BIOL 1411 1st Edition Lecture 22 Outline of Last Lecture I DNA Structure II DNA Replication Outline of Current Lecture III DNA Replication and Function a Repairs b Genes Code for Proteins Lecture How is a chromosome replicated o Small circular prokaryotic chromosomes generally have a single origin or replication o Large linear eukaryotic chromosomes have many hundreds of origins of replication o Replication complexes bind to the sites at the some time and catalyze simultaneous replication Repair of errors in DNA synthesis o DNA polymerases make mistakes in replication and DNA can be damaged in living cells o Cells have 3 repair mechanisms Proofreading by DNA polymerase As DNA polymerase adds a nucleotide to a growing strand it has a proofreading function if bases are paired incorrectly the nucleotide is removed by DNA polymerase Mismatch repair correct new strand The newly replicated DNA is scanned for mismatched bases The mismatch repair mechanism recognizes old and new strands by modifications present on the template strand If mismatch repair fails to distinguish old and new strands The DNA sequence may change Excision repair remove damaged bases DNA can be damaged by radiation chemicals in the environment and random spontaneous chemical reactions These notes represent a detailed interpretation of the professor s lecture GradeBuddy is best used as a supplement to your own notes not as a substitute Excision repair enzymes constantly scan DNA for damaged basesthey are excised and DNA polymerase I adds the correct ones important to the cell recognizing damaged bases before replication Genes code for proteins o The early one gene to one enzyme relationship is now most commonly expressed as the one gene one polypeptide relationship o Some genes are involved in controlling other genes some genes code for functional RNA and are not translated into polypeptides Information flow from gene to protein o Gene expression to form a specific polypeptide occurs in 2 steps Transcription copies information from a DNA sequence a gene to a complementary RNA sequence Translation converts RNA sequence to amino acid sequence of a polypeptide The central dogma of molecular biology o Eukaryotes the nuclear envelope separates transcription and translation DNA in nucleus site of transcription Ribosomes in cytoplasm ER site of translation mRNA is the intermediate messenger o Prokaryotes translation occurs on growing nRNA o 3 kinds of RNA in protein synthesis Messenger RNA mRNA carries copy of a DNA sequence to site of protein synthesis at the ribosome Transfer RNA tRNA carries amino acids for polypeptide assembly Ribosomal RNA rRNA catalyzes peptide bonds and provides structure o RNA differs from DNA Usually a single polynucleotide strands The sugar is ribose Contains U instead of thymine o RNA polymerase catalyzes synthesis of RNA from a DNA template Like DNA polymerases RNA polymerases are processive a single enzyme template binding results in polymerization of hundreds of RNA bases RNA polymerases can only add new nucleotides to the 3 end of a growing stand 5 3 Unlike DNA polymerase RNA polymerase can initiate synthesis without an existing 3 OH so do not need primers RNA polymerases lack a proofreading function o Transcription components DNA template for base pairings one of the two strands of DNA Nucleotide triphosphates ATP GTP CTP UTP as substrates An RNA polymerase enzyme Transcription occurs in 3 phases Initiation o Requires a promoter a special sequence of DNA o RNA polymerase binds to the promoter o Promoter directs RNA polymerase where to start and which direction to transcribe o Part of each promoter is the initiation site of transcription Elongation o RNA polymerase unwinds DNA about 10 base pairs at a time reads template in 3 5 direction o The RNA transcript is antiparallel to the template DNA strand nucleotides added at 3 end o RNA polymerases do not proofread and correct mistakes Termination o Is specified by a base sequence in DNA that destabilizes the transcription complex o For some genes the transcript falls away from the RNA polymerase and DNA template for others a helper protein pulls it away