Question

structure of dna: chargaffs rules
chargaff discovered that the total number of purines (adenine and guanine) is always
equal to the total number of pyrimidines (thymine and cytosine).
lets understand why - answer these questions to unpack chargaffs rule:

1. which base does adenine pair with?
2. how many adenines are there on strand a?

there are 2 adenines

1. how many thymines are there on strand b?

there are 2 thymines

1. the number of cytosines on strand a is equal

to the number of what on strand b?

1. count the number of purines (a and g) and the number

of pyrimidines (t and c) and then write these as a ratio.
2 guanine

Explanation:

Step1: Identify base - pairing rules

In DNA, adenine (A) pairs with thymine (T), and cytosine (C) pairs with guanine (G).

Step2: Answer question 1

Since adenine (A) pairs with thymine (T), but looking at the base - pairing in the given DNA strands (in DNA double - strand, A pairs with T in complementary strand. But if we consider the question about which base adenine pairs with in terms of Chargaff's rules (more about quantity in double - strand, but in base - pairing, A pairs with T). However, if we assume it's about complementary base in double - strand, from the strand A (with bases A, C, A, C, T) and strand B (T, G, T, G, A), adenine (A) pairs with thymine (T).

Step3: Answer question 2

Count adenines in strand A. In strand A (sequence: A, C, A, C, T), the number of adenines is 2.

Step4: Answer question 3

Since A pairs with T in double - strand DNA. The number of thymines in strand B is equal to the number of adenines in strand A. Since there are 2 adenines in strand A, there are 2 thymines in strand B.

Step5: Answer question 4

Count cytosines in strand A (sequence: A, C, A, C, T). The number of cytosines in strand A is 2. Since C pairs with G, the number of guanines on strand B is 2.

Step6: Answer question 5

Count purines (A and G) and pyrimidines (T and C).

• Purines: Adenine (A) count in strand A is 2, guanine (G) count in strand B (paired with C in strand A) is 2. Total purines \(=2 + 2=4\)
• Pyrimidines: Thymine (T) count in strand B (paired with A in strand A) is 2, cytosine (C) count in strand A is 2. Total pyrimidines \(=2+2 = 4\)
• Ratio of purines to pyrimidines \(\frac{4}{4}=1\)

Answer:

1. Thymine (T)
2. 2
3. 2
4. 2
5. The ratio of purines (A + G) to pyrimidines (T + C) is \(1\) (since \( (2 + 2)\div(2 + 2)=1\))