3 HIPOTESIS TEORI REPLIKASI DNA

Summary

This transcript delves into the role of DNA in determining genetic traits, explaining how its sequence varies between individuals. It covers the concept of DNA replication, describing three hypotheses: conservative, dispersive, and semi-conservative. The semi-conservative hypothesis, supported by Watson and Crick, is further validated by Meselson and Stahl's experiment using *Escherichia coli*. Their study, which involved nitrogen isotopes, showed that DNA replication results in hybrid DNA molecules with varying densities. This experiment supports the semi-conservative model, where each strand of the double helix serves as a template for a new strand, ensuring accurate genetic replication.

Takeaways

• 😀 DNA determines an individual's genetic traits due to the unique sequence of base pairs in each living organism.
• 😀 The differences in base pair sequences are identified during DNA analysis, such as sequencing processes.
• 😀 DNA functions both as a heterocatalytic and autocatalytic machine, meaning it helps in the synthesis of other molecules and can replicate itself.
• 😀 DNA replication produces new DNA, following hypotheses that explain the process in detail.
• 😀 The conservative hypothesis suggests that the old DNA double helix stays intact while producing a new one.
• 😀 The dispersive hypothesis proposes that the double helix breaks into segments, which then form new DNA by combining with old segments.
• 😀 The semi-conservative hypothesis suggests that the double helix separates and each strand pairs with a new partner to form new DNA.
• 😀 The semi-conservative replication hypothesis is strongly supported by Watson and Crick’s model, which shows that one strand is retained while the other is newly synthesized.
• 😀 Meselson and Stahl's experiment with Escherichia coli using isotopic nitrogen proved that DNA replication occurs semi-conservatively.
• 😀 In their experiment, E. coli cells were first grown in a medium with heavy nitrogen (15N), and later in normal nitrogen (14N), which showed the DNA had a weight between the two types after one division.
• 😀 The results of the experiment demonstrated that the DNA does not break down or shrink during replication, reinforcing the semi-conservative replication hypothesis.

Q & A

What is the role of DNA in determining genetic traits?

DNA determines genetic traits by having a unique sequence of base pairs that differ between individuals. These differences can be detected through DNA analysis methods like sequencing.

What are the main types of hypotheses regarding DNA replication?

There are three main hypotheses: the conservative hypothesis, where the old DNA double helix remains intact and produces a new one; the dispersive hypothesis, where the double helix is broken and new segments combine with old segments; and the semi-conservative hypothesis, where the two strands of the double helix separate and each binds to its complementary strand.

Which DNA replication hypothesis did Watson and Crick propose?

Watson and Crick proposed that DNA replication occurs semi-conservatively, where each strand of the original DNA molecule serves as a template for the synthesis of a new complementary strand.

How did Meselson and Stahl's experiment support the semi-conservative model?

Meselson and Stahl's experiment involved growing *Escherichia coli* in a medium with heavier nitrogen isotopes (15N) and then transferring the bacteria to a medium with normal nitrogen isotopes (14N). The resulting DNA showed intermediate weight after one division, supporting the semi-conservative model of replication.

What did Meselson and Stahl use in their experiment to label the DNA?

They used ammonium ions containing the heavier nitrogen isotope, 15N, in the growth medium, which allowed them to distinguish the DNA in the bacteria from the normal DNA that used the more common 14N isotope.

Why did Meselson and Stahl's experiment use *Escherichia coli*?

*Escherichia coli* was chosen because it is a well-known bacterium that can live on inorganic salts and can be easily cultured to observe changes in DNA replication, making it ideal for their experiment.

What did Meselson and Stahl observe after the bacteria were transferred to normal ammonium ions?

After transferring the bacteria to a medium with normal ammonium ions (14N), they observed that the DNA from the new generation had an intermediate weight, suggesting that each DNA molecule consisted of one old strand and one newly synthesized strand.

What did the intermediate weight of the DNA after one cell division indicate?

The intermediate weight of the DNA after one cell division indicated that DNA replication was semi-conservative, meaning each new DNA molecule consisted of one old strand and one newly synthesized strand.

How did the use of different nitrogen isotopes help distinguish between old and new DNA strands?

The use of different nitrogen isotopes (15N and 14N) allowed Meselson and Stahl to distinguish between old DNA strands (with 15N) and newly synthesized strands (with 14N), helping to demonstrate how DNA replication works.

What was the significance of the two types of DNA that Meselson and Stahl observed in their experiment?

The two types of DNA—one with normal weight and one with intermediate weight—demonstrated that DNA replication does not involve complete breakdown or shrinkage of the original DNA strands. Instead, the semi-conservative model was confirmed, where each parent strand is conserved and paired with a newly synthesized strand.