To copy the partition table and MBR from a disk to create the same definition of partitions as in other, you have to copy the first 512 blocks from the header of the disks like with the next command to copy it to another disk for mirroring two disks without create the partition in both disks:
# dd if=/dev/sda of=/dev/sdb bs=512 count=1
This is correct for EXACTLY the same partition sizes. In the case you want the MBR only on a drive that has a root partition of a different size then issue the same command with a smaller block size.
# dd if=/dev/sda of=/dev/sdb bs=446 count=1
The first 446 blocks of the disks is where the MBR information is stored, so to do a backup of it instead of another disks, use a file like of=/path/to/backup_mbr.