If a 10 cm long piece of string was packed into a 5 cm long tube, the

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If a 10 cm long piece of string was packed into a 5 cm long tube, the packing ratio would be 2 (2 cm of string per cm of tube). The packing ratio is a useful measure of the degree of compactness achieved. The same idea can be applied to the problem of packing DNA into chromosomes.

a. A chromosome 10 μm long was found to contain 8.7 cm of DNA. What is the packing ratio of DNA in this chromosome? Show your working.

b. There are 46 chromosomes in an adult human cell. Their average length is about 6 μm. The total length of DNA in the 46 chromosomes is 1.8 m. What is the overall packing ratio for DNA in human chromosomes? Show your working.

c. A nucleosome packages 50 nm of DNA into a diameter of 11 nm. What is the packing ratio of DNA in nucleosomes?

d. The linker DNA between nucleosomes is 53 base pairs long. Each base pair occupies a 0.34 nm length of the DNA double helix. Nucleosomes plus their linker DNA form a 10 nm wide fibre (the ‘beads-on-a string’ form of chromatin). What is the packing ratio of DNA in this form of chromatin? Show your working.

e. Nucleosomes and their linker DNA (the beads-on-a-string form of chromatin) are coiled into a helix to produce a 30 nm wide fibre called a solenoid. This has about six nucleosomes per turn of the helix. 40 000 nm of DNA are packed into a 1 μm length of this solenoid. What is the packing ratio of 30 nm fibres?

f. Each nucleosome is made of eight histone protein molecules. These molecules are highly conserved during evolution, meaning there are extremely few changes in their amino acid sequences over time (far fewer than is usual). What does this suggest about their functioning?

g. As stated in Question 8b, the total length of DNA in the 46 chromosomes of an adult human cell is 1.8 m. The average adult human contains about 100 trillion cells (100 million million, or 1014).

i. What is the total length of DNA in a human? Express your answer in km.

ii. How many times would this stretch to the Sun and back? (Average distance to Sun = 149 600 000 km = about 150 million km = 1.5 × 10⁸ km.)

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Cambridge International AS And A Level Biology

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Authors: Mary Jones, Richard Fosbery, Jennifer Gregory, Dennis Taylor

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Question Posted: Sep 01, 2021 01:46 AM

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If a 10 cm long piece of string was packed into a 5 cm long tube, the

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