Let's understand Chargaff's rule and determine the correct base pairing in DNA.
Chargaff's rule states that in a double-stranded DNA molecule:
1. The amount of adenine (A) is equal to the amount of thymine (T).
2. The amount of cytosine (C) is equal to the amount of guanine (G).
These pairings are based on the structure of the DNA molecule, where:
- Adenine (A) pairs with Thymine (T) through two hydrogen bonds.
- Cytosine (C) pairs with Guanine (G) through three hydrogen bonds.
Now let's evaluate the given options:
1. [tex]$A = T ; C = G$[/tex]
- This option accurately reflects Chargaff's rule.
- Adenine (A) pairs with Thymine (T).
- Cytosine (C) pairs with Guanine (G).
2. [tex]$A=C ; T=G$[/tex]
- This option is incorrect.
- Adenine (A) does not pair with Cytosine (C).
- Thymine (T) does not pair with Guanine (G).
3. [tex]$A = G ; T = U$[/tex]
- This option is incorrect for DNA.
- Adenine (A) does not pair with Guanine (G).
- Thymine (T) pairs with Adenine (A), not Uracil (U), which is found in RNA.
4. [tex]$A=U ; C=G$[/tex]
- This option is incorrect for DNA.
- Adenine (A) pairs with Thymine (T), not Uracil (U) which is found in RNA.
- Cytosine (C) correctly pairs with Guanine (G) but this is not the complete answer.
Given this analysis, the correct answer is:
[tex]\[ A = T ; C = G \][/tex]
from the first option. Thus, the index of the correct pairing from the given options is 1.
