Basic Structure of DNA

DNA, or deoxyribonucleic acid, is the molecule that carries the genetic instructions for life. It comprises two long strands that coil around each other to form a double helix. 

1. Double Helix Formation

• Shape: DNA has a twisted ladder-like structure known as a double helix.

• Strands: It consists of two long strands running in opposite directions, or anti-parallel, coiled around each other.

2. Nucleotides: Building Blocks

• Components: Each strand is made up of repeating units called nucleotides.

• Nucleotide Structure:

  • Phosphate Group: Links the nucleotides together, forming the backbone of the DNA strand.

  • Deoxyribose Sugar: A five-carbon sugar to which the phosphate group and the nitrogenous base are attached.

  • Nitrogenous Base: There are four types of nitrogenous bases in DNA:

    • Adenine (A)

    • Thymine (T)

    • Cytosine (C)

    • Guanine (G)

3. Complementary Base Pairing

• Base Pairs: The nitrogenous bases form pairs, linking the two DNA strands together through hydrogen bonds.

  • Adenine pairs with Thymine (A-T) with two hydrogen bonds.

  • Cytosine pairs with Guanine (C-G) with three hydrogen bonds.

• Complementarity: The sequence of bases on one strand determines the sequence on the complementary strand, making DNA's replication possible.

4. DNA Backbone

• Sugar-Phosphate Backbone: The sides of the DNA ladder are formed by alternating sugar (deoxyribose) and phosphate groups.

• Directionality: One end of the strand is the 5' (five-prime) end, with a phosphate group attached to the fifth carbon of the sugar, and the other end is the 3' (three-prime) end, with a hydroxyl group attached to the third carbon.

5. Antiparallel Orientation

• Strands Run Opposite: The two DNA strands run in opposite directions; one strand runs from 5' to 3' and the other from 3' to 5'.

• Importance for Replication: This orientation is crucial for DNA replication and function.

6. Major and Minor Grooves

• Grooves in Helix: The twisting of the DNA strands creates a major groove and a minor groove, which are critical for the binding of proteins that regulate gene expression.