7. The purpose of using dideoxynucleotides in Sanger's classic method for DNA sequencing is to terminate the DNA synthesis reaction at specific points. Dideoxynucleotides lack a 3' hydroxyl group, which is necessary for the formation of phosphodiester bonds between nucleotides. By incorporating dideoxynucleotides into the DNA synthesis reaction, the chain termination occurs at random positions, resulting in a mixture of DNA fragments of different lengths. These fragments can then be separated by size using gel electrophoresis, allowing for the determination of the DNA sequence.
8. The CRISPR-Cas9 gene editing system is used by bacteria to protect themselves from invading bacteriophages. When a bacteriophage infects a bacterium, it introduces its DNA into the bacterial cell. The bacteria's CRISPR system recognizes and captures a small piece of the phage's DNA and incorporates it into its own genome as a spacer. This spacer is transcribed into a small RNA molecule called CRISPR RNA (crRNA), which guides the Cas9 protein to the complementary sequence in the phage's DNA. Once bound, Cas9 cuts the phage's DNA, disabling it and preventing further infection. This adaptive immune system allows bacteria to remember and defend against specific phages in the future.