The replication or The replication or duplication of DNA duplication of DNA depends on one main idea, depends on one main idea, namely that the nitrogen namely that the nitrogen bases of the nucleotides bases of the nucleotides are are complementarycomplementary to each to each other on opposite halves of other on opposite halves of the molecule as shown the molecule as shown here. Thus here. Thus the base the base adenine(A) in DNA always adenine(A) in DNA always pairs with thymine(T) and pairs with thymine(T) and guanine(G) with guanine(G) with cytosine(C).  cytosine(C).  This pairing is due to the This pairing is due to the shapes of the nucleotide shapes of the nucleotide bases which allow bases which allow hydrogen bonds to form hydrogen bonds to form between the between the complementary bases, complementary bases, holding the two halves of holding the two halves of the DNA molecule the DNA molecule togethertogether. 

This idea provided an immediate solution to how DNA replicated. All that should have to happen is the following sequence: 1. The parent DNA molecule unzips exposing the two halves of the DNA molecule 2. Each half of the parent DNA molecule serves as a template for the complementarity bases to be brought into the correct position to make a new complementary half for each of the original parent halves. This theoretical consideration suggested to Watson and Crick that DNA replication is what we now call semiconservative.

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שלבי הניסוי

1. These scientists grew the bacteria E-coli in media containing nutrients with the isotope Nitrogen 15, 15N as opposed to the normal isotope of nitrogen, 14N. After a time some of the DNA containing the normal isotope is replaced with DNA containing ,15N as new DNA synthesis proceeds.

They cultured the bacteria for a sufficient amount of time that so that they could be sure that all the DNA contained just 15N.

כל הדנ"א של החיידקים מורכב מחנקן15N

שלב :ראשון

:שלב שני

2. they took these bacteria and set up cultures using growth medium containing 14N and followed the bacteria through a number of replication cycles.

שלב :שלישי

3. they could separate DNA's with differing amounts of 15N by using a process called cesium density gradient centrifugation which separates molecules by weight in a solution of cesium chloride. These scientists noted that one would expect a different distribution of DNA's by weight depending on whether the type of replication involved.

גידול החיידקים למשך מספר מחזורי חלוקה

14N על מצע גידול המכיל

אחרי כל מחזור חלוקה ) דור( -אחרי כל מחזור חלוקה ) דור( -בידוד הדנ"א והפרדתו למקטעים על

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