How to lock few cells based on a selection from dropdown

Hi

I have a dropdown list which has six selection. below the drop down i have many coloums where user can enter the data. Now what i am looking is based on a certain selection in the drop down few coloums should get locked from data entry and only few should be enabled.

Ex: A1 has the dropdown (Values 1,2,3,4,5,6)

User entry are A3, A4, B3,B4, C3, C4, D3, D4, E3, E4, F3, F4.

Now if i select 1 in A1 then only A3 and A4 should be enabled for under entry and B, C, D, E, F coloum for row 3 and row 4 should get locked from entry

If i select 2 then B3 and B4 for entry and rest A,C,D,E,F colum and row 3 and 4 should get locked.

Can some one help me with the stsp by step and the code to achieve this

Regards

highlite A3 & A4 and go to Data Validation / in Allow go to Custom and formula =$A$1=1
highlite B3 & B4 and go to Data Validation / in Allow go to Custom and formula =$A$1=2
etc.etc.
you can only type in A3 & A4 when there is a 1 in A1