I need jquery code which can copy the data from one column(look up column) and populate into other column (single text line) in the same list in sharepoint 2010.
Should you need to execute that script outside of that SharePoint site, you could use the SPServices library which uses SharePoint webservices in the background: http://spservices.codeplex.com/
May I ask what the purpose of this copy is though? Depending on what you need it for, you could also just use a calculated column to "display" the value of the other column, but I also wouldn't see much purpose of that. :)