Question

Give the sequence on the opposite strand for ACGTAT, ACATCT, and ATCGTA (all read \(5^{\prime} \rightarrow 3^{\prime}\) ).

Short answer

TGCTAT, TGTAGA, TAGCAT

Step-by-step solution

- Understand the Problem

The problem requires finding the complementary DNA sequence for each given sequence. Each DNA strand runs in a 5' to 3' direction, and the complementary strand will also run in the 5' to 3' direction.

- Know the Base Pairing Rules

In DNA, adenine (A) pairs with thymine (T), and cytosine (C) pairs with guanine (G). For each nucleotide on the given strand, find its complementary base.

- Apply Base Pairing to the First Sequence (ACGTAT)

For the given sequence ACGTAT, find the complementary bases: A → T, C → G, G → C, T → A, A → T, T → A. The complementary sequence is TGCTAT.

- Apply Base Pairing to the Second Sequence (ACATCT)

For the given sequence ACATCT, find the complementary bases: A → T, C → G, A → T, T → A, C → G, T → A. The complementary sequence is TGTAGA.

- Apply Base Pairing to the Third Sequence (ATCGTA)

For the given sequence ATCGTA, find the complementary bases: A → T, T → A, C → G, G → C, T → A, A → T. The complementary sequence is TAGCAT.

Key concepts

Complementary DNA Strands

In DNA, the two strands are known as complementary strands. This means each base on one strand pairs with a specific base on the opposite strand.
Adenine (A) always pairs with thymine (T), and cytosine (C) always pairs with guanine (G).
This pairing is due to hydrogen bonds between the bases.
Creating complementary DNA strands is crucial for DNA replication.
During replication, each original strand serves as a template for a new, complementary strand.
This ensures that the genetic information is accurately copied and passed on.
If we look at a sequence like ACGTAT, the complementary strand would be found by pairing each base:

• A pairs with T
• C pairs with G
• G pairs with C
• T pairs with A
• A pairs with T
• T pairs with A

Thus, the complementary strand to ACGTAT would be TGCTAT.

Nucleotide Base Pairing

Base pairing is a key principle in DNA structure.
The DNA molecule is composed of four nucleotides: adenine (A), thymine (T), cytosine (C), and guanine (G).
These nucleotides pair in a specific way:

• A pairs with T
• C pairs with G

This pairing is called nucleotide base pairing.
DNA base pairs are held together by hydrogen bonds.
A and T form two hydrogen bonds, while C and G form three.
These bonds are strong enough to hold the strands together, but weak enough to allow them to separate when needed.
For example, in the sequence ACATCT, the complementary strand is found by applying base pairing rules:

• A pairs with T
• C pairs with G
• A pairs with T
• T pairs with A
• C pairs with G
• T pairs with A

The result is the complementary strand TGTAGA.

5' to 3' Direction

DNA strands have directionality, indicated as 5' (5-prime) to 3' (3-prime).
This direction is important for several cellular processes, including DNA replication and transcription.
In DNA replication, new nucleotides are added to the 3' end of the growing strand.
This means DNA is synthesized in a 5' to 3' direction.
The 5' end has a phosphate group attached to the fifth carbon of the sugar ring, while the 3' end has a hydroxyl group attached to the third carbon.
For example, if we take the sequence ATCGTA:

• A is at the 5' end
• G is at the 3' end

When writing or reading DNA sequences, it is conventional to go from 5' to 3'.
The complementary sequence is written in the same 5' to 3' direction.
So, for ATCGTA, the complementary sequence is TAGCAT.
Even though we determine the complementary bases in the 3' to 5' direction, we write them in 5' to 3' as well.