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Fig. 16-16b6 3 5 5 Template strand 3 3 RNA primer 3 1 5 5 3 1 5 3 5 2 3 3 5 Okazaki fragment 3 5 1 5 3 5 2 1 5 3 1 2 Overall direction of replication 3 5 The Central Dogma of Molecular Biology DNA RNA Protein Trait RNA vs. DNA Study the images of the two molecules and observe their similarities & differences. 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 The Central Dogma of Molecular Biology DNA RNA Protein Trait Transferring genetic information into protein. 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 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 mRNA is Spliced • Introns: Non-coding regions of DNA • Exons: Coding regions of DNA 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 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 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 Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings • 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 • 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 Fig. 17-4 DNA molecule Gene 2 Gene 1 Gene 3 DNA template strand TRANSCRIPTION mRNA Codon TRANSLATION Protein Amino acid The Genetic Code Translation: The RNA directed synthesis of a polypeptide • 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 Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Translation mRNA codons are translated into a polypeptide chain 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) Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings • 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 Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Fig. 17-18-4 Amino end of polypeptide E 3 mRNA Ribosome ready for next aminoacyl tRNA P A site site 5 GTP GDP E E P A P A GDP GTP E P A • During and after synthesis, a polypeptide chain spontaneously coils and folds into its three-dimensional shape • Proteins may also require posttranslational 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 Fig. 17-25 DNA TRANSCRIPTION 3 RNA polymerase 5 RNA transcript RNA PROCESSING Exon RNA transcript (pre-mRNA) Intron Aminoacyl-tRNA synthetase NUCLEUS Amino acid CYTOPLASM AMINO ACID ACTIVATION tRNA mRNA Growing polypeptide 3 A Activated amino acid P E Ribosomal subunits 5 TRANSLATION E A Codon Ribosome Anticodon