 Part One Poodles make more poodles. Sheep make more sheep. Replication is a basic fact of life. All living things make other living things that are to one degree or another duplicates of themselves. What is the mechanism behind all this? The answer lies in a molecule called DNA. In 1869, Friedrich Miescher extracted a substance, which he called nuclein from the nuclei of white blood cells. Nuclein later became known as nucleic acid. Living cells contain two kinds of nucleic acids—ribonucleic acid (RNA) which contains the sugar, ribose and deoxyribonucleic (DNA) which contains the sugar, deoxyribose. Nucleic acids are found in all living things, from the simplest protozoan to the most complex forms of animal and plant life. Early clues about the function nucleic acids in the cell came from the study of the strange behavior of pneumonia bacteria. In 1928, during experiments performed by Fred Griffith, harmless uncoated bacteria were transformed into deadly disease causing organisms. Dr. Oswald T. Avery and his colleagues cleared up the mystery of how this happened in 1944. The dramatic change was due to DNA. And, since the transformation was permanent—passed on from generation to generation—DNA was somehow also responsible for the transmission of hereditary information. DNA is made up of several different units. The sugar, deoxribose; a phosphate; and four organic compounds called nitrogen bases, which contain nitrogen: adenine, guanine, thymine and cytosine. Each nitrogen base, together with deoxyribose and phosphate, forms a unit called a nucleotide. The precise structure of the molecule was explored by a technique called x-ray crystallography. X-ray pictures taken by two scientists named Maurice Wilkins and Rosalind Franklin provided important clues about the arrangement of atoms inside the molecule. Another important piece of information concerning the relative amounts of nitrogen bases, which DNA contained, was provided by a scientist named Erwin Chargaff. Two young scientists—James Watson and Francis Crick—finally pieced together the precise structure of DNA. The model proposed by Watson and Crick for the structure of DNA is shaped like a twisted ladder. This type of figure is known as a double helix. The sides of the twisted ladder are made up of alternating units of deoxyribose and phosphate. The rungs of the ladder are composed of paired nitrogen bases. Adenine always pairs with thymine, and guanine always pairs cytosine. The bases are held together by hydrogen bonds. Watson and Crick’s model also suggested a way in which DNA could make copies of itself. First, the ladder untwists. Then the bases break apart. Since and adenine nucleotide can only bond with thymine, and guanine can only bond with cytosine, new units are assembled in precisely the same order as old. When the splitting and pairing processes are competed, two identical DNA molecules stand in the place of one. The process by which DNA makes copies of itself is called replication. • Discussion Questions and Suggested Answers 1. WHAT SUGAR IS FOUND IN DNA? IN RNA? HOW ARE THESE SUGARS DIFFERENT? The sugar found in DNA is deoxyribose. In RNA, the sugar is ribose. The sugars are different only in that deoxyribose contains one less atom of oxygen than does ribose. 2. WHAT SUBSTANCE CAUSED THE TRANSFORMATION OF HARMLESS BACTERIA INTO DEADLY ONES IN GRIFFITH’S AND AVERY’S EXPERIMENT? The substance responsible for the change was DNA. 3. WHERE IN THE NUCLEUS IS DNA FOUND? WHY IS IT NECESSARY FOR DNA TO REPLICATE? DNA is found in nuclear structures called chromosomes. DNA must replicate so that, during the process of cell division, all of the genetic information which was contained in the nucleus of the cell is passed on equally to the two new daughter cells. • For Further Discussion 1. What are the pyrimidine bases in DNA? The purine bases? 2. What is a nucleotide? 3. What is a helix? A double helix? 4. What units make up the rungs of the DNA “ladder”? What units make up the sides of the DNA “ladder”? 5. Why must a purine base always be paired with a pyrimidine base in the rungs of the DNA ladder? 6. What role do hydrogen bonds play in the structure of DNA? In the replication of DNA? 7. What was the unique contribution of Watson and Crick in the series of discoveries that led to the structure of DNA? How were their investigations similar to those of many other scientists? How were they unique? • Discussion Questions and Suggested Answers 1. WHY ARE PROTEINS IMPORTANT? WHAT UNITS MAKE UP PROTEINS? Proteins are important because they perform many functions which are necessary for the growth and maintenance of living things. Proteins are made of units called amino acids. 2. WHAT FOUR SYMBOLS FORM THE DNA CODE? HOW MANY SYMBOLS CODE ONE AMINO ACID? The four symbols of the DNA code are the abbreviations for the four bases: A (adenine), T (thymine), G (guanine) and C (cytosine). Three bases (symbols) code for one amino acid. 3. WHAT HOLDS THE MESSENGER RNA IN PLACE DURING PROTEIN ASSEMBLY? The messenger RNA is held in place by the ribosomes. 4. WHAT IS ONE CODING UNIT ON MESSENGER RNA CALLED? One coding unit on messenger RNA is called a codon. 5. WHAT DOES TRANSFER RNA DO? Transfer RNA is the “taxi service” which is responsible for bringing the proper amino acid into position on the growing protein chain. 6. HOW DOES TRANSFER RNA RECOGNIZE ITS PLACE ON THE MESSENGER RNA? Three bases at one end of the transfer RNA, called the “anti-codon,” match up with their complementary bases on the messenger RNA. So, each anti-codon on transfer RNA lines up with the appropriate codon on messenger RNA. 7. WHY IS THE ORDER OF THE BASES IMPORTANT? The bases must be in order to convey correct genetic information. 8. WHAT ARE SOME OF THE POTENTIAL HAZARDS OF RECOMBINANT DNA RESEARCH? THE BENEFITS? The potential hazards of recombinant DNA research include the possibility of accidentally creating and releasing potentially hazardous organisms. Some of the benefits that may be reaped from recombinant DNA research include producing corn and other crops without expensive fertilizing agents and producing large quantities of substances such as insulin needed for survival by some persons. • For Further Discussion 1. Describe some of the important uses of proteins. 2. In which part of the DNA molecule is genetic information stored? 3. What is transcription? 4. What are three differences between DNA and messenger RNA? 5. What are chromosome puffs? 6. What is the cause of sickle-cell anemia? • Glossary Adenine—one of the four nitrogen bases found in DNA and RNA. Pairs with thymine in DNA or uracil in RNA. Amino acids—building blocs of proteins. Chromosomes—structures in the nucleus which contains DNA. Codon—a sequence of three nitrogen bases on the messenger RNA which forms a coding unit for protein assembly. Cytosine—a nitrogen base which pairs with guanine—found in DNA and RNA. DNA (deoxyribonuleic acid)—the nucleic acid which contains the genetic code. Deoxyribose—the sugar found in DNA. Genetic code—the sixty-four different combinations of three nitrogen bases which form the language of heredity. Genes—segments along a DNA molecule which code for specific proteins. Guanine—a nitrogen base which links with cytosine. Found in DNA and RNA. Helix—a spiral structure with a repeating pattern. Hydrogen bond—a relatively weak chemical bond which holds the complementary bases together in the DNA ladder. Messenger RNA—the RNA molecule which carries the code for protein synthesis into the cytoplasm. Nitrogen bases—organic compounds, which contain nitrogen. The nitrogen bases found in DNA and RNA are adenine thymine, guanine, cytosine and uracil. Nucleotides—the building blocks of DNA and RNA. Each nucleotide contains a sugar, a phosphate and a base. Nucleic acids—general term for DNA and RNA Proteins—large molecules made up of long chains of amino acids. Purines—adenine and guanine—ling with pyrimidines. Pyrimidines—thymine, cytosine and uracil—link with purines. Recombinant DNA—DNA molecule made up of segments of DNA from two different organisms. Replication—the process during which DNA makes copies of itself. RNA (Ribonucleic acid)—a type of nucleic acid which translates the genetic code into specific proteins. Ribose—the sugar found in RNA. Ribosomes—small structures in the cytoplasm which are the sites of protein syntheses in the cell. Thymine—a nitrogen base which links with adenine in DNA. Transcription—the process during which messenger RNA is formed on a master strand of DNA. Transfer RNA—an RNA molecule which transports specific amino acids to their proper place on the ribosome during protein synthesis. Triplet code—sequence of three nitrogen bases on DNA which carries the code for a particular amino acid. Uracil—a nitrogen base found in RNA; chemically similar to thymine. Links to adenine. X-ray crystallography—a technique which uses x-rays to produce shadow pictures of the arrangement of atoms in molecules. • Suggested Research and Discussion Topics 1. Discuss the contribution that Gregor Mendel made to the modern understanding of heredity. 2. Research and report on the significance of Hershey and Chase’s experiments with bacterial viruses. 3. Write to your local state and/or federal representatives individually or as a class to find out what legislation (if any) is being considered relative to recombinant DNA research. |
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