DNA Structure and Replication Chapter 16
You must know The structure of DNA The knowledge about DNA gained from the work of Griffith; Avery, MacLeod, and McCarty; Hershey and Chase; Wilkins and Franklin; and Watson and Crick Replication is semi-conservative and occurs 5’ to 3’ The roles of DNA polymerase III, DNA polymerase I, ligase, helicase, primase, and topoisomerase in replication The general differences between bacterial chromosomes and eukaryotic chromosomes How DNA packaging can affect gene expression
Frederick Griffith - 1928 Next question: Was that substance protein or DNA?
Avery, mccarty, & mcleod - 1944
Hershey & chase - 1952 DNA is the genetic material!
Franklin & Wilkins – early 1950s Franklin deduced that DNA was helical and consisted of 2 or 3 chains
Watson & crick - 1953 Used models to determine the structure of DNA
Dna structure Sugar-phosphate backbone with rungs of nitrogenous bases Strands are antiparallel 1 nucleotide
Dna structure Sugar-phosphate backbone with rungs of nitrogenous bases Strands are antiparallel 1 nucleotide
Nitrogenous base pairing Nitrogenous Bases: -Purines – Adenine (A) and Guanine (G) -Pyrimidines – Cytosine (C), Thymine (T), Uracil (U)
Semi-Conservative Replication A. New strands are composed of 1 strand of parental DNA and 1 strand of newly formed DNA B. Free-floating nucleotides Can be DNA or RNA Triphosphates – reactions to remove extra two phosphates are exergonic – provide the energy to build the new strand
Replication Enzymes Enzyme Function Helicase Unzips & unwinds DNA Topoisomerase Relieves strain of unwound DNA SSBs Help hold DNA open and stabilize it Primase Builds RNA Primer DNA Polymerase III Builds new DNA strand DNA Polymerase I Replaces RNA primer with DNA DNA Ligase Joins Okazaki fragments together https://www.youtube.com/watch?v=kTbeC7e2kKA
1. Helicase unzips DNA (breaks hydrogen bonds) creating a replication bubble at the origin of replication Multiple origins per chromosome in eukaryotes Each side of bubble has replication fork Bubble enlarges as replication proceeds until bubbles meet
2. Primase builds a short primer of RNA nucleotides (5 – 10 bases)
3. DNA Polymerase III builds the complimentary strand of DNA in the 5’ 3’ direction Free-floating DNA nucleotides move in to match up with parent strand, DNA polymerase III moves along and binds them together (forms covalent bonds between phosphate of one nucleotide and 3’carbon of next)
4. DNA Polymerase I replaces RNA primer with DNA nucleotides
Leading and Lagging Strands New nucleotides must be added on to the 3’ end Leading strand – Bases easily added as DNA is unzipped
Lagging Strand – has a delay Section unzips, then strand is built back towards origin Results in chunks called Okazaki fragments DNA ligase bonds Okazaki fragments together after primer is replaced https://www.youtube.com/watch?v=kTbeC7e2kKA
Speed and Accuracy ~4000 nucleotides per second Nucleotide excision repair (mismatch repair)– enzymes called nucleases cut out incorrect nucleotides and then gap is filled in with correct nucleotide
telomeres Lose a small portion of the chromosome every time it is replicated Telomeres consist of highly repetitive sequences in order to protect coding genes Cancer cells (ex. HeLa) – telomerase is activated – prevents degradation of telomeres and renders cells “immortal” Interesting article on HeLa cells: http://berkeleysciencereview.com/article/good-bad-hela/
DNA packaging Prokaryotes – one circular chromosome associated with very few proteins Eukaryotes – linear chromosomes associated with many proteins Histones – proteins that associate with DNA to help it coil DNA is negatively charged, histones are positively charged
Chromatin – the packaged DNA and proteins The more tightly coiled the DNA is, the less accessible it is to transcription enzymes = coils control gene expression Euchromatin – very extended and accessible to transcription – form of most DNA during interphase Heterochromatin – more condensed (like during mitosis) and generally not transcribed (Barr bodies are also an example)