Introduction DN Blueprints contain the instructions for building a house. Your cells also contain blueprints known as DN, or deoxyribonucleic acid. DN must do two things: 1. supply instructions for cell processes and the building cell structures. 2. be able to be copied each time a cell divides so that each cell contains an identical set of genes. he Structure of DN DN is made of smaller pieces called nucleotides. Each nucleotide is made of three parts: a sugar, a phosphate, and a nitrogenous (nitrogen) base. Nucleotides are identical except for the bases, which come in one of four varieties adenine (), thymine (), guanine (), or cytosine (). In the 1950 s, a biochemist named Erwin hargaff found that the amount of adenine in a DN strand always equals the amount of thymine and that the amount of guanine always equals the amount of cytosine. From this, he concluded that the nucleotides with particular bases must pair together. We abbreviate hargaff s rules like this: = and =. Rosalind Franklin, who was the first person to create images of DN molecules, made the next contribution to the study of DN. She used a process called X-ray diffraction to discover that DN had a spiral shape. wo other young scientists expanded on Franklin s discovery James Watson and Francis rick built models of DN and concluded that DN resembles a twisted ladder, which they called a double helix. From DN to ene Each cell has 46 chromosomes, which are made of DN and protein. section of a chromosome for a particular trait, such as height, is called a gene. gene may contain thousands of loops of DN. he loops are made of the DN ladder, which looks like a spiral staircase. Making opies of DN Because always binds with and always binds with, the two sides of the ladder are said to be complementary to the other. his allows DN to make a copy of itself, or replicate. When this occurs, the DN molecule splits (unzips) down the middle where the two bases meet. he existing bases are used as the pattern for the complementary side. 96
How DN Works 1. copy of a portion of the DN molecule where a particular gene is located is made and transferred outside of a cell s nucleus. 2. his single strand is a copy of one strand of the original DN. 3. Each group of three bases codes for one amino acid. 4. copy of the DN strand is fed through the ribosome. 5. ransfer molecules deliver amino acids from the cytoplasm of the cell to the ribosome. 6. he amino acids are dropped off at the ribosome. 7. he amino acids join together to make a protein. 8. he order of the bases on the copy of DN determines which amino acids are transferred to the ribosome and which protein is eventually made. hanges In enes Damage to DN can be caused by physical and chemical changes. mutagen is anything that causes a mutation in DN. Mutagens can be UV light, x-rays, asbestos, cigarette smoke, and many others. Mutations occur when there is a change in the order of bases in an organism s DN. When a base is left out, it is called deletion. When a base is added, it is called insertion. he most common error occurs when an incorrect base replaces a correct base. his is called substitution. Fortunately, repair enzymes are continuously on the job, patrolling the DN for errors. Usually the errors are repaired, but sometimes the mistake becomes part of the genetic message. hese changes can be either helpful, harmful, or have no consequence. Review of DN Reading 1. What must DN be able to do for an organism? 2. Describe the smaller parts of DN and how they fit together. 97
3. Describe the contributions of scientists to the study of DN. 4. What is the relationship between DN, chromosomes, and genes? 5. Describe the types of mutation. 98
mounts DN Hierarchy Notes mounts re four types of... hese come together to make... Word Bank...are the building blocks of......coils up to make... denine Bases hromosomes ytosine DN enes uanine Nucleotides Phosphates Sugars hymine...are segments of... 99
DN he Double Helix Recall that the nucleus is a small spherical, dense body in a cell. It is often called the "control center" because it controls all the activities of the cell including cell reproduction, and heredity. hromosomes are microscopic, threadlike strands composed of the chemical DN (short for deoxyribonucleic acid). In simple terms, DN controls the production of proteins within the cell. hese proteins form the structural units of cells and control all chemical processes within the cell. hink of proteins as the building blocks for an organism, because proteins make up your skin, your hair, and parts of the cell membrane. How you look is largely determined by the proteins that are made, which is determined by the sequence of DN in the nucleus. hromosomes are composed of genes, which are segments of DN that code for a particular protein which in turn codes for a trait. Hence you hear it commonly referred to as the gene for baldness or the gene for blue eyes. Meanwhile, DN is the chemical that genes and chromosomes are made of. DN is called a nucleic acid because it was first found in the nucleus. We now know that DN is also found in organelles, the mitochondria and chloroplasts, though it is the DN in the nucleus that actually controls the cell's workings. In 1953, James Watson and Francis rick established the structure of DN. he shape of DN is a double helix, which is like a twisted ladder. he DN helix is actually made of repeating units called nucleotides. Each nucleotide consists of three molecules: a sugar (deoxyribose), a phosphate, and then one of the four nitrogenous bases. he sides of the ladder are made of alternating sugar and phosphate molecules. he sugar in DN is deoxyribose. he rungs of the ladder are pairs of 4 types of nitrogen bases. he bases are known by their coded letters:,,, and. hese bases always bond in a certain way. denine will only bond to thymine. uanine will only bond with cytosine. his is known as the "Base-Pair Rule" or hargaff s Rule. he bases can occur in any order along a strand of DN. he order of these bases is the code that contains the instructions. For instance would code for a different gene than. strand of DN contains millions of bases. (For simplicity, the image only contains a few.) he two sides of the DN ladder are held together loosely by hydrogen bonds. he DN can actually "unzip" when it needs to replicate - or make a copy of itself. DN needs to copy itself when a cell divides, so that the new cells each contain a copy of the DN. Without these instructions, the new cells wouldn't have the correct information. he hydrogen bonds are represented by small circles. he Blueprint of Life Every cell in your body has the same "blueprint" or the same DN. Like the blueprints of a house tell the builders how to construct a house, the DN "blueprint" tells the cell how to build the organism. Yet, how can a heart be so different from a brain if all the cells contain the same instructions? lthough much work remains in genetics, it has become apparent that a cell has the ability to turn off most genes and only work with the genes necessary to do a job. We also know that a lot of DN apparently is nonsense and codes for nothing. hese regions of DN that do not code for proteins are called "introns", or sometimes "junk DN". he sections of DN that do actually code from proteins are called "exons". 100
Questions: 1. Write out the full name for DN. 2. What is a gene? 3. Where in the cell are chromosomes located? 4. DN can be found in what two organelles? 5. Which two scientists established the structure of DN? 6. What is the shape of DN? 7. What are the sides, or rails, of the DN double helix made of? 8. What are the "rungs" of the DN double helix made of? 9. What sugar is found in DN? 10. How do the bases bond together in DN? bonds with and bonds with 11. DN is made of repeating units called 12. How do some cells become brain cells and others become skin cells, when the DN in LL the cells is exactly the same. In other words, if the instructions are exactly the same, how does one cell become a brain cell and another a skin cell? 13. Why is DN called the "Blueprint of Life"? 101
oloring: olor all the phosphates LIH BLUE (one is labeled with a "P"). olor all the deoxyribose sugars DRK BLUE (one is labeled with a "D"). olor the thymine bases RED. olor the adenine bases REEN. olor the guanine bases PURPLE. olor the cytosine bases YELLOW. olor the hydrogen bonds grey. 102
Mutation Practice mutation is a change in DN sequence. his can be caused by a mutagen, anything that causes a mutation, or by an error in DN replication. Mutations usually do not cause any visible change in an organism. While there is a change in DN sequence, there may not be a change in the function of the protein made by the sequence. here are three types of mutations: Insertion: n extra base is added to the DN sequence. Deletion: base is removed from the DN sequence. Substitution: base is switched with another base. If a complementary base is switched with its partner, there is no change in the function. 1. Below is half a section of DN that has been split apart and is ready to copy itself. Write the appropriate letter in the space provided to build the DN s new complementary strand............................ 2. Below are two DN sequences, the original and the mutated strands. Identify where the mutation occurred and also the type of mutation it is. YPE OF MUION Base Sequence in Original ell DN Base Sequence in Mutated ell DN 103
3. Below are two DN sequences, the original and the mutated strands. Identify where the mutation occurred and also the type of mutation it is. YPE OF MUION Base Sequence in Original ell DN Base Sequence in Mutated ell DN 4. Ribosomes read a complementary copy of DN in order to make proteins. Each group of three bases (called a triplet or codon ) forms the code for an amino acid. When mutations occur, they can change the information that the DN carries. o understand this process better, look at the sentence below, which uses only three-letter words. MY O HE RED HO PO OFF HE LO If one letter is deleted from this sentence, it can become: MY HER EDH OP OO FF HEL O How is this similar to what can happen when a mutation occurs in DN? 104
Lab: DN & Proteins Investigating Mutations Objective: Study how mutations in DN may or may not lead to changes in a trait s structure or function. Instructions: 1. For each spoon, transfer water from one container to the other by dipping it into the water, lifting, and pouring the remainder into the empty bucket. One trial = 20 transfers of water. 2. For each trial, record the amount (in milliliters) that has successfully been transferred (round to the nearest tenth) by using the graduated cylinder provided. Warm-Up Questions: 1. What is DN? 2. What is a mutation? 3. List and describe the three different types of mutations. 4. What is the difference between a trait s structure and a trait s function? Variables: Independent Variable Dependent Variable Hypotheses: 1. If a mutation in DN occurs, then the structure of the trait will (sometimes, always, never) change. 2. If a mutation in DN occurs, then the function of the trait will (sometimes, always, never) change. 105
Pre-Lab Practice: Study the table below. ircle where the mutation occurs in each of the Mutants, identify which type of mutation has occurred, and indicate whether there is a change in structure (using pictures of the hands). Protein DN Strand ype of Mutation Is there a change in structure? Normal None None Mutant Mutant B Mutant Mutant D How It Works: mutation changes a in a DN sequence. Each set of three bases, called a, codes for a particular. When strung together, the amino acids code for a particular, which determines the of your traits. So, a change in the protein structure may or may not be visible and may or may not cause a change in, or how well the trait works. Data able: Protein ype of Mutation Volume (ml) ransferred YOUR SPOON Volume (ml) ransferred LSS VERE hange from Normal Normal None None Mutant Mutant B Mutant Mutant D Insertion Substitution Substitution Deletion 106
Quick Review: change in DN sequence is called a mutation. change in structure means that the strand of DN, once paired with its other half, will be different from the original DN and cause the trait to be built differently. If there is a change in function, it means the trait will work in a different way than the original trait. Protein Was there a change in the DN sequence? Was there a change in structure (appearance)? Was there a change in function (amount of water transferred)? Mutant Yes Mutant B Yes Mutant Yes Mutant D Yes 107