html5-img
1 / 53

Genes & Chromosomes

Genes & Chromosomes. Part III, Chapters 24, 25. Central Dogma . DNA replicates  more DNA for daughters (Gene w/in) DNA transcribed  RNA Gene = segment of DNA Encodes info to produce funct’l biol product RNA translated  protein. Genome. Sum of all DNA

adonica
Download Presentation

Genes & Chromosomes

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  1. Genes & Chromosomes Part III, Chapters 24, 25

  2. Central Dogma • DNA replicates  more DNA for daughters • (Gene w/in) DNA transcribed  RNA • Gene = segment of DNA • Encodes info to produce funct’l biol product • RNA translated  protein

  3. Genome • Sum of all DNA • Genes + noncoding regions • Chromosomes • Each w/ single, duplex DNA helix • Contain many genes • Historical: One gene = one enzyme • Now: One gene = one polypeptide • Some genes code for tRNAs, rRNAs • Some DNA sequences (“genes”) = recognition sites for beginning/ending repl’n, transcr’n

  4. Most gene products are “proteins” • Made of aa’s in partic sequence • Each aa encoded in DNA as 3 nucleotide seq along 1 strand of dbl helix • How many nucleotides (or bp’s) needed for prot of 350 aa’s?

  5. Prokaryotic DNA • Viruses • Rel small amt DNA • 5K to 170K base pairs (bp’s) • One chromosome • Chromosome = “packaged” DNA • Many circular

  6. Bacterial DNA -- larger than viral • E. coli ~4.6 x 106 bp’s • Both chromosomal, extrachromosomal • Usually 1 chromosome/cell • Extrachromosomal = plasmid • 103-105 bp’s • Replicate • Impt to antibiotic resistance

  7. Chromosomes Complex Packaging reduces E.coli DNA 850x

  8. Eukaryotic DNA • Many chromosomes • Single human cell DNA ~ 2 m • Must be efficiently packaged

  9. Euk Chromosomes • Prok’s – usually only 1 cy of each gene (but exceptions) • Euk’s (ex: human) • Book: coding region (genes coding for prot’s) ~ 1.5% total human genome • Exons

  10. Euk’s (ex: mouse): ~30% repetitive • “Junk”? • Non-transcribed seq’s • Centromeres – impt during cell division • Telomeres – help stabilize DNA • Introns – “intervening” seq’s • Function unclear • May be longer than coding seq’s (= exons)

  11. Supercoiling • DNA helix is coil • Relaxed coil not bent • BUT can coil upon itself  supercoil • Due to packing; constraints; tension • Superhelical turn = crossover • Impt to repl’n, transcr’n • Helix must relax so can open, expose bp’s • Must unwind from supercoiling

  12. Topoisomerases • Enz’s in bacteria, euk’s • Cleave phosphodiester bonds in 1/both strands • Where are these impt in nucleic acids? • Type I – cleaves 1 strand • Type II – cleaves both strands • After cleavage, rewind DNA + reform phosphodiester bond(s) • Result – supercoil removed

  13. Type I

  14. Type II

  15. DNA Packaging • Chromosomes = packaged DNA • Common euk “X”- “Y”-looking structures • Each = single, uninterrupted mol DNA • Chromatin = chromosomal material • Equiv amts DNA + protein • Some RNA also assoc’d

  16. 1st Level Pakaging in Euk’s Around Histones • DNA bound tightly to histones

  17. Histones • Basic prot’s • About 50% chromosomal mat’l • 5 types • All w/ many +-charged aa’s • Differ in size, amt +/- charged aa’s • What aa’s are + charged? • Why might + charged prot be assoc’d w/ DNA helix? • 1o structures well conserved across species

  18. Must remove 1 helical turn in DNA to wind around histone • Topoisomerases impt

  19. Histones bind @ specific locations on DNA • Mostly AT-rich areas

  20. Nucleosome • Histone wrapped w/ DNA •  7x compaction of DNA • Core = 8 histones (2 copies of 4 diff histone prot’s) • ~140 bp DNA wraps around core • Linker region -- ~ 60 bp’s extend to next nucleosome • Another histone prot may“sit” outside • Stabilizes

  21. Chromatin • Further-structured chromosomal mat’l • Repeating units of nucleosomes • “Beads on a string” • Flexibly jointed chain

  22. 30 nm Fiber • Further nucleosome packing • ~100x compaction • Some nucleosomes not inc’d into tight structure

  23. Rosettes • Fiber loops around nuclear scaffold • Proteins + topoisomerases incorporated • 20-100K bp’s per loop • Related genes in loop • Book ex: Drosophila loop w/ complete set genes coding for histones • ~6 loops per rosette = ~ 450K bp’s/ rosette • Further coiling, compaction  10,000X compaction total

  24. Semiconservative Replication • 2 DNA strands/helix • Nucleotide seq of 1 strand automatically specifies complementary strand seq • Base pairing rule: A w/ T and G w/ C ONLY in healthy helix • Each strand serves as template for partner • “Semiconservative” • Semi – partly • Conserved parent strand

  25. DNA repl’n  daughter cell w/ own helix • 1 strand is parental (served as template) • 2nd strand is newly synth’d

  26. Definitions • Template • DNA strand w/ precise info for synth complementary strand • = parental strand during repl’n • Origin • Unique point on DNA helix (strand) @ which repl’n begins • Replication Fork • Site of unwinding of parental strand and synth of daughter strand • NOTE: helix unwinding crucial to repl’n success

  27. Repl’n Fork – cont’d • Bidirectional repl’n • 2 repl’n forks simultaneously synth daughter strands

  28. At Replication Fork • Both parental strands serve as templates • Simultaneous synth of daughter cell dbl helices • Expected • Helix unwinds  repl’n fork • Get 2 free ends • 1 end 5’ –PO4, 1 end 3’ –PO4 • REMEMBER: paired strands of helix antiparallel

  29. Expected -- cont’d • Repl’n each strand at end of parent • One strand will replicate 5’  3’ • Direction of active repl’n 5’  3’ • Happens @ parent strand w/ 3’ end • Yields 2nd antiparallel dbl helix • One strand will replicate 3’  5’ • Direction of active repl’n 3’  5’ • Happens @ parent strand w/ 5’ end • Yields antiparallel dbl helix

  30. But, exper’l evidence: • Repl’n ALWAYS 5’  3’ • Can envision at parental strand w/ 3’ end • What happens at other parental strand??

  31. Okazaki Fragments • Discovered by Dr. Okazaki • Found near repl’n fork • Small segments daughter strand DNA synth’d 5’  3’ • Along parental template strand w/ 5’ end • Get series small DNA segments/fragments • So synth along this strand in opp direction of overall replication (or of unwinding of repl’n fork)

  32. “Lagging strand” • Takes longer to synth fragments + join them • Other parental strand, w/ continuous synth “leading strand” • W/ repl’n, fragments joined enzymatically  complete daughter strand • Overall, repl’n on both strands in 5’  3’ direction (w/ respect to daughter)

More Related