Concept 29 DNA is packaged in a chromosome.
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Hi! 200 base pairs of DNA are wrapped around a histone core to make a nucleosome. If one base pair of DNA is 0.3 nm long, and the nucleosome has a diameter of 11 nm, how much has the DNA been "packed" along the axis of the 10-nm fiber? Choices: 3X 30X none 6X 60X Answers: No, this is true if 100 bp of DNA is wrapped around each histone core. No, this is true if 1000 bp of DNA is wrapped around each histone core. No, the DNA has been 'shrunken' and packaged by wrapping around the nucleosome. That is correct. No, this is true if 2000 bp of DNA is wrapped around each histone core. Since 200 base pairs of DNA are wrapped around each histone core, that means about 60 nm of DNA have been reduced to the width of 11 nm — a factor of six. This is the packing ratio. In the 30-nm fiber, the nucleosomes are arranged in groups of six. A group of six nucleosomes has 1,200 bp of DNA. What is the packing ratio for the 30-nm fiber along its axis? Choice: 6X 60X 12X none of the above Answers: No, this is the packing ratio for the 10 nm fiber. No, this is not correct. That is correct. No, there is a correct answer. There are six nucleosomes along the axis of the 30-nm fiber. Each nucleosome has a packing ratio of six, so the packing ratio of the 30-nm fiber is 6 X 6 = 36. In December 1999, the first human chromosome was completely sequenced. Chromosome #22 is one of the smallest human chromosomes and has 33.5 million base pairs of DNA. Without packaging, what is the length of chromosome 22? Remember 1 base pair = 0.3 nm. Choices: 1mm 10mm 100mm Answers: No, check your calculations. That is correct. No, check your calculations. Without packaging, the DNA in a small chromosome stretches out to 10 mm. 1 base pair = 0.3 nm 33,500,000 base pairs = 10,050,000 nm = 10 mm The nucleus in most cells averages 5 mm in diameter. Even packaging into the 30-nm fiber is not enough to fit one chromosome's worth of DNA into the nucleus. Higher-order packaging — looping chromosomes onto protein scaffolds — completes the process.