcription.
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30Transcription takes place in the nucleus of a cell. It involves the decoding of a gene’s DNA sequence to identify the amino acids from which specific proteins will be built. Amino acids are the building blocks of proteins. Proteins are responsible for the cell’s architecture, structure, and a multitude of chemical reactions. They are the body’s main building material.
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32The first step in transcription is RNA synthesis. RNA synthesis is the creation of an RNA molecule to complement a DNA sequence in a gene. The cell uses an enzyme called RNA polymerase to pull apart the two strands of DNA. One of the DNA strands becomes the template from which a strand of RNA will be created.
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34Researchers peering through specialized microscopes discovered that RNA polymerase acts like “battery-powered spiders.” The enzyme crawls along the template DNA strand adding complementary RNA nucleotides one at a time to the growing RNA strand.
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36It is important to remember that during transcription, RNA polymerase adds the complementary nucleotide guanine to the growing RNA strand when it encounters the nucleotide cytosine. However, when it gets to the DNA nucleotide adenine, it adds a uracil nucleotide.
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38Specific base sequences on the DNA template strand called promoter sites tell the RNA polymerase where to begin transcription. Promoter sites are located at the beginning of genes.
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40A molecule of RNA is complete when the RNA polymerase reaches another base sequence called a terminator site. Terminator sites stop transcription by causing the enzyme to drop off the DNA template strand. The newly created RNA strand is a copy of the DNA gene.
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42It is also important to understand that only two to three percent of the material contained in a DNA molecule is genetic information coded to produce proteins. That means that the RNA copy of the gene contains some material that needs to be trimmed out. RNA splicing is the process of trimming out segments of non-protein producing instructions.
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44Scientists call the segments of a gene that code for proteins exons. Segments that do not code for proteins are called introns. RNA splicing removes introns from the RNA strand and stitches the remaining exons together. This cutting and stitching process allows exons to be spliced together in a variety of patterns. Each unique pattern can create a different protein. Alternative splicing is what allows approximately 20,000 genes in a human to produce the hundreds of thousands of proteins necessary for life.
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46Once the gene has been read by RNA polymerase and the RNA has been spliced, the result is messenger RNA or mRNA. The strand of mRNA is now released to make its way out of the cell’s nucleus and into the cytoplasm where the coded information it contains will be interpreted in a process called translation.
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48The discussion of DNA replication and transcription is not complete without addressing mutations. DNA mutations are changes to the genetic code that may happen naturally or be the direct result of mistakes occurring during DNA replication or transcription. A mutation may be the result of one nucleotide base being substituted for another or the deletion or insertion of a base in a DNA sequence. Some mutations can be harmless; others change the amino acid structure resulting in an inactive protein.
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51>>> Translation
52After transcription, when the RNA polymerase has read the gene and the RNA has been spliced, the messenger RNA or mRNA moves out of the cell’s nucleus into the cytoplasm. The final step in the process of creating those all important proteins is translation. Translation takes place in the cell’s cytoplasm.
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54The purpose of the translation process is protein synthesis. There are two major players in the process: amino acids and ribosomes. Let’s first examine the role of amino acids. Long chains of amino acids are called polypeptides. Polypeptides make proteins. Every mRNA molecule in mammals is translated into about 900 protein molecules. The shape and function of a protein depends on which amino acids it is made from. That is determined by the nucleotide base sequences found in the mRNA.
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56The four RNA nucleotide bases are:
57Adenine (A)
58Cytosine (C)
59Guanine (G)
60Uracil (U)
61The universal genetic code for almost every organism is made of three-letter combinations of these four bases. These three-letter words are called codons. There are only 64 possible codons. (4 x 4 x 4 = 64)
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63The circular graphic on this screen identifies the 20 amino acids produced by 61 of the 64 codons. Can you determine the amino acid produced by the codon AGU?
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65Begin decoding by locating the first letter of the codon, A, in the center of the circle. Next, find the second letter, G, in the “A” section of the table. Finally, locate the third letter, U, on the ring of small letters.
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67The name of the amino acid AGU codes for Serine and it is found in the outer-most ring. Notice that the codon AGC also codes for Serine. Most amino acids can be coded for by multiple codons.
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69In addition to specifying a particular amino acid, some codons function as START and STOP signals during translation. The codon AUG stands for the amino acid methionine. AUG is also known as the START codon and is the first codon on all mRNA. That means that methionine is the first amino acid present when building all proteins but may be removed at a later time. There are three codons that do not produce amino acids. These codons are UAA, UAG, and UGA. They function as STOP signals and end the translation process.
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71Ribosomes are located in the cell’s cytoplasm and are made of ribosomal RNA or rRNA and many proteins. They are some of the largest and most complex structures in the cell. Ribosomes hold the mRNA thread-like strand in position so that when the START codon is reached, transfer RNA or tRNA can bind to it.
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73tRNA molecules also have sets of three-unpaired bases that complement mRNA codons. tRNA bases are called anticodons. The pairing of codons with anticodons is the mechanism by which tRNA binds to mRNA.
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75Each tRNA molecule carries a single amino acid. The ribosome joins amino acids brought by tRNAs creating an ever-growing polypeptide chain. The translation process for the gene ends when the tRNA reaches a STOP codon. At that point, the ribosome releases the mRNA and the polypeptide chain. Both the ribosome and mRNA may be used again.
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77Once released, the amino acids that make up the protein begin to interact with enzymes and cause the long polypeptide chain to begin folding and changing its shape into a well-defined, three-dimensional structure. A protein’s function depends on it folding into the correct three-dimensional shape. Misshapen protein structures may produce toxic agents or diseases. For example, some allergies are the result of incorrectly folded proteins for which the immune system cannot produce antibodies.
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79Some proteins will remain in the cell. A multitude of processes within the cell ensure the protein makes it to the correct organelle. Other proteins, like digestive enzymes and hormones, leave the cell destined for other parts of the body.