DNA Structure Function Replication



DNA Function, Structure and Replication Function and Structure of DNA1. Everyone has heard of DNA, but what is it and what does it do? Briefly summarize what you already know about DNA.All organisms, including humans, other animals, plants, and bacteria have DNA inside their cells. Each DNA molecule contains many genes. Each gene is a segment of DNA that provides the instructions to make a protein. To stay alive, a cell needs many different types of proteins. For example, a cell needs:protein enzymes to carry out the chemical reactions that sustain lifetransport proteins to move ions and molecules into and out of the cell structural proteins.2. Why do all living things need to have DNA in their cells?Two people may have different versions of a gene in their cells. As shown in this chart, different versions of a gene give the instructions to make different versions of a protein, which can result in different characteristics. Gene in DNA ProteinCharacteristicOne version of this gene gives the instructions to make functional protein enzyme.Functional enzyme makes melanin, the pigment molecule in skin and hair.Normal skin and hair colorAnother version of this gene gives the instructions to make a nonfunctional version of this protein.Nonfunctional protein does not make melanin.Albinism (very pale skin and hair)3a. Based on what you know about human skin color, are these two versions of a gene the only factors that influence skin color? no ___ yes ___3b. Explain your reasoning.To understand how the instructions for making a protein are encoded in a DNA molecule, you need to understand the structure of DNA.Each DNA molecule consists of two strands of nucleotides twisted together in a long spiral called a double helix.This figure shows a small part of a much longer DNA molecule. On the right, the figure shows more detail of the nucleotides in a tiny part of the DNA molecule. (In question 7 you will draw the missing nucleotide at the top of the right-hand strand.) Each nucleotide contains:a phosphate (P) and a sugar one of the four bases (A = adenine, C = cytosine, G = guanine, or T = thymine) 4a. Covalent bonds link the phosphate of one nucleotide to the sugar of the next nucleotide. These linked sugars and phosphates form the backbone of each DNA strand. Draw a rectangle around the backbone of the DNA strand on the right.4b. For the DNA strand on the right, use a dashed line to draw a rectangle around the top nucleotide. Circle the base in that nucleotide. 5. Each base in one strand of the DNA double helix is joined by hydrogen bonds with a base in the other strand. Use the figure to determine which bases pair together in a DNA double helix. A in one strand always pairs with _____ in the other strand. C in one strand always pairs with _____ in the other strand. These are the base-pairing rules. The bases that pair together are called complementary bases. 6. Each type of nucleotide is named after the base it contains, A, C, G, or T. Thus, A and T can represent complementary bases or complementary nucleotides. The other pair of complementary nucleotides are ___ and ___.7. Draw an additional nucleotide at the top of the right-hand strand. Explain how you know which nucleotide should be added at the top of this strand.8. In the double helix diagram, the backbone of each strand is shown as a helical ribbon. What is represented by each horizontal bar in the double helix diagram? In the DNA, each gene has a sequence of nucleotides that spell out the instructions for a sequence of amino acids that make a protein.The sequence of nucleotides in the DNA of a gene determines the sequence of amino acids in a protein which determines the structure and function of the protein which influences the characteristics or traits of the organism.9. Explain how a difference in the sequence of nucleotides in a gene can result in one boy being albino and the other boy having normal skin and hair color. (A complete answer will include these terms: nucleotides, sequence, amino acids, protein, enzyme, pigment.)Each DNA molecule combines with proteins to form a chromosome. This figure shows how DNA winds around proteins in a chromosome. Each human cell has 46 chromosomes, and each human chromosome has hundreds or thousands of genes.DNA ReplicationOur bodies need to make new cells for growth or to replace damaged cells. This figure shows how new cells are formed by cell division. Before a cell divides into two daughter cells, the cell makes a copy of all of its DNA (DNA replication). Then, cell division separates the two copies of each chromosome, so each daughter cell gets a complete set of chromosomes with all the genes. 10. Explain why a cell needs to replicate its DNA before the cell divides into two daughter cells.This figure shows how DNA replication produces two new DNA molecules that are identical to the original DNA molecule.First, the two strands of the DNA double helix are separated (shown as the dark strands of nucleotides in the figure). Then, complementary nucleotides are added to form the second strand in each new DNA double helix molecule (shown as the light strands of nucleotides in the figure). 11a. This figure shows two free nucleotides, A and G. Which of these will be the next nucleotide to be added at the top of the growing DNA strand on the left? ___11b. How do you know?The top part of this drawing shows a short segment of DNA, and the bottom part shows the two strands of this DNA segment after they have been separated to begin the process of DNA replication. 12. For each of the single DNA strands, create a second DNA strand to produce two double-stranded pieces of DNA. Use the base-pairing rules to add complementary nucleotides. Your teacher will give you additional instructions.13a. Compare the two new double-stranded pieces of DNA with the original double-stranded piece of DNA. Do they all have the exact same sequence of nucleotides in both strands? yes ___ no ___ 13b. If no, describe any differences.14. Why is it important for both copies of the DNA molecule to have the exact same sequence of nucleotides as the original DNA molecule?Enzymes play an important role in DNA replication. For example, the enzyme DNA polymerase makes a covalent bond between each added complementary nucleotide and the previous nucleotide in a growing DNA strand.15. Based on the function of DNA polymerase, explain why each part of the name DNA polymerase (DNA, polymer, -ase) makes sense. 16a. During DNA replication, the double helix structure, the base-pairing rules, and DNA polymerase work together to make two DNA molecules that are identical to the original DNA molecule. How does the double helix structure help to produce two new DNA molecules that are identical to the original DNA molecule?16b. How do the base-pairing rules help to produce two new DNA molecules that are identical to the original DNA molecule?16c. Explain why DNA polymerase is needed. ................
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