DNA is present in such large amounts in many tissues that

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1 12 2 hromosomes and Replication Section 12 2 is present in such large amounts in many tissues that it s easy to extract and analyze. But where is found in the cell? How is it organized? Where are the genes that Mendel first described a century and a half ago? and hromosomes Prokaryotic cells lack nuclei and many of the organelles found in eukaryotes. heir molecules are located in the cytoplasm. Most prokaryotes have a single circular molecule that contains nearly all of the cell s genetic information. his large molecule is usually referred to as the cell s chromosome, as shown in Figure Eukaryotic is a bit more complicated. Many eukaryotes have as much as 1000 times the amount of as prokaryotes. his is not found free in the cytoplasm. Eukaryotic is generally located in the cell nucleus in the form of a number of chromosomes. he number of chromosomes varies widely from one species to the next. For example, diploid human cells have 46 chromosomes, Drosophila cells have 8, and giant sequoia tree cells have 22. Key oncept What happens during replication? Vocabulary chromatin histone replication polymerase Reading Strategy: sking Questions Before you read, study the diagram in Figure Make a list of questions about the diagram. s you read, write down the answers to your questions. 1 FOUS Objectives Summarize the events of replication Relate the molecule to chromosome structure. Vocabulary Preview Explain that the suffix -ase is used to denote molecules that are enzymes, proteins that catalyze biochemical reactions. hallenge students to look carefully at the term polymerase. sk: Based on the root of the word, what do you think polymerase does? (It synthesizes the molecule by adding individual nucleotides to the polymer.) Length molecules are surprisingly long. he chromosome of the prokaryote E. coli, which can live in the human colon (large intestine), contains 4,639,221 base pairs. he length of such a molecule is roughly 1.6 mm, which doesn t sound like much until you think about the small size of a bacterium. o fit inside a typical bacterium, the molecule must be folded into a space only one one-thousandth of its length. Figure 12 8 Most prokaryotes, such as this E. coli bacterium, have only a single circular chromosome. his chromosome holds most of the organism s. Print: hromosome E. coli Bacterium SEION RESOURES eaching Resources, Lesson Plan 12 2, dapted Section Summary 12 2, Section Summary 12 2, Worksheets 12 2, Section Review 12 2 Reading and Study Workbook, Section 12 2 Lab Worksheets, hapter 12 Exploration echnology: Bases on the hromosome iext, Section 12 1 nimated Biological oncepts DVD, 21 ransparencies Plus, Section 12 2 Lab Simulations D-ROM, Structure and Replication Reading Strategy Encourage students to preview all diagrams in this section before reading. For Figure 12 11, have students list questions for the diagram and write answers to those questions as they read. 2 INSRU and hromosomes Demonstration o give students an idea of how large a molecule is compared to the cell into which it is packed, show students a ball formed from a piece of string that is about 1 meter long. hen, draw a circle on the board that has a diameter of about 1 mm. If needed, review the metric equivalents so that students can see the relationship between micrometers (diameter of a bacterium) and millimeters (10-3 to 1) and millimeters to meters (also 10-3 to 1). Explain that the string represents the molecule that must fit inside the circle drawn on the board. Encourage students to speculate about how the molecule is able to fit into the cell and why it is so long. and RN 295

2 12 2 (continued) Build Science Skills Using Models ive students a very long piece of yarn (30 50 cm), some beads, short wooden dowels, and a film canister. hallenge students to use the materials to help them fit the yarn inside the container, making sure that the yarn can be unwound fairly easily. Encourage students to make comparisons between their models and chromatin. Review the cell cycle with students. Remind them that interphase is the phase that occurs before mitosis, or cell division. fter the cell has divided, its daughter cells are in interphase until they divide. Discuss the three phases of interphase: 1, S, and 2. Remind students that cell growth occurs during 1, replication occurs during S, and the cell prepares for mitosis in 2. nswers 1. (a) No radioactivity is incorporated. (b) Very large amounts of radioactivity are incorporated. (c) No additional radioactivity is incorporated. 2. No, is synthesized at one point in the cell cycle about halfway between the two cell divisions. synthesis lasts about an hour. 3. he chromosomes in the cell nucleus would contain the most radioactivity because thymine is part of the molecule and chromosomes are composed of. Figure 12 9 he in a bacterium is about 1000 times as long as the bacterium itself. It must therefore be very tightly folded. Using nalogies ompare in a bacterium to a rope jammed into a backpack. Synthesis of New Molecules How can you investigate when and where cells synthesize? Scientists have done this by briefly adding radioactively labeled thymine to the medium in which a cell grows. cell that is synthesizing will take the labeled thymine nucleotide into its. he graph shows the total amount of radioactive label taken into from thymine during an eight-hour period between two cell divisions. 1. Interpreting raphics ontrast the amounts of radioactivity incorporated during the following times: (a) the first four hours of the experiment, (b) the next two hours, and (c) the final two hours. 2. Drawing onclusions Is synthesized continually during the cell cycle (the period o get a rough idea of what this means, think of a large school backpack. hen, imagine trying to pack a 300-meter length of rope into the backpack! Figure 12 9, which shows spilling out from a ruptured bacterium, indicates how dramatically the must be folded to fit within the cell. hromosome Structure he in eukaryotic cells is packed even more tightly. human cell contains almost 1000 times as many base pairs of as a bacterium. he nucleus of a human cell contains more than 1 meter of. How is so much folded into tiny chromosomes? he answer can be found in the composition of eukaryotic chromosomes. Eukaryotic chromosomes contain both and protein, tightly packed together to form a substance called chromatin. hromatin consists of that is tightly coiled around proteins called histones, as shown in Figure ogether, the and histone molecules form a beadlike structure called a nucleosome. Nucleosomes pack with one another to form a thick fiber, which is shortened by a system of loops and coils. During most of the cell cycle, these fibers are dispersed in the nucleus so that individual chromosomes are not visible. During mitosis, however, the fibers of each individual chromosome are drawn together, forming the tightly packed chromosomes you can see through a light microscope in dividing cells. he tight packing of nucleosomes may help separate chromosomes during mitosis. here is also some evidence that changes in chromatin structure and histone- binding are associated with changes in gene activity and expression. Radioactive Label Mitosis Synthesis Mitosis ime (hours) between cell divisions)? If not, during what phase is it synthesized? How long is that phase? 3. Predicting Which organelle or cell structure would you expect to contain the most radioactivity after this experiment? Explain. UNIVERSL ESS Inclusion/Special Needs Show students a picture of a bacterial cell and an animal cell. alk about the location of in each cell and where replication occurs. hen, discuss situations in which the cell would need copies of its molecule. Elicit from students that replication occurs when new cells are needed, either during healing and growth (mitosis) or during the production of sex cells (meiosis). Less Proficient Readers Encourage students to outline this section as they read it. Students should include at least two main points for each green head in the section. Students can also use illustrations in their outlines to help describe the main points. heck the outlines to make sure that students have included all Vocabulary terms and Key oncepts. 296 hapter 12

3 hromosome Supercoils Nucleosome oils What do nucleosomes do? Nucleosomes seem to be able to fold enormous lengths of into the tiny space available in the cell nucleus. his is such an important function that the histone proteins themselves have changed very little during evolution probably because mistakes in folding could harm a cell s ability to reproduce. What is chromatin? Replication Histones When Watson and rick discovered the double helix structure of, there was one more remarkable aspect that they recognized immediately. he structure explained how could be copied, or replicated. Each strand of the double helix has all the information needed to reconstruct the other half by the mechanism of base pairing. Because each strand can be used to make the other strand, the strands are said to be complementary. If you could separate the two strands, the rules of base pairing would allow you to reconstruct the base sequence of the other strand. In most prokaryotes, replication begins at a single point in the chromosome and proceeds, often in two directions, until the entire chromosome is replicated. In the larger eukaryotic chromosomes, replication occurs at hundreds of places. Replication proceeds in both directions until each chromosome is completely copied. he sites where separation and replication occur are called replication forks. double helix Figure Eukaryotic chromosomes contain wrapped around proteins called histones. he strands of nucleosomes are tightly coiled and supercoiled to form chromosomes. Interpreting raphics What is each -histone complex called? NS For: Links on replication Visit: Web ode: cbn-4122 Use Visuals Figure Use Figure to reinforce the structure of chromosomes. sk: What are chromosomes composed of? (Proteins and ) Discuss the role of histones in compacting the molecule. sk: What is a nucleosome? ( beadlike structure composed of wrapped around a histone molecule) Why would it be advantageous to the cell to have its molecules tightly condensed into nucleosomes during mitosis? (o prevent the chromatin from tangling and to make separation and division of the chromatin more efficient during mitosis) hen, challenge students to infer why changes in chromatin structure are associated with gene expression. Help students realize that the molecule must be unwound from the histone molecule so that the individual nucleotide bases can be exposed if the molecule is to be copied or translated into protein. Replication NS Download a worksheet on replication for students to complete, and find additional teacher support from NS SciLinks. FS ND FIURES Straight talk about nomenclature hromatin is a general term that describes a substance that is made up of, histone and nonhistone proteins, and some RN that is identified by its special staining properties. hromosomes are composed of chromatin. hromosomes have specific structures. Bacteria usually have a single circular chromosome. Multicellular plants and animals have rod-shaped chromosomes. chromatid is found only in replicated chromosomes after replication has occurred in the S phase of the cell cycle. hromatids are the two highly visible chromosome replicas held together at their centromeres during prophase and metaphase of mitosis and meiosis. nswers to... hromatin is and protein tightly packed together. Figure 12 9 Both situations involve packing a very long, narrow material (the or rope) into a compact space (the bacterium or backpack). Figure nucleosome and RN 297

4 REPLIION 12 2 (continued) Use Visuals Figure s students examine the diagram of replication, have volunteers explain what a complementary strand of is and what happens when the molecule unzips. (Hydrogen bonds break.) Discuss how the strand being copied acts as a template for the new strand being synthesized. sk: Why is the new strand complementary to the original strand? (It is made up of a base sequence of purines and pyrimidines that match, or base pair, with the sequence of purines and pyrimidines of the original strand.) Remind students of the tightly wound chromosome structure. Have students infer how chromosome structure might change during replication. (he nucleosomes unwind.) Figure During replication, the molecule produces two new complementary strands. Each strand of the double helix of serves as a template for the new strand. he electron micrograph shows a double strand of human. Original strand New strand rowth rowth Nitrogenous bases Replication fork For: Replication activity Visit: PHSchool.com Web ode: cbp-4122 polymerase Original strand New strand Demonstration Demonstrate that each new molecule is composed of one original strand and one new strand. Use two strands of red yarn to represent the molecule of a chromosome. s you separate the strands of yarn for replication, lay down a new strand of green yarn to represent the replicated strand of. When replication is complete, the result will be two sets of yarn molecules composed of one red strand and one green strand. Explain that these two sets of yarn molecules are connected by centromeres (represent this with a paper clip) until they are separated during mitosis. KEY denine () hymine () ytosine () uanine () polymerase Replication fork For: Replication activity Visit: PHSchool.com Web ode: cbe-4122 Students learn about replication by producing complementary strands of. FS ND FIURES Where other genes are located ll the genes in a cell are not located in the nucleus, although for many years scientists thought they were. However, researchers began to notice that some inherited traits such as Leber disorder, a type of blindness seem to follow an unusual inheritance pattern. ould the genes for such traits be located somewhere else in the cell? In the 1970s, researchers found that mitochondria and chloroplasts have their own genes that code for proteins. In the 1980s, Douglas Wallace of Emory University first demonstrated that human mitochondrial is inherited from the mother and later showed that most cases of Leber disorder are due to a mutation in mitochondrial. Other mitochondrial genetic disorders have since been discovered. 298 hapter 12

5 Duplicating Before a cell divides, it duplicates its in a copying process called replication. his process ensures that each resulting cell will have a complete set of molecules. During replication, the molecule separates into two strands, and then produces two new complementary strands following the rules of base pairing. Each strand of the double helix of serves as a template, or model, for the new strand. Figure shows the process of replication. he two strands of the double helix have separated, allowing two replication forks to form. s each new strand forms, new bases are added following the rules of base pairing. In other words, if the base on the old strand is adenine, thymine is added to the newly forming strand. Likewise, guanine is always paired to cytosine. For example, a strand that has the bases produces a strand with the complementary bases. he result is two molecules identical to each other and to the original molecule. Note that each molecule resulting from replication has one original strand and one new strand. How Replication Occurs replication is carried out by a series of enzymes. hese enzymes unzip a molecule of. he unzipping occurs when the hydrogen bonds between the base pairs are broken and the two strands of the molecule unwind. Each strand serves as a template for the attachment of complementary bases. replication involves a host of enzymes and regulatory molecules. You may recall that enzymes are highly specific. For this reason, they are often named for the reactions they catalyze. he principal enzyme involved in replication is called polymerase (PHL-ih-mur-ayz) because it joins individual nucleotides to produce a molecule, which is, of course, a polymer. polymerase also proofreads each new strand, helping to maximize the odds that each molecule is a perfect copy of the original Section ssessment 1. Key oncept Explain how is replicated. 2. Where and in what form is eukaryotic found? 3. How are the long molecules found in eukaryotes packed into short chromosomes? 4. How are histones related to nucleosomes? 5. What is the role of polymerase in replication? 6. ritical hinking omparing and ontrasting How is the structure of chromosomes in eukaryotes different from the structure of chromosomes in prokaryotes? reating a Venn Diagram Make a Venn diagram that compares the process of replication in prokaryotes and eukaryotes. ompare the location, steps, and end products of the process in each kind of cell. (For more on Venn diagrams, see ppendix.) Build Science Skills pplying oncepts Divide the class into pairs. Instruct each student to write a base pair sequence for one strand of. Partners should exchange their sequences with each other and write the sequence of the complementary strand. Monitor students to be sure they understand the process. 3 SSESS Evaluate Understanding sk students to describe the steps that occur in the process of replication. sk other students to explain how is packaged to fit inside cells. Reteach Have students review Figure and Figure Encourage them to draw a diagram that combines the ideas described in each figure. Remind students to clearly label their diagrams. In the Venn diagrams, students should describe prokaryotic replication as beginning at a single point and occurring in the cytoplasm. Eukaryotic replication begins in hundreds of places at once and occurs in the nucleus. replication in both eukaryotes and prokaryotes proceeds in both directions, results in two identical strands of, and so on. You might use the diagrams to emphasize that the process of replication is very similar in the two types of cells Section ssessment 1. he molecule separates into two strands, which serve as templates against which the new strands are made, following the rules of base pairing. 2. In the cell nucleus as chromosomes 3. is tightly wound around histones, forming nucleosomes. Nucleosomes are tightly coiled and supercoiled to form chromosomes. 4. Nucleosomes are composed of wound around histones. 5. Polymerizes individual nucleotides to produce 6. Prokaryotes: single, circular molecule; eukaryotes: many chromosomes composed of tightly coiled and proteins called histones If your class subscribes to the iext, use it to review the Key oncepts in Section and RN 299