Genes and How They Work. Chapter 15

Transcription

1 Genes and How They Work Chapter 15

2 The Nature of Genes They proposed the one gene one enzyme hypothesis. Today we know this as the one gene one polypeptide hypothesis. 2

3 The Nature of Genes The central dogma of molecular biology states that information flows in one direction: DNA RNA protein Transcription is the flow of information from DNA to RNA. Translation is the flow of information from RNA to protein. 3

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5 The Genetic Code Deciphering the genetic code required determining how 4 nucleotides (A, T, G, C) could encode more than 20 amino acids. Francis Crick and Sydney Brenner determined that the DNA is read in sets of 3 nucleotides for each amino acid. 5

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7 The Genetic Code codon: set of 3 nucleotides that specifies a particular amino acid reading frame: the series of nucleotides read in sets of 3 (codon) only 1 reading frame is correct for encoding the correct sequence of amino acids 7

8 The Genetic Code Marshall Nirenberg identified the codons that specify each amino acid. RNA molecules of only 1 nucleotide and of specific 3-base sequences were used to determine the amino acid encoded by each codon. The amino acids encoded by all 64 possible codons were determined. 8

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10 The Genetic Code stop codons: 3 codons (UUA, UGA, UAG) in the genetic code used to terminate translation start codon: the codon (AUG) used to signify the start of translation The remainder of the code is degenerate meaning that some amino acids are specified by more than one codon. 10

11 Gene Expression Overview template strand: strand of the DNA double helix used to make RNA coding strand: strand of DNA that is complementary to the template strand RNA polymerase: the enzyme that synthesizes RNA from the DNA template 11

12 Gene Expression Overview Transcription proceeds through: initiation RNA polymerase identifies where to begin transcription elongation RNA nucleotides are added to the 3 end of the new RNA termination RNA polymerase stops transcription when it encounters terminators in the DNA sequence 12

13 Gene Expression Overview Translation proceeds through initiation mrna, trna, and ribosome come together elongation trnas bring amino acids to the ribosome for incorporation into the polypeptide termination ribosome encounters a stop codon and releases polypeptide 13

14 Gene Expression Overview Gene expression requires the participation of multiple types of RNA: messenger RNA (mrna) carries the information from DNA that encodes proteins ribosomal RNA (rrna) is a structural component of the ribosome transfer RNA (trna) carries amino acids to the ribosome for translation 14

15 Gene Expression Overview Gene expression requires the participation of multiple types of RNA: small nuclear RNA (snrna) are involved in processing pre-mrna signal recognition particle (SRP) is composed of protein and RNA and involved in directing mrna to the RER micro-rna (mirna) are very small and their role is not clear yet 15

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17 Eukaryotic Transcription RNA polymerase I transcribes rrna. RNA polymerase II transcribes mrna and some snrna. RNA polymerase III transcribes trna and some other small RNAs. Each RNA polymerase recognizes its own promoter. 17

18 Eukaryotic Transcription Initiation of transcription of mrna requires a series of transcription factors transcription factors proteins that act to bind RNA polymerase to the promoter and initiate transcription 18

19 Eukaryotic pre-mrna Splicing In eukaryotes, the primary transcript must be modified by: addition of a 5 cap addition of a 3 poly-a tail removal of non-coding sequences (introns) 19

20 Eukaryotic pre-mrna Splicing The spliceosome is the organelle responsible for removing introns and splicing exons together. introns non-coding sequences exons sequences that will be translated 20

21 trna and Ribosomes trna molecules carry amino acids to the ribosome for incorporation into a polypeptide the anticodon loop contains 3 nucleotides complementary to mrna codons 21

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24 trna and Ribosomes The ribosome has multiple trna binding sites: P site binds the trna attached to the growing peptide chain A site binds the trna carrying the next amino acid E site binds the trna that carried the last amino acid 24

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26 trna and Ribosomes The ribosome has two primary functions: decode the mrna form peptide bonds peptidyl transferase is the enzymatic component of the ribosome which forms peptide bonds between amino acids 26

27 Translation In prokaryotes, initiation of translation requires the formation of the initiation complex including an initiator trna charged with Nformylmethionine the small ribosomal subunit mrna strand The ribosome binding sequence of mrna is complementary to part of rrna 27

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29 Translation Elongation of translation involves the addition of amino acids a charged trna binds to the A site if its anticodon is complementary to the codon at the A site peptidyl transferase forms a peptide bond the ribosome moves down the mrna in a 5 to 3 direction 29

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32 Translation There are fewer trnas than codons. Wobble pairing allows less stringent pairing between the 3 base of the codon and the 5 base of the anticodon. This allows fewer trnas to accommodate all codons. 32

33 Translation Elongation continues until the ribosome encounters a stop codon. Stop codons are recognized by release factors which release the polypeptide from the ribosome. 33

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35 Mutation: Altered Genes Point mutations alter a single base. base substitution mutations substitute one base for another transitions or transversions also called missense mutations nonsense mutations create stop codon frameshift mutations caused by insertion or deletion of a single base 35

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38 Mutation: Altered Genes Too much genetic change (mutation) can be harmful to the individual. However, genetic variation (caused by mutation) is necessary for evolutionary change of the species. 38