If you are applying for a science job in the US, you should follow certain conventions employers expect from applicants. Although I usually give advice on writing scientific manuscripts, I have been getting queries lately on resumes and cover letters. I am not familiar with conventions for applying for jobs in other countries, but I have seen people applying for jobs in the US without using a cover letter. If you simply send a resume for a job without a cover letter, it is highly unlikely that an employer will respond to you. Introducing yourself with a cover letter is standard practice in the US. If you are not sure about the form of the cover letter, you will need to write a letter in which you point out 2-3 things in your resume that are relevant to the job you are applying to. Also, point out 2-3 things in your personality or educational history that is relevant to this job. You can use phrases like, "As can be seen in my resume, I worked for 2 years as a....". The cover letter serves as a bridge to connect your general lists of skills and experience found on your resume by pointing out how you are particularly suited for a certain job.
While it's critical to use a cover letter to point out your skills, work history and expertise, I also believe it is critical to mention personality features that make you suitable for the job. You may be surprised to know that many companies and HR directors are very interested in your personality and whether it will fit in with their team. While it's common for all applicants to have impressive technical skills, not everyone has the right personality for particular companies. Most companies can train you on the job if your skills don't fully match, but changing a personality is much trickier. So if you have the right personality, point this out too. Companies are fearful of hiring someone who is difficult, someone who may make others dread coming to work, so you should reassure them of how your personality is right for the job. Phrases like "I work well in teams", "I am considerate of others", "I am a good team player", "I work hard to be respectful and get along with my colleagues", etc., can be helpful.
I should also mention that errors in cover letters or CVs, no matter how minor they may seem, are considered unprofessional; consequently, I would polish and improve your CV as much as possible.For expert advice on writing a cover letter for a job in science, I recommend Writing a Winning Cover Letter by John K Borchardt on Science Careers from Science Magazine. Below is an excerpt: