fig. 16-16b6 template strand 5 5 3 3 rna primer 3 5 5 3 1 1 3 3 5 5 okazaki fragment 1 2 3 3 5 5 1 2...
TRANSCRIPT
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Fig. 16-16b6
Template strand
5
53
3
RNA primer 3 5
5
3
1
1
3
35
5
Okazaki fragment
12
3
3
5
5
12
3
3
5
5
12
5
5
3
3
Overall direction of replication
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The Central Dogma of Molecular Biology
DNA RNA Protein Trait
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RNA vs. DNAStudy the images of the two molecules and observe their
similarities & differences.
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Basic Principles of Transcription and Translation
• RNA is the intermediate between genes and the proteins for which they code
• Transcription is the synthesis of RNA under the direction of DNA
• Transcription produces messenger RNA (mRNA)
• Translation is the synthesis of a polypeptide, which occurs under the direction of mRNA
• Ribosomes are the sites of translation
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The Central Dogma of Molecular Biology
DNA RNA Protein TraitTransferring genetic information into protein.
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Transcription: Rewriting DNA into mRNA
•Enzymes add nucleotides to mRNA from 5’ to 3’
•Transcription is initiated at a of DNA that promoter, a sequence signals the start of a gene
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Enzymes Modify the “pre-mRNA”
• Enzymes attach a “cap” to the mRNA which binds the mRNA to the ribosome
• Enzymes attach a tail of nucleotides to the mRNA, this controls the “lifespan” of the mRNA
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mRNA is Spliced
• Introns: Non-coding regions of DNA
• Exons: Coding regions of DNA
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The Functional and Evolutionary Importance of Introns
• Some genes can encode more than one kind of polypeptide, depending on which segments are treated as exons during RNA splicing
• The number of different proteins an organism can produce is much greater than its number of genes
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
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The Genetic Code
• How are the instructions for assembling amino acids into proteins encoded into DNA?
• There are 20 amino acids, but there are only four nucleotide bases in DNA
• How many bases correspond to an amino acid?
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
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Codons: Triplets of Bases
• The flow of information from gene to protein is based on a triplet code: a series of nonoverlapping, three-nucleotide words
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• During transcription, one of the two DNA strands called the template strand provides a template for ordering the sequence of nucleotides in an RNA transcript
• During translation, the mRNA base triplets, called codons, are read in the 5 to 3 direction
• Each codon specifies the amino acid to be placed at the corresponding position along a polypeptide
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
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• Codons along an mRNA molecule are read by translation machinery in the 5 to 3 direction
• Each codon specifies the addition of one of 20 amino acids
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
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Fig. 17-4
DNAmolecule
Gene 1
Gene 2
Gene 3
DNAtemplatestrand
TRANSCRIPTION
TRANSLATION
mRNA
Protein
Codon
Amino acid
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The Genetic Code
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Translation: The RNA directed synthesis of a polypeptide
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• Molecules of tRNA are not identical:– Each carries a specific amino acid on one end– Each has an anticodon on the other end; the
anticodon base-pairs with a complementary codon on mRNA
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Translation
mRNA codons are translated into a polypeptide chain
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Ribosomes
• Ribosomes facilitate specific coupling of tRNA anticodons with mRNA codons in protein synthesis
• The two ribosomal subunits (large and small) are made of proteins and ribosomal RNA (rRNA)
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• A ribosome has three binding sites for tRNA:– The P site holds the
tRNA that carries the growing polypeptide chain
– The A site holds the tRNA that carries the next amino acid to be added to the chain
– The E site is the exit site, where discharged tRNAs leave the ribosome
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Fig. 17-18-4
Amino endof polypeptide
mRNA
5
3E
Psite
Asite
GTP
GDP
E
P A
E
P A
GDPGTP
Ribosome ready fornext aminoacyl tRNA
E
P A
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• During and after synthesis, a polypeptide chain spontaneously coils and folds into its three-dimensional shape
• Proteins may also require post-translational modifications before doing their job
• Some polypeptides are activated by enzymes that cleave them
• Other polypeptides come together to form the subunits of a protein
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
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Fig. 17-25
TRANSCRIPTION
RNA PROCESSING
DNA
RNAtranscript
3
5RNApolymerase
Poly-A
Poly-A
RNA transcript(pre-mRNA)
Intron
Exon
NUCLEUS
Aminoacyl-tRNAsynthetase
AMINO ACID ACTIVATIONAminoacid
tRNACYTOPLASM
Poly-A
Growingpolypeptide
3
Activatedamino acid
mRNA
TRANSLATION
Cap
Ribosomalsubunits
Cap
5
E
P
A
AAnticodon
Ribosome
Codon
E