Biology - DNA Structure and Replication-S

DNA Structure and Replication How is genetic information stored and copied? Why? Deoxyribonucleic acid or DNA is the molecule of heredity. It contains the genetic blueprint for life. For organisms to grow and repair damaged cells, each cell must be capable of accurately copying itself. So how does the structure of DNA allow it to copy itself so accurately? Model 1 – The Structure of DNA La Nucleotide Model of DNA C G T A A T G C Helix Model of DNA Phosphate Deoxyribose sugar Nitrogencontaining base Nitrogen Bases Adenine Thymine C G Guanine Cytosine T A 1. Refer to the diagram in Model 1. a. What are the three parts of a nucleotide? Phosphate, deoxyribose sugar, nitrogen containing base b. What kind of sugar is found in a nucleotide? deoxyribose sugar c. Which nucleotide component contains nitrogen? nitrogen containing base d. Name the four nitrogen bases shown in Model 1. Adenine, thymine, guanine, cytosine 2. DNA is often drawn in a “ladder model.” Locate this drawing in Model 1. a. Circle a single nucleotide on each side of the ladder model of DNA. DNA Structure and Replication 1 b. What part(s) of the nucleotides make up the rungs of the “ladder”? nitrogen bases c. What parts of the nucleotides make up the sides (backbone) of the “ladder”? phosphate d. Look at the bottom and top of the “ladder” in Model 1. Are the rungs parallel (the ends of the strands match) or antiparallel (the ends of the strands are opposites)? one side ends in sugar and the other one ends in phosphate 3. On the ladder model of DNA label each of the bases with the letter A, T, C or G. 4. Refer to Model 1. When one nucleotide contains adenine, what type of base is the adenine attached to on the opposite nucleotide strand? thymine 5. The two strands of DNA are held together with hydrogen bonds between the nitrogen bases. These are weak bonds between polar molecules. How many hydrogen bonds connect the two bases from Question 4? 2 6. Refer to Model 1. When one nucleotide contains cytosine, what type of base is the cytosine attached to on the opposite nucleotide strand? guanine 7. How many hydrogen bonds connect the two bases from Question 6? 3 8. With your group, use a complete sentence to write a rule for how the bases are arranged in the ladder model of DNA. the adenine and thymine bases always pair together. Guanine and cytosis also pair up. Read This! Erwin Chargaff (1905–2002), an Austrian-American biochemist, investigated the ratio of nucleotide bases found in the DNA from a variety of organisms. From his research, as well as research by Rosalind Franklin and Maurice Wilkins, Watson and Crick developed the complementary base-pair rule during their race to discover the structure of DNA. The complementary base-pair rule states that adenine and thymine form pairs across two strands, and guanine and cytosine form pairs across two strands. 9. Fill in the complementary bases on the strand below according to the base-pair rule. A T T A C G C G A T G C 10. The ladder model of DNA is a simplified representation of the actual structure and shape of a DNA molecule. In reality, the strands of DNA form a double helix. Refer to the double helix diagram in Model 1 and describe its shape using a complete sentence. a double helix is two twisted backbones made of alternating sugar and phosphate. The base pairs connect the backbones in the middle of the helix. 2 Model 2 – DNA Replication Direction of DNA helicase Free Nucleotides DNA helicase 11. Examine Model 2. Number the steps below in order to describe the replication of DNA in a cell. 4 ______ Hydrogen bonds between nucleotides form. 1 ______ Hydrogen bonds between nucleotides break. 2 ______ Strands of DNA separate. ______ Free nucleotides are attracted to exposed bases on the loose strands of DNA. 3 12. Locate the DNA helicase on Model 2. a. What type of biological molecule is DNA helicase? enzyme b. What is the role of DNA helicase in the replication of DNA? unzip the double helix 13. What rule is used to join the free nucleotides to the exposed bases of the DNA? base pairing rule 14. This type of replication is called semi-conservative replication. Considering the meaning of these words (semi—half; conserve—to keep), explain why DNA replication is called semi-conservative. the new DNA is built off of the old half DNA Structure and Replication 3 15. DNA molecules can be tens of thousands of base pairs in length. Mistakes in DNA replication lead to mutations, which may or may not be harmful to an organism. How does semi-conservative replication help prevent mutations during DNA replication? the old DNA series as a guide or template so that the DNA is copied exactly by the base pairing. 16. The proportions of the bases are consistent within a species; however they do vary between species. Using the base-pair rules, complete the following table to show the percentage of each type of base in the five different organisms. Organism 4 Percentage of each type of base Adenine Human 31 Cow 28 Salmon 29 Guanine Cytosine 19 19 22 22 21 Thymine 31 28 21 Wheat 27 23 Yeast 31 19 23 29 27 19 31 Extension Questions Model 3 – Timing of DNA Replication If the chromatin were condensed, replicated chromosomes would be visible. (End of S) S G2 If the chromatin were condensed, homologous chromosomes would be visible. (End of G1) Chromosomes are condensed. Matching sisgter chromatids are connected by a centromere. (Beginning of M) M G1 17. According to Model 3, what term refers to loose DNA inside of a nucleus? chromatin 18. During what part of the cell cycle is the DNA in a cell’s nucleus replicated? synthesis phase 19. During what part of the cell cycle is the DNA in a cell condensed into chromosomes? gap 2 20. Replicated chromosomes are often illustrated as an X shape to match how they look in real life just before cell division. a. According to Model 3, which of the following diagrams correctly show an original set of homologous chromosomes (grey) and their sister chromatids (black)—the replicated portion? b. What structure holds the two sister chromatids together as they prepare for cell division? centromere DNA Structure and Replication 5