Problem 10 - Biology

Question

If the sequence of the \(5^{\prime}-3^{\prime}\) strand is AATGCTAC, then the complementary sequence has the following sequence: a. \(3^{\prime}\) -AATGCTAC-5' b. 3 '-CATCGTAA-5' c. 3'-TTACGATG-5' d. 3'-GTAGCATT-5'

Step-by-Step Solution

Answer

The complementary sequence is 3'-TTACGATG-5'.

Step 1 - Understand the Concept of Complementary Bases

In DNA, adenine (A) pairs with thymine (T), and cytosine (C) pairs with guanine (G). Therefore, if you know the sequence of one strand, you can determine the sequence of its complementary strand by replacing each base with its complement.

Step 2 - Write the Given Sequence

The given sequence of the 5'-3' strand is AATGCTAC.

Step 3 - Determine the Complements

Find the complementary bases for each base in the sequence: A -> T, T -> A, G -> C, and C -> G.

Step 4 - Construct the Complementary Sequence

Using the complements found in Step 3, construct the sequence: A -> T, A -> T, T -> A, G -> C, C -> G, T -> A, A -> T, and C -> G, resulting in the sequence: TTACGATG.

Step 5 - Form the Correct Orientation

Since the complementary strand is oriented from 3' to 5', the final complementary sequence is 3'-TTACGATG-5'.

Step 6 - Match the Answer

Compare the constructed complementary sequence to the given choices. The correct answer is c. 3'-TTACGATG-5'.

Key Concepts

Complementary DNA Strands

DNA is made up of two strands that coil around each other to form a double helix. Each strand consists of a sequence of nucleotides: adenine (A), thymine (T), cytosine (C), and guanine (G). These strands are considered complementary because each nucleotide on one strand pairs with a specific complementary nucleotide on the opposite strand:

• Adenine (A) pairs with thymine (T)
• Cytosine (C) pairs with guanine (G)

This complementary base pairing is key to DNA replication and function. By knowing the sequence of nucleotides on one strand, you can always determine the sequence on the opposite strand through these base-pairing rules. For instance, if one DNA strand has the following sequence: 5'-AATGCTAC-3', the complementary strand would need to have the following sequence to pair correctly: 3'-TTACGATG-5'. This arrangement of nucleotides ensures the DNA's structural stability and enables accurate replication during cell division.

Nucleotide Pairs

Nucleotides are the building blocks of DNA, consisting of a phosphate group, a sugar molecule, and a nitrogenous base. The nitrogenous bases are what pair to form the rungs of the DNA ladder structure.

• Adenine (A) pairs with thymine (T) through two hydrogen bonds
• Cytosine (C) pairs with guanine (G) through three hydrogen bonds

These pairs are consistent due to the size, shape, and hydrogen bonding capabilities of the bases. The pairing is always between a purine (A, G) and a pyrimidine (T, C):

• Pyrimidines: Thymine (T), Cytosine (C)
• Purines: Adenine (A), Guanine (G)

This precise pairing is critical for DNA replication and transcription since it provides a template for creating an exact copy of the DNA sequence. Without these specific pairs, the DNA molecule would not be able to maintain its structural integrity and function.

5' to 3' Orientation

The DNA strands have a directionality, often referred to as 5' to 3' end orientation. Each strand has a 5' (five prime) end, which has a phosphate group attached to the fifth carbon of the sugar molecule, and a 3' (three prime) end, which has a hydroxyl group attached to the third carbon of the sugar molecule.
During DNA replication and synthesis, enzymes always add new nucleotides to the 3' end of the growing strand, which means DNA grows in a 5' to 3' direction. This directionality is crucial:

• 5' end typically bears a phosphate group
• 3' end has a hydroxyl group that can form bonds with incoming nucleotides

For example, in the exercise, the given DNA sequence is oriented from 5' to 3', reading as 5'-AATGCTAC-3'. Consequently, the complementary strand must be aligned from 3' to 5'. Only by understanding these orientations can you correctly identify or construct a strand's complementary sequence.