BIOC0001 Practicals

BIOC0001 Practicals How many base pairs long is a typical gene? - ~10^4bp. What is supercoiling? What is its purpose? - The twisting of a DS DNA helix around itself allowing it to be packaged efficiently into a cell. Why might two sections of DNA which are the same length travel at different rates during gel electrophoresis? - Some of the DNA, but not all, becomes relaxed during laboratory preparation (the state of a circular, non-supercoiled plasmid). Some of the DNA remains supercoiled and so is more compact meaning it migrates faster than its equally sized linear or relaxed counterpart. How are restriction endonucleases made? - They are made naturally by bacteria as a defence against viral infection. What are all restriction sites recognised by restriction endonucleases known as? - Palindromic. Why are all restriction sites palindromic? - It allows the restriction endonuclease to cut both strands at the same point. What is the gel used in gel electrophoresis? Why? - Agarose. What type of molecule is agarose? - Polysaccharide. Describe the movement of DNA fragments in agarose gel electrophoresis. - They move towards the anode (positive end) of the gel by moving through pores in the gel. The larger the fragment, the more drag it will experience whilst moving through the gel and so the slower it will move. What chemical is used to visualise DNA and how does it work? - Ethidium bromide which intercalates (inserts) between bases and will fluoresce a pink-orange colour when illuminated with UV light (300nm). What wavelength is the UV light used to show the ethidium bromide? - 300nm. What does a restriction map of a DNA fragment show? - The positions of the different target sites for various restriction endonucleases. Draw the locations of three restriction sites P, Q and R on a 9kb plasmid given the following information: