1 Genes and How They Work Chapter 15

2 Outline Cells Use RNA to Make Protein Gene Expression Genetic Code Transcription Translation Spliced Genes – Introns and Exons Prokaryotic and Eukaryotic Gene Expression

3 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

4 Central Dogma of Gene Expression

5 Gene Expression Transcription – DNA sequence is transcribed into RNA sequence  initiated when RNA polymerase binds to promoter binding site  moves along DNA strand and adds corresponding complementary RNA nucleotide  disengages at stop signal

6 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

7 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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9 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

10 Transcription Bubble

11 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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13 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.

14 Transcription Eukaryotic transcription differs from prokaryotic transcription: – three RNA polymerase enzymes – initiation complex forms at promoter – RNAs are modified after transcription

15 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

16 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

17 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

18 Formation of Initiation Factor

19 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.

20 Translation Translocation – ribosome moves nucleotides along mRNA molecule

21 Translation Termination – Nonsense codons are recognized by release factors that release the newly made polypeptide from the ribosome.

22 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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24 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

25 Eukaryotic Genes are Fragmented

26 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.

27 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.

28 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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