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?

Verified step by step guidance

1. Identify the key components of DNA replication, focusing on the role of Okazaki fragments in the lagging strand synthesis.
2. Understand that during DNA replication, the leading strand is synthesized continuously, while the lagging strand is synthesized in short segments known as Okazaki fragments.
3. Recognize that the short pulse of ³H-thymidine labels newly synthesized DNA, allowing the tracking of DNA synthesis over time.
4. Consider that initially, the labeled DNA will appear as short fragments because the lagging strand is synthesized in these short segments.
5. As time progresses, these short fragments are joined together by DNA ligase, resulting in longer DNA strands, which explains the observed shift from short to longer labeled DNA strands.

Key Concepts

DNA Replication

DNA replication is the biological process by which a cell duplicates its DNA before cell division. It involves unwinding the double helix and synthesizing new strands complementary to the original ones. This process is crucial for genetic inheritance and is facilitated by enzymes such as DNA polymerase, which adds nucleotides to form new strands.

Okazaki Fragments

Okazaki fragments are short sequences of DNA nucleotides synthesized on the lagging strand during DNA replication. They are formed because DNA polymerase can only synthesize DNA in the 5' to 3' direction, leading to the discontinuous synthesis of the lagging strand. These fragments are later joined together by the enzyme DNA ligase to create a continuous strand.

Centrifugation and Denaturing Gradients

Centrifugation is a technique used to separate components of a mixture based on their density by spinning them at high speeds. In the context of DNA analysis, denaturing gradients can be employed to separate DNA fragments of different lengths. As DNA strands are subjected to denaturing conditions, shorter fragments migrate differently than longer ones, allowing researchers to analyze the size distribution of synthesized DNA.