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Name ________________________________________________ Date ____________________ DNA Article Every time a cell divides, it must duplicate its DNA (Deoxyribose Nucleic Acid) so Tab NOTES that each of the two new daughter cells has all of the genetic material of the original parent cell. The process of DNA duplication is called replication. DNA replication occurs in ALL CELLS (somatic cells and gametes) before the cell reproduces. Structure of DNA First, let us begin by examining the structure of a DNA molecule. DNA is in the shape of a double helix, which looks like a twisted ladder or spiral staircase. The two sides of the DNA ladder are made of alternating molecules of sugar and phosphate. The rungs of the DNA ladder are made up of a pair of molecules called nitrogen bases. These molecules contain the element nitrogen and other elements. DNA has four kinds of nitrogen bases: adenine (A), thymine (T), guanine (G), and cytosine (C). The capital letters, A, T, G, and C are used to represent the four bases. The bases on one side of the ladder pair with bases on the other side in a very specific way. Adenine (A) only pairs with thymine (T), while guanine (G) only pairs with cytosine (C). The nitrogen bases are held together by hydrogen bonds, which are a chemical attraction between the hydrogen atoms in each nitrogen base. This pairing pattern, referred to as complementarity, is the key to understanding how DNA replication occurs. Replication of DNA DNA replication occurs in two steps: Unwinding and Rebuilding. It begins when the two sides of the DNA molecule unwind and separate, somewhat like a zipper unzipping. An enzyme, DNA Helicase, is responsible for unzipping the DNA molecule. An enzyme is a protein that makes chemical processes happen faster; think of enzymes like workers in the cell. They put different elements together to make molecules as well as break molecules apart very efficiently. Once the DNA molecule has been unzipped, there are exposed base pairs on either side of the ladder. This creates a template for new nitrogen bases to fill in. There are free-­‐floating nitrogen base pairs in the nucleus and they are attached to their partner (A with T and C with G) on the exposed DNA strand by another enzyme DNA Polymerase. Because of the complementary way in which the nitrogen bases pair with one another (A always with T and C always with G) the order of the bases in each new DNA molecules exactly matches the order in the original DNA molecule. Once the new bases are attached, two identical DNA molecules are created. What if there was a mistake? Suppose that a mistake occurred during replication and the wrong base sequence was copied down. For example, instead of the base A, the DNA molecule might have the base G. Such a mistake is one type of mutation that can occur in a cell’s genetic material. The word mutation comes from a Latin word – mutatio – which means, “change.” There are several different types of mutations. A base pair or series of base pairs can be substituted, removed, or added to DNA segments. Check out the diagram to the right for an illustration of these mutation examples. Often, the cell has a way of catching these errors and repairing them after DNA replication is complete and before mitosis begins. If a mutation is not “caught” by the cell, it can affect an organism’s physiological traits. A physiological trait is a characteristic or feature of an organism. Mutations can be helpful, harmful, or neutral. When the mutation increases an organism’s ability to survive and reproduce, then it is helpful. When the mutation makes it harder for an organism to survive and reproduce, then that mutation is harmful. When a mutation does not affect an organism’s chances of survival, then it is a neutral mutation. We will talk more about mutations during our unit on Natural Selection and Evolution, but in the meantime check out the examples of mutations below: Antibiotic Resistant Bacteria Antibiotics are chemicals that kill bacteria; you probably have taken an Helpful Mutation antibiotic for an ear infection or Strep throat. Gene mutations have enabled some bacteria to become resistant to certain antibiotics – that is, the antibiotics do not kill the bacteria that have mutations. Sickle Cell Anemia This mutation causes blood cells to grow in a crescent moon shape instead of a Harmful Mutation circular shape. These misshaped blood cells can get stuck in blood vessels of humans and cause dangerous clots. Cilantro: love it or loathe it Some people love cilantro, which is a lemony herb found in salsas. Other people CANNOT STAND cilantro, because it tastes like soap to them. This has to do Neutral Mutation with a gene that helps build olfactory (smell) receptors. About 14% of the population have this mutation and, for them, cilantro tastes super soapy. This mutation is neither harmful nor helpful when it comes to survival and reproduction, so we call it a “neutral” mutation. 1. In YOUR OWN WORDS: DNA Structure and Replication HW Questions What does “complementarity” mean with regards to the base pairs of DNA? Why does complementarity enable a cell to perfectly copy DNA? _________________________________________________________________________________________________________________________ _________________________________________________________________________________________________________________________ _________________________________________________________________________________________________________________________ _________________________________________________________________________________________________________________________ _________________________________________________________________________________________________________________________ _________________________________________________________________________________________________________________________ 2. Summarize the two steps of DNA Replication – be sure to include the enzyme names: Step 1: ________________________________________________________________________________________________________ ________________________________________________________________________________________________________________ ________________________________________________________________________________________________________________ Step 2: ________________________________________________________________________________________________________ ________________________________________________________________________________________________________________ ________________________________________________________________________________________________________________ 3. What is the end result of replication? _________________________________________________________________________________________________________________________ _________________________________________________________________________________________________________________________ 4. What are “mutations?” _________________________________________________________________________________________________________________________ _________________________________________________________________________________________________________________________ 5. What are the different kinds of mutations? _________________________________________________________________________________________________________________________ _________________________________________________________________________________________________________________________ _________________________________________________________________________________________________________________________ _________________________________________________________________________________________________________________________