Biology - Student Reader & Workbook Unit 3, Chapter 4: Molecular Genetics - DNA Structure and Protein Synthesis

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1 Biology - Student Reader & Workbook Unit 3, Chapter 4: Molecular Genetics - DNA Structure and Protein Synthesis UNIT 3, CHAPTER 4: MOLECULAR GENETICS: DNA STRUCTURE AND... 3 PROTEIN SYNTHESIS... 3 LESSON 4.1: DNA AND RNA... 4 Lesson Objectives... 4 Lesson Vocabulary... 4 Lesson Introduction... 4 Central Dogma of Molecular Biology... 5 DNA... 5 Griffith Searches for the Genetic Material... 5 Avery s Team Makes a Major Contribution... 6 Hershey and Chase Seal the Deal... 7 Chargaff Write the Rules... 7 The Double Helix... 8 Graphic Organizer: Contributions to the Understanding of Genetics DNA Replication Graphic Organizer: DNA Replication RNA RNA vs. DNA Types of RNA Lesson Summary Lesson Review Questions Recall Apply Concepts Think Critically Points to Consider Multimedia Links LESSON 4.2: PROTEIN SYNTHESIS...17 Lesson Objectives Lesson Vocabulary Lesson Introduction Graphic Organizer: Protein Synthesis Transcription Steps of Transcription The Genetic Code Reading the Genetic Code Characteristics of the Genetic Code Translation Graphic Organizer: Process of Translation Graphic Organizer: Transcription and Translation How Protein Production is Regulated Regulation of Gene Expression

2 Lesson Summary Lesson Review Questions Recall Apply Concepts Think Critically Points to Consider Multimedia Links

3 Unit 3, Chapter 4: Molecular Genetics: DNA Structure and Protein Synthesis The spiral structure in the picture is a large organic molecule. Can you guess what it is? Here s a hint: molecules like this one determine who you are. They contain genetic information that controls your characteristics. They determine your eye color, facial features, and other physical attributes. What molecule is it? You probably answered DNA. Today, it is commonly known that DNA is the genetic material. For a long time, scientists knew such molecules existed. They were aware that genetic information was contained within organic molecules. However, they didn t know which type of molecules play this role. In fact, for many decades, scientists thought that proteins were the molecules that carry genetic information. In this chapter, you will learn how scientists discovered that DNA carries the code of life. 3

4 Lesson 4.1: DNA and RNA Lesson Objectives State the central dogma of molecular biology. Outline discoveries that led to knowledge of DNA s structure and function. Describe the structure of RNA, and identify the three main types of RNA. Lesson Vocabulary central dogma of molecular biology Chargaff s rules messenger RNA (mrna) ribosomal RNA (rrna) transfer RNA (trna) Lesson Introduction Your DNA, or deoxyribonucleic acid, contains the genes that determine who you are. How can this organic molecule control your characteristics? DNA contains instructions for all the proteins your body makes. Proteins, in turn, determine the structure and function of all your cells. What determines a protein s structure? It begins with the sequence of amino acids that make up the protein. Instructions for making proteins with the correct sequence of amino acids are encoded in DNA. Before Reading: Recall DNA controls your characteristics because it instructs your body s production of protein. Below are a few conditions you learned about in the last chapter on Human Genetics. Each one is a condition caused by a mutation in the DNA, which altered the protein. Can you draw a line to match the correct condition with the problem? Condition Sickle cell anemia Hemophilia A Marfan syndrome Albinism Problem Defective connective tissue protein Lack of production of pigment protein melanin Abnormal hemoglobin protein in red blood cells Decreased activity of blood clotting protein 4

5 Central Dogma of Molecular Biology DNA is found in chromosomes. In eukaryotic cells, chromosomes always remain in the nucleus, but proteins are made at ribosomes in the cytoplasm. How do the instructions in DNA get to the site of protein synthesis outside the nucleus? Another type of nucleic acid is responsible. This nucleic acid is RNA, or ribonucleic acid. RNA is a small molecule that can squeeze through pores in the nuclear membrane. It carries the information from DNA in the nucleus to a ribosome in the cytoplasm and then helps assemble the protein. In short: DNA RNA Protein Discovering this sequence of events was a major milestone in molecular biology. It is called the central dogma of molecular biology. Reading Check: 1. What is the central dogma of molecular biology? 2. DNA is the instruction for making. However, in eukaryotic cells DNA is trapped inside the while protein production occurs at ribosomes in the. Therefore is needed to carry the information from the inside the nucleus to the in the cytoplasm in order to produce the final product - protein. 3. What else is the production of proteins similar to? Can you think of other situations where a messenger is required to get information turned into a final product? DNA DNA is the genetic material in your cells. It was passed on to you from your parents and determines your characteristics. The discovery that DNA is the genetic material was another important milestone in molecular biology. Griffith Searches for the Genetic Material Many scientists contributed to the identification of DNA as the genetic material. In the 1920s, Frederick Griffith made an important discovery. He was studying two different strains of a bacterium, called R (rough) strain and S (smooth) strain. He injected the two strains into mice. The S strain killed the mice, but the R strain did not (see Figure 7.1). Griffith also injected mice with S-strain bacteria that had been killed by heat. As expected, the killed bacteria did not 5

6 harm the mice. Next, the dead S-strain bacteria were mixed with live R-strain bacteria and injected in the mice. PREDICT: What would you expect to happen to the mice injected with the dead S-strain bacteria and live R-strain bacteria mix? Why? Look at Figure 4.1 to see the surprising results of Griffith s study. Figure 4.1 Griffith showed that a substance could be transferred to harmless bacteria and make them deadly. Griffith s Experimental Results Based on his observations, Griffith deduced that something in the killed S strain was transferred to the previously harmless R strain, making the R strain deadly. What was that something? What type of substance could change the characteristics of the organism that received it? Avery s Team Makes a Major Contribution In the early 1940s, a team of scientists led by Oswald Avery tried to answer the question raised by Griffith s results what was transferred from one stain of bacteria to the other? Scientists believed it was either protein or DNA. To find out which one it was, they inactivated either protein or DNA in the S-strain bacteria before killing them and transferring them to the R 6

7 strain. When they inactivated proteins, the R strain was deadly to the injected mice. This ruled out proteins as the genetic material. Why? Even without the S-strain proteins, the R strain was transformed. However, when the researchers inactivated DNA in the S strain, the R strain remained harmless. This led to the conclusion that DNA is the substance that controls the characteristics of organisms. In other words, DNA is the genetic material. Hershey and Chase Seal the Deal The conclusion that DNA is the genetic material was not widely accepted at first. It had to be confirmed by other research. In the 1950s, Alfred Hershey and Martha Chase did experiments with viruses and bacteria. Viruses are not cells. They are basically DNA inside a protein coat. To reproduce, a virus must insert its own genetic material into a cell (such as a bacterium). Then it uses the cell s machinery to make more viruses. The researchers used different radioactive elements to label the DNA and proteins in viruses. This allowed them to identify which molecule the viruses inserted into bacteria. DNA was the molecule they identified. This confirmed that DNA is the genetic material. Reading Check: 1. When Griffith injected mice with a mix of rough strain and heat-killed smooth strain bacteria and saw that the mice died, he came to the conclusion that a substance must be passing between the two bacteria strains. Explain why. 2. Avery s team and Hershey and Chase came to the same conclusion using two different experiments. A) What conclusion did they come to? B) How did Avery s team come to this conclusion? C) How did Hershey and Chase come to this conclusion? Chargaff Write the Rules Other important discoveries about DNA were made in the mid-1900s by Erwin Chargaff. He studied DNA from many different species. He was especially interested in the four different nitrogen bases of DNA: adenine, guanine, cytosine, and thymine (see Figure 4.2). Chargaff found that concentrations of the four bases differed from one species to another. However, within each species, the concentration of adenine was always about the same as the concentration of thymine. The same was true of the concentrations of guanine and cytosine. These observations came to be known as Chargaff s rules. The significance of the rules would not be revealed until the structure of DNA was discovered. 7

8 Figure 4.2: The DNA of all species has the same four nitrogen bases. Nitrogen Bases in DNA RECALL: DNA is an example of the biomolecule nucleic acid. One monomer, or unit, of a nucleic acid is a nucleotide. A nitrogen base is just one part of a nucleotide. Do you remember the three parts of a nucleotide? Label the sugar (deoxyribose), phosphate group, and nitrogen base on the nucleotide below: The Double Helix After DNA was found to be the genetic material, scientists wanted to learn more about it. James Watson and Francis Crick are usually given credit for discovering that DNA has a double helix shape like a spiral staircase (see Figure 4.3). The discovery was based on the prior work of Rosalind Franklin and other scientists, who had used X rays to learn more about DNA s structure. Franklin and these other scientists have not always been given credit for their contributions. Word Work The word helix refers to a spiral shape and double means twice, or twofold. DNA is referred to as a double helix because one molecule of DNA is made of two nucleotide chains that connect and twist to form a spiral. 8

9 Figure 4.3: The DNA molecule has a double helix shape. This is the same basic shape as a spiral staircase. Do you see the resemblance? Which parts of the DNA molecule are like the steps of the spiral staircase? DNA Molecule Spiral Staircase 9

10 Graphic Organizer: Contributions to the Understanding of Genetics The biotechnology in available to us now would not be possible without our detailed understanding of DNA. Many researchers built upon each other s work to reveal more and more about the secrets of heredity. Complete the graphic organizer below showing the contributions of a few key scientists to our understanding of genetics. Scientists Research Done Conclusions Drawn Mendel Studied inheritance in pea plants Griffith Avery Hershey and Chase Chargaff Watson, Crick and Franklin Injected mice with different variations of virulent and non-virulent bacteria Inactived substances in the bacteria studied by Griffith to see what was being transferred Followed what substance was being transferred by viruses into cells Studied concentrations of DNA nitrogen bases in many different species Used x-rays to learn about the structure of DNA The Structure of DNA The double helix shape of DNA, together with Chargaff s rules, led to a better understanding of DNA. Scientists concluded that the double helix formed as a result of two chains of nucleotides bonding together in a way that causes a twist, like a spiral staircase or twisted ladder. The sides of the ladder are made of sugar and phosphate groups attaching one nucleotide to the other. The middle of the ladder, or the rungs, are the nitrogen bases of one chain of nucleotides bonding with the nitrogen bases of the other chain of nucleotides. Chargaff s rules regarding nitrogen base concentrations helped to determine how the two chains of nucleotides were held together. The reason why the concentration of the nitrogen bases adenine and thymine is always equal is because adenine and thymine are complementary and always bond together. This means that adenine and thymine are counterparts, and their molecular shape allows them to bond together like two puzzle pieces might fit together. Likewise, cytosine is complementary to guanine. If you look at the nitrogen bases in Figure 4.2, you will see why. Adenine and guanine have a two-ring structure. Cytosine and thymine have just one ring. If adenine were to bind with guanine and cytosine with thymine, the 10

11 distance between the two DNA chains would vary. However, when a one-ring molecule always binds with a two-ring molecule, the distance between the two chains is kept constant. This maintains the uniform shape of the DNA double helix. Reading Check: 1. Adenine is complementary to. a) adenine b) cytosine c) guanine d) thymine 2. Adenine, cytosine, guanine, and thymine are all. a) nucleic acids b) nitrogen bases c) nucleotides d) sugars 3. DNA is described as a double helix because it is made of two chains of. a) nitrogen bases b) phosphate groups c) nucleotides d) sugars 4. Describe what would happen to the shape of DNA if cytosine could bind with thymine and adenine could bind with guanine. Before Reading: RECALL: DNA must be copied before a eukaryotic cell undergoes mitosis so that both new cells have a complete set of DNA. Do you remember during which phase of the cell cycle this happens? DNA Replication DNA replication is the process in which DNA is copied. It occurs during the synthesis (S) phase of the eukaryotic cell cycle in preparation for the division of the cell into two new cells. This ensures that both new cells have the same DNA. Knowledge of DNA s structure helped scientists understand how DNA replicates. Remember that DNA is made of two chains of nucleotides bonding together by their nitrogen bases. If DNA is described as a twisted ladder, the rungs of the ladder are made of the nitrogen bases. DNA replication begins when an enzyme breaks the bonds between complementary nitrogen bases in DNA, separating the two chains of nucleotides (see Figure 4.4). This exposes the bases inside the molecule so they can be read by another enzyme and used to build two new DNA strands with complementary bases. The two daughter (new) molecules that result each contain one strand from the parent (original) molecule and one new strand that is complementary to it. As a result, the two daughter molecules are both identical to the parent 11

12 molecule. This is a simple summary of DNA replication; the process of DNA replication is actually much more complex. Figure 4.4: DNA replication is a semi-conservative process. Half of the parent DNA molecule is conserved in each of the two daughter DNA molecules. DNA Replication The parent DNA molecule consists of two polynucleotide chains held together by bonds between complementary nitrogen bases. An enzyme breaks the bonds between the two polynucleotide chains of the parent molecule. Another enzyme pairs new, complementary nucleotides with those in the two parental chains. Two daughter DNA molecules form, each containing one new chain (green) and one parental chain (aqua). Word Work The prefix semi- means half. The word conserve means to preserve, or save. Why is DNA replication described as a semiconservative process? 12

13 Reading Check: Two different enzymes help with the process of DNA replication. The job of the first enzyme is to. The job of the second enzyme is to. The final product of DNA replication will be two new DNA molecules, each with one new strand and one old strand. Graphic Organizer: DNA Replication Use the text and Figure 4.4 to help complete the graphic organizer below showing the steps of DNA replication. Step 1: Step 2: Step 3: 13

14 RNA DNA alone cannot tell your cells how to make proteins. Since it is trapped inside the nucleus it needs to be able to send its protein instructions out to the ribosomes in the cytoplasm. It needs the help of RNA, the other main player in the central dogma of molecular biology. RNA vs. DNA RNA, like DNA, is a nucleic acid. However, RNA differs from DNA in several ways. In addition to being smaller than DNA, RNA also consists of one nucleotide chain instead of two. contains the nitrogen base uracil instead of thymine. contains the sugar ribose instead of deoxyribose. Types of RNA There are three main types of RNA, all of which are involved in making proteins. 1. Messenger RNA (mrna) copies the genetic instructions from DNA in the nucleus and carries them to the cytoplasm. 2. Ribosomal RNA (rrna) helps form ribosomes, where proteins are assembled. 3. Transfer RNA (trna) brings amino acids to ribosomes, where they are joined together to form proteins. In the next lesson, you can read in detail how these three types of RNA help cells make proteins. Reading Check: Circle whether the following statements apply to DNA or RNA or both. 1. Consists of only one strand of nucleotides. DNA RNA 2. Contains the nitrogen base cytosine. DNA RNA 3. Contains the sugar deoxyribose. DNA RNA 4. Helps form ribosomes. DNA RNA 5. Contains the nitrogen base uracil. DNA RNA 6. Is a nucleic acid. DNA RNA 7. In eukaryotes, is found only inside the nucleus. DNA RNA 14

15 Lesson Summary The central dogma of molecular biology states that DNA contains instructions for making a protein, which are copied by RNA; and RNA uses the instructions to make a protein. In short: DNA RNA Protein. The work of several researchers led to the discovery that DNA is the genetic material. Other researchers discovered that DNA has a double helix shape, consisting of two polynucleotide chains held together by bonds between complementary bases. RNA differs from DNA in several ways. There three main types of RNA: messenger RNA (mrna), ribosomal RNA (rrna), and transfer RNA (trna). Each type plays a different in role in making proteins. Lesson Review Questions Recall 1. State the central dogma of molecular biology. 2. Outline research that determined that DNA is the genetic material. 3. What are Chargaff s rules? 4. Identify the structure of the DNA molecule. 5. Why is DNA replication said to be semi-conservative? Apply Concepts 6. Create a diagram that shows how DNA replication occurs. Think Critically 7. Explain why complementary base pairing is necessary to maintain the double helix shape of the DNA molecule. 8. Compare and contrast DNA and RNA. Points to Consider All three types of RNA are needed by cells to make proteins. Can you develop a model in which the three types of RNA interact to make a protein? How do you think mrna copies the genetic instructions in DNA? How are these instructions encoded in the DNA molecule? Multimedia Links You can watch a video about the central dogma and other concepts in this lesson at the link below. You can watch an animation about the research of both Griffith and Avery at this link: 15

16 You can learn more about Franklin s work by watching the video at this link: You can see a detailed animation of the process of DNA replication at this link: 16

17 Lesson 4.2: Protein Synthesis Lesson Objectives Give an overview of transcription. Describe the genetic code. Explain how translation occurs. Lesson Vocabulary protein synthesis transcription promoter genetic code codon translation Lesson Introduction The process in which cells make proteins is called protein synthesis. It actually consists of two processes: transcription and translation. Transcription takes place in the nucleus. It uses DNA to make an RNA molecule. RNA then leaves the nucleus and goes to a ribosome in the cytoplasm, where translation occurs. Translation reads the genetic code in mrna and makes a protein. Before Reading: RECALL: The central dogma of molecular biology actually outlines protein synthesis. Do you remember what the central dogma is? Write it below: Word Work Let s see why the two steps of protein synthesis are called transcription and translation by looking at the meanings of the two words. Transcription is based on the word transcribe, which means to make a written copy. In protein synthesis, first a DNA copy is created in the form of. Translation means to change something from one language to another. The message from DNA is a code that needs to be translated to know the correct order of amino acids for making the. 17

18 Graphic Organizer: Protein Synthesis Central Dogma: Name of Process: Transcription Transcription is the first part of the central dogma of molecular biology: DNA RNA. It is the transfer of genetic instructions in DNA to mrna. During transcription, a strand of mrna is made that is complementary to a strand of DNA. Figure 4.5 shows how this occurs. Figure 4.5: Transcription uses the sequence of bases in a strand of DNA to make a complementary strand of mrna. Triplets are groups of three successive nucleotide bases. Codons are triplet groups of bases in mrna Overview of Transcription b. Single Coding Strand of DNA c. Complementary Strand of mrna 18

19 Steps of Transcription Transcription takes place in three steps: initiation, elongation, and termination. The steps are illustrated in Figure Initiation is the beginning of transcription. It occurs when the enzyme RNA polymerase binds to a region of a gene called the promoter. This signals the DNA to unwind so the enzyme can read the bases in one of the DNA strands. The enzyme is ready to make a strand of mrna with a complementary sequence of bases. 2. Elongation is the addition of nucleotides to the mrna strand. 3. Termination is the ending of transcription. The mrna strand is complete, and it detaches from DNA. Figure 4.6: Transcription occurs in the three steps shown here. DIRECTIONS: Write in a summary of what is occurring in the box next to each step using the reading above. Steps of Transcription STEP 1: STEP 2: STEP 3: 19

20 Reading Check: 1. Transcription begins when the enzyme binds to a region of DNA known as the. This causes the DNA to unwind so it can be read. This step is called. The second step is called and the enzyme adds nucleotides to create a strand of. The final step is called and the completed mrna detaches from the. 2. Transcription Exercise: Using the DNA sequence below, write the nitrogen bases for the complementary strand of mrna. Remember that RNA contains uracil instead of thymine so uracil will complement adenine for mrna. DNA C G T G A T C C A mrna Before Reading: RECALL: From the last chapter you learned that humans only have 20,000 to 25,000 genes. Genes are sections of DNA that code for proteins, but we have more than 25,000 different proteins. How do you think this is possible? Processing mrna In eukaryotes, the new mrna is not yet ready for translation. It must go through more processing before it leaves the nucleus. This may include splicing, editing, and polyadenylation. These processes modify (change) the mrna in various ways. Such modifications allow a single gene to be used to make more than one protein. Splicing removes introns from mrna (see Figure 4.7). Introns are regions that do not code for proteins (see Figure 4.6). The remaining mrna consists only of regions that do code for proteins, which are called exons. Editing changes some of the nucleotides in mrna. For example, the human protein called APOB, which helps transport lipids in the blood, has two different forms because of editing. One form is smaller than the other because editing adds a premature (early) stop signal in mrna. 20

21 Polyadenylation adds a tail to the mrna. The tail consists of a string of As (adenine bases). It signals the end of mrna. It is also involved in exporting mrna from the nucleus. In addition, the tail protects mrna from enzymes that might break it down. Figure 4.7: Splicing removes introns from mrna. Splicing Reading Check: 1. We can have more proteins than we do genes because a single gene can be used to make more than one protein. How is this possible? 2. These are regions of mrna that code for proteins. (circle one) introns exons 3. This process modifies mrna by changing some of the nucleotides. (circle one) splicing editing polyadenylation 4. This process removes noncoding regions of RNA. (circle one) splicing editing polyadenylation 5. The tail of mrna consists of a string of bases. (circle one) thymine guanine adenine 6. Besides signaling the stop for mrna, name two other purposes for adding a tail to mrna. The Genetic Code How is the information in a gene encoded? The answer is the genetic code. The genetic code consists of the sequence of nitrogen bases A, C, G, T (or U for RNA) in a polynucleotide chain. The four bases make up the letters of the genetic code. The letters are combined in groups of three to form code words, called codons. Each codon stands for (encodes) one amino acid, unless it codes for a start or stop signal. 21

22 There are 20 common amino acids in proteins. When considering how many codon combinations can exist you calculate the permutations. There are four nitrogen bases to choose from and they are in groups of three to form a codon. Therefore there are 4 3, or 64, possible codon combinations - more than enough to code for the 20 amino acids. The genetic code is shown in Figure 4.8. Think About It The coding for amino acids is beautiful in its simplicity and functionality. Why is the genetic code read in groups of three nitrogen bases? What would happen if a codon were only two nitrogen bases? Figure 4.8: To find the amino acid for a particular codon, find the cell in the table for the first and second bases of the codon. Then, within that cell, find the codon with the correct third base. The Genetic Code 22

23 Reading the Genetic Code As shown in Figure 4.8, the codon AUG codes for the amino acid methionine. This codon is also the start codon that begins translation. The start codon establishes the reading frame of mrna. The reading frame is the way the letters are divided into codons. After the AUG start codon, the next three letters are read as the second codon. The next three letters after that are read as the third codon, and so on. This is illustrated in Figure 4.9. The mrna molecule is read, codon by codon, until a stop codon is reached. UAG, UGA, and UAA are all stop codons. They do not code for any amino acids. Figure 4.9: The genetic code is read three bases at a time. Codons are the code words of the genetic code. Reading the Genetic Code 23

24 Reading Check: Use Figure 4.8 and 4.9 to answer the following questions. 1. Which amino acid does codon 2 in Figure 4.9 code for? 2. Which amino acid does codon 4 in Figure 4.9 code for? 3. What amino acid does every protein start with? Why? 4. Is it possible to have two different codons code for the same amino acid? Explain. 5. Are there codons that do not code for an amino acid? What are they and what do they code for? Characteristics of the Genetic Code The genetic code has a number of important characteristics. The genetic code is universal. All known living things have the same genetic code. The same codons code for the same amino acids. This shows that all organisms share a common evolutionary history. The genetic code is unambiguous. Each codon codes for just one amino acid (or start or stop). The genetic code is redundant. Most amino acids are encoded by more than one codon. In Figure 4.8, how many codons code for the amino acid threonine? Reading Check: 1. Why would the fact that all known living things share the same genetic code be evidence that all organisms share a common evolutionary history? 2. What might happen if codons encoded more than one amino acid? 3. What might be an advantage of having more than one codon for the same amino acid? 24

25 Translation Translation is the second part of the central dogma of molecular biology: RNA Protein. It is the process in which the genetic code in mrna is read to make a protein. Figure 4.10 shows how this happens. After mrna leaves the nucleus, it moves to a ribosome, which consists of rrna and proteins. The ribosome reads the sequence of codons in mrna. Molecules of trna bring amino acids to the ribosome in the correct sequence. To understand the role of trna, you need to know more about its structure. Molecules of trna are able to place amino acids in the correct sequence because the trna has nitrogen bases that complement the mrna codons. This part of the trna is called an anticodon. The matching trna carries with it the amino acid that corresponds to the mrna codon. For example, the amino acid lysine is coded for by the mrna codon AAG, so the trna anticodon is UUC. Therefore, lysine would be carried by a trna molecule with the anticodon UUC. Wherever the codon AAG appears in mrna, a UUC anticodon of trna temporarily binds. While bound to mrna, trna gives up its amino acid. Bonds form between the amino acids as they are brought one by one to the ribosome. The chain of amino acids keeps growing until a stop codon is reached. Graphic Organizer: Process of Translation Complete the blanks in the graphic organizer below showing the steps of translation. Step 1: leaves the nucleus and attaches to a in the cytoplasm. Step 2: A trna binds to the complementary codon. The trna carries with it the correct coded for by the codon. Step 3: releases its amino acid while still bound to mrna. Bonds form between as new ones are brought in one by one. Step 4: The chain of amino acids continues to grow until a codon is reached. 25

26 Figure 4.10: Translation of the codons in mrna to a chain of amino acids occurs at a ribosome. Transcription and Translation TRANSCRIPTION TRANSLATION (consists of rrna and proteins) A chain of amino acids forms a protein and is also known as a polypeptide chain because peptide bonds form between the amino acids. After a polypeptide chain (protein) is created, it may undergo additional processes. For example, it may assume a folded shape due to interactions among its amino acids. The shape of a protein is particularly important and unique to the job it needs to do. It may also bind with other polypeptides or with different types of molecules, such as lipids or carbohydrates. Many proteins travel to a Golgi apparatus to be modified for the specific job they will do. 26

27 Reading Check: 1. Where does translation take place? 2. Find the three types of RNA in Figure What are their roles in translation? 3. What is an anticodon and what is its function? 4. Describe three processes that can happen to an amino acid chain (protein) after it has been completed. Graphic Organizer: Transcription and Translation Contrast the processes of transcription and translation in the organizer below. Part of the central dogma of molecular biology Location in (eukaryotic) cell where it occurs Types of RNA involved Transcription Translation 27

28 Before Reading: Think Aloud All of your cells contain the exact same DNA. Since your DNA codes for protein, does this mean that all of your cells make the same proteins? How Protein Production is Regulated All of your cells have the same genes. However, not all cells make the same proteins. If they did, then all your cells would be alike. Instead, you have cells with different structures and functions. This is because different cells make different proteins. They do this by using, or expressing, different genes. Regulation of Gene Expression Gene expression is regulated to ensure that the correct proteins are made when and where they are needed. It is similar to being able to turn on and off genes. Regulation may occur at any point in the expression of a gene, from the start of transcription to the processing of a protein after translation. For instance, transcription of a gene can either be promoted (started) or prevented by the use of regulatory proteins that interact with RNA polymerase, the enzyme that transcribes DNA to mrna. The regulation of proteins is especially important during the development of an organism. Regulatory proteins must turn on certain genes in particular cells at just the right time so the organism develops normal organs and organ systems. 28

29 Lesson Summary Transcription is the DNA RNA part of the central dogma of molecular biology. It occurs in the nucleus. During transcription, a copy of mrna is made that is complementary to a strand of DNA. In eukaryotes, mrna may be modified before it leaves the nucleus. The genetic code consists of the sequence of bases in DNA or RNA. Groups of three bases code form codons, and each codon stands for one amino acid (or start or stop). The codons are read in sequence following the start codon until a stop codon is reached. The genetic code is universal, unambiguous, and redundant. Translation is the RNA protein part of the central dogma. It occurs at a ribosome. During translation, a protein is synthesized using the codons in mrna as a guide. All three types of RNA play a role in translation. Lesson Review Questions Recall 1. Describe transcription. 2. How may mrna be modified before it leaves the nucleus? 3. What is the genetic code? What are codons? 4. Outline the steps of translation. Apply Concepts 5. Use the genetic code in Figure 7.8 to translate the following segment of RNA into a sequence of five amino acids: GUC-GCG-CAU-AGC-AAG Think Critically 6. The genetic code is universal, unambiguous, and redundant. Explain what this means and why it is important. 7. How are transcription and translation related to the central dogma of molecular biology? Points to Consider When DNA is replicated or transcribed, accidents can happen, leading to a change in the base sequence. What do you think could cause such accidents to occur? How might the changes affect the reading frame? How might the encoded protein be affected? 29

30 Multimedia Links You can watch an animation of the process of transcription at this link: A more detailed video about transcription is available at this link: You can watch a video showing splicing in more detail at this link: To see how scientists cracked the genetic code, go to this link: To see how translation happens, go the link below. You can see how proteins travel to a Golgi apparatus to be modified by watching the animation at this link: 30

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