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This 5 page Class Notes was uploaded by Vrena Puentes on Sunday October 25, 2015. The Class Notes belongs to BSC2010 at Florida State University taught by Steven Marks in Summer 2015. Since its upload, it has received 59 views. For similar materials see Biological Science 1 in Biology at Florida State University.
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Date Created: 10/25/15
102015 Ch 16 The Molecular Basis of Inheritance Involves 3 Major Steps DNA replication 1 Initiation RNA transcription 2 Elongation Protein translation 3 Termination A cell must copy its DNA because it contains more information than RNA or Protein and because it is the only biological molecule that can copy itself DNA GENE AGENE BGENE If one strand of a DNA molecule has the sequence of bases 539ATTGCA339 the other complementary antiparallel strand would have the sequence 5 TGCAAT 3 BECAUSE 5 A39ITGCA 3 match w the one they go with 3 TAACGT 5 bonded by hydrogen bonds Matthew Meselson and Franklin Stahl at Cal Tech supported the semiconservative model They labeled the nucleotides of the old strands w a heavy isotope of Nitrogen N15 while any new nucleotides were labeled w a lighter isotope N14 Used bacteria to produce DNA s presence of N15 Move bacteria into new growth vial with N14 Used centrifugation to separate these 2 things Semiconservative model WAS correct Control DNA Replication Dipoid cells 2 copies of each chromosome Cells do not tolerate more or less DNA Cels must only replicate their DNA in preparation for cell division Origin of Replication Strands separate forming a replication bubble Helicase is the enzyme responsible for separating DNA At the end of the replication bubble you have replication forks This results in two daughter molecules Why do you think that prokaryotic cells have only one origin of replication on their chromosomes They have less DNA to copy 102215 DNA Replication Copy all of genomic DNA in a cell Only in preparation for cell division Helicase separating the two DNA strands One helicase at each replication fork Single strand binding protein keep the separated DNA from rejoining its complimentary strand Primase enzyme that adds RNA primers to single stranded DNA Primer short piece of RNA with complimentary sequence to the DNA strand Short 1520 nucleotides Helicase unwinds DNA SSB stabilizes ssDNA Primase adds an RNA primer which will be the basis for elongation Unwinding DNA gets difficult because as you separate that twisted DNA the areas in the middle and ends begin to twist more and more and causes supercoils which ultimately create a knot Topoisomerase travels ahead of the replication fork and turns the DNA in the opposite direction in order to counteract the supercoils It has to knick the DNA by breaking the sugar phosphate and unwind it in order to do this Synthesizing a new DNA strand DNA polymerases enzymes that catalyze the elongation of new DNA at a replication fork Most DNA pols require a primer and a DNA template strand DNA pols add nucleotides only to the free 3 end of a growing strand A new DNA strand can elongate only to the 5 to 3 direction Cannot add nucleotides to naked template DNA they need an exposed 3 hydroxyl group on the nucleotide strand Leading strand synthesis Synthesis of leading strands is continuous in the 5 to 3 direction We copy the other strand in the opposite direction by making lagging strands Lagging strand synthesis Lagging strand elongates the other new strand DNA polymerase must work in the direction away from the replication fork Primase must bring together new primers The lagging strand is synthesized by DNA pol III as a series of segments called Okazaki fragments which are short newly synthesized DNA fragments that are formed on the lagging template strand during DNA replication Despite their names leading and lagging strands happen at the same time Nothing is added to the 5 end nucleotides are added to the 3 end always How do the leading and the lagging strands differ The leading strand is synthesized in the same direction as the movement of the replication fork whereas the lagging strand is synthesized in the opposite direction We are left with DNARNA hybrids RNA primers must be replaced with DNA DNA pol I can remove RNA nucleotides and ll in the gap w DNA nucleotides The ends of Okazaki fragments must be joined together DNA ligase can join the 5 end of one DNA molecule w the 3 end of another Coses the gap btwn fragments when DNA pol 1 is done replacing nucleotides If DNA pol I was nonfunctional how would that affect the leading strand during DNA synthesis in a eukaryotic cell The leading strand would not be able to connect to the lagging strand approaching it from the next origin of replication Termination of Replication Termination sequences in prokaryotes Ter sites 23bp sequences Causes termination in vitro Protein bounded to the ter site stops the replication fork from proceeding You don t need termination in eukaryotes Their chromosomes don t have termination sequences Bacteria prok have circular genomes Humans euk have linear genomes Eukaryotic DNA Replication Results in shorter and shorter daughter molecules This is a problem because that organism starts depleting its own genetic material We protect against chromosome shortening by adding telomeres to the end of chromosomes Telomeres eukaryotic chromosomal DNA molecules that have special repetitive and noncoding nucleotide sequences Teomeres do not prevent the shortening of DNA molecules but they do postpone the erosion of genes near the ends of DNA molecules Somatic cells are nonreproductive cells in the body Do not express telomerase Teomeres are generally shorter in older individuals Germline cells are reproductive cells in the body Express a protein called telomerase Teomerase builds telomeres so that it maintains telomere length from one cell division to the next Proofreading Replication Errors About once every 100k nucleotides DNA polymerase will add the wrong base DNA pol l and pol lll have 3 to 5 exonuclease activity which allows them to remove the mistaken nucleotide as soon as it is added This allows mutations to NOT occur Bacterial cells don t have this which causes many more mutations Without the activity of the primase enzyme what would happen during DNA replication Neither strand would form because new DNA cannot be synthesized without a primer Heicase Unwinds DNA to create two single strands Single Strand Binding Protein SSB Binds to and stabilizes the single stranded DNA Primase Provides an RNA primer to start new DNA strand synthesis Topoisomerase Travels ahead of helicase quotnickingquot and swiveling the DNA to relieve torsional strain on the molecule Poymerase lll Assembles new DNA strand in the 5 3 direction Poymerase Can remove RNA nucleotides as it adds new DNA in a 5 3 direction Ligase Joins two DNA strands together at their ends Teomerase Maintains telomere length in germline cells to protect the health of gametes Chromatin complex of DNA and protein is found in the nucleus of euk cells Chromosomes t into the nucleus thru and elaborate multilevel system of packing Most chromatin is loosely packed in the nucleus during interphase and condenses prior to mitosis Euchromatin loosely packed chromatin DNA is accessible Can undergo replication and transcription Heterochromatin tightly packed chromatin which makes it dif cult for the cell to express genetic info DNA is inaccessible Can t undergo replication and transcription