Figure 16.5 The double helix Strands “anti-parallel” 5’ end 3’ end 3’ end 5’ end
J.D. Watson, F.H.Crick, “Molecular Structure of Nucleic Acids: A Structure for Deoxyribonucleic Acids.” Nature 171 (1953): 738 “It has not escaped our notice that the specific pairing we have postulated immediately suggests a possible copying mechanism for the genetic material.”
DNA REPLICATION Helicase separates strands and unwinds DNA DNA polymerase reads “template” and incorporates proper nucleotide. DNA Ligase seals newly made fragments.
DNA polymerase incredible fast and accurate! 50-500 nt/second! Error rate 1/10,000 DNA Polymerase “Proofreading” Excision/Repair enzymes
Nucleotide excision repair of DNA damage (Caused by U.V. irradiation)
Mutations which affect DNA replication and DNA repair have been associated with some cancers and in premature aging diseases. Xeroderma Pigmentosum
Gene Expression: Genotype to Phenotype HOW IS THE INFORMATION STORED IN THE DNA UTILIZED? DNA RNA PROTEIN WHAT IS THE GENETIC CODE? Transcription Translation
By the mid-1960s the entire code was deciphered. • 61 of 64 triplets code for amino acids. • The codon AUG not only codes for the amino acid methionine but also indicates the start of translation. • Three codons do not indicate amino acids but signal the termination of translation.
The Process of Transcription Production of an RNA copy (transcript) of a gene. 3 main types of RNAs (both proks. and euks.) - mRNA (messenger RNA) - rRNA (ribosomal RNA) - tRNA (transfer RNA) How are these RNAs made? What are their functions in the cell?
Transcriptioncan beseparatedinto threestages:initiation, elongation, andtermination.
As RNA polymerase moves along the DNA, it untwists the double helix, 10 to 20 bases at time. • The enzyme addsnucleotides to the3’ end of thegrowing strand. • Behind the pointof RNA synthesis,the double helixre-forms and theRNA moleculepeels away.
Eukaryotic RNA is processed: Exon Intron Exon Intron Exon DNA Transcription Addition of cap and tail Cap RNA transcript with cap and tail Introns removed Tail Exons spliced together mRNA Coding sequence Nucleus Cytoplasm Figure 10.10
The Process of Translation RNA (mRNA) Protein (phenotype)
0 Amino acid attachment site Hydrogen bond RNA polynucleotide chain Anticodon Figure 10.11A 2. Transfer RNA molecules serve as interpreters during translation
Amino acid attachment site Anticodon • Each tRNA molecule • Is a folded molecule bearing a base triplet called an anticodon on one end • A specific amino acid • Is attached to the other end Example: Codon UUU Anticodon AAA
3. A ribosome attaches to the mRNA • And translates its message into a specific polypeptide aided by transfer RNAs (tRNAs)
How does Translation work? Start of genetic message Second base U C A G U UAU UGU UGC UGA Stop UUU UCU Cys Phe Tyr UUC UAC C UCC Ser U UCA UUA UAA Stop A Leu UCG UAG Stop UGG Trp G U CAU CGU CUU CCU His C CAC CGC CUC CCC C Pro End Arg Leu CUA CCA CAA CGA A Gln CAG CGG CUG CCG G Third base First base U ACU AUU AAU AGU Ser Asn ACC AGC AUC AAC Ile C A Thr AUA AGA ACA AAA A Lys Arg Met or start ACC AGG AAG AUG G U GUU GAU GGU GCU Asp C GGC GCC GUC GAC Gly Ala G Val GUA GCA GGA GAA A Glu GUG GCG GGG GAG G Figure 10.8A Figure 10.13A • A start codon marks the beginning of an mRNA message
Large ribosomalsubunit Met Met Initiator tRNA P site A site U C U A C A A U G AUG Startcodon Small ribosomalsubunit mRNA 1 2 • Initiation: mRNA, a specific tRNA, and the ribosome subunits assemble • - Once initiation is complete, amino acids are added one by one to the first amino acid
-Elongation adds amino acids to the polypeptide chain until a stop codon terminates translation
Second base U C A G U UAU UGU UGC UGA Stop UUU UCU Cys Phe Tyr UUC UAC C UCC Ser U UCA UUA UAA Stop A Leu UCG UAG Stop UGG Trp G U CAU CGU CUU CCU His C CAC CGC CUC CCC C Pro Arg Leu CUA CCA CAA CGA A Gln CAG CGG CUG CCG Third base G First base U ACU AUU AAU AGU Ser Asn ACC AGC AUC AAC Ile C A Thr AUA AGA ACA AAA A Lys Arg Met or start ACC AGG AAG AUG G U GUU GAU GGU GCU Asp C GGC GCC GUC GAC Gly Ala G Val GUA GCA GGA GAA A Glu GUG GCG GGG GAG G Figure 10.8A • Termination • Elongation continues until a stop codon reaches the ribosome’s A site, terminating translation
Summary of transcription and translation Direction of transcription 5’-GGCTTTAACGGGTAT-3’ 3’-CCGAAATTGCCCATA-5’ mRNA 5’-GGCUUUAACGGGUAU-3’ Codons? Anti-codons?
Normal hemoglobin DNA Mutant hemoglobin DNA C A T T T C mRNA mRNA G A A G U A Normal hemoglobin Sickle-cell hemoglobin Glu Val • Mutations can change the meaning of genes • Mutations are changes in the DNA base sequence • Caused by errors in DNA replication or recombination, or by mutagens
Normal gene U G C U U C A G A A U G A G G mRNA Met Lys Gly Protein Phe Ala Base substitution A A G A U G C A U G A G U U C Lys Met Phe Ser Ala Missing U Base deletion G G C G A C A U A U G A G U U Lys Ala His Met Leu • Substituting, inserting, or deleting nucleotides alters a gene • With varying effects on the organism Results in “silent mutations”, “missense mutations” or “nonsense” mutations. Results in “frameshift mutations” THECATATETHERAT THECATTETHERAT
Sickle Cell • Tay Sachs • Cystic Fibrosis • Hemophilia • PKU • Cancer
If all the cells in our body have the same genes, why do we have such diverse types of cells??? Not all genes are utilized (expressed) in all cells, so the proteins found in different cell types differ. Gene expression is regulated!
Gene Regulation in Prokaryotes Colorized SEM 7,000 Figure 11.1A 0 • Proteins interacting with DNA turn prokaryotic genes on or off in response to environmental changes • Early understanding of gene control • Came from studies of the bacterium Escherichia coli
Regulatory proteins bind to control sequences in the DNA • And turn operons on or off in response to environmental changes Lactose Operon
Promoter Operator Genes DNA Activerepressor Activerepressor Tryptophan Inactiverepressor Inactiverepressor Lactose Figure 11.1C lac operon trp operon 0 • The trp operon • Is similar to the lac operon, but functions somewhat differently
Regulation of Gene Expression in Eukaryotes Not all genes in an organism are “turned on” in all cells or at all times in particular cell. All cells of an organism have the same set of genes, but cells from different tissues look very different and function very different from one another. -Development, Stem cells, differentiated cells - In an adult organism, certain genes are active or inactive at different times.
Levels of Eukaryotic Gene Regulation Transcriptional Regulation: Inactivation of large regions of chromosomes Control of affinity of RNA polymerase for the promoter (transcription efficiency) Post-transcriptional Regulation: RNA processing Transport out of nucleus mRNA stability Translation efficiency
Production of Stem Cells for Therapeutic Medicine http://www.youtube.com/watch?v=AZ6J4Q-m2Tk
VIRUSES What are viruses? Are they alive? How do they work and how do they harm us?