Question

if a strand of dna has the nitrogen base sequence 5-atttgc-3, what will be the sequence of the matching strand? 3-uaaacg-5 3-atttgc-5 3-taaacg-5 3-tuuucg-5 3-gcaaat-5 part b if a dna double helix is 100 nucleotide pairs long and contains 25 adenine bases, how many guanine bases does it contain? 50 25 200 75 150

Explanation:

Step1: Recall base - pairing rules in DNA

In DNA, adenine (A) pairs with thymine (T), and cytosine (C) pairs with guanine (G). The two strands are anti - parallel. Given the sequence 5'-ATTTGC-3', the complementary strand will be 3'-TAAACG-5'.

Step2: Use Chargaff's rules for Part B

Chargaff's rules state that in DNA, the amount of A = T and the amount of G = C. A 100 - nucleotide - pair long DNA has 200 nucleotides in total. If there are 25 adenine bases, then there are also 25 thymine bases (A = T). So the combined number of A and T bases is 25+25 = 50. The remaining number of bases is 200 - 50=150. Since G = C, the number of guanine bases is $\frac{150}{2}=75$.

Answer:

Part A: 3'-TAAACG-5'
Part B: 75