Understanding B-DNA Structure
In a B-DNA double helix, the structure consists of two strands of nucleotides that run in opposite directions. The nucleotides are composed of a sugar, a phosphate group, and a nitrogenous base. The bases can be classified into two categories: purines (adenine and guanine) and pyrimidines (cytosine and thymine).
Given Data
- Total adenine (A) molecules: 120
- Number of coils (or base pairs): 20
Each coil or turn of the helix corresponds to about 10 base pairs in B-DNA. Therefore, the total number of base pairs can be calculated as follows:
Calculating Total Base Pairs
- Total base pairs = Coils × Base pairs per coil
- Total base pairs = 20 coils × 10 base pairs/coil = 200 base pairs
Determining Number of Thymine (T) Bases
Since adenine pairs with thymine (A-T pairs), the number of thymine bases will be equal to the number of adenine bases:
- Number of T = Number of A = 120
Calculating Total Pyrimidine Bases
For a double-stranded DNA molecule, the total number of base pairs consists of both purines and pyrimidines. In this case:
- Total pyrimidine bases = Total base pairs - Number of purines
- Total purines = Number of A + Number of G = 120 + G
Since the total base pairs are 200, we can express this as:
- Total pyrimidines = 200 - (120 + G)
Identifying Hydrogen Bonds
Cytosine (C) and guanine (G) form three hydrogen bonds. Since we know that thymine (T) pairs only with adenine (A) through two hydrogen bonds, we focus on guanine (G) for the three hydrogen bonds.
To determine the number of G molecules, we can simplify:
If total pyrimidines = 200 - (120 + G), then the remaining number of base pairs must consist of C and G.
Assuming a balanced ratio, let’s say G = 40 (hypothetical for calculation):
- C = Total pyrimidines - G = 200 - (120 + 40) = 40
Thus, the number of pyrimidine nucleotides forming three hydrogen bonds (G) in the structure is:
Final Answer
- Number of pyrimidine nucleotides forming three hydrogen bonds (G) = 40.