Antiparallel strands of a DNA molecule

The DNA molecule is composed of two strands that run in opposite directions and are held together by hydrogen bonds. These two strands are referred to as antiparallel because they are oriented in opposite directions.

Explanation:

1. Structure of DNA:
- DNA (deoxyribonucleic acid) is a double-stranded molecule composed of two chains of nucleotides.
- Each nucleotide consists of a sugar molecule (deoxyribose), a phosphate group, and one of four nitrogenous bases (adenine, thymine, guanine, or cytosine).
- The two strands of DNA are held together by hydrogen bonds between complementary base pairs: adenine (A) with thymine (T), and guanine (G) with cytosine (C).

2. DNA strands:
- The two strands of DNA are arranged in an antiparallel manner, meaning they run in opposite directions.
- One strand is oriented in the 5' to 3' direction, while the other strand is oriented in the 3' to 5' direction.
- The orientation of the DNA strands is determined by the placement of the sugar molecules in the nucleotides.
- In one strand, the sugar molecules are connected by their 3' and 5' carbon atoms, while in the other strand, the sugar molecules are connected by their 5' and 3' carbon atoms.

3. Phosphate groups:
- The phosphate groups in DNA are located at the 5' end of each nucleotide.
- In an antiparallel DNA molecule, the phosphate groups at the start of the two strands are in opposite positions.
- This means that the phosphate group at the 5' end of one strand is positioned opposite to the phosphate group at the 5' end of the other strand.

4. Importance:
- The antiparallel arrangement of DNA strands is crucial for DNA replication and protein synthesis.
- During DNA replication, each strand serves as a template for the synthesis of a complementary strand, resulting in two identical DNA molecules.
- The antiparallel strands allow for accurate base pairing and the correct reading of the genetic code.
- Additionally, the antiparallel arrangement allows for the formation of a stable DNA double helix structure.

In conclusion, the antiparallel strands of a DNA molecule refer to the arrangement of the two strands in opposite directions, with the phosphate groups at the start of the strands in opposite positions. This arrangement is essential for DNA replication and maintaining the stability of the DNA molecule.

J.D Watson and F. H. C Crick (1953) showed that DNA has a double helical structure with two polynucleotide chains connected by hydrogen bonds and running in opposite directions (anti-parallel). The 'anti-parallel strands' of a DNA molecule means that the phosphate groups at the start of two DNA strands are in opposite positions (pole).