BIOL 1411 1st Edition Lecture 22Outline of Last Lecture I. DNA StructureII. DNA ReplicationOutline of Current Lecture III. DNA Replication and Function a. Repairsb. 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 replicationo Replication complexes bind to the sites at the some time and catalyze simultaneous replication - Repair of errors in DNA synthesiso DNA polymerases make mistakes in replication, and DNA can be damaged in living cellso 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 reactionsThese 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 bases-- they 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 proteino 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 messengero Prokaryotes: translation occurs on growing nRNAo 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:- Initiationo Requires a promoter- a special sequence of DNAo RNA polymerase binds to the promotero Promoter directs RNA polymerase where to start and which direction to transcribeo Part of each promoter is the initiation site of transcription- Elongationo RNA polymerase unwinds DNA about 10 base pairs at a time; reads template in 3’  5’ directiono The RNA transcript is antiparallel to the template DNA strand; nucleotides added at 3’ endo RNA polymerases do not proofread and correct mistakes - Termination o Is specified by a base sequence in DNA that destabilizes the transcription complexo For some genes the transcript falls away from the RNA polymerase and DNA template- for others a helper proteinpulls it