Chapter 26 Nucleic Acids and Protein Synthesis
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Nucleic Acids Nucleic acids are: Molecules that store information for cellular growth and reproduction. Deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). Large molecules consisting of long chains of monomers called nucleotides.
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Nucleic Acids
Nucleic acids are: Made from Nucleotides Nucleotides are made from Nucleosides Nucleosides are made from Nitrogen bases and pentose (five-carbon) sugar.
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Nucleosides and nucleotides
HO
Nucleoside
Nucleotide 4
The nucleotides adenosine 5´-monophosphate (AMP) and deoxycytidine 5´-monophosphate (dCMP) are shown below. Nucleotides are formed from nucleosides by
1. 2. 3. 4.
addition of a heterocyclic nitrogen base to a sugar. changing the heterocyclic nitrogen base. changing the monsaccharide. conversion of an alcohol functional group on the monosaccharide to a phosphate ester. 5
The nucleotides adenosine 5´-monophosphate (AMP) and deoxycytidine 5´-monophosphate (dCMP) are shown below. Nucleotides are formed from nucleosides by
1. 2. 3. 4.
addition of a heterocyclic nitrogen base to a sugar. changing the heterocyclic nitrogen base. changing the monsaccharide. conversion of an alcohol functional group on the monosaccharide to a phosphate ester. 6
Nitrogen Bases
The nitrogen bases in nucleic acids consist of the: Pyrimidines C, T, and U And purines A and G.
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The hetereocyclic nitrogen base in the nucleotide shown below is the
1. 2. 3.
4.
purine base adenine. purine base cytosine. pyrimidine base adenine. pyrimidine base cytosine.
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The hetereocyclic nitrogen base in the nucleotide shown below is the
1. 2. 3.
4.
purine base adenine. purine base cytosine. pyrimidine base adenine. pyrimidine base cytosine.
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Pentose Sugars
The pentose (fivecarbon) sugar: In RNA is ribose. In DNA is deoxyribose. Has carbon atoms numbered with primes to distinguish them from the nitrogen bases. 10
Nucleosides A nucleoside: Has a nitrogen base linked by a glycosidic bond to C1’ of a ribose or deoxyribose. Is named by changing the nitrogen base ending to -osine for purines and –idine for pyrimidines
HO
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Nucleotides A nucleotide: Is a nucleoside that forms a phosphate ester with the C5’ OH group of ribose or deoxyribose. Is named using the name of the nucleoside followed by 5’-monophosphate. 12
Nucleic acids are polynucleotides. Nucleic acids are classified as either DNA or RNA. The nucleotide shown below might be found in
1. 2.
3. 4.
either DNA or RNA. neither DNA nor RNA. only DNA. only RNA.
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Nucleic acids are polynucleotides. Nucleic acids are classified as either DNA or RNA. The nucleotide shown below might be found in
1. 2.
3. 4.
either DNA or RNA. neither DNA nor RNA. only DNA. only RNA.
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Names of Nucleosides and Nucleotides
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DNA and RNA differ in the nature of the monsaccharide unit and the nature of the heterocyclic base. In a nucleic acid, which heterocyclic base is never combined with the sugar shown below?
1. 2. 3. 4.
Adenine Guanine Thymine Uracil
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DNA and RNA differ in the nature of the monsaccharide unit and the nature of the heterocyclic base. In a nucleic acid, which heterocyclic base is never combined with the sugar shown below?
1. 2. 3. 4.
Adenine Guanine Thymine Uracil
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Nucleosides and Nucleotides with Purines
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Nucleosides and Nucleotides with Pyrimidines
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AMP, ADP, and ATP
Adding phosphate groups to AMP forms the diphosphate ADP and the triphosphate ATP. 20
Learning Check Give the name and abbreviation for the following and list its nitrogen base and sugar:
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Solution Guanosine 5’-monophosphate; GMP nitrogen base: guanine sugar: ribose
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Primary Structure of Nucleic Acids In the primary structure of nucleic acids: Nucleotides are joined by phosphodiester bonds. The 3’-OH group of the sugar in one nucleotide forms an ester bond to the phosphate group on the 5’-carbon of the sugar of the next nucleotide.
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Primary Structure of Nucleic Acids
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Structure of Nucleic Acids A nucleic acid polymer: Has a free 5’-phosphate group at one end and a free 3’-OH group at the other end. Is read from the free 5’end using the letters of the bases. This example reads 5’—A—C—G—T—3’. 25
Example of RNA
In RNA, A, C, G, and U are linked by 3’-5’ ester bonds between ribose and phosphate.
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Example of DNA
In DNA, A, C, G, and T are linked by 3’-5’ ester bonds between deoxyribose and phosphate.
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DNA Double Helix In DNA: There are two strands of nucleotides that wind together in a double helix. Two hydrogen bonds form between the complementary base pairs A-T. Three hydrogen bonds form between the complementary base pairs G-C.
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DNA Double Helix Structure
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How many hydrogen bonds link the two complementary strands of DNA shown below?
1. 2. 3. 4.
10 11 12 13
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How many hydrogen bonds link the two complementary strands of DNA shown below?
1. 2. 3. 4.
10 11 12 13
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Learning Check Write the complementary base sequence for the matching strand in the following DNA section: 5’—A—G—T—C—C —A—A—T—C— 3’
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Solution Write the complementary base sequence for the matching strand in the following DNA section: 5’—A—G—T—C—C—A—A—T—C—3’
3’—T—C—A—G—G—T—T—A—G—5’
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What is the sequence of the DNA strand on the rightside of the picture below?
1. 2. 3. 4.
A–G–T–C C–T–G–A G–A–C–T T–C–A–G
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What is the sequence of the DNA strand on the rightside of the picture below?
1. 2. 3. 4.
A–G–T–C C–T–G–A G–A–C–T T–C–A–G
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Animation_26.1
DNA Replication DNA replication involves: Unwinding the DNA Pairing the bases in each strand with new bases to form new complementary strands. Producing two new DNA strands that exactly duplicate the original DNA. 36
Hydrolysis Energy
Energy from the hydrolysis of each nucleoside triphosphate adding to the complementary strand is used to form the phosphodiester bond. 37
Direction of Replication
During replication, helicase unwinds the parent DNA at several sections. At each open DNA section called a replication fork, DNA polymerase catalyzes the formation of 5’-3’ester bonds of the leading strand. The lagging strand, which grows in the 3’-5’ direction, is synthesized in short sections called Okazaki fragments. The Okazaki fragments are joined by DNA ligase to give a single 3’-5’ DNA strand. 38
Direction of Replication
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Learning Check Match the following: 1) helicase 2) DNA polymerase 3) replication fork 4) Okazaki fragments A. Short segments formed by the lagging strand. B. The starting point for synthesis in unwound DNA sections. C. The enzyme the unwinds the DNA double helix. D. The enzyme the catalyzes the formation of phosphodiester bonds of complementary bases. 40
Solution Match the following: 1) helicase 2) DNA polymerase 3) replication fork 4) Okazaki fragments A. 4 Short segments formed by the lagging strand. B. 3 The starting point for synthesis in unwound DNA sections. C. 1 The enzyme the unwinds the DNA double helix. D. 2 The enzyme the catalyzes the formation of phosphodiester bonds of complementary bases. 41
26.7 DNA and RNA
The three differences in structure between DNA and RNA are
DNA bases are A, G, C, and T; the RNA bases are A, G, C, and U the sugar in DNA is 2-deoxy-D-ribose; ribose in RNA it is D-ribose DNA is always double stranded; stranded there are several kinds of RNA, all of which are singlestranded 42
Animation_26.10
Transfer RNA and Protein Synthesis
The following two processes are involved in transfer, and use of genetic information: Transcription: The process by which the genetic messages contained in DNA are read and copied. Translation: The process by which the genetic messages carried by RNA are decoded and used to build proteins. Animation_26.10.1
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26.7 Structure and Function of RNA RNA is similar to DNA – both are sugarphosphate polymers and both have nitrogencontaining bases attached – but there are differences between them. DNA has only one kind of function-storing genetic information. By contrast, the different kinds of RNA perform different functions.
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The following three RNA make it possible for the encoded information carried by the DNA to be put to use in the synthesis of proteins. Ribosome RNA: The granular organelles in the cell where protein synthesis takes place. These organelles are composed of protein and ribosomal RNA (rRNA). Messenger RNA (mRNA): The RNA that carries the code transcribed from DNA and directs protein synthesis.
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Transfer RNA (tRNA): The smaller RNA that delivers amino acids one by one to protein chains growing at ribosomes. Each tRNA recognizes and carries only one amino acid.
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26.8 Transcription: RNA Synthesis
RNA are synthesized in the cell nucleus. Before leaving the nucleus, all types of RNA are modified in the various ways needed for their various functions. In transcription, a small section of the DNA double helix unwinds, the bases on the two strands are exposed, and one by one the complementary nucleotides are attached. 47
rRNA, tRNA, and mRNA are all synthesized in essentially the same manner. Only one of the two DNA strands is transcribed during RNA synthesis. The DNA strand that is transcribed is the template strand; while the its complementary strand is the informational strand. The messenger RNA produced is a duplicate of the DNA informational strand, but with U base wherever the DNA has a T base. 48
The process of forming mRNA from DNA is called 1. 2. 3. 4.
reduction. replication. transcription. translation.
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The process of forming mRNA from DNA is called 1. 2. 3. 4.
reduction. replication. transcription. translation.
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26.9 The Genetic Code
The ribonucleotide sequence in an mRNA chain is like a coded sentence that species the order in which amino acid residues should be joined to form a protein. Each word, or codon in the mRNA sentence is a series of three ribonucleotides that code for a specific amino acid. 51
For example, the series uracil-uracil-guanine (UUG) on an mRNA chain is a codon directing incorporation of the amino acid leucine into a growing protein chain. Of the 64 possible three-base combinations in RNA, 61 code for specific amino acids and 3 code for chain termination.
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26.10 Translation: Transfer RNA and Protein Synthesis
The synthesis of proteins occur at ribosomes, which are outside the nucleus and within the cytoplasm of cells. The mRNA connects with the ribosome, and the amino acids attached to transfer RNA (tRNA) are delivered one by one. 53
Three stages in protein Synthesis are initiation, elongation, and termination. A diagram of translation is shown in the Fig 26.8. 54
Chapter Summary
Nucleic acids are polymers of nucleotides. Each nucleotide contains a sugar, a base, and a phosphate group. The sugar is D-ribose in ribonucleic acid (RNA) and 2-deoxy-D-ribose in deoxyribonucleic acid (DNA). A nucleoside contains a sugar and a base, but not the phosphate group. The DNA in each chromosome consists of two polynucleotides strands twisted together in a double helix. 55
Chapter Summary Contd.
The bases on the two strands are complementary – opposite every thimine is an adenine, opposite every guanine is a cytisine. The base pairs are connected by hydrogen bonds – two between T and A; three between G and C. Messenger RNA (mRNA) carries the genetic information out of the nucleus to the ribosomes where protein synthesis occurs. 56
Chapter Summary Contd.
Transfer RNA (tRNA) bond to amino acids that they deliver to protein synthesis. Ribosomal RNA (rRNA) are incorporated into ribosomes. In transcription, one strand of the DNA is copied (the template) and the other is the informational strand, and is not copied. The genetic information is read as a sequence of codons-triplets of bases in DNA that give the sequence of amino acids in a protein. 57
End of Chapter 26
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Animations
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