DRAW IT Copy the polynucleotide strand in Figure 5.23a and label the bases G, T, C, and T, starting from the 5′ end. Assuming this is a DNA polynucleotide, now draw the complementary strand, using the same symbols for phosphates (circles), sugars (pentagons), and bases. Label the bases. Draw arrows showing the 5'→3' direction of each strand. Use the arrows to make sure the second strand is antiparallel to the first. Hint: After you draw the first strand vertically, turn the paper upside down; it is easier to draw the second strand from the 5′ toward the 3′ direction as you go from top to bottom.

Verified step by step guidance

1. First, draw the original polynucleotide strand vertically with the 5′ end at the top. Represent the phosphate groups as circles and the sugar molecules as pentagons. Connect them alternately to form the sugar-phosphate backbone. Label the bases G, T, C, and T attached to the sugars, starting from the 5′ end at the top.
2. Next, to draw the complementary strand, start by flipping your paper upside down. This will help you ensure that the new strand is antiparallel to the original strand.
3. Begin drawing the complementary strand from what is now the top of the page (originally the bottom), which represents the 5′ end of the new strand. Use the same symbols for phosphates and sugars as you did for the first strand.
4. Label the bases on the complementary strand according to the base-pairing rules: C pairs with G, A pairs with T. Since the original strand from top to bottom reads G, T, C, T, the complementary bases from top to bottom will be C, A, G, A.
5. Finally, draw arrows on both strands to indicate the 5′ to 3′ direction. The arrow on the original strand should point downwards, and the arrow on the complementary strand should point upwards, confirming that the strands are antiparallel.

Key Concepts

DNA Structure

DNA is composed of two long strands forming a double helix, with each strand made up of nucleotides. Each nucleotide consists of a phosphate group, a sugar (deoxyribose), and a nitrogenous base (adenine, thymine, cytosine, or guanine). The bases pair specifically (A with T and C with G) through hydrogen bonds, which is crucial for the stability and replication of DNA.

Antiparallel Orientation

The two strands of DNA run in opposite directions, referred to as antiparallel orientation. One strand runs from the 5' to 3' direction, while the complementary strand runs from 3' to 5'. This orientation is essential for DNA replication and the formation of complementary base pairs, ensuring accurate genetic information transfer.

Base Pairing Rules

Base pairing rules dictate how nitrogenous bases pair in DNA: adenine (A) pairs with thymine (T), and cytosine (C) pairs with guanine (G). This specificity is vital for the fidelity of DNA replication and transcription processes, as it ensures that genetic information is accurately copied and expressed in cells.