Chromatin structure | Anatomy2Medicine
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Chromatin structure

 

  • Chromatin structure

 

    • DNA exists in the condensed, chromatin form in order to fit into the nucleus.
    • Negatively charged DNA loops twice around positively charged histone octamer to form nucleosome “bead.”
    • Histones are rich in the amino acids lysine and arginine.
    • H1 binds to the nucleosome and to “linker DNA,” thereby stabilizing the chromatin fiber.
    • H1 is the only histone that is not in the nucleosome core.
    • In mitosis, DNA condenses to form chromosomes.
    • DNA and histone  synthesis occur during S Phase.
      • HYF for exam – The single finding of high autoantibody titers to histones, without any other autoantibodies, is characteristic of drug-induced lupus. The most commonly implicated drugs are procainamide, hydralazine  and isoniazid.
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      Presentation1

      Ans A
      Exam MCQ
      In DNA tertiary structure, what is a histone octamer?
      a) A complex consisting of eight positively charged histone proteins (two of each H2A, H2B, H3 and H4) that aid in the packaging of DNA
      b) A complex consisting of eight negatively charged histone proteins (two of each H2A, H2B, H3 and H4) that aid in the packaging of DNA
      c) A complex consisting of nine positively charged histone proteins (H1 and two of each H2A, H2B, H3 and H4) that aid in the packaging of DNA.
      d) A complex consisting of nine negatively charged histone proteins (H1 and two of each H2A, H2B, H3 and H4) that aid in the packaging of DNA.
      Ans A
      A histone octamer is a complex of eight positively charged histone proteins (two of each H2A, H2B, H3 and H4) that aid in the packaging of DNA. Negatively charged DNA wraps around these histone octamers to form the nucleosome. The DNA is held there by ionic bonds. Linker histone H1 binds to each nucleosome where the DNA enters and exits and this draws a string of nucleosomes closer together to form the 10nm fibre
      Exam MCQ
      Which statement is false?
      A)  In eukaryotes, histones are poorly conserved.
      B)  There are at least five different classes of histones.
      C)   Histones are richer in lysine and arginine residues than most other proteins.
      D)  The 30nm fiber form of chromatin is more compact than the beads on a string form.
      Ans a
      Exam MCQ
      A core nucleosomes is composed of (choose the best answer)
      A)  two copies each of histone H2A, H2B, H3 and H4 and 147 bp of DNA
      B)  two copies each of histone H2A, H2B, H3 and H4 and 1470 bp of DNA.
      C)  four copies each of histone H2A, H2B, H3 and H4 and 147 bp of DNA.
      D)  four copies each of histone H2A, H2B, H3 and H4 and 1470 bp of DNA.
      Ans A
      Exam MCQ
      When histone H1 is incorporated into a nucleosome
      A)  the nucleosome is about to be moved by a remodeling enzyme.
      B)  it means that replication of the DNA has just occured.
      C)  it increases the probability that condensation of the chromatin will occur.
      D)  it means that the underlying DNA is probably a core promoter.
      Ans C
      Exam MCQ
      In vitro, the rate of transcription from DNA is
      A)  higher when histones are present than when histones are absent.
      B)  lower when histones are present than when histones are absent.
      C)  proceeds in the wrong direction unless histones are present
      Ans B
      Exam MCQ
      Where would you most likely find chromatin in the beads on a string conformation?
      A)  Heterochromatin
      B)  Actively transcribed chromatin
      C)  Silenced chromatin
      D)  Deacetylated chromatin
      E)  30 nm fiber chromatin
      Ans B
      HeterochromatinCondensed, transcriptionally inactive,sterically inaccessible.
      HeteroChromatinis Highly Condensed.
      Euchromatin
      Less condensed, transcriptionally active, sterically accessible.
      What is “truly transcribed.” Is Euwhich is true
      DNA methylationTemplate strand cytosine and adenine are methylated in DNA replication, which allows mismatch repair enzymes to distinguish between old and new strands in prokaryotes
      DNA methylation at CpG islands represses transcription.
      Histone methylation
      Usually reversibly represses DNA transcription, but can activate it in some cases.
      Histone Methylation makes DNA Mute.
      CpG Methylation makes DNA Mute.
      Histone acetylation
      Relaxes DNA coiling, allowing for transcription
      Histone Acetylation makes DNA Active.
      Exam MCQ
      Ans A
      Exam MCQ
      Ans A
      Exam MCQ
      Ans A
      Exam MCQ
      Two potential devices that eukaryotic cells use to regulate transcription are
      DNA methylation and histone amplification.
      DNA amplification and histone methylation.
      DNA acetylation and methylation.
      DNA methylation and histone acetylation.
      histone amplification and DNA acetylation.  

      Presentation1