Took me a minute to figure out what you meant by "dot", but now I think i know. Not sure if the "dot" for the preview pane is actually missing, or just off screen in your case. But if it truly is gone you could try deleting the ~/Library/Preferences/com.apple.mail.plist file and then rebooting. After the reboot the system will recreate the file and hopefully fix your problem. Good luck.