The Genetic Code Quiz

PDF page 605: The flow of genetic information in cells from DNA to mRNA to protein is described by the Central Dogma (Figure 15.3), which states that genes specify the sequence of mRNAs, which in turn specify the sequence of proteins. The decoding of one molecule to another is performed by specific proteins and RNAs. Because the information stored in DNA is so central to cellular function, it makes intuitive sense that the cell would make mRNA copies of this information for protein synthesis, while keeping the DNA itself intact and protected. The copying of DNA to RNA is relatively straightforward, with one nucleotide being added to the mRNA strand for every nucleotide read in the DNA strand. The translation to protein is a bit more complex because three mRNA nucleotides correspond to one amino acid in the polypeptide sequence. However, the translation to protein is still systematic and colinear, such that nucleotides 1 to 3 correspond to amino acid 1, nucleotides 4 to 6 correspond to amino acid 2, and so on.

PDF page 603: Since the rediscovery of Mendel’s work in the 1900s, scientists have learned much about how the genetic blueprints stored in DNA are capable of replication, expression, and mutation. Just as the 26 letters of the English alphabet can be arranged into what seems to be a limitless number of words, with new ones added to the dictionary every year, the four nucleotides of DNA—A, T, C, and G—can generate sequences of DNA called genes that specify tens of thousands of polymers of amino acids. In turn, these sequences can be transcribed into mRNA and translated into proteins which orchestrate nearly every function of the cell. The genetic code refers to the DNA alphabet (A, T, C, G), the RNA alphabet (A, U, C, G), and the polypeptide alphabet (20 amino acids). But how do genes located on a chromosome ultimately produce a polypeptide that can result in a physical phenotype such as hair or eye color—or a disease like cystic fibrosis or hemophilia? The Central Dogma describes the normal flow of genetic information from DNA to mRNA to protein: DNA in genes specify sequences of mRNA which, in turn, specify amino acid sequences in proteins. The process requires two steps, transcription and translation. During transcription, genes are used to make messenger RNA (mRNA). In turn, the mRNA is used to direct the synthesis of proteins during the process of translation. Translation also requires two other types of RNA: transfer RNA (tRNA) and ribosomal RNA (rRNA). The genetic code is a triplet code, with each RNA codon consisting of three consecutive nucleotides that specify one amino acid or the release of the newly formed polypeptide chain; for example, the mRNA codon CAU specifies the amino acid histidine. The code is degenerate; that is, some amino acids are specified by more than one codon, like synonyms you study in your English class (different word, same meaning). For example, CCU, CCC, CCA, and CCG are all codons for proline. It is important to remember the same genetic code is universal to almost all organisms on Earth. Small variations in codon assignment exist in mitochondria and some microorganisms. Deviations from the simple scheme of the central dogma are discovered as researchers explore gene expression with new technology. For example the human immunodeficiency virus (HIV) is a retrovirus which stores its genetic information in single stranded RNA molecules. Upon infection of a host cell, RNA is used as a template by the virally encoded enzyme, reverse transcriptase, to synthesize DNA. The viral DNA is later transcribed into mRNA and translated into proteins. Some RNA viruses such as the influenza virus never go through a DNA step. The RNA genome is replicated by an RNA dependent RNA polymerase which is virally encoded.

PDF page 603: Since the rediscovery of Mendel’s work in the 1900s, scientists have learned much about how the genetic blueprints stored in DNA are capable of replication, expression, and mutation. Just as the 26 letters of the English alphabet can be arranged into what seems to be a limitless number of words, with new ones added to the dictionary every year, the four nucleotides of DNA—A, T, C, and G—can generate sequences of DNA called genes that specify tens of thousands of polymers of amino acids. In turn, these sequences can be transcribed into mRNA and translated into proteins which orchestrate nearly every function of the cell. The genetic code refers to the DNA alphabet (A, T, C, G), the RNA alphabet (A, U, C, G), and the polypeptide alphabet (20 amino acids). But how do genes located on a chromosome ultimately produce a polypeptide that can result in a physical phenotype such as hair or eye color—or a disease like cystic fibrosis or hemophilia? The Central Dogma describes the normal flow of genetic information from DNA to mRNA to protein: DNA in genes specify sequences of mRNA which, in turn, specify amino acid sequences in proteins. The process requires two steps, transcription and translation. During transcription, genes are used to make messenger RNA (mRNA). In turn, the mRNA is used to direct the synthesis of proteins during the process of translation. Translation also requires two other types of RNA: transfer RNA (tRNA) and ribosomal RNA (rRNA). The genetic code is a triplet code, with each RNA codon consisting of three consecutive nucleotides that specify one amino acid or the release of the newly formed polypeptide chain; for example, the mRNA codon CAU specifies the amino acid histidine. The code is degenerate; that is, some amino acids are specified by more than one codon, like synonyms you study in your English class (different word, same meaning). For example, CCU, CCC, CCA, and CCG are all codons for proline. It is important to remember the same genetic code is universal to almost all organisms on Earth. Small variations in codon assignment exist in mitochondria and some microorganisms. Deviations from the simple scheme of the central dogma are discovered as researchers explore gene expression with new technology. For example the human immunodeficiency virus (HIV) is a retrovirus which stores its genetic information in single stranded RNA molecules. Upon infection of a host cell, RNA is used as a template by the virally encoded enzyme, reverse transcriptase, to synthesize DNA. The viral DNA is later transcribed into mRNA and translated into proteins. Some RNA viruses such as the influenza virus never go through a DNA step. The RNA genome is replicated by an RNA dependent RNA polymerase which is virally encoded.

PDF page 605: The flow of genetic information in cells from DNA to mRNA to protein is described by the Central Dogma (Figure 15.3), which states that genes specify the sequence of mRNAs, which in turn specify the sequence of proteins. The decoding of one molecule to another is performed by specific proteins and RNAs. Because the information stored in DNA is so central to cellular function, it makes intuitive sense that the cell would make mRNA copies of this information for protein synthesis, while keeping the DNA itself intact and protected. The copying of DNA to RNA is relatively straightforward, with one nucleotide being added to the mRNA strand for every nucleotide read in the DNA strand. The translation to protein is a bit more complex because three mRNA nucleotides correspond to one amino acid in the polypeptide sequence. However, the translation to protein is still systematic and colinear, such that nucleotides 1 to 3 correspond to amino acid 1, nucleotides 4 to 6 correspond to amino acid 2, and so on.

PDF page 603: Since the rediscovery of Mendel’s work in the 1900s, scientists have learned much about how the genetic blueprints stored in DNA are capable of replication, expression, and mutation. Just as the 26 letters of the English alphabet can be arranged into what seems to be a limitless number of words, with new ones added to the dictionary every year, the four nucleotides of DNA—A, T, C, and G—can generate sequences of DNA called genes that specify tens of thousands of polymers of amino acids. In turn, these sequences can be transcribed into mRNA and translated into proteins which orchestrate nearly every function of the cell. The genetic code refers to the DNA alphabet (A, T, C, G), the RNA alphabet (A, U, C, G), and the polypeptide alphabet (20 amino acids). But how do genes located on a chromosome ultimately produce a polypeptide that can result in a physical phenotype such as hair or eye color—or a disease like cystic fibrosis or hemophilia? The Central Dogma describes the normal flow of genetic information from DNA to mRNA to protein: DNA in genes specify sequences of mRNA which, in turn, specify amino acid sequences in proteins. The process requires two steps, transcription and translation. During transcription, genes are used to make messenger RNA (mRNA). In turn, the mRNA is used to direct the synthesis of proteins during the process of translation. Translation also requires two other types of RNA: transfer RNA (tRNA) and ribosomal RNA (rRNA). The genetic code is a triplet code, with each RNA codon consisting of three consecutive nucleotides that specify one amino acid or the release of the newly formed polypeptide chain; for example, the mRNA codon CAU specifies the amino acid histidine. The code is degenerate; that is, some amino acids are specified by more than one codon, like synonyms you study in your English class (different word, same meaning). For example, CCU, CCC, CCA, and CCG are all codons for proline. It is important to remember the same genetic code is universal to almost all organisms on Earth. Small variations in codon assignment exist in mitochondria and some microorganisms. Deviations from the simple scheme of the central dogma are discovered as researchers explore gene expression with new technology. For example the human immunodeficiency virus (HIV) is a retrovirus which stores its genetic information in single stranded RNA molecules. Upon infection of a host cell, RNA is used as a template by the virally encoded enzyme, reverse transcriptase, to synthesize DNA. The viral DNA is later transcribed into mRNA and translated into proteins. Some RNA viruses such as the influenza virus never go through a DNA step. The RNA genome is replicated by an RNA dependent RNA polymerase which is virally encoded.

PDF page 603: Since the rediscovery of Mendel’s work in the 1900s, scientists have learned much about how the genetic blueprints stored in DNA are capable of replication, expression, and mutation. Just as the 26 letters of the English alphabet can be arranged into what seems to be a limitless number of words, with new ones added to the dictionary every year, the four nucleotides of DNA—A, T, C, and G—can generate sequences of DNA called genes that specify tens of thousands of polymers of amino acids. In turn, these sequences can be transcribed into mRNA and translated into proteins which orchestrate nearly every function of the cell. The genetic code refers to the DNA alphabet (A, T, C, G), the RNA alphabet (A, U, C, G), and the polypeptide alphabet (20 amino acids). But how do genes located on a chromosome ultimately produce a polypeptide that can result in a physical phenotype such as hair or eye color—or a disease like cystic fibrosis or hemophilia? The Central Dogma describes the normal flow of genetic information from DNA to mRNA to protein: DNA in genes specify sequences of mRNA which, in turn, specify amino acid sequences in proteins. The process requires two steps, transcription and translation. During transcription, genes are used to make messenger RNA (mRNA). In turn, the mRNA is used to direct the synthesis of proteins during the process of translation. Translation also requires two other types of RNA: transfer RNA (tRNA) and ribosomal RNA (rRNA). The genetic code is a triplet code, with each RNA codon consisting of three consecutive nucleotides that specify one amino acid or the release of the newly formed polypeptide chain; for example, the mRNA codon CAU specifies the amino acid histidine. The code is degenerate; that is, some amino acids are specified by more than one codon, like synonyms you study in your English class (different word, same meaning). For example, CCU, CCC, CCA, and CCG are all codons for proline. It is important to remember the same genetic code is universal to almost all organisms on Earth. Small variations in codon assignment exist in mitochondria and some microorganisms. Deviations from the simple scheme of the central dogma are discovered as researchers explore gene expression with new technology. For example the human immunodeficiency virus (HIV) is a retrovirus which stores its genetic information in single stranded RNA molecules. Upon infection of a host cell, RNA is used as a template by the virally encoded enzyme, reverse transcriptase, to synthesize DNA. The viral DNA is later transcribed into mRNA and translated into proteins. Some RNA viruses such as the influenza virus never go through a DNA step. The RNA genome is replicated by an RNA dependent RNA polymerase which is virally encoded.

PDF page 605: (4 ), which is far more than the number of amino acids. Scientists theorized that amino acids were encoded by nucleotide triplets and that the genetic code was degenerate. In other words, a given amino acid could be encoded by more than one nucleotide triplet. This was later confirmed experimentally; Francis Crick and Sydney Brenner used the chemical mutagen proflavin to insert one, two, or three nucleotides into the gene of a virus. When one or two nucleotides were inserted, protein synthesis was completely abolished. When three nucleotides were inserted, the protein was synthesized and functional. This

PDF page 603: Since the rediscovery of Mendel’s work in the 1900s, scientists have learned much about how the genetic blueprints stored in DNA are capable of replication, expression, and mutation. Just as the 26 letters of the English alphabet can be arranged into what seems to be a limitless number of words, with new ones added to the dictionary every year, the four nucleotides of DNA—A, T, C, and G—can generate sequences of DNA called genes that specify tens of thousands of polymers of amino acids. In turn, these sequences can be transcribed into mRNA and translated into proteins which orchestrate nearly every function of the cell. The genetic code refers to the DNA alphabet (A, T, C, G), the RNA alphabet (A, U, C, G), and the polypeptide alphabet (20 amino acids). But how do genes located on a chromosome ultimately produce a polypeptide that can result in a physical phenotype such as hair or eye color—or a disease like cystic fibrosis or hemophilia? The Central Dogma describes the normal flow of genetic information from DNA to mRNA to protein: DNA in genes specify sequences of mRNA which, in turn, specify amino acid sequences in proteins. The process requires two steps, transcription and translation. During transcription, genes are used to make messenger RNA (mRNA). In turn, the mRNA is used to direct the synthesis of proteins during the process of translation. Translation also requires two other types of RNA: transfer RNA (tRNA) and ribosomal RNA (rRNA). The genetic code is a triplet code, with each RNA codon consisting of three consecutive nucleotides that specify one amino acid or the release of the newly formed polypeptide chain; for example, the mRNA codon CAU specifies the amino acid histidine. The code is degenerate; that is, some amino acids are specified by more than one codon, like synonyms you study in your English class (different word, same meaning). For example, CCU, CCC, CCA, and CCG are all codons for proline. It is important to remember the same genetic code is universal to almost all organisms on Earth. Small variations in codon assignment exist in mitochondria and some microorganisms. Deviations from the simple scheme of the central dogma are discovered as researchers explore gene expression with new technology. For example the human immunodeficiency virus (HIV) is a retrovirus which stores its genetic information in single stranded RNA molecules. Upon infection of a host cell, RNA is used as a template by the virally encoded enzyme, reverse transcriptase, to synthesize DNA. The viral DNA is later transcribed into mRNA and translated into proteins. Some RNA viruses such as the influenza virus never go through a DNA step. The RNA genome is replicated by an RNA dependent RNA polymerase which is virally encoded.

PDF page 603: Since the rediscovery of Mendel’s work in the 1900s, scientists have learned much about how the genetic blueprints stored in DNA are capable of replication, expression, and mutation. Just as the 26 letters of the English alphabet can be arranged into what seems to be a limitless number of words, with new ones added to the dictionary every year, the four nucleotides of DNA—A, T, C, and G—can generate sequences of DNA called genes that specify tens of thousands of polymers of amino acids. In turn, these sequences can be transcribed into mRNA and translated into proteins which orchestrate nearly every function of the cell. The genetic code refers to the DNA alphabet (A, T, C, G), the RNA alphabet (A, U, C, G), and the polypeptide alphabet (20 amino acids). But how do genes located on a chromosome ultimately produce a polypeptide that can result in a physical phenotype such as hair or eye color—or a disease like cystic fibrosis or hemophilia? The Central Dogma describes the normal flow of genetic information from DNA to mRNA to protein: DNA in genes specify sequences of mRNA which, in turn, specify amino acid sequences in proteins. The process requires two steps, transcription and translation. During transcription, genes are used to make messenger RNA (mRNA). In turn, the mRNA is used to direct the synthesis of proteins during the process of translation. Translation also requires two other types of RNA: transfer RNA (tRNA) and ribosomal RNA (rRNA). The genetic code is a triplet code, with each RNA codon consisting of three consecutive nucleotides that specify one amino acid or the release of the newly formed polypeptide chain; for example, the mRNA codon CAU specifies the amino acid histidine. The code is degenerate; that is, some amino acids are specified by more than one codon, like synonyms you study in your English class (different word, same meaning). For example, CCU, CCC, CCA, and CCG are all codons for proline. It is important to remember the same genetic code is universal to almost all organisms on Earth. Small variations in codon assignment exist in mitochondria and some microorganisms. Deviations from the simple scheme of the central dogma are discovered as researchers explore gene expression with new technology. For example the human immunodeficiency virus (HIV) is a retrovirus which stores its genetic information in single stranded RNA molecules. Upon infection of a host cell, RNA is used as a template by the virally encoded enzyme, reverse transcriptase, to synthesize DNA. The viral DNA is later transcribed into mRNA and translated into proteins. Some RNA viruses such as the influenza virus never go through a DNA step. The RNA genome is replicated by an RNA dependent RNA polymerase which is virally encoded.

PDF page 603: Since the rediscovery of Mendel’s work in the 1900s, scientists have learned much about how the genetic blueprints stored in DNA are capable of replication, expression, and mutation. Just as the 26 letters of the English alphabet can be arranged into what seems to be a limitless number of words, with new ones added to the dictionary every year, the four nucleotides of DNA—A, T, C, and G—can generate sequences of DNA called genes that specify tens of thousands of polymers of amino acids. In turn, these sequences can be transcribed into mRNA and translated into proteins which orchestrate nearly every function of the cell. The genetic code refers to the DNA alphabet (A, T, C, G), the RNA alphabet (A, U, C, G), and the polypeptide alphabet (20 amino acids). But how do genes located on a chromosome ultimately produce a polypeptide that can result in a physical phenotype such as hair or eye color—or a disease like cystic fibrosis or hemophilia? The Central Dogma describes the normal flow of genetic information from DNA to mRNA to protein: DNA in genes specify sequences of mRNA which, in turn, specify amino acid sequences in proteins. The process requires two steps, transcription and translation. During transcription, genes are used to make messenger RNA (mRNA). In turn, the mRNA is used to direct the synthesis of proteins during the process of translation. Translation also requires two other types of RNA: transfer RNA (tRNA) and ribosomal RNA (rRNA). The genetic code is a triplet code, with each RNA codon consisting of three consecutive nucleotides that specify one amino acid or the release of the newly formed polypeptide chain; for example, the mRNA codon CAU specifies the amino acid histidine. The code is degenerate; that is, some amino acids are specified by more than one codon, like synonyms you study in your English class (different word, same meaning). For example, CCU, CCC, CCA, and CCG are all codons for proline. It is important to remember the same genetic code is universal to almost all organisms on Earth. Small variations in codon assignment exist in mitochondria and some microorganisms. Deviations from the simple scheme of the central dogma are discovered as researchers explore gene expression with new technology. For example the human immunodeficiency virus (HIV) is a retrovirus which stores its genetic information in single stranded RNA molecules. Upon infection of a host cell, RNA is used as a template by the virally encoded enzyme, reverse transcriptase, to synthesize DNA. The viral DNA is later transcribed into mRNA and translated into proteins. Some RNA viruses such as the influenza virus never go through a DNA step. The RNA genome is replicated by an RNA dependent RNA polymerase which is virally encoded.

PDF page 604: The Science Practice Challenge Questions contain additional test questions for this section that will help you prepare for the AP exam. These questions address the following standards: [APLO 3.4][APLO 3.25] The cellular process of transcription generates messenger RNA (mRNA), a mobile molecular copy of one or more genes with an alphabet of A, C, G, and uracil (U). Translation of the mRNA template converts nucleotide-based genetic information into a protein product. Protein sequences consist of 20 commonly occurring amino acids; therefore, it can be said that the protein alphabet consists of 20 letters (Figure 15.2). Each amino acid is defined by a three-nucleotide sequence called the triplet codon. Different amino acids have different chemistries (such as acidic versus basic, or polar and nonpolar) and different structural constraints. Variation in amino acid sequence gives rise to enormous variation in protein structure and function.

PDF page 604: The Science Practice Challenge Questions contain additional test questions for this section that will help you prepare for the AP exam. These questions address the following standards: [APLO 3.4][APLO 3.25] The cellular process of transcription generates messenger RNA (mRNA), a mobile molecular copy of one or more genes with an alphabet of A, C, G, and uracil (U). Translation of the mRNA template converts nucleotide-based genetic information into a protein product. Protein sequences consist of 20 commonly occurring amino acids; therefore, it can be said that the protein alphabet consists of 20 letters (Figure 15.2). Each amino acid is defined by a three-nucleotide sequence called the triplet codon. Different amino acids have different chemistries (such as acidic versus basic, or polar and nonpolar) and different structural constraints. Variation in amino acid sequence gives rise to enormous variation in protein structure and function.

PDF page 603: Since the rediscovery of Mendel’s work in the 1900s, scientists have learned much about how the genetic blueprints stored in DNA are capable of replication, expression, and mutation. Just as the 26 letters of the English alphabet can be arranged into what seems to be a limitless number of words, with new ones added to the dictionary every year, the four nucleotides of DNA—A, T, C, and G—can generate sequences of DNA called genes that specify tens of thousands of polymers of amino acids. In turn, these sequences can be transcribed into mRNA and translated into proteins which orchestrate nearly every function of the cell. The genetic code refers to the DNA alphabet (A, T, C, G), the RNA alphabet (A, U, C, G), and the polypeptide alphabet (20 amino acids). But how do genes located on a chromosome ultimately produce a polypeptide that can result in a physical phenotype such as hair or eye color—or a disease like cystic fibrosis or hemophilia? The Central Dogma describes the normal flow of genetic information from DNA to mRNA to protein: DNA in genes specify sequences of mRNA which, in turn, specify amino acid sequences in proteins. The process requires two steps, transcription and translation. During transcription, genes are used to make messenger RNA (mRNA). In turn, the mRNA is used to direct the synthesis of proteins during the process of translation. Translation also requires two other types of RNA: transfer RNA (tRNA) and ribosomal RNA (rRNA). The genetic code is a triplet code, with each RNA codon consisting of three consecutive nucleotides that specify one amino acid or the release of the newly formed polypeptide chain; for example, the mRNA codon CAU specifies the amino acid histidine. The code is degenerate; that is, some amino acids are specified by more than one codon, like synonyms you study in your English class (different word, same meaning). For example, CCU, CCC, CCA, and CCG are all codons for proline. It is important to remember the same genetic code is universal to almost all organisms on Earth. Small variations in codon assignment exist in mitochondria and some microorganisms. Deviations from the simple scheme of the central dogma are discovered as researchers explore gene expression with new technology. For example the human immunodeficiency virus (HIV) is a retrovirus which stores its genetic information in single stranded RNA molecules. Upon infection of a host cell, RNA is used as a template by the virally encoded enzyme, reverse transcriptase, to synthesize DNA. The viral DNA is later transcribed into mRNA and translated into proteins. Some RNA viruses such as the influenza virus never go through a DNA step. The RNA genome is replicated by an RNA dependent RNA polymerase which is virally encoded.

PDF page 603: Since the rediscovery of Mendel’s work in the 1900s, scientists have learned much about how the genetic blueprints stored in DNA are capable of replication, expression, and mutation. Just as the 26 letters of the English alphabet can be arranged into what seems to be a limitless number of words, with new ones added to the dictionary every year, the four nucleotides of DNA—A, T, C, and G—can generate sequences of DNA called genes that specify tens of thousands of polymers of amino acids. In turn, these sequences can be transcribed into mRNA and translated into proteins which orchestrate nearly every function of the cell. The genetic code refers to the DNA alphabet (A, T, C, G), the RNA alphabet (A, U, C, G), and the polypeptide alphabet (20 amino acids). But how do genes located on a chromosome ultimately produce a polypeptide that can result in a physical phenotype such as hair or eye color—or a disease like cystic fibrosis or hemophilia? The Central Dogma describes the normal flow of genetic information from DNA to mRNA to protein: DNA in genes specify sequences of mRNA which, in turn, specify amino acid sequences in proteins. The process requires two steps, transcription and translation. During transcription, genes are used to make messenger RNA (mRNA). In turn, the mRNA is used to direct the synthesis of proteins during the process of translation. Translation also requires two other types of RNA: transfer RNA (tRNA) and ribosomal RNA (rRNA). The genetic code is a triplet code, with each RNA codon consisting of three consecutive nucleotides that specify one amino acid or the release of the newly formed polypeptide chain; for example, the mRNA codon CAU specifies the amino acid histidine. The code is degenerate; that is, some amino acids are specified by more than one codon, like synonyms you study in your English class (different word, same meaning). For example, CCU, CCC, CCA, and CCG are all codons for proline. It is important to remember the same genetic code is universal to almost all organisms on Earth. Small variations in codon assignment exist in mitochondria and some microorganisms. Deviations from the simple scheme of the central dogma are discovered as researchers explore gene expression with new technology. For example the human immunodeficiency virus (HIV) is a retrovirus which stores its genetic information in single stranded RNA molecules. Upon infection of a host cell, RNA is used as a template by the virally encoded enzyme, reverse transcriptase, to synthesize DNA. The viral DNA is later transcribed into mRNA and translated into proteins. Some RNA viruses such as the influenza virus never go through a DNA step. The RNA genome is replicated by an RNA dependent RNA polymerase which is virally encoded.

PDF page 605: because there are only 16 possible two-nucleotide combinations (4 ). In contrast, there are 64 possible nucleotide triplets

PDF page 605: (4 ), which is far more than the number of amino acids. Scientists theorized that amino acids were encoded by nucleotide triplets and that the genetic code was degenerate. In other words, a given amino acid could be encoded by more than one nucleotide triplet. This was later confirmed experimentally; Francis Crick and Sydney Brenner used the chemical mutagen proflavin to insert one, two, or three nucleotides into the gene of a virus. When one or two nucleotides were inserted, protein synthesis was completely abolished. When three nucleotides were inserted, the protein was synthesized and functional. This

PDF page 605: because there are only 16 possible two-nucleotide combinations (4 ). In contrast, there are 64 possible nucleotide triplets