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

2. 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. מנגנון השכפול כולל אם כך שני שלבים :

3. שלב ראשון פתיחת הקשרים שבין שני הגדילים

4. שלב שני כל גדיל משלים את עצמו לסליל כפול

5. קבלת שני סלילים כפולים. בכל סליל מחצית אחת מקורית ( כחול כהה ) ומחצית שנייה ( תכלת ) חדשה. http://highered.mcgraw-hill.com/sites/0072437316/student_view0/chapter14/animations.html#

6. ניסוי מסלזון ושטאל להוכחת מנגנון שכפול הדנ " א http://highered.mcgraw-hill.com/sites/0072437316/student_view0/chapter14/animations.html#

7. שלבי הניסוי 1.These scientists grew the bacteria E-coli in media containing nutrients with the isotope Nitrogen 15, 15 N as opposed to the normal isotope of nitrogen, 14 N. After a time some of the DNA containing the normal isotope is replaced with DNA containing, 15 N as new DNA synthesis proceeds. isotope They cultured the bacteria for a sufficient amount of time that so that they could be sure that all the DNA contained just 15 N. כל הדנ " א של החיידקים מורכב מחנקן 15 N שלב ראשון :

8. שלב שני : 2. they took these bacteria and set up cultures using growth medium containing 14 N 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 14 N על מצע גידול המכיל אחרי כל מחזור חלוקה ( דור ) - אחרי כל מחזור חלוקה ( דור ) - בידוד הדנ " א והפרדתו למקטעים על פי מסה סגולית

9. תוצאות

10. מסקנות מהי הסכמה המתארת נכון את מנגנון שכפול הדנ " א לאור הניסוי ?

11. דור הורים דור צאצאים ראשון דור צאצאים שני