Question

1. A Which DNA replication model is the correct model and shows this best

   2 A) Suppose that you have a chemical substance, how do you test if this substance is a mutagen?

   3 A) What are the differences of transcription and translation in prokaryotes and eukaryotes?

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Answer 1

A. DNA replication is the biological process of making copies of DNA occurring in all living organisms. There are three types of DNA replication, conservative, semiconservative and dispersive. In conservative DNA replication, the parent DNA strands stay associated together, while the new daughter DNA forms its own double-helix. In dispersive mode, the two daughter DNA strands have alternating segments of both parental and newly synthesized DNA. In semi-conservative DNA replication, half of the parent DNA double helix is conserved in each of the two daughter DNA molecules. So these daughter cells are genetically identical to one another and genetically identical to the parent cell. This is why semi-conservative DNA replication model is choose the correct model. Meselson and Stahl showed that DNA follows the semi-conservative model.

2.A.

Many tests have been developed for detecting chemical substances as mutagens. In in vitro methods bacteria, bacteriophages, human or other mammalian cells are using to test the mutagenicity of a chemical substance. Many in vivo tests are also conducted in fruit flies or mammals to test the genotoxicity of a chemical substance. Mutagenesity and genotoxicity are often used because they are synonymous.

The Ames test is a widely employed biological assay to detect whether a given chemical can cause mutations in the DNA of the test organism (bacteria). This assay assesses the mutagenic potential of the tested chemical compounds. In Ames test the bacteria (Salmonella typhimurium) are spread on an agar plate with small amount of histidine in the agar. Salmonella typhimurium carry mutations in genes involved in histidine synthesis. So the histidine in the growth medium allows the bacteria to grow and mutate. When the histidine is depleted in the growth media, only mutated bacteria gain the ability to produce its own histidine will survive. The mutagenicity of a tested chemical substance is proportional to the number of colonies observed in the growth media after 48 hours of incubation.

3.A. Transcription and translation are coupled in prokaryotes; when translation begins the mRNA is still being synthesized. When bacterial genes are transcribed there is no nucleus to separate the processes of transcription and translation, so their transcripts can immediately be translated. But in a eukaryotic cell, the transcription and translation are not coupled, transcription occurs in the nucleus, and translation occurs in the cytoplasm.