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Genes and How They Work

Chapter 15

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Outline

• Cells Use RNA to Make Protein• Gene Expression• Genetic Code• Transcription• Translation• Spliced Genes

– Introns and Exons• Prokaryotic and Eukaryotic Gene Expression

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Cells Use RNA to Make Protein

• During polypeptide synthesis, ribosomal RNA (rRNA) is the site of polypeptide assembly.

– Transfer RNA (tRNA) transports and positions amino acids.

– Messenger RNA (mRNA) directs which amino acids are assembled into polypeptides.

• Central Dogma– DNA RNA Protein

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Central Dogma of Gene Expression

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Gene Expression

• Transcription– DNA sequence is transcribed into RNA

sequence initiated when RNA polymerase binds to

promoter binding sitemoves along DNA strand and adds

corresponding complementary RNA nucleotide

disengages at stop signal

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Gene Expression

• Translation– nucleotide sequence of mRNA transcript is

translated into amino acid sequence in the polypeptide

rRNA recognizes and binds to start sequence

moves three nucleotides at a time disengages at stop signal

• Gene expression - collective of transcription and translation

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Genetic Code

• Genetic code consists of a series of information blocks called codons.

– reading frame (triplet) each codes for one amino acid

genetic code is nearly universal mitochondria chloroplasts

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Transcription

• RNA polymerase– only one of two DNA strands (template or

antisense strand) is transcribed– non-transcribed strand is termed coding

strand or sense strand– In both bacteria and eukaryotes, the

polymerase adds ribonucleotides to the growing 3’ end of an RNA chain.

synthesis proceeds in 5’3’ direction

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Transcription Bubble

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Transcription

• Promoter– Transcription starts at RNA polymerase

binding sites called promoters on DNA template strand.

• Initiation– Other eukaryotic factors bind, assembling

a transcription complex. RNA polymerase begins to unwind DNA

helix.

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Transcription Complex

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Transcription

• Elongation– Transcription bubble moves down DNA at

constant rate leaving growing RNA strands protruding from the bubble.

• Termination– Stop sequences at the end of the gene

cause phosphodiester bond formation to cease, transcription bubble to dissociate, and RNA polymerase to release DNA.

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Transcription

• Eukaryotic transcription differs from prokaryotic transcription:

– three RNA polymerase enzymes– initiation complex forms at promoter– RNAs are modified after transcription

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Translation

• Begins when initial portion of mRNA molecule binds to rRNA in a ribosome

– tRNA molecule with complimentary anticodon binds to exposed codon on mRNA

some tRNA molecules recognize more than one codon

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Translation

• Activating enzymes– tRNA molecules attach to specific amino

acids through the action of activating enzymes (aminoacyl-tRNA syntheases).

must correspond to specific anticodon sequences on a tRNA molecule as well as particular amino acids

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Translation

• Start and stop signals– start signal coded by AUG codon– stop signal coded by one of three

nonsense codons: UAA - UAG - UGA• Initiation

– Polypeptide synthesis begins with the formation of an initiation complex.

initiation factors

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Formation of Initiation Factor

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Translation

• Elongation– After initiation complex forms, large

ribosome subunit binds, exposing mRNA codon adjacent to the initiating codon, positioning it for interaction with another amino acid-bearing tRNA molecule.

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Translation

• Translocation– ribosome moves nucleotides along mRNA

molecule

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Translation

• Termination– Nonsense codons are recognized by

release factors that release the newly made polypeptide from the ribosome.

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Spliced Gene Transcripts

• DNA sequence specifying a protein is broken into segments (exons) scattered among longer noncoding segments (introns).

• Initially, primary RNA transcript is produced for the entire gene.

– Small nuclear ribonuclearproteins (snRNPs) associate with proteins to form spliceosomes.

Lariat forms, excising introns and splicing exons to form mature mRNA.

alternative splicing

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Spliceosome

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RNA Splicing

• During RNA processing, intron sequences are cut out of primary transcript before it is used in polypeptide synthesis.

– remaining sequences are not translated remaining exon sequences are spliced

together to form final processed mRNA

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Eukaryotic Genes are Fragmented

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Differences Between Prokaryotic and Eukaryotic Gene Expression

• Most eukaryotic genes possess introns.• Individual bacterial mRNA molecules often

contain transcripts of several genes.• Eukaryotic mRNA molecules must be

completely formed and must pass across the nuclear membrane before translation.

• In prokaryotes, translation begins at the AUG codon preceded by a special nucleotide sequence.

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Differences Between Prokaryotic and Eukaryotic Gene Expression

• Eukaryotic mRNA molecules have introns cut out and exons joined together before translation.

• Eukaryotic ribosomes are larger than prokaryotic ribosomes.

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Summary

• Cells Use RNA to Make Protein• Gene Expression• Genetic Code• Transcription• Translation• Spliced Genes

– Introns and Exons• Prokaryotic and Eukaryotic Gene Expression

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