The introduction should provide the context in which the experiments were done and the major objectives of the research. It should begin with general background that develops the theoretical framework for the specific questions under investigation. A good introduction begins with a broad perspective and then focuses the readers attention to a series of specific questions or hypotheses.
Start off with the "big picture". Why is this question worth investigating? Use published literature to provide background information and to develop the relevant conceptual framework. Consider that you are writing for a scientifically literate audience. Material that would be found in an introductory text should not be included. Also avoid being too general or giving too many details. At this point you are trying to provide a focus for your study. When you summarize the literature, include details such as the organisms used, the questions asked, and the relationship of the results of this study to your project. Try to distill the pertinent information into 1-3 sentences. If you are reviewing several papers, try to show how they are related and lead to your study.
Focus in on the your experimental system. Describe the system you are working on and why it is good for addressing the questions of interest. Try to develop a logical flow from the "big picture" to your objectives.
Finish your introduction with the specific questions/hypotheses your study addresses. What is the purpose of your study? Give a summary of the questions that you are addressing or the hypotheses that you are testing. Make some predictions about what you expect based on the information that you provided earlier in the introduction. In general hypotheses are presented as expectations.
Marginal: The hypothesis of this study is ..........
Better: It is expected that ...[prediction]. or If ... [mechanism] ... then ... [prediction].