Chapter 12 DNA organization in chromosomes

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  1. Bacterial and viral chromosomes are usually
    • a single nucleic acid molecule
    • largely devoid of associated proteins
    • much smaller than eukaryotic chromosomes
    • single or double-stranded DNA or RNA
    • viral can be circular or linear molecules
    • material is inert until released into host cell
  2. Nucleoid
    • bacterial chromosomes are circular, double stranded DNA compacted into nucleoid region
    • bacterial chromosomes can be readily replicated and transcribed
  3. HU and H1 DNA-binding proteins
    DNA in bacteria may be associated with these proteins
  4. Supercoiling
    • compacts DNA
    • most circular DNA molecules in bacteria are slightly underwound and supercoiled
  5. Topoisomerases
    • cut one or both DNA strands and wind or unwind the helix before resealing the ends
    • found in eukaryotes along with supercoiled DNA
  6. What creates supercoils downstream as the double helix unwinds in pro and eukaryotes?
    DNA replication and transcription
  7. Polytene chromosomes and lampbrush chromosomes
    • very large eukaryotic chromosomes that can be visualized by light microscopy
    • these chromosomes are unusual and not usually found in eukaryotic cells
  8. Polytene chromosomes
    • found in various tissues in the larvae of some flies and several species of protozoans and plants
    • have distinctive banding patterns
    • represent paired homologs
    • composed of large numbers of identical DNA strands
  9. Chromomeres
    distinctive banding patterns
  10. what is special about paired homologs of polytene chromosomes?
    the DNA of the paired homologs of polytene chromosomes undergoes many rounds of replication without strand separation or cytoplasmic dividsion
  11. Puff regions
    occurs in polytene chromosomes where the DNA has uncoiled that are visible manifestations of a high level of gene activity (transcription that produces RNA)
  12. Lampbrush
    • chromosomes are large and have extensive DNA looping
    • found in most vertebrate oocytes as well as spermatocytes of some insects
    • found in the diplotene stage of prophase I of meiosis
    • lampbrush loops are similar to puffs in polytene chromosomes and are sites of geen activity
  13. Chromatin
    • eukaryotic chromosomes complexed into a nucleoprotein structure
    • during interphase, chromatin is dispersed throughout nucleus
    • during cell division, chromatin condenses and becomes visible as chromosomes
    • bound up in nucleosomes with a positive charged proteins called histones or nonhistones
  14. Histones
    positive charged proteins
  15. Five main types of histones
    H1,H2A,H2B,H3, and H4
  16. Nucleosomes
    • electronic microscopic observations of chromatin have revealed its fibers are composed of a linear array of spherical particles (nucleosomes)
    • concensed several times to form the intact chromatids
  17. To allow replication and gene expression, chromatin must
    relax its compact structure and expose regions of DNA to regulatory proteins
  18. Histone tails
    • unstructured tails that are not packed into the folded histone domains with the nucleosome but protrude from them
    • provide potential targets along the chromatin fiber for chemical modifications that may include acetylation, methylation, phosphorylation
  19. Euchromatin
    uncoiled and active
  20. Heterochromatin
    • condensed areas and are inactive because they either lack genes or contain genes that are repressed
    • telomere maintains chromosome integrity
    • centromere is involved in chromosome movement
  21. C-banding
    only the centromeres (heterochromatin are stained)
  22. G-banding
    due to differential staining along the length of each chromosome
  23. Differential staining reactions reflect
    • heterogeneity and complexity of the chromosome
    • unique banding pattern allows the distinction of identical-sized chromosomes and centromere placement
    • precision allows homologs to be distinguished from one another, including translocated segments
  24. Repetitive DNA
    • sequences are repeated many times within eukaryotic chromosomes
    • there are a number of categories of repetitive DNA
  25. Satellite DNA
    • highly repetitive and consists of short repeated sequences
    • found in the heterochromatic regions of chromosomes
    • not found in prokaryotes
  26. Centromeres
    • primary constrictions along eukaryotic chromosomes
    • mediate chromosomal migration during mitosis and meiosis
  27. Kinetochore proteins
    region that bind to the spindle fibers during cell division
  28. Telomeric DNA
    sequences consist of short tandem repeats that contribute to the stability and integrity of the chromosome
  29. Moderately repetitive DNA
    • variable number tandem repeats (VNTRs)
    • minisatellites
    • Microsatellites (short tandem repeats, STRs)
  30. Short interspersed elements (SINES) and long interspersed elements (LINES)
    dispersed throughout the genome rather than tandemly repeated and constitute over 1/3 of the human genome
  31. Retrotransposons
    transposable elements are generated via an RNA intermediate
  32. Pseudogenes
    • large number of single-copy noncoding regions
    • these are DNA sequences that have undergone significant mutational alterations (inserations and deletions) and are not transcribed
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Chapter 12 DNA organization in chromosomes
2013-10-28 21:29:09

Chapter 12 genetics notes
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