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DNA

DNA. Consists of Deoxyribose sugar Phosphate group A, T, C, G Double stranded molecule (Double Helix) Two strands of DNA run antiparallel to each other (opposite direction) 5’ to 3’ 5’ is the end with the phosphate group 3’ is where deoxyribose sugar is located Nitrogenous bases

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DNA

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  1. DNA • Consists of • Deoxyribose sugar • Phosphate group • A, T, C, G • Double stranded molecule (Double Helix) • Two strands of DNA run antiparallel to each other (opposite direction) • 5’ to 3’ • 5’ is the end with the phosphate group • 3’ is where deoxyribose sugar is located • Nitrogenous bases • Held together by hydrogen bonds • A pairs with T ( forms double bond) • C pairs with G (forms a triple bond)

  2. DNA • 46 chromosomes • 6 billion base pairs • ~25,000 genes

  3. Four Requirements for DNA to be Genetic Material • 1. Must carry information • Cracking the genetic code • 2. Must replicate • DNA replication • 3. Must allow for information to change • Mutation • 4. Must govern the expression of the phenotype • Gene function

  4. DNA Replication S phase G1 G2 interphase Mitosis -prophase -metaphase -anaphase -telophase • Process of duplication of the entire genome prior to cell division • Biological significance • extreme accuracy of DNA replication is necessary in order to preserve the integrity of the genome in successive generations • In eukaryotes , replication only occurs during the S phase of the cell cycle.

  5. Rules of Replication • Semi-conservative • Parent strands serve as templates for daughter strands • Starts at the ‘origin’ • Site(s) of parent strand separation • Synthesis always in the 5-3’ direction • Problems with anti-parallel strands • Semi-discontinuous • Solution to anti-parallel strands • RNA primers required

  6. Mechanism of DNA Replication 1. Strand Separation • Proteins bind to DNA and open up double helix • Prepare DNA for complementary base pairing 2. Building Complementary Strands • Proteins connect the correct sequences of nucleotides into a continuous new strand of DNA 3. Dealing With Errors During DNA Replication • Proteins release the replication complex

  7. DNA Replication is Semi-Conservative • Separating the two parent strands and building new complementary strand for each • New DNA has one new strand and one old strand

  8. Strand Separation • Double Helix • Unwound at replication origins (many origins on DNA) • Helicase • binds to origins and unwinds the two strands creating replication bubbles • Two strands separating creates a replication fork

  9. Enzyme Enzyme DNA Strand Separation Problems 1. Unwinding DNA creates tension (supercoil) • Topoisomerases • relieves tension by cutting strands near the replication fork 2. Single strands want to join back together (annealing) • Single-stranded binding proteins (SSBs) • attachtothe DNA strands stabilizing them

  10. Strand Separation • Multiple replication origins decrease the overall time of DNA replication to about 1 hour

  11. Building Complementary Strands • RNA primasebegins the replication process • Builds small complementary RNA segments on strand at beginning of replication fork (RNA primers) • DNA polymerase III • Builds new strand using nucleoside triphosphates • Adds nucleotides to the 3’ end of a parent strand • New strands are always assembled 5’ to 3’

  12. DNA polymerase III 3 3 3 3 3 3 3 3 3 3 3 growing replication fork growing replication fork 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 Building Complementary Strands • Leading Strand • DNA that is copied in the direction toward the replication fork • Lagging Strand • DNA that is copied in the direction away from the replication fork leading strand lagging strand leading strand lagging strand leading strand lagging strand

  13. Building Complementary Strands • Anti parallel strands replicated simultaneously • Leading strand synthesis continuously in 5’– 3’ • Lagging strand synthesis in fragments in 5’-3’ • semi-discontinuous

  14. Leading Strand • Single RNA primer is used to start strand • DNA polymerase III moves towards replication fork 5’ to 3’ direction • Continuous

  15. Lagging Strand • DNA polymerase III moves away from replication fork • Discontinuous • Okazaki fragments are used to solve problem • 1000 – 2000 base pairs long • Multiple RNA primers are used • DNA polymerase I • removes RNA primers and replaces with DNA nucleotide • Fills the gaps

  16. Building Complementary Strands • DNA ligase • Links last nucleotide to Okazaki fragment • Formation of phosphodiester bond

  17. Dealing With Errors • DNA polymerase III • Proofread and correct errors • Errors are usually base pair mismatches • After replication • Average of 1 error per million base pairs • DNA polymerase II • Repairs damage after strands have been synthesized

  18. 3 3 3 3 5 5 5 5 Chromosome Erosion • DNA polymerases can only add to 3 end of an existing DNA strand • Loss of bases at 5 ends in every replication • DNA polymerase I cannot replace final RNA primer DNA polymerase I growing replication fork DNA polymerase III

  19. Does it Create a Problem?

  20. Telomeres 3 3 3 3 5 5 5 5 • Repeating, non-coding sequences at the end of chromosomes • Protective cap • TTAGGG • Limit to ~50 cell divisions • Telomerase extends telomeres • Can add DNA bases at 5’ end • Shorten gradually after each cell division growing replication fork telomerase TTAAGGG TTAAGGG TTAAGGG

  21. Cells Aging Process • Cell senescence • Cells loses ability to function properly as a person ages • Hayflick limit • Telomeres length decrease with age • No longer provide protection for the chromosome • Possibly links to age-related diseases • Dementia, atherosclerosis, macular degeneration • Gene expression changes • Key to living forever?

  22. Packing of Eukaryotic DNA • Organization • Negative DNA wraps around positive histones • Nucleosome – cluster of 8 histones • Solenoids – coiled strings of nucleosomes (chromatin fibres)

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