Verify with your department or faculty the writing style and any specifications that must be adhered to in your dissertation. Generally, dissertations in the biology major use the Council of Science Editors writing style. Unless directed otherwise, this is the style you should expect to use for your overall dissertation and conclusion.
Re-read the body and introduction of your paper. Write down anything that is significant from your research or from the analysis of your research. Your conclusion will need to address the significance of your dissertation and may be the first thing that is read by peers. The conclusion should stimulate a reader's interest in reading the paper's body, by doing an excellent job of emphasizing why your research is important, significant points and possibilities for further research.
Analyze your dissertation from a critical perspective. Ask peers or faculty to read your dissertation and further analyze it. Ask for feedback about the significance of the research, and ask readers what stands out most to them in the paper's body. The feedback you get from others is important and can help you identify what should be in your conclusion. Also ask others why they, after reading your paper, would say the research is important. Your conclusion is written for an audience of peers and should reflect their thoughts.
Write a transition statement that breaks the conclusion away from the body. The dissertation has separate parts, and the conclusion is not a direct flow from the body. It should be written as an obvious transition to an overall summary and analysis. The first sentence should be interesting and ground your conclusion as a wrap-up of the research. A great conclusion begins with a sentence that grabs the reader's attention. While scientific dissertations are not necessarily written to be entertaining, they can still grab the reader's attention with a statement that is relevant and pressing.
Read your introduction and your conclusion together. The questions and significance you propose to your readers in your introduction should be addressed in the conclusion. Ideally, a reader should be able to read your introduction and conclusion and be able to sum up the research in your dissertation, including the theoretical framework and research findings.
Ask peers or faculty to review your conclusion and make suggestions for improvement. Ask them to read it critically and seriously consider their suggestions.