Question

. A 10 decimeter long DNA is packed inside a nucleus whose diameter is 4um. What is the level of DNA compaction in this case? Please show calculations to get full credit. (1 pt) Why don't living cells have DNA replication in the 3 to 5' direction? Be very specific

Answer 1

Diameter of the nucleus = 4 uM = 4 X 10^-6 m
Length of DNA = 10 decimeter = 1 m

1 m condensed to 1 um = 10⁶ compaction
1 m condensed to 4 um = (10⁶/4) compaction
= 1000000/4
= 250000

DNA strand extension can only occur in 5'-3' direction. This is due to the fact that DNA polymerase can only add an incoming nucleotide to the 3'-OH group. The -OH group of the existing nucleotide can perform an SN2 nucleophilic attack on the incoming nucleotide to form the phosphodiester bond. This also enables the mismatch repair to be energetically feasible. There is no nucleic acid polymerase in any living system that can polymerize in 3'-5' direction. The need for proofreading explains why DNA replication occurs only in 5'-3' direction.

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