DNA polymerases in all organisms add only 5' nucleotides to the 3' end of a growing DNA strand, never to the 5' end. One possible reason for this is the fact that most DNA polymerases have a proofreading function that would not be energetically possible if DNA synthesis occurred in the 3' to 5' direction.
Sketch the reaction that DNA polymerase would have to catalyze if DNA synthesis occurred in the 3' to 5' direction.

Bloom syndrome (OMIM 210900) is an autosomal recessive disorder caused by mutation of a DNA helicase. Among the principal symptoms of the disease are chromosome instability and a propensity to develop cancer. Explain these symptoms on the basis of the helicase mutation.

How does rolling circle replication differ from bidirectional replication?

The genome of D. melanogaster consists of approximately 1.7x10⁸ base pairs. DNA synthesis occurs at a rate of 30 base pairs per second. In the early embryo, the entire genome is replicated in five minutes. How many bidirectional origins of synthesis are required to accomplish this feat?

DNA polymerases in all organisms add only 5' nucleotides to the 3' end of a growing DNA strand, never to the 5' end. One possible reason for this is the fact that most DNA polymerases have a proofreading function that would not be energetically possible if DNA synthesis occurred in the 3' to 5' direction.

Consider the information in your sketch and speculate as to why proofreading would be problematic.

Assume that the sequence of bases shown below is present on one nucleotide chain of a DNA duplex and that the chain has opened up at a replication fork. Synthesis of an RNA primer occurs on this template starting at the base that is underlined.

In the intact RNA primer, which nucleotide has a free 3'-OH terminus?

3'.......GGCTACCTGGATTCA....5'

Assume that the sequence of bases shown below is present on one nucleotide chain of a DNA duplex and that the chain has opened up at a replication fork. Synthesis of an RNA primer occurs on this template starting at the base that is underlined.

If the RNA primer consists of eight nucleotides, what is its base sequence?

3'.......GGCTACCTGGATTCA....5'

Reiji and Tuneko Okazaki conducted a now classic experiment in 1968 in which they discovered a population of short fragments synthesized during DNA replication. They introduced a short pulse of ³H-thymidine into a culture of E. coli and extracted DNA from the cells at various intervals. In analyzing the DNA after centrifugation in denaturing gradients, they noticed that as the interval between the time of ³H-thymidine introduction and the time of centrifugation increased, the proportion of short strands decreased and more labeled DNA was found in larger strands. What would account for this observation?

DNA replication in early Drosophila embryos occurs about every 5 minutes. The Drosophila genome contains approximately 1.8×10⁸ base pairs. Eukaryotic DNA polymerases synthesize DNA at a rate of approximately 40 nucleotides per second. Approximately how many origins of replication are required for this rate of replication?