1. Lesson Overview 12.1 Identifying the Substance of Genes

2. Lesson Overview Lesson Overview Identifying the Substance of Genes THINK ABOUT IT How do genes work? To answer that question, the first thing you need to know is what genes are made of. How would you go about figuring out what molecule or molecules go into making a gene?

3. Lesson Overview Lesson Overview Identifying the Substance of Genes The Role of DNA What is the role of DNA in heredity?

4. Lesson Overview Lesson Overview Identifying the Substance of Genes The Role of DNA What is the role of DNA in heredity? The DNA that makes up genes must be capable of storing, copying, and transmitting the genetic information in a cell.

5. Lesson Overview Lesson Overview Identifying the Substance of Genes The Role of DNA The DNA that makes up genes must be capable of storing, copying, and transmitting the genetic information in a cell. These three functions are analogous to the way in which you might share a treasured book, as pictured in the figure.

6. Lesson Overview Lesson Overview Identifying the Substance of Genes Storing Information The foremost job of DNA, as the molecule of heredity, is to store information. Genes control patterns of development, which means that the instructions that cause a single cell to develop into an oak tree, a sea urchin, or a dog must somehow be written into the DNA of each of these organisms.

7. Lesson Overview Lesson Overview Identifying the Substance of Genes Copying Information Before a cell divides, it must make a complete copy of every one of its genes, similar to the way that a book is copied.

8. Lesson Overview Lesson Overview Identifying the Substance of Genes Copying Information To many scientists, the most puzzling aspect of DNA was how it could be copied. Once the structure of the DNA molecule was discovered, a copying mechanism for the genetic material was soon put forward.

9. Lesson Overview Lesson Overview Identifying the Substance of Genes Transmitting Information When a cell divides, each daughter cell must receive a complete copy of the genetic information. Careful sorting is especially important during the formation of reproductive cells in meiosis. The loss of any DNA during meiosis might mean a loss of valuable genetic information from one generation to the next.

10. Lesson Overview 12.2 The Structure of DNA

11. Lesson Overview Lesson Overview Identifying the Substance of Genes THINK ABOUT IT The DNA molecule must somehow specify how to assemble proteins, which are needed to regulate the various functions of each cell. What kind of structure could serve this purpose without varying from cell to cell? Understanding the structure of DNA has been the key to understanding how genes work.

12. Lesson Overview Lesson Overview Identifying the Substance of Genes The Components of DNA What are the chemical components of DNA?

13. Lesson Overview Lesson Overview Identifying the Substance of Genes The Components of DNA What are the chemical components of DNA? DNA is a nucleic acid made up of nucleotides joined into long strands or chains by covalent bonds.

14. Lesson Overview Lesson Overview Identifying the Substance of Genes Nucleic Acids and Nucleotides Nucleic acids are long, slightly acidic molecules originally identified in cell nuclei. Nucleic acids are made up of nucleotides, linked together to form long chains. The nucleotides that make up DNA are shown.

15. Lesson Overview Lesson Overview Identifying the Substance of Genes Nucleic Acids and Nucleotides DNA’s nucleotides are made up of three basic components: a 5-carbon sugar called deoxyribose, a phosphate group, and a nitrogenous base.

16. Lesson Overview Lesson Overview Identifying the Substance of Genes Nitrogenous Bases and Covalent Bonds The nucleotides in a strand of DNA are joined by covalent bonds formed between their sugar and phosphate groups.

17. Lesson Overview Lesson Overview Identifying the Substance of Genes Nitrogenous Bases and Covalent Bonds DNA has four kinds of nitrogenous bases: adenine (A), guanine (G), cytosine (C), and thymine (T). The nitrogenous bases stick out sideways from the nucleotide chain.

18. Lesson Overview Lesson Overview Identifying the Substance of Genes Nitrogenous Bases and Covalent Bonds The nucleotides can be joined together in any order, meaning that any sequence of bases is possible.

19. Lesson Overview Lesson Overview Identifying the Substance of Genes Solving the Structure of DNA What clues helped scientists solve the structure of DNA?

20. Lesson Overview Lesson Overview Identifying the Substance of Genes Solving the Structure of DNA What clues helped scientists solve the structure of DNA? The clues in Franklin’s X-ray pattern enabled Watson and Crick to build a model that explained the specific structure and properties of DNA.

21. Lesson Overview Lesson Overview Identifying the Substance of Genes Chargaff’s Rules Erwin Chargaff discovered that the percentages of adenine [A] and thymine [T] bases are almost equal in any sample of DNA. The same thing is true for the other two nucleotides, guanine [G] and cytosine [C]. The observation that [A] = [T] and [G] = [C] became known as one of “Chargaff’s rules.”

22. Lesson Overview Lesson Overview Identifying the Substance of Genes Franklin’s X-Rays In the 1950s, British scientist Rosalind Franklin used a technique called X-ray diffraction to get information about the structure of the DNA molecule.

23. Lesson Overview Lesson Overview Identifying the Substance of Genes Franklin’s X-Rays X-ray diffraction revealed an X-shaped pattern showing that the strands in DNA are twisted around each other like the coils of a spring. The angle of the X-shaped pattern suggested that there are two strands in the structure. Other clues suggest that the nitrogenous bases are near the center of the DNA molecule.

24. Lesson Overview Lesson Overview Identifying the Substance of Genes The Work of Watson and Crick At the same time, James Watson, an American biologist, and Francis Crick, a British physicist, were also trying to understand the structure of DNA. They built three-dimensional models of the molecule.

25. Lesson Overview Lesson Overview Identifying the Substance of Genes The Work of Watson and Crick Early in 1953, Watson was shown a copy of Franklin’s X-ray pattern. The clues in Franklin’s X-ray pattern enabled Watson and Crick to build a model that explained the specific structure and properties of DNA.

26. Lesson Overview Lesson Overview Identifying the Substance of Genes The Work of Watson and Crick Watson and Crick’s breakthrough model of DNA was a double helix, in which two strands were wound around each other.

27. Lesson Overview Lesson Overview Identifying the Substance of Genes The Double-Helix Model What does the double-helix model tell us about DNA?

28. Lesson Overview Lesson Overview Identifying the Substance of Genes The Double-Helix Model What does the double-helix model tell us about DNA? The double-helix model explains Chargaff’s rule of base pairing and how the two strands of DNA are held together.

29. Lesson Overview Lesson Overview Identifying the Substance of Genes The Double-Helix Model A double helix looks like a twisted ladder. In the double-helix model of DNA, the two strands twist around each other like spiral staircases. The double helix accounted for Franklin’s X-ray pattern and explains Chargaff’s rule of base pairing and how the two strands of DNA are held together.

30. Lesson Overview Lesson Overview Identifying the Substance of Genes Antiparallel Strands In the double-helix model, the two strands of DNA are “antiparallel”— they run in opposite directions. This arrangement enables the nitrogenous bases on both strands to come into contact at the center of the molecule. It also allows each strand of the double helix to carry a sequence of nucleotides, arranged almost like letters in a four-letter alphabet.

31. Lesson Overview Lesson Overview Identifying the Substance of Genes Hydrogen Bonding Watson and Crick discovered that hydrogen bonds could form between certain nitrogenous bases, providing just enough force to hold the two DNA strands together. Hydrogen bonds are relatively weak chemical forces that allow the two strands of the helix to separate. The ability of the two strands to separate is critical to DNA’s functions.

32. Lesson Overview Lesson Overview Identifying the Substance of Genes Base Pairing Watson and Crick’s model showed that hydrogen bonds could create a nearly perfect fit between nitrogenous bases along the center of the molecule. These bonds would form only between certain base pairs— adenine with thymine, and guanine with cytosine. This nearly perfect fit between A–T and G–C nucleotides is known as base pairing, and is illustrated in the figure.

33. Lesson Overview Lesson Overview Identifying the Substance of Genes Base Pairing Watson and Crick realized that base pairing explained Chargaff’s rule. It gave a reason why [A] = [T] and [G] = [C]. For every adenine in a double- stranded DNA molecule, there had to be exactly one thymine. For each cytosine, there was one guanine.

34. Lesson Overview 12.3 DNA Replication

35. Lesson Overview Lesson Overview Identifying the Substance of Genes THINK ABOUT IT Before a cell divides, its DNA must first be copied. How might the double-helix structure of DNA make that possible?

36. Lesson Overview Lesson Overview Identifying the Substance of Genes Copying the Code What role does DNA polymerase play in copying DNA?

37. Lesson Overview Lesson Overview Identifying the Substance of Genes Copying the Code What role does DNA polymerase play in copying DNA? DNA polymerase is an enzyme that joins individual nucleotides to produce a new strand of DNA.

38. Lesson Overview Lesson Overview Identifying the Substance of Genes Copying the Code Base pairing in the double helix explained how DNA could be copied, or replicated, because each base on one strand pairs with only one base on the opposite strand. Each strand of the double helix has all the information needed to reconstruct the other half by the mechanism of base pairing. Because each strand can be used to make the other strand, the strands are said to be complementary.

39. Lesson Overview Lesson Overview Identifying the Substance of Genes The Replication Process Before a cell divides, it duplicates its DNA in a copying process called replication. This process ensures that each resulting cell has the same complete set of DNA molecules.

41. Lesson Overview Lesson Overview Identifying the Substance of Genes The Replication Process During replication, the DNA molecule separates into two strands and then produces two new complementary strands following the rules of base pairing. Each strand of the double helix of DNA serves as a template, or model, for the new strand.

42. Lesson Overview Lesson Overview Identifying the Substance of Genes The two strands of the double helix separate, or “unzip,” allowing two replication forks to form. The Replication Process

43. Lesson Overview Lesson Overview Identifying the Substance of Genes The Replication Process As each new strand forms, new bases are added following the rules of base pairing. If the base on the old strand is adenine, then thymine is added to the newly forming strand. Likewise, guanine is always paired to cytosine.

44. Lesson Overview Lesson Overview Identifying the Substance of Genes The result of replication is two DNA molecules identical to each other and to the original molecule. Each DNA molecule resulting from replication has one original strand and one new strand. The Replication Process

45. Lesson Overview Lesson Overview Identifying the Substance of Genes The Role of Enzymes DNA replication is carried out by a series of enzymes. They first “unzip” a molecule of DNA by breaking the hydrogen bonds between base pairs and unwinding the two strands of the molecule. Each strand then serves as a template for the attachment of complementary bases.

46. Lesson Overview Lesson Overview Identifying the Substance of Genes The Role of Enzymes The principal enzyme involved in DNA replication is called DNA polymerase. DNA polymerase is an enzyme that joins individual nucleotides to produce a new strand of DNA. DNA polymerase also “proofreads” each new DNA strand, ensuring that each molecule is a perfect copy of the original.

48. Lesson Overview 13.1 RNA

49. Lesson Overview Lesson Overview Identifying the Substance of Genes THINK ABOUT IT DNA is the genetic material of cells. The sequence of nucleotide bases in the strands of DNA carries some sort of code. In order for that code to work, the cell must be able to understand it. What, exactly, do those bases code for? Where is the cell’s decoding system?

51. Lesson Overview Lesson Overview Identifying the Substance of Genes The Role of RNA How does RNA differ from DNA?

52. Lesson Overview Lesson Overview Identifying the Substance of Genes The Role of RNA How does RNA differ from DNA? There are three important differences between RNA and DNA: (1) the sugar in RNA is ribose instead of deoxyribose, (2) RNA is generally single- stranded and not double-stranded, and (3) RNA contains uracil in place of thymine.

53. Lesson Overview Lesson Overview Identifying the Substance of Genes The Role of RNA Genes contain coded DNA instructions that tell cells how to build proteins. The first step in decoding these genetic instructions is to copy part of the base sequence from DNA into RNA. RNA, like DNA, is a nucleic acid that consists of a long chain of nucleotides. RNA then uses the base sequence copied from DNA to direct the production of proteins.

54. Lesson Overview Lesson Overview Identifying the Substance of Genes Comparing RNA and DNA Each nucleotide in both DNA and RNA is made up of a 5-carbon sugar, a phosphate group, and a nitrogenous base. There are three important differences between RNA and DNA: (1) The sugar in RNA is ribose instead of deoxyribose. (2) RNA is generally single-stranded and not double-stranded. (3) RNA contains uracil in place of thymine. These chemical differences make it easy for the enzymes in the cell to tell DNA and RNA apart.

55. Lesson Overview Lesson Overview Identifying the Substance of Genes Comparing RNA and DNA The roles played by DNA and RNA are similar to the master plans and blueprints used by builders.

56. Lesson Overview Lesson Overview Identifying the Substance of Genes Comparing RNA and DNA A master plan has all the information needed to construct a building. Builders never bring a valuable master plan to the building site, where it might be damaged or lost. Instead, they prepare inexpensive, disposable copies of the master plan called blueprints.

57. Lesson Overview Lesson Overview Identifying the Substance of Genes Comparing RNA and DNA Similarly, the cell uses DNA “master plan” to prepare RNA “blueprints.” The DNA molecule stays safely in the cell’s nucleus, while RNA molecules go to the protein-building sites in the cytoplasm—the ribosomes.

58. Lesson Overview Lesson Overview Identifying the Substance of Genes Functions of RNA You can think of an RNA molecule, as a disposable copy of a segment of DNA, a working copy of a single gene. RNA has many functions, but most RNA molecules are involved in protein synthesis only. RNA controls the assembly of amino acids into proteins. Each type of RNA molecule specializes in a different aspect of this job.

59. Lesson Overview Lesson Overview Identifying the Substance of Genes Functions of RNA The three main types of RNA are messenger RNA, ribosomal RNA, and transfer RNA.

60. Lesson Overview Lesson Overview Identifying the Substance of Genes Messenger RNA Most genes contain instructions for assembling amino acids into proteins. The RNA molecules that carry copies of these instructions are known as messenger RNA (mRNA): They carry information from DNA to other parts of the cell.

61. Lesson Overview Lesson Overview Identifying the Substance of Genes Ribosomal RNA Proteins are assembled on ribosomes, small organelles composed of two subunits. These ribosome subunits are made up of several ribosomal RNA (rRNA) molecules and as many as 80 different proteins.

62. Lesson Overview Lesson Overview Identifying the Substance of Genes Transfer RNA When a protein is built, a transfer RNA (tRNA) molecule transfers each amino acid to the ribosome as it is specified by the coded messages in mRNA.

63. Lesson Overview Lesson Overview Identifying the Substance of Genes RNA Synthesis How does the cell make RNA?

64. Lesson Overview Lesson Overview Identifying the Substance of Genes RNA Synthesis How does the cell make RNA? In transcription, segments of DNA serve as templates to produce complementary RNA molecules.

65. Lesson Overview Lesson Overview Identifying the Substance of Genes Transcription Most of the work of making RNA takes place during transcription. During transcription, segments of DNA serve as templates to produce complementary RNA molecules. The base sequences of the transcribed RNA complement the base sequences of the template DNA.

67. Lesson Overview Lesson Overview Identifying the Substance of Genes Transcription In prokaryotes, RNA synthesis and protein synthesis take place in the cytoplasm. In eukaryotes, RNA is produced in the cell’s nucleus and then moves to the cytoplasm to play a role in the production of proteins. Our focus will be on transcription in eukaryotic cells.

68. Lesson Overview Lesson Overview Identifying the Substance of Genes Transcription Transcription requires an enzyme, known as RNA polymerase, that is similar to DNA polymerase.

69. Lesson Overview Lesson Overview Identifying the Substance of Genes Transcription RNA polymerase binds to DNA during transcription and separates the DNA strands.

70. Lesson Overview Lesson Overview Identifying the Substance of Genes Transcription RNA polymerase then uses one strand of DNA as a template from which to assemble nucleotides into a complementary strand of RNA.

72. Lesson Overview 13.2 Ribosomes and Protein Synthesis

74. Lesson Overview Lesson Overview Identifying the Substance of Genes THINK ABOUT IT How would you build a system to read the messages that are coded in genes and transcribed into RNA? Would you read the bases one at a time, as if the code were a language with just four words—one word per base? Perhaps you would read them as individual letters that can be combined to spell longer words.

75. Lesson Overview Lesson Overview Identifying the Substance of Genes The Genetic Code What is the genetic code, and how is it read?

76. Lesson Overview Lesson Overview Identifying the Substance of Genes The Genetic Code What is the genetic code, and how is it read? The genetic code is read three “letters” at a time, so that each “word” is three bases long and corresponds to a single amino acid.

77. Lesson Overview Lesson Overview Identifying the Substance of Genes The Genetic Code The first step in decoding genetic messages is to transcribe a nucleotide base sequence from DNA to RNA. This transcribed information contains a code for making proteins.

78. Lesson Overview Lesson Overview Identifying the Substance of Genes The Genetic Code Proteins are made by joining amino acids together into long chains, called polypeptides. As many as 20 different amino acids are commonly found in polypeptides.

79. Lesson Overview Lesson Overview Identifying the Substance of Genes The Genetic Code The specific amino acids in a polypeptide, and the order in which they are joined, determine the properties of different proteins. The sequence of amino acids influences the shape of the protein, which in turn determines its function.

80. Lesson Overview Lesson Overview Identifying the Substance of Genes The Genetic Code RNA contains four different bases: adenine, cytosine, guanine, and uracil. These bases form a “language,” or genetic code, with just four “letters”: A, C, G, and U.

81. Lesson Overview Lesson Overview Identifying the Substance of Genes The Genetic Code Each three-letter “word” in mRNA is known as a codon. A codon consists of three consecutive bases that specify a single amino acid to be added to the polypeptide chain.

82. Lesson Overview Lesson Overview Identifying the Substance of Genes How to Read Codons Because there are four different bases in RNA, there are 64 possible three-base codons (4 × 4 × 4 = 64) in the genetic code. This circular table shows the amino acid to which each of the 64 codons corresponds. To read a codon, start at the middle of the circle and move outward.

83. Lesson Overview Lesson Overview Identifying the Substance of Genes How to Read Codons Most amino acids can be specified by more than one codon. For example, six different codons—UUA, UUG, CUU, CUC, CUA, and CUG—specify leucine. But only one codon—UGG— specifies the amino acid tryptophan.

84. Lesson Overview Lesson Overview Identifying the Substance of Genes Start and Stop Codons The genetic code has punctuation marks. The methionine codon AUG serves as the initiation, or “start,” codon for protein synthesis. Following the start codon, mRNA is read, three bases at a time, until it reaches one of three different “stop” codons, which end translation.

85. Lesson Overview Lesson Overview Identifying the Substance of Genes Translation What role does the ribosome play in assembling proteins?

86. Lesson Overview Lesson Overview Identifying the Substance of Genes Translation What role does the ribosome play in assembling proteins? Ribosomes use the sequence of codons in mRNA to assemble amino acids into polypeptide chains.

87. Lesson Overview Lesson Overview Identifying the Substance of Genes Translation The sequence of nucleotide bases in an mRNA molecule is a set of instructions that gives the order in which amino acids should be joined to produce a polypeptide. The forming of a protein requires the folding of one or more polypeptide chains. Ribosomes use the sequence of codons in mRNA to assemble amino acids into polypeptide chains. The decoding of an mRNA message into a protein is a process known as translation.

88. Lesson Overview Lesson Overview Identifying the Substance of Genes Steps in Translation Messenger RNA is transcribed in the nucleus and then enters the cytoplasm for translation.

89. Lesson Overview Lesson Overview Identifying the Substance of Genes Steps in Translation Translation begins when a ribosome attaches to an mRNA molecule in the cytoplasm. As the ribosome reads each codon of mRNA, it directs tRNA to bring the specified amino acid into the ribosome. One at a time, the ribosome then attaches each amino acid to the growing chain.

90. Lesson Overview Lesson Overview Identifying the Substance of Genes Steps in Translation Each tRNA molecule carries just one kind of amino acid. In addition, each tRNA molecule has three unpaired bases, collectively called the anticodon—which is complementary to one mRNA codon. The tRNA molecule for methionine has the anticodon UAC, which pairs with the methionine codon, AUG.

91. Lesson Overview Lesson Overview Identifying the Substance of Genes Steps in Translation The ribosome has a second binding site for a tRNA molecule for the next codon. If that next codon is UUC, a tRNA molecule with an AAG anticodon brings the amino acid phenylalanine into the ribosome.

92. Lesson Overview Lesson Overview Identifying the Substance of Genes Steps in Translation The ribosome helps form a peptide bond between the first and second amino acids— methionine and phenylalanine. At the same time, the bond holding the first tRNA molecule to its amino acid is broken.

93. Lesson Overview Lesson Overview Identifying the Substance of Genes Steps in Translation That tRNA then moves into a third binding site, from which it exits the ribosome. The ribosome then moves to the third codon, where tRNA brings it the amino acid specified by the third codon.

94. Lesson Overview Lesson Overview Identifying the Substance of Genes Steps in Translation The polypeptide chain continues to grow until the ribosome reaches a “stop” codon on the mRNA molecule. When the ribosome reaches a stop codon, it releases both the newly formed polypeptide and the mRNA molecule, completing the process of translation.

95. Lesson Overview Lesson Overview Identifying the Substance of Genes The Roles of tRNA and rRNA in Translation Ribosomes are composed of roughly 80 proteins and three or four different rRNA molecules. These rRNA molecules help hold ribosomal proteins in place and help locate the beginning of the mRNA message. They may even carry out the chemical reaction that joins amino acids together.

96. Lesson Overview Lesson Overview Identifying the Substance of Genes The Molecular Basis of Heredity What is the “central dogma” of molecular biology?

97. Lesson Overview Lesson Overview Identifying the Substance of Genes The Molecular Basis of Heredity What is the “central dogma” of molecular biology? The central dogma of molecular biology is that information is transferred from DNA to RNA to protein.

98. Lesson Overview Lesson Overview Identifying the Substance of Genes The Molecular Basis of Heredity Most genes contain instructions for assembling proteins.

99. Lesson Overview Lesson Overview Identifying the Substance of Genes The Molecular Basis of Heredity Many proteins are enzymes, which catalyze and regulate chemical reactions. A gene that codes for an enzyme to produce pigment can control the color of a flower. Another gene produces proteins that regulate patterns of tissue growth in a leaf. Yet another may trigger the female or male pattern of development in an embryo. Proteins are microscopic tools, each specifically designed to build or operate a component of a living cell.

100. Lesson Overview Lesson Overview Identifying the Substance of Genes The Molecular Basis of Heredity Molecular biology seeks to explain living organisms by studying them at the molecular level, using molecules like DNA and RNA. The central dogma of molecular biology is that information is transferred from DNA to RNA to protein. There are many exceptions to this “dogma,” but it serves as a useful generalization that helps explain how genes work.

101. Lesson Overview Lesson Overview Identifying the Substance of Genes The Molecular Basis of Heredity Gene expression is the way in which DNA, RNA, and proteins are involved in putting genetic information into action in living cells. DNA carries information for specifying the traits of an organism. The cell uses the sequence of bases in DNA as a template for making mRNA.

102. Lesson Overview Lesson Overview Identifying the Substance of Genes The Molecular Basis of Heredity The codons of mRNA specify the sequence of amino acids in a protein. Proteins, in turn, play a key role in producing an organism’s traits.

103. Lesson Overview Lesson Overview Identifying the Substance of Genes The Molecular Basis of Heredity One of the most interesting discoveries of molecular biology is the near- universal nature of the genetic code. Although some organisms show slight variations in the amino acids assigned to particular codons, the code is always read three bases at a time and in the same direction. Despite their enormous diversity in form and function, living organisms display remarkable unity at life’s most basic level, the molecular biology of the gene.

104. Lesson Overview 13.3 Mutations

105. Lesson Overview Lesson Overview Identifying the Substance of Genes THINK ABOUT IT The sequence of bases in DNA are like the letters of a coded message. What would happen if a few of those letters changed accidentally, altering the message? What effects would you predict such changes to have on genes and the polypeptides for which they code?

106. Lesson Overview Lesson Overview Identifying the Substance of Genes Types of Mutations What are mutations?

107. Lesson Overview Lesson Overview Identifying the Substance of Genes Types of Mutations What are mutations? Mutations are heritable changes in genetic information.

108. Lesson Overview Lesson Overview Identifying the Substance of Genes Types of Mutations Now and then cells make mistakes in copying their own DNA, inserting the wrong base or even skipping a base as a strand is put together. These variations are called mutations, from the Latin word mutare, meaning “to change.” Mutations are heritable changes in genetic information.

109. Lesson Overview Lesson Overview Identifying the Substance of Genes Types of Mutations All mutations fall into two basic categories: Those that produce changes in a single gene are known as gene mutations. Those that produce changes in whole chromosomes are known as chromosomal mutations.

110. Lesson Overview Lesson Overview Identifying the Substance of Genes Gene Mutations Mutations that involve changes in one or a few nucleotides are known as point mutations because they occur at a single point in the DNA sequence. They generally occur during replication. If a gene in one cell is altered, the alteration can be passed on to every cell that develops from the original one.

111. Lesson Overview Lesson Overview Identifying the Substance of Genes Gene Mutations Point mutations include substitutions, insertions, and deletions.

112. Lesson Overview Lesson Overview Identifying the Substance of Genes Substitutions In a substitution, one base is changed to a different base. Substitutions usually affect no more than a single amino acid, and sometimes they have no effect at all.

113. Lesson Overview Lesson Overview Identifying the Substance of Genes Substitutions In this example, the base cytosine is replaced by the base thymine, resulting in a change in the mRNA codon from CGU (arginine) to CAU (histidine). However, a change in the last base of the codon, from CGU to CGA for example, would still specify the amino acid arginine.

114. Lesson Overview Lesson Overview Identifying the Substance of Genes Insertions and Deletions Insertions and deletions are point mutations in which one base is inserted or removed from the DNA sequence. If a nucleotide is added or deleted, the bases are still read in groups of three, but now those groupings shift in every codon that follows the mutation.

115. Lesson Overview Lesson Overview Identifying the Substance of Genes Insertions and Deletions Insertions and deletions are also called frameshift mutations because they shift the “reading frame” of the genetic message. Frameshift mutations can change every amino acid that follows the point of the mutation and can alter a protein so much that it is unable to perform its normal functions.

116. Lesson Overview Lesson Overview Identifying the Substance of Genes Chromosomal Mutations Chromosomal mutations involve changes in the number or structure of chromosomes. These mutations can change the location of genes on chromosomes and can even change the number of copies of some genes. There are four types of chromosomal mutations: deletion, duplication, inversion, and translocation.

117. Lesson Overview Lesson Overview Identifying the Substance of Genes Chromosomal Mutations Deletion involves the loss of all or part of a chromosome.

118. Lesson Overview Lesson Overview Identifying the Substance of Genes Chromosomal Mutations Duplication produces an extra copy of all or part of a chromosome.

119. Lesson Overview Lesson Overview Identifying the Substance of Genes Chromosomal Mutations Inversion reverses the direction of parts of a chromosome.

120. Lesson Overview Lesson Overview Identifying the Substance of Genes Chromosomal Mutations Translocation occurs when part of one chromosome breaks off and attaches to another.

121. Lesson Overview Lesson Overview Identifying the Substance of Genes Effects of Mutations How do mutations affect genes?

122. Lesson Overview Lesson Overview Identifying the Substance of Genes Effects of Mutations How do mutations affect genes? The effects of mutations on genes vary widely. Some have little or no effect; and some produce beneficial variations. Some negatively disrupt gene function. Mutations often produce proteins with new or altered functions that can be useful to organisms in different or changing environments.

123. Lesson Overview Lesson Overview Identifying the Substance of Genes Effects of Mutations Genetic material can be altered by natural events or by artificial means. The resulting mutations may or may not affect an organism. Some mutations that affect individual organisms can also affect a species or even an entire ecosystem.

124. Lesson Overview Lesson Overview Identifying the Substance of Genes Effects of Mutations Many mutations are produced by errors in genetic processes. For example, some point mutations are caused by errors during DNA replication. The cellular machinery that replicates DNA inserts an incorrect base roughly once in every 10 million bases. Small changes in genes can gradually accumulate over time.

125. Lesson Overview Lesson Overview Identifying the Substance of Genes Effects of Mutations Stressful environmental conditions may cause some bacteria to increase mutation rates. This can actually be helpful to the organism, since mutations may sometimes give such bacteria new traits, such as the ability to consume a new food source or to resist a poison in the environment.

126. Lesson Overview Lesson Overview Identifying the Substance of Genes Mutagens Some mutations arise from mutagens, chemical or physical agents in the environment. Chemical mutagens include certain pesticides, a few natural plant alkaloids, tobacco smoke, and environmental pollutants. Physical mutagens include some forms of electromagnetic radiation, such as X-rays and ultraviolet light.

127. Lesson Overview Lesson Overview Identifying the Substance of Genes Mutagens If these mutagens interact with DNA, they can produce mutations at high rates. Some compounds interfere with base-pairing, increasing the error rate of DNA replication. Others weaken the DNA strand, causing breaks and inversions that produce chromosomal mutations. Cells can sometimes repair the damage; but when they cannot, the DNA base sequence changes permanently.

128. Lesson Overview Lesson Overview Identifying the Substance of Genes Harmful and Helpful Mutations The effects of mutations on genes vary widely. Some have little or no effect; and some produce beneficial variations. Some negatively disrupt gene function. Whether a mutation is negative or beneficial depends on how its DNA changes relative to the organism’s situation. Mutations are often thought of as negative because they disrupt the normal function of genes. However, without mutations, organisms cannot evolve, because mutations are the source of genetic variability in a species.

129. Lesson Overview Lesson Overview Identifying the Substance of Genes Harmful Effects Some of the most harmful mutations are those that dramatically change protein structure or gene activity. The defective proteins produced by these mutations can disrupt normal biological activities, and result in genetic disorders. Some cancers, for example, are the product of mutations that cause the uncontrolled growth of cells.

130. Lesson Overview Lesson Overview Identifying the Substance of Genes Harmful Effects Sickle cell disease is a disorder associated with changes in the shape of red blood cells. Normal red blood cells are round. Sickle cells appear long and pointed. Sickle cell disease is caused by a point mutation in one of the polypeptides found in hemoglobin, the blood’s principal oxygen- carrying protein. Among the symptoms of the disease are anemia, severe pain, frequent infections, and stunted growth.

131. Lesson Overview Lesson Overview Identifying the Substance of Genes Beneficial Effects Some of the variation produced by mutations can be highly advantageous to an organism or species. Mutations often produce proteins with new or altered functions that can be useful to organisms in different or changing environments. For example, mutations have helped many insects resist chemical pesticides. Some mutations have enabled microorganisms to adapt to new chemicals in the environment.

132. Lesson Overview Lesson Overview Identifying the Substance of Genes Beneficial Effects Plant and animal breeders often make use of “good” mutations. For example, when a complete set of chromosomes fails to separate during meiosis, the gametes that result may produce triploid (3N) or tetraploid (4N) organisms. The condition in which an organism has extra sets of chromosomes is called polyploidy.

133. Lesson Overview Lesson Overview Identifying the Substance of Genes Beneficial Effects Polyploid plants are often larger and stronger than diploid plants. Important crop plants—including bananas and limes—have been produced this way. Polyploidy also occurs naturally in citrus plants, often through spontaneous mutations.