copyright © 2003 pearson education, inc. publishing as benjamin cummings the flow of genetic...
TRANSCRIPT
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
THE FLOW OF GENETIC INFORMATION FROM DNA TO RNA TO PROTEINA specific gene specifies a polypeptide
– The DNA is transcribed into RNA, which is translated into the polypeptide
DNA
TRANSCRIPTION
RNA
TRANSLATION
Protein
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
• The “words” of the DNA “language” are triplets of bases called codons
– The codons in a gene specify the amino acid sequence of a polypeptide
CODONS
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Figure 10.7
DNA molecule
Gene 1
Gene 2
Gene 3
DNA strand
TRANSCRIPTION
RNA
Polypeptide
TRANSLATIONCodon
Amino acid
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
• Virtually all organisms share the same genetic code
• 64 Codons total (4 x 4 x 4)
• AUG = Start (Met)
• 3 Stops
• Redundant, but NOT ambiguous
CODON TRANSLATION
Figure 10.8A
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
• An exercise in translating the genetic code
Figure 10.8B
Startcodon
RNA
Transcribed strand
StopcodonTranslation
Transcription
DNA
Polypeptide
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Where does RNA come from: TRANSCRIPTION
Figure 10.9A
RNApolymerase
RNA nucleotide
Direction oftranscription
Newly made RNA
Templatestrand of DNA
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
• In transcription, the DNA helix unzips– Promoter: turns a
gene on/off
– RNA nucleotides line up along one strand of the DNA following the base-pairing rules
– The single-stranded messenger RNA peels away and the DNA strands rejoin
RNA polymerase
DNA of gene
PromoterDNA Terminator
DNAInitiation
Elongation
Termination
Area shownin Figure 10.9A
GrowingRNA
RNApolymerase
Completed RNA
Figure 10.9B
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
• Noncoding segments called introns are spliced out
• A cap and a tail are added to the ends
RNA Processing: Splicing Exons & Discarding Introns
Figure 10.10
DNA
RNAtranscriptwith capand tail
mRNA
Exon Intron IntronExon Exon
TranscriptionAddition of cap and tail
Introns removed
Exons spliced together
Coding sequence
NUCLEUS
CYTOPLASM
Tail
Cap
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
• In the cytoplasm, a ribosome attaches to the mRNA and translates its message into a polypeptide
• The process is aided by transfer RNAs
TRANSLATION: tRNA helps assemble the polypeptide chain (amino acids)
Figure 10.11A
Hydrogen bond
Amino acid attachment site
RNA polynucleotide chain
Anticodon
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
• Each tRNA molecule has a triplet anticodon on one end and an amino acid attachment site on the other
Figure 10.11B, C
Anticodon
Amino acidattachment site
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Ribosomes build polypeptides
Figure 10.12A-C
Codons
tRNAmolecules
mRNA
Growingpolypeptide
Largesubunit
Smallsubunit
mRNA
mRNAbindingsite
P site A site
P A
Growingpolypeptide
tRNA
Next amino acidto be added topolypeptide
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
• mRNA, a specific tRNA, and the ribosome
Figure 10.13B
1
Initiator tRNA
mRNA
Startcodon Small ribosomal
subunit
2
P site
Largeribosomalsubunit
A site
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
• The mRNA moves a codon at a time
• A tRNA pairs with each codon, adding an amino acid to the growing polypeptide
Amino acids are added to the polypeptide chain until a stop Codon is reached
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Figure 10.14
1 Codon recognition
Amino acid
Anticodon
AsiteP site
Polypeptide
2 Peptide bond formation
3 Translocation
Newpeptidebond
mRNAmovement
mRNA
Stopcodon
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
• The sequence of codons in DNA spells out the primary structure of a polypeptide
– Polypeptides form proteins that cells and organisms use
Review: The flow of genetic information in the cell is DNARNAprotein
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
• Summary of transcription and translation
Figure 10.15
1Stage mRNA istranscribed from aDNA template.
Anticodon
DNA
mRNARNApolymerase
TRANSLATION
Enzyme
Amino acid
tRNA
InitiatortRNA
Largeribosomalsubunit
Smallribosomalsubunit
mRNA
Start Codon
2Stage Each amino acid attaches to its proper tRNA with the help of a specific enzyme and ATP.
3Stage Initiation of polypeptide synthesis
The mRNA, the first tRNA, and the ribosomal subunits come together.
TRANSCRIPTION
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Figure 10.15 (continued)
4Stage ElongationGrowingpolypeptide
Codons
5Stage Termination
mRNA
Newpeptidebondforming
Stop Codon
The ribosome recognizes a stop codon. The poly-peptide is terminated and released.
A succession of tRNAs add their amino acids to the polypeptide chain as the mRNA is moved through the ribosome, one codon at a time.
Polypeptide
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
• Mutations are changes in the DNA base sequence
– These are caused by errors in DNA replication or by mutagens
– The change of a single DNA nucleotide causes sickle-cell disease
Mutations can change the meaning of genes
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Figure 10.16A
Normal hemoglobin DNA
mRNA
Normal hemoglobin
Glu
Mutant hemoglobin DNA
mRNA
Sickle-cell hemoglobin
Val
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
• Types of mutations
Figure 10.16B
mRNA
NORMAL GENE
BASE SUBSTITUTION
BASE DELETION
Protein Met Lys Phe Gly Ala
Met Lys Phe Ser Ala
Met Lys Leu Ala His
Missing