The two polynucleotide chains of DNA are complementary, means

Options

A. if one starts with 5' end the other must start with 3' end
B. if the sequence of bases of one chain is known, that of other can be determined
C. two chains are held up by hydrogen bonds
D. all of the above

Answer

To understand the concept of complementary polynucleotide chains in DNA, let's break down the explanation step by step.

### Step-by-Step Solution:

1. **Definition of Complementary Chains**:
- The two polynucleotide chains of DNA are described as complementary because the sequence of bases on one strand determines the sequence of bases on the other strand.

**Hint**: Think of complementary as a matching relationship, similar to how puzzle pieces fit together.

2. **Directionality of DNA Strands**:
- DNA strands have directionality, meaning they have a 5' (five-prime) end and a 3' (three-prime) end. One strand runs in the 5' to 3' direction, while the other runs in the 3' to 5' direction.

**Hint**: Remember that the 5' and 3' ends refer to the carbon positions in the sugar molecule of the DNA backbone.

3. **Base Pairing Rules**:
- The bases on the two strands pair specifically: adenine (A) pairs with thymine (T) through two hydrogen bonds, and guanine (G) pairs with cytosine (C) through three hydrogen bonds.

**Hint**: Use the acronym "A-T, G-C" to remember which bases pair together.

4. **Determining Sequences**:
- If the sequence of bases on one strand is known, the sequence of the other strand can be easily determined due to these specific pairing rules. For example, if one strand has the sequence ACGT, the complementary strand will have the sequence TGCA.

**Hint**: Visualize the strands as mirrors of each other, where knowing one side gives you the other.

5. **Hydrogen Bonds**:
- The two strands are held together by hydrogen bonds formed between the paired bases. This bonding is crucial for the stability of the DNA double helix structure.

**Hint**: Think of hydrogen bonds as the glue that holds the two strands together.

6. **Overall Structure**:
- The two polynucleotide chains are wound around the same axis, forming the characteristic double helix structure of DNA.

**Hint**: Picture a twisted ladder, where the rungs are the base pairs and the sides are the sugar-phosphate backbones.

### Conclusion:
The complementary nature of the two polynucleotide chains in DNA is essential for its function in genetic information storage and transmission. Understanding this concept is fundamental in molecular biology.