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Chapter 9 Nucleic Acids and Proteins The Molecules That Make You What You Are Copyright W. H. Freeman and Company · New York Chemistry Applied •DNA and RNA: carrying genetic information •RNA: translating genetic information •Proteins •Human Genetics Copyright W. H. Freeman and Company · New York Nucleic Acids: DNA and RNA Structures and Properties 1. The basic units of DNA and RNA are nucleotides. • DNA and RNA are large polymer molecules called nucleic acids. • The subunits in DNA and RNA are called nucleotides. • A nucleotide is made up of an acidic phoshate unit, a sugar, and a nitrogen base. Copyright W. H. Freeman and Company · New York Nucleic Acids: DNA and RNA Structures and Properties • DNA and RNA differ in the type of sugar and in one type of nitrogen base. • DNA contains the sugar deoxyribose while RNA contains the sugar ribose. • Each sugar is condensed with a nitrogenase base to form a nucleoside. Copyright W. H. Freeman and Company · New York Nucleic Acids: DNA and RNA Structures and Properties • Three different nitrogenous bases occur in both DNA and RNA. The fourth possible base is different in DNA and RNA. • The sequences of bases do not form any regular repeating pattern. Their sequence represents a code from which all information about the living system can be obtained. • The five bases are: Copyright W. H. Freeman and Company · New York Nucleic Acids: DNA and RNA Structures and Properties • Guanine and adenine are classified as purines. Cytosine, thymine, and uracil are classified as pyrimidines. • The bases are assigned the codes: •A adenine •C cytosine •G guanine •T thymine •U uracil • A, G, and C are found in both DNA and RNA. • T is found only in DNA and U is found only in RNA. • DNA stands for the chemical name: deoxyribonucleic acid, while RNA stands for the chemical name: ribonucleic acid. Copyright W. H. Freeman and Company · New York Nucleic Acids: DNA and RNA Structures and Properties 2. Nucleotides are joined together to form biological polymers. • Individual nucleotides are connected in DNA and RNA through condensation reactions: + 2 H 2O Copyright W. H. Freeman and Company · New York Nucleic Acids: DNA and RNA Structures and Properties • A short section of DNA: Copyright W. H. Freeman and Company · New York Nucleic Acids: DNA and RNA Structures and Properties 3. DNA is a double helix. • Based on the work of James Watson, Francis Crick, and Rosalind Franklin in the 1950’s, we now know DNA is a double helix of two separate DNA chains running in opposite directions. Copyright W. H. Freeman and Company · New York Nucleic Acids: DNA and RNA Structures and Properties • Hydrogen bonding between the bases, which form base pairs stacked on top of each other at the center of the helix, holds the chains together. Copyright W. H. Freeman and Company · New York Nucleic Acids: DNA and RNA Structures and Properties • In the double helix, an A base on one strand is always hydrogen bonded to a T base on the other strand and vise-versa. The same is true for a G base and a C base. This is called the principle of complementarity. • The principle of complementarity is basis for replication, the process where a copy of the DNA double helix is made. Copyright W. H. Freeman and Company · New York Nucleic Acids: DNA and RNA Structures and Properties • In replication, the DNA double helix unwinds and each strand provides a template along which complementary bases align and are polymerized. • Replication is fast and almost always accurate. Occasional inaccuracies, when the occur, represent mutations. Copyright W. H. Freeman and Company · New York DNA: The Genetic Message 4. The structure of DNA carries information as a linear sequence of nucleotides. • The language in DNA is carried by the sequence of four bases: A, C, T, and G. Words of three letters represent the basic vocabulary in DNA. • There are 64 possible “words” contained in the 3 letter DNA genetic code. • Most of these words specify the identity of one of twenty amino acids contained in protein molecules. • A few words specify the position in the “text” where the code for a particular protein starts and stops. Copyright W. H. Freeman and Company · New York DNA: The Genetic Message 5. Specific sequences of nucleotides are genes. • Long strands of DNA are stored within the nucleus of most cells in the form of chromosomes. • The number of chromosomes varies with species (Human = 26 or 23 pairs) • The DNA within a chromosome is divided into many segments, many of which carry specific information in the form of genes. • Genes are separated from each other by intergenic regions. • Each gene is preceded by a regulatory region in the DNA which allows the gene to be turned on and off. • When a gene is turned on, its information is transferred to RNA which is then “translated” into specific proteins. Copyright W. H. Freeman and Company · New York DNA: The Genetic Message 6. The Human Genome Project • Since 1990, researchers have been determining the DNA sequence of the entire human genome. • They have produced a working draft which identifies individual gene sequences and maps their locations on the 23 pairs of human chromosomes. • More than 30,000 gene have been identified. Copyright W. H. Freeman and Company · New York RNA: The Genetic Message Translator 7. Genetic information goes from DNA to RNA to protein. • Genetic information in the form of DNA is contained in the nucleus of most cells. The site of protein synthesis is outside the nucleus in the cytoplasm of the cell, however. • DNA cannot leave the nucleus, so a copy of the relevant segment of the DNA called mRNA is made and exported to the cytoplasm. The synthesis of this mRNA is called transcription. • The mRNA provides the instructions for ribosomes in the along with transfer RNA’s (tRNA) in the cytoplasm to construct the protein. Copyright W. H. Freeman and Company · New York RNA: The Genetic Message Translator 8. Information on DNA is transferred by transcription of a complementary sequence of nucleotides on mRNA. • To build a complementary mRNA sequence from DNA, the DNA must first unwind to expose the bases. • A protein enzyme called RNA polymerase assists in bringing unattached nucleotide bases and correctly paring them to the exposed DNA bases. The RNA polymerase then catalyzes the polymerization of the assembled nucleotides. • The RNA produced is a complementary copy of the segment of DNA transcribed. Copyright W. H. Freeman and Company · New York RNA: The Genetic Message Translator Copyright W. H. Freeman and Company · New York RNA: The Genetic Message Translator 9. RNA codons dictate the sequence of amino acids in a protein. • A sequence of three successive nucleotides on a mRNA molecule is called a codon. • The correlation of codons to amino acids is: Copyright W. H. Freeman and Company · New York RNA: The Genetic Message Translator • Note that for most amino acids there is more than one codon assigned. • In addition, several codons specify start and stop signals specifying the beginning and end of the corresponding protein. Copyright W. H. Freeman and Company · New York RNA: The Genetic Message Translator 10. Ribosomes build polypeptides. • Translation is the process of converting the three letter codes specified in the mRNA to the 20 amino acid alphabet of proteins. • This process is carried out by large structures called ribosomes which are built from several segments of rRNA and a group of ribosomal proteins. • Ribosomes contain sites where mRNA and two tRNA’s can bind. • There is at least one tRNA for each of the 20 amino acids. Each tRNA contains three bases in its sequence called the anticodon, which is complementary to the codon for the amino acid in question. Copyright W. H. Freeman and Company · New York RNA: The Genetic Message Translator • At the other end of the tRNA is a site where the amino acid itself is attached by an enzyme. The amino acid corresponding to the anticodon of the tRNA is always attached correctly. • As the ribosome reads the codons on the mRNA, it assembles tRNAs with matching anticodons in the correct sequence and polymerizes the amino acids they carry. Copyright W. H. Freeman and Company · New York RNA: The Genetic Message Translator Copyright W. H. Freeman and Company · New York RNA: The Genetic Message Translator 11. Cells regulate gene activity by turning genes on and off. • The production of functioning proteins is call gene expression. • The genes that are functioning at a given time depend upon the type of cell (liver, heart, skin, etc.), the developmental stage of the cell, and sometimes upon varying conditions outside the cell. • Proteins and other molecules carry out the gene regulation process. • Housekeeping gene are “on” all the time and manage the basic functions of the cell Copyright W. H. Freeman and Company · New York RNA: The Genetic Message Translator • When genes need to be turned on, activator proteins which recognize and bind to specific DNA sequences are often involved. • Once bound, they help the RNA polymerase bind to the DNA. • Repressor proteins bind to DNA sequences and prevent RNA polymerase from binding, thus shutting off the gene. • The signals to activator and repressor proteins that cause them to bind or not bind to the DNA are often small signal molecules. These molecules bind to the proteins and change their shape. • The signal molecules can be anything from nutrients (glucose, lactose, maltose) to carcinogens, to drugs. Copyright W. H. Freeman and Company · New York RNA: The Genetic Message Translator 1. Information from genes on DNA is transcribed to an mRNA. 2. mRNA leaves the nucleus through nuclear pores. 3. mRNA binds to a site on a ribosome. 4. mRNA and the first tRNA molecules are brought together within the ribosome, initiating construction of a polypeptide. 5. mRNA moves through the ribosome. As each codon is “read”, a new amino acid is added to the growing polypeptide chain. 6. A stop signal on the mRNA is encountered and the peptide chain is released. Copyright W. H. Freeman and Company · New York The Genetic Message Expressed I: Protein Form 12. Proteins are polyamides. • Proteins are non-branched condensation polymers of alpha amino acids. • Shorter proteins are often called polypeptides. • There are twenty different alpha amino acids in human proteins, formed by varying the –R group above. Copyright W. H. Freeman and Company · New York The Genetic Message Expressed I: Protein Form Copyright W. H. Freeman and Company · New York The Genetic Message Expressed I: Protein Form 13. Polypeptides are relatively short chains of amino acids. • Two amino acids are joined in a polypeptide or protein by a peptide bond. • Greek prefixes designate the number of amino acids for the first few numbers and then the term polypeptide is used (for instance, dipeptide, tripeptide, etc.) Copyright W. H. Freeman and Company · New York The Genetic Message Expressed I: Protein Form • The artificial sweetener, aspartame, is the di-peptide: Asp-Phe. • Note that this is a different chemical substance than Phe-Asp. Copyright W. H. Freeman and Company · New York The Genetic Message Expressed I: Protein Form • Because some people with an inborn condition called phenylketonuria (PKU) cannot metabolize phenylalanine, those people must avoid using aspartame as a sweetener and avoid foods cooked using aspartame. • Endorphins are naturally occurring polypeptides in the body in the chemical group called opioids. These molecules naturally relieve pain and produce pleasant sensation. Copyright W. H. Freeman and Company · New York The Genetic Message Expressed I: Protein Form 14. Protein shapes are determined by interactions of backbones and side chains. • Proteins fold up to have specific shapes containing regions that interact in a very specific way with various smaller molecules, and sometimes other protein molecules. • Protein structure is viewed at four levels of complexity: •Primary structure •Secondary structure •Tertiary structure •Quaternary structure Copyright W. H. Freeman and Company · New York The Genetic Message Expressed I: Protein Form • Primary structure is simply the sequence of amino acids making up the polypeptide chain. • The R groups of amino acids can be classified as polar, non-polar. The polar amino acids are hydrophylic (water loving) and the non-polar amino acids are hydrophobic (water hating). • The completed protein chain will fold up in such a way to place the hydrophobic side chains at the center of the protein, away from contact with the solvent, and the hydrophylic side chains at the surface, where they can interact with the solvent. Copyright W. H. Freeman and Company · New York The Genetic Message Expressed I: Protein Form • Secondary structure is ordered hydrogen bonding between one part of the polypeptide chain to another part. Two of the most important patterns of secondary structure are the alpha helix and the beta pleated sheet. Copyright W. H. Freeman and Company · New York The Genetic Message Expressed I: Protein Form • In the alpha helix, the N-H unit of each amino acid hydrogen bonds with the C=O in the fourth amino acid further along in the sequence. • The R groups all protrude outward from the alpha helix and the center is hollow. • The alpha helix predominates in alpha keratins (hair, skin, nails) • In the beta pleated sheet, the polypeptide chain zigs back and forth within a plane. Hydrogen bonds between N-H and C=O groups on adjacent strands hold the peptide in a more or less planar structure. • The beta pleated sheet predominates in silk. Copyright W. H. Freeman and Company · New York The Genetic Message Expressed I: Protein Form • Tertiary structure is the overall folding of a protein, including its secondary structure into a globular three dimensional shape. • The folding is determined by: •Salt bridge attractions between –COO- and –NH3+ groups •Hydrogen bonds •Covalent S-S (disulfide) bonds •Weak attractions between hydrophobic groups Copyright W. H. Freeman and Company · New York The Genetic Message Expressed I: Protein Form • Quaternary structure is the arrangement of subunits within a protein which consists of more than one separate polypeptide chain. • An well studied example of a protein having a quaternary structure is hemoglobin. • This protein is found in red blood cells and is responsible for carrying oxygen from the lungs to the tissues. Copyright W. H. Freeman and Company · New York The Genetic Message Expressed I: Protein Form 15. Your hair curls or doesn’t curl due to disulfide bridges and hydrogen bonds. • In human hair, two alpha helical proteins coil around each other, forming a supercoil. The supercoil structure is locked in place by –S-S- bonds between adjacent proteins. • Whether your hair is straight or curly, the shape is locked into place by these –S-S- bonds. • In permanent waving or straightening of hair, a chemical treatment breaks the –S-S- bonds and the hair is placed in the desired shape. • A second chemical is then applied which remakes the –S-Sbonds, this time locking the hair into the new desired shape. Copyright W. H. Freeman and Company · New York The Genetic Message Expressed I: Protein Form Copyright W. H. Freeman and Company · New York The Genetic Message Expressed II: Protein Function 16. You need protein in your diet. • The human body requires about a gram of protein per kilogram of body weight per day. This protein is digested to provide a pool of alpha amino acid needed for human protein synthesis. • Digestive enzymes speed up the reverse polymerization of proteins by catalyzing the addition of water to the amide bonds holding the amino acids together. • Sources of protein include fish, meat, and beans. • Out of the 20 amino acids needed, 10 are essential amino acids. The human body cannot synthesize these amino acids or produce them by rearranging amino acids of another type contained in your diet. Copyright W. H. Freeman and Company · New York The Genetic Message Expressed II: Protein Function 17. What is a complete protein? • A dietary protein is a complete protein if it contains all of the essential amino acids in the correct proportions required for human proteins. • Examples of complete proteins are meat, seafood, poultry, milk, human milk, cheese, and eggs. • Examples of incomplete proteins include wheat, corn, rice and oats, all low in lysine and some in tryptophan, both essential amino acids. • Legumes, such as beans and peas are high in lysine and tryptophan but low in other essential amino acids. • Combining grains and legumes at a meal does provide adequate amounts of all of the essential amino acids. Copyright W. H. Freeman and Company · New York The Genetic Message Expressed II: Protein Function • If only one essential amino acid is missing from the diet, creation of dependant proteins will cease to some extent. • Dietary protein transformation into human protein can be summarized: broken down essential to make dietary protein -- 20 amino acids -- protein required by body • Excess amino acids in the diet are decomposed. The nitrogen is excreted as urea, and the carbon skeletons are converted to glucose or stored as fat. Copyright W. H. Freeman and Company · New York The Genetic Message Expressed II: Protein Function 18. Proteins have a wide variety of roles in the body. • Structural Proteins • Contractile Proteins • Regulatory Proteins • Protective (or defense) Proteins • Transport Proteins • Catalytic Proteins • Storage Proteins Copyright W. H. Freeman and Company · New York The Genetic Message Expressed II: Protein Function 19. Enzymes are catalysts for specific chemical reactions that occur in living mater. • Enzymes are involved in almost every process occurring in the body. • Enzymes are the catalysts that speed up chemical reactions. Without enzymes, most biological reactions would proceed at an imperceptibly slow rate. • In a chemical reaction, bonds in the reactants must first be broken and then re-made as the products are formed. • If one views this as one continuous process, somewhere in the middle the energy of the system must be very high. That is, an energy hill must be climbed in order to get to the products. Copyright W. H. Freeman and Company · New York The Genetic Message Expressed II: Protein Function • If this were not the case, the reaction would be very fast under all conditions. • Without a catalyst (enzyme) the energy to climb this hill comes from the kinetic energy of the colliding molecules. The rate of the reaction can be increased by heating the solution. • In the human body, heating the reaction is not an option. Instead, an enzyme effectively lowers the amount of energy to climb the hill by changing the way the reaction occurs in some way. • The top of the energy hill is called the transition state in chemical kinetics and an enzyme is said to lower the energy of the transition state. Copyright W. H. Freeman and Company · New York The Genetic Message Expressed II: Protein Function • Enzymes are globular proteins with pockets or groves on their surface called active sites, where the reactant molecules bind. • After the reaction is over, the product molecules will be located in these same sites and are subsequently released. • The active sites are very specific for which reactant molecules can bind. • The forces binding a reactant molecule to an enzyme can include: •Hydrogen bonding •Hydrophobic interactions •Ionic attractions between ions of unlike charge •The binding process positions the reactants in exactly the correct orientation to each other for the reaction to occur. Copyright W. H. Freeman and Company · New York The Genetic Message Expressed II: Protein Function Copyright W. H. Freeman and Company · New York The Genetic Message Expressed II: Protein Function • All of the enzymes in the body are created in reactions that themselves require enzymes. • About 1/3 off all known enzymes require one or more metal ions such as iron, copper, zinc, manganese, magnesium, etc. in order to function. Copyright W. H. Freeman and Company · New York The Genetic Message Expressed II: Protein Function 20. Proteins may require additional molecules to function. • Simple proteins contain only amino acids. • Conjugated proteins possess other groups besides amino acids. These other groups include metal ions, phosphate groups, sugars, lipids, or other small molecules. • Some proteins require small complex organic or organometallic components called coenzymes which cannot be synthesized by the organism itself. • Many of the vitamins in our diet supply the coenzymes or precursors to the coenzymes needed by enzymes in our body. Copyright W. H. Freeman and Company · New York The Genetic Message Expressed II: Protein Function • Conjugated proteins containing a sugar unit are called glycoproteins. • Glycoproteins are important parts of cell membranes and are responsible for cell surface properties such as blood group specificity (ABO). • Conjugated proteins containing a lipid molecule are called lipoproteins. Copyright W. H. Freeman and Company · New York The Genetic Message Expressed II: Protein Function 21. Proteins combined with lipids can contribute to heart disease risk. • Cholesterol and other lipids are synthesized in the liver or absorbed in the intestines and must then be transported to the places in the body where they are needed. • Lipids themselves are not very soluble in water, so they are bound to lipoproteins in the blood in order to be transported. • Two classes of lipoproteins are involved: low-density lipoproteins, LDL’s, and high-density lipoproteins, HDL’s. Copyright W. H. Freeman and Company · New York The Genetic Message Expressed II: Protein Function • LDL particles are very cholesterol rich and carry cholesterol from the liver where it is synthesized to various membranes. • Too much cholesterol in the body can accumulate in the blood vessels causing atherosclerosis, which may eventually lead to a heart attack. • HDL particles are initially rich in protein and low in cholesterol. They can bind cholesterol and transport it back to the liver where it can be broken down. A lack of HDL is correlated to a higher incidence of heart disease. • HDL is often called “good cholesterol” and LDL “bad cholesterol”. A ratio of LDL to HDL of 5 in men indicates an increased risk for heart disease. The corresponding ratio in women is 4.5. • People with high cholesterol levels may be prescribed a class of drugs called statins which block cholesterol synthesis. Copyright W. H. Freeman and Company · New York The Genetic Message Expressed II: Protein Function 22. If a protein’s shape is altered, its ability to function is affected. • When a protein loses its functional shape it is said to be denatured. • An example of denaturation occurs in the stomach where the strong acidic conditions denature the proteins in the food allowing rapid hydrolysis by digestive enzymes. • Both heat and extremes in pH will denature most proteins. Copyright W. H. Freeman and Company · New York The Genetic Message in Action 23. Mutations: When things go wrong • A mutation is a permanent change in the sequence of bases in a DNA molecule. • Mutations are passed on to offspring. • Mutations can occur naturally (at a very low rate) or be induced by environmental effects such as radiation or toxic chemicals. • Chemical agents that cause mutations are called mutagens. • Teratogens are mutagens that cause birth defects. The most infamous of these was thalidomide in the late 1950’s. Thalidomide occurs in left handed and right handed forms. Unknown at the time, one form prevents miscarriages, but the other produces birth defects. Copyright W. H. Freeman and Company · New York The Genetic Message in Action • Mutations can change DNA in several ways: •Substitution mutation - When a single base changes in the DNA sequence, a single amino acid may change in the protein sequence. •Insertion mutation – Three bases are inserted into the DNA, an additional amino acid is inserted in the protein sequence. •Multiple Insertion - A multiple of three bases are inserted into the DNA; multiple amino acids are inserted into the protein sequence. •Single Deletion – Three bases are deleted from the DNA, a amino acid is deleted from the protein. Copyright W. H. Freeman and Company · New York The Genetic Message in Action •Multiple Deletion – A multiple of three bases is deleted from the DNA, multiple amino acids are deleted from the protein. •Small Inversion – A few groups of three bases reverse their order, a few amino acids in the protein in reversed order. •Large Inversion – Many groups of three bases reverse their order, large number of amino acids in the protein are in reversed order. •Not all mutations produce an observable effect. Silent mutations replace one amino acid with another of similar properties so the protein is not affected much. Copyright W. H. Freeman and Company · New York The Genetic Message in Action •Humans have two sets of chromosomes, one set from each parent. Except for sex linked traits (XY chromosomes) there are two copies of every gene. •Usually at least one unmutated copy of each gene is inherited from one of the parents which “masks” any mutated copy. In cases such as this the mutation is said to be recessive. •In certain cases, mutations provide an advantage to an organism. The recessive gene for sickle cell anemia confers an advantage to people infected with malaria. •When a mutated gene is inherited from both parents, disease usually results. This often occurs in children from close relatives and in certain ethnic group. Copyright W. H. Freeman and Company · New York The Genetic Message in Action 24. Cloning • Clones are individuals having exactly the same DNA. Natural clones are represented by identical twins. • Many animals have now been cloned, however laboratory production of cloned humans is not yet possible. • Cloning of an animal basically involves replacing the nucleus of an egg cell from a donor with the nucleus of a body cell from a second donor. • The science is imperfect and most cloned cells do not survive. Many of those that do are not healthy. • Cloning raises ethical and religious questions for many people. Copyright W. H. Freeman and Company · New York The Genetic Message in Action 25. Proteins and DNA can be used as evidence in legal proceedings. • Except for the case of identical twins, DNA is unique to a single person. • It can theoretically be used to identify an individual with a very high degree of certainty. • DNA is very long lasting and older samples from almost any biological specimen can be successfully analyzed. • Very small samples can be “amplified” using the PCR or polymerase chain reaction to obtain a sample large enough to be analyzed. • The actual sequences in the DNA that are used to identify an individual are the repetitive sequence segments between the segments coding for proteins. Copyright W. H. Freeman and Company · New York The Genetic Message in Action • Long strands of DNA are first chopped into shorter pieces using restriction nucleases. • These fragments are then sorted by size and electrical charge using a special gel and a strong electric field. • Further treatment an transfer to a nylon membrane results in a DNA fingerprint which can be compared to similar fingerprints from known individuals. Copyright W. H. Freeman and Company · New York Summarizing the Main Ideas • The basic units of DNA and RNA are nucleotides. • Each nucleotide consists of a sugar unit connected to a phosphate group and a nitrogenous base. • In DNA the sugar is deoxyribose and the bases are A,T,C,and G. • In RNA the sugar is ribose and the bases are A,U,C, and G. • Alternating phosphate and sugar groups form the backbone structure of DNA with the bases extending from the backbone. • The 3D structure of DNA is a double helix of two chains running in opposite directions, held together by hydrogen bonds between the base pairs. • The principle of complementarity requires that A always hydrogen bonds to T and C to G. This is the basis for replication and information transfer. Copyright W. H. Freeman and Company · New York Summarizing the Main Ideas • The linear sequence of bases in DNA carries information in the form of genes. • Genes encode the sequences of proteins, which are involve in all life functions. • Information in DNA is transcribed into a single complementary mRNA molecule. • Triplets of bases on the mRNA called codons specify amino acids. • Translation of mRNA base sequence information into protein amino acid sequences requires ribosomes and tRNA’s. • Each tRNA has an anticodon which is complementary to a codon on the mRNA at one end and an amino acid at the other end. • The ribosome matches codons on the mRNA to anticodons on tRNA molecules one codon at a time and extends the growing polypeptide chain. Copyright W. H. Freeman and Company · New York Summarizing the Main Ideas • Proteins are polyamides produced via condensation reactions of amino acids. • Alpha amino acids have a –COOH and a –NH2 group bonded to a central –CHR- group. • Short polymers of amino acids are called peptides; longer ones are called proteins. • Some conjugated proteins contain groups other than amino acids. • The primary structure of a protein is its sequence of amino acids. • The secondary structure of a protein is the shape adopted the polymer’s backbone. Examples are the alpha helix and the beta pleated sheet. • The tertiary structure of a protein is it overall folding. • The quaternary structure of a protein is its arrangement of subunits relative to each other. Copyright W. H. Freeman and Company · New York Summarizing the Main Ideas • The unfolding of a protein from its native form is called denaturation. • Different proteins have different functions in the body. • Enzymes are proteins that catalyze chemical reactions. • A reactant molecule, the substrate, fits into the active site of an enzyme. • During the reaction, partial bonds form between the substrate and the enzyme which lowers the amount of energy required before the substrate can react. • Lipoproteins are a collection of protein molecules surrounding a lipid particle. They provide a mechanism for the transport of insoluble hydrophobic molecules in the bloodstream. • Mutations are changes in the DNA sequence coding for particular proteins. Copyright W. H. Freeman and Company · New York Summarizing the Main Ideas • Mutations can alter the shape and function of protein molecules. • Silent mutations do not alter overall protein function. • Insertions or deletions may affect protein function minimally or may render it inactive. • An mutation rendering a protein inactive is lethal if the protein is required for life. • Mutations are the basis for many diseases. • Understanding of the genetic code has made cloning and DNA analysis possible. • Cloning uses genetic material to make exact copies of organisms. • DNA analysis allows forensic investigators o use DNA from biological samples to identify, convict, or exonerate individuals. Copyright W. H. Freeman and Company · New York