Question

Here is a portion of the nucleotide sequence in phenylalanine tRNA. $$ \text { 3'-ACCACCUGCUCAGGCCUU-5' } $$ Write the nucleotide sequence of its DNA complement.

Short answer

Answer: The complementary DNA sequence to the given phenylalanine tRNA molecule is 5'-TGGTGGACGAGTCCGGAA-3'.

Step-by-step solution

Note the tRNA sequence

Write down the given tRNA sequence: $$ \text{3'-ACCACCUGCUCAGGCCUU-5'} $$

Replace uracil with thymine

Since DNA has thymine (T) instead of uracil (U), replace all the U's in the tRNA sequence with T's: $$ \text{3'-ACCACCTGCTCAGGCCTT-5'} $$

Write the complementary DNA sequence

Now apply the base pairing rules (A-T, C-G) to find the complementary DNA sequence. Remember that we need to write this sequence in the 5' to 3' direction: $$ \text{5'-TGGTGGACGAGTCCGGAA-3'} $$ So, the nucleotide sequence of the DNA complement to the given phenylalanine tRNA is: $$ \text{TGGTGGACGAGTCCGGAA} $$

Key concepts

Nucleotide Sequence

When we examine the genetic code, the nucleotide sequence becomes a focal point. These sequences are the language of DNA and RNA, where nucleotides serve as the alphabets. A nucleotide comprises a sugar molecule (deoxyribose in DNA and ribose in RNA), a phosphate group, and a nitrogenous base. There are four types of bases in DNA—adenine (A), thymine (T), cytosine (C), and guanine (G); while in RNA, uracil (U) replaces thymine. The order of these bases dictates the genetic instructions used in the development and functioning of all living organisms.
In practical exercises, such as determining the DNA complement of a tRNA sequence, we must accurately transcribe the nucleotide sequence. The sequence needs to be read in the 3' to 5' direction to align with the standard 5' to 3' orientation of the complementary DNA strand. This orientation is essential because DNA and RNA synthesis occurs in this direction, a critical concept in understanding the replication and transcription processes in cells.

Base Pairing Rules

One must grasp the base pairing rules to understand how nucleotides interact with each other. These rules form the foundation of DNA structure and function. Adenine always pairs with thymine (A-T) in DNA, while cytosine pairs with guanine (C-G). In RNA sequences, adenine pairs with uracil (A-U).
Applying these rules helps decipher the complementary strand in DNA transcription exercises. For instance, given a tRNA sequence with uracil, we first replace it with thymine to reflect the DNA structure and then determine the opposite base using the pairing rules. This knowledge is not just useful for academic exercises but also for understanding molecular biology at a more fundamental level, such as how genes are copied and read.

Exercise Improvement Advice

To ensure better understanding while performing these exercises, one can practice by physically writing out the sequences and highlighting or color-coding the base pairs. Additionally, developing mnemonic devices to remember which bases pair together can be an effective study tool.

DNA Transcription

DNA transcription is a pivotal biological process where a segment of DNA is copied into RNA by the enzyme RNA polymerase. Begin this process by unwinding the DNA helix, exposing the nucleotide sequence of a gene. The RNA polymerase reads the DNA strand in the 3' to 5' direction and constructs a complementary RNA strand in the 5' to 3' direction, using the base pairing rules.
This process not only copies the DNA's message into a form that can leave the nucleus (as mRNA), but also includes editing and splicing steps, resulting in the final messenger RNA that can be translated into protein at the ribosome. In the case of tRNA, transcription follows similar base substitution rules, which the exercise demonstrates to predict the complementary DNA sequence. Understanding transcription helps bridge the concepts from nucleotide sequences and the rules of base pairing to the synthesis and function of different RNA molecules, which play crucial roles in cellular function and protein synthesis.