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2012-11-29 07:11:40

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  1. What are polytene chromsosomes? Lampbrush chromsomes?
    • Polytene chromsomes: paired homologues that are comprised of large numbers of identical DNA strands.  They undergo replication without strand separation or cytoplasmic division.  Puffs are created when the bands separate during transcription.
    • Lampbrush chromosomes: meiotic chromsomes.  Tend to extend (not condense) during meiosis.  Loops, similar to polytene puffs, indicate DNA being used in transcription.
  2. What is a nucleosome? Give details of the nucleosome structure
    • Histones: positively charged proteins associated with DNA
    • nonhistone proteins: less positively charged...
    • Histones are able to bond electrostatically to the negatively charged phosphate groups of nucleotides, and occur regularly (~every 200 bp) along DNA. 
    • Nucleosome: DNA coils around an octamer of histones (H2A)2+(H2B)2 and (H3)2+(H4)2. This is the first level of packing, reducing DNA to ~1/3 its original length.
  3. What are the different types of histone proteins associated with eukaryotic chromatin
    • H1: further packs nucleosomes into solenoids (second level of packing, 6x DNA compaction)
    • (H2A)2+(H2B)2: tetramer responsible for 1/2 of the nucleosome histone complex
    • (H3)2+(H4)2: tetramer responsible for 1/2 of the nucleosome histone complex
  4. What is the difference between heterochromatin and euchromatin?
    • heterochromatin: genetically inactive due to lack of genes or repressed genes.  Replicate later during the S phase than euchromatin.  eg. telomere, centromere, much of Y chromosome
    • euchromatin: genetically active, less dense areas of the chromosome ("normal chromatin")
  5. What is satellite DNA?
    • Differs from main-band DNA in its molecular composition.
    • Consists of short repetitive sequences.
    • Found in heterochromatic centromeric regions of chromsomes.
    • Shows as an additional peak when analzying DNA composition.
    • Not present in prokaryotes
  6. What are the different types of repetitive DNA found in eukaryotic chromsosmes?
    • Highly repetitive sequences: (Centromeric and Telomeric DNA)
    • Centromeric DNA: found in the centromere
    • Telomeric DNA: found in the telomere
    • Middle repetitive sequences: (VNTRs and STRs) most prominent are noncoding sequences
    • Variable number tandem repeats: (minisatellites) found within and between genes, variations in length originally used for DNA fingerprinting
    • Short tandem repeats: (microsatellites) disperesed throughout genome, vary based on number of repeats at a given site
    • Repetitive Transposed Sequences: (SINEs and LINEs) interspersed individually throughout the genome, can be short or long, are transposable
    • Short interspersed elements: <500bp long, may be present 500000 times in the human genome. 
    • Long interspersed elements: ~6kb long, may be present 850000 times
  7. What are transposable sequences?
    • Transposable sequences are mobile sequences that have the potential to move to different locations within the genome.  1) transcription into RNA molecule
    • 2) RNA serves as template for DNA complement using reverse transcriptase
    • 3) integration into chromsome at new site