24 Jun BRIEFLY OUTLINE TWO MECHANISMS BY WHICH DOUBLE-STRAND BREAKS ARE REPAIRED BY A CELL.
1. The three components of the eukaryotic cytoskeleton are microtubules, microfilaments and intermediate filaments. a) State the principle protein component of the microtubules, and briefly describe the structure of a microtubule. b) Describe how microfilaments polymerize from their monomer components. Include a description of the protein component(,0) treadmilling, and the role of ATP and capping proteins. c) The following proteins all interact with microfilaments: ezrin, ARP complex, cofilin, myosin, fimrin. Describe the functions of three of these proteins within the cell. d) Give one example of a protein that may form intermediate filaments and describe the type of structures that intermediate filaments produce. 3. a) Briefly describe the main differences between integral and peripheral membrane proteins in terms of their location and interactions with the lipid bilayer. b) Describe the arrangement of secondary structural elements in one type of multipass transmembrane protein. c) How can antibodies raised against segments of amino acid in a membrane protein be used to provide information on the structure of that protein? 4. a) Briefly describe how the core eukaryotic nucleosome is formed from its protein components and the location of the DNA strand, in relation to the nucleosome. b) How is the local structure of chromatin maintained during genomic DNA replication? c) Describe what occurs to nuclear DNA in a cell that has undergone apoptosis, and explain the characteristic pattern shown by DNA from an apoptotic cell, when it is examined by agarase gel electrophoresis. 5. a) List three ways in which mature mRNA differs from the primary transcript. b) Which of the features of mature eukaryotic mRNA, identified in )a), influence the stability of mRNA and how? c) Briefly explain what is meant by RNA editing, how this process can occur and how it affects the protein that the RNA encodes. 8. a) Briefly describe what is meant by the term ‘growth cone.’ b) What effects do the molecules netrin and semaphorin have on the process of neurit outgrowth? c) What type of molecules are cadherins, and what effect do they have on neurite outgrowth? d) Describe how netrin, semaphorin and slit control the growth of the axons of commissural neurons across the midline and towards the brain during development. 10. a) Briefly describe the G2 checkpoint which is sensitive to DNA damage. Use diagrams as appropriate to explain how the activities of the phosphatase Cdc25 and kinase Weee1 play a role in the control of entry into M phase. b) Explain how the positive feedback loop in this checkpoint functions. c) What are the two principal external causes of double-strand breaks in DNA? d) Briefly outline two mechanisms by which double-strand breaks are repaired by a cell. 11. a) Give one example of a cell-type that may be mobile within the tissues of an adult mammal, and explain how motility contributes to the normal function of that cell b) Define the term ‘chemotaxis’ and give one example of a molecule that can induce chemotaxis. c) Explain with the aid of a labelled diagram how molecules that induce chemotaxis can act on a cell to cause its polarization. The diagram should include information on the receptor for the chemotactic molecule and the intracellular signals and events that follow activation of the receptor. 12. a) Explain the difference between reactive oxygen species and free radicals, and give one example of each. b) What cellular process is the main source of free radicals in eukaryotes and where does it take place? c) What biological molecules can be damaged by interaction with free radicals? Give one example of the way that a free radical can interact with a biological molecule to change its structure. d) Explain the different between primary and secondary antioxidant defenses, giving one example of each.
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