7.1 dna structure and replication
- 1. 7.1 DNA Structure and Replication
- 2. Guidance Details of DNA replication between prokaryotes and eukaryotes differ. Only the prokaryotic system is expected. The protein enzymes involved in DNA replication should include helicase, DNA gyrase, single strand binding proteins, DNA primase, and DNA polymerases I and III The regions of DNA that do not code for proteins should be limited to regulators of gene expression, introns, telomeres, and genes for tRNAs. Understandings • Nucleosomes help to supercoil the DNA • DNA structure suggested a mechanism for DNA replication • DNA polymersases can only add nucleotides to the 3’ end of a primer • DNA replication is continuous on the leading strand and discontinuous on the lagging strand • DNA replication is carried out by a complex system of enzymes • Some regions of DNA do not code for proteins but have other important functions Applications/Skills • A: Rosalind Franklin and Maurice Wilkins investigation of DNA by x-ray diffraction • A: Use of nucleotides containing dideoxyribonucleic acids to stop DNA replication in preparation of samples for base sequencing • A: Tandem repeats are used in DNA profiling • S: Analysis of results of the Hershey and Chase experiment to provide evidence that DNA is the genetic material • S: Utilization of molecular visualization software to analyze the association between protein and DNA within a nucleosome
- 3. DNA is the Genetic Material • Alfred Hershey and Martha Chase showed DNA contains the genetic information, not protein • Two cultures of bacteriophages: one with radioactive sulfur (component of protein) and one with radioactive phosphorous (component of DNA) http://www.hyglos.de/typo3temp/pics/f248049cfa.jpg
- 4. • The bacteriophages were then used to infect E. coli • One culture of E. coli contained radioactive P within the cell wall; the other contained no radioactive S within the cell wall • This showed that DNA was transferred from the bacteriophage to the bacteria and must be the substance containing genetic information http://www.hyglos.de/typo3temp/pics/f248049cfa.jpg
- 5. DNA Structure • Nucleotides are attached by phophodiester bonds • Condensation reaction • New nucleotides are always attached to the 3’ end of the previous nucleotide http://usmle.biochemistryformedics.com/wp-content/uploads/3-5phosphodiester-linkage.jpg
- 6. Structure continued • Purines are double ringed structures (Guanine and Adenine) • Pyrimidines are single ringed structures (Cytosine and Thymine) • A-T: two hydrogen bonds • C-G: three hydrogen bonds
- 7. DNA Packaging • DNA is organized into nucleosomes • 8 histone proteins (positively charged) • DNA (negatively charged) wraps around twice • H1 histone to help maintain structure of nucleosome • Further wrapping of DNA into supercoiled chromosomes http://www.mit.edu/~kardar/teaching/projects/dna_packing_website/histone_dna.jpg
- 9. DNA Sequences • Highly repetitive sequences • About 45% of total DNA • 5-300 bp’s • Up to 100 000 replicates • Sometimes called jumping genes due to the fact that they can move around the chromosome that they are a part of • No coding functions
- 10. DNA Sequences • Protein-coding genes • Less than 2% of DNA • Single copy genes with coding function • Contains code for producing a protein • Interspersed among noncoding fragments (introns)
- 11. DNA Sequences • Structural DNA • About 20% of DNA • No coding function • Around the centromere and telomeres
- 12. Summary of Replication Helicase binds at the origin and breaks H bonds between nitrogenous bases Replication forks 1 2
- 13. Primase attaches at the fork and produces an RNA primer 3 DNA polymerase III adds nucleotides in the 5’ to 3’ direction4 DNA polymerase I removes the RNA primer and replaces it with DNA nucleotides 5
- 14. Assembled continuously toward the fork Assembled discontinuously toward the fork (Okazaki fragments) DNA ligase seals Okazaki fragments together
- 15. PCR