How the null subcarriers are inserted in OFDM spectrum?

For example: For bandwidth 10MHz, fft size Nfft=1024 that needs a sampling rate Fs≥1024×15kHz=15.36MHz. The useful subcarriers are the 600. There are 424 null subcarriers inserted at the middle of the spectrum. Please i need some one correct for me, Isn't each one of these 1024 subcarriers ,including null subcarriers, is transmitted over its specified bandwidth ( in OFDM each subcarrier has a bandwidth equal to Δf=15kHz what ever its content) If that is true (each one of the 1024 has its bandwidth, that mean the total bandwidth required for these subcarriers equal to 15MHz not 10MHz)