1. Describe the Hershey-Chase experiment and indicate its findings.
2. Differentiate between B-DNA and Z-DNA. Which form is more amenable to binding of transcription factors? How might this affect gene expression?
3. Outline the process of DNA replication, and describe the roles of the enzymes involved.
The Correct Answer and Explanation is :
1. Hershey-Chase Experiment and Findings
The Hershey-Chase experiment, conducted in 1952 by Martha Chase and Alfred Hershey, provided crucial evidence supporting DNA as the genetic material. Using the T2 bacteriophage (a virus that infects bacteria), they tested whether DNA or protein was responsible for carrying genetic information. They labeled the DNA of the virus with radioactive phosphorus (P-32) and the protein coat with radioactive sulfur (S-35). The virus was then allowed to infect E. coli bacteria.
After infection, the researchers used a blender to separate the viral protein coats from the bacterial cells. They then measured the radioactivity inside the bacteria. The results showed that the phosphorus-labeled DNA entered the bacterial cells, while the sulfur-labeled protein did not. This indicated that DNA, not protein, was the genetic material that the virus used to replicate within the bacteria.
2. B-DNA vs Z-DNA
B-DNA and Z-DNA are two forms of the DNA double helix.
- B-DNA is the most common and stable form of DNA under physiological conditions. It has a right-handed helix with 10 base pairs per turn, which is the typical structure of DNA in most organisms.
- Z-DNA, on the other hand, is a left-handed helix with 12 base pairs per turn. It occurs under certain conditions, like high salt concentrations or in sequences with alternating purines and pyrimidines (such as GC-rich regions). Z-DNA is considered a transient form of DNA and less stable than B-DNA.
Binding of Transcription Factors:
B-DNA is more amenable to binding transcription factors because of its more stable and open structure, making it accessible for interactions with regulatory proteins. Z-DNA, being more tightly wound and less stable, is less likely to bind transcription factors easily.
Effect on Gene Expression:
The preferential binding of transcription factors to B-DNA can facilitate the initiation of transcription. On the other hand, Z-DNA may play a role in regulating gene expression by forming in regions of active transcription or DNA replication, influencing gene activity indirectly by making the DNA more flexible or creating signals that can modulate transcription factor binding.
3. DNA Replication Process
DNA replication is a semi-conservative process that occurs during the S-phase of the cell cycle, where a new DNA strand is synthesized using an existing strand as a template. The process involves several key steps:
- Initiation: Replication begins at specific locations called origins of replication. The enzyme helicase unwinds the double helix, separating the two strands and creating replication forks. The enzyme single-strand binding proteins (SSBs) then stabilize these single strands.
- Priming: The enzyme primase synthesizes a short RNA primer to provide a starting point for DNA synthesis. This primer is essential because DNA polymerase, the enzyme that synthesizes DNA, can only add nucleotides to an existing strand.
- Elongation: The enzyme DNA polymerase III adds nucleotides to the 3’ end of the RNA primer, extending the new DNA strand. On the leading strand, replication is continuous. On the lagging strand, replication occurs in fragments known as Okazaki fragments. DNA polymerase I then removes the RNA primers and fills in the gaps with DNA nucleotides.
- Termination: Replication ends when the replication forks meet or when the end of the chromosome is reached. The enzyme ligase seals the nicks between the Okazaki fragments, completing the process.
This highly coordinated series of enzymatic activities ensures the accurate and faithful duplication of the DNA, allowing genetic information to be passed on to daughter cells.