Can I write a bootloader to Atmega using serial port
The answer is that it's very easy to bootload your Arduino directly from your computer, just using a simple USB to serial interface. When I bootloaded my own Arduino for a Reprap, I used a USBTinyISP to connect to the Arduino (it requires a bit of circuit, but it's not much) and then used AVRDUDE to upload the bootloader directly to the Arduino. It's obviously not the only way, but it's one which has worked for me in the past, and which is much cheaper than using another Arduino.