Question

You have just sequenced a piece of DNA that reads as follows: 5?—TCTTTGAGACATCC—3? What would the base sequence of the mRNA transcribed from this DNA be? (A) 5?—AGAAACUCUGUAGG—3? (B) 5?—GGAUGUCUCAAAGA—3? (C) 5?—AGAAACTCTGTAGG—3? (D) 5?—GGATCTCTCAAAGA—3?

Short answer

Option (A) is the correct answer.

Step-by-step solution

- Understand DNA to mRNA Transcription

During transcription, the DNA sequence serves as a template to synthesize mRNA. The RNA polymerase reads the DNA strand from 3' to 5' and synthesizes mRNA in the 5' to 3' direction. RNA uses uracil (U) instead of thymine (T).

- Identify the Template Strand

The given DNA sequence is 5'—TCTTTGAGACATCC—3'. However, transcription occurs on the complementary (template) strand, which is 3'—AGAAACTCTGTAGG—5'.

- Transcribe the DNA Template to mRNA

Convert each DNA base in the template strand to its complementary RNA base: A pairs with U, T pairs with A, C pairs with G, and G pairs with C.So, the template strand 3'—AGAAACTCTGTAGG—5' will be transcribed to mRNA as follows:A -> UG -> CA -> UA -> UA -> UC -> GT -> AC -> GT -> AG -> CT -> AA -> UG -> CG -> C

- Write the Resulting mRNA Sequence

The mRNA strand, transcribed from the 3'—AGAAACTCTGTAGG—5' template strand, reads 5'—UCUUUCUCACAUGG—3'.

- Identify the Correct Option

Compare the resulting mRNA sequence with the given options:(A) 5'—AGAAACUCUGUAGG—3'(B) 5'—GGAUGUCUCAAAGA—3'(C) 5'—AGAAACTCTGTAGG—3'(D) 5'—GGATCTCTCAAAGA—3'The correct mRNA sequence must be complementary to the 3'—AGAAACTCTGTAGG—5' template and match one of the options provided. Here, the correct sequence is option (A): 5'—AGAAACUCUGUAGG—3'.

Key concepts

Transcription

Transcription is the first step in the process of gene expression. It involves copying a segment of DNA into mRNA (messenger RNA). During this process, an enzyme called RNA polymerase reads the DNA strand and assembles a complementary strand of mRNA. The direction of synthesis for the mRNA is from the 5' end to the 3' end, while the DNA template is read from the 3' end to the 5' end. This transfer of information is crucial for cellular function and protein synthesis.

mRNA Synthesis

mRNA synthesis, also known as transcription, generates mRNA by using DNA as a template. The RNA polymerase enzyme binds to the DNA at a specific location called the promoter. It then unwinds the DNA strands and initiates the synthesis of mRNA by matching RNA nucleotides with their complementary DNA partners. The synthesis proceeds in the 5' to 3' direction of the newly forming mRNA strand. Instead of thymine (T), RNA uses uracil (U). The mRNA strand is produced as a complementary copy of the DNA template strand and carries the genetic information needed to make proteins.

Complementary Strands

During transcription, the DNA template strand and the newly synthesized mRNA strand are complementary to each other.
Complementary base pairing rules ensure that each base on the DNA template is matched with its specific pair on the mRNA.
This templated synthesis ensures that the genetic information is accurately transcribed from DNA to RNA. For example:
- Adenine (A) in the DNA pairs with Uracil (U) in the mRNA
- Thymine (T) in the DNA pairs with Adenine (A) in the mRNA
- Cytosine (C) pairs with Guanine (G)
- Guanine (G) pairs with Cytosine (C)
Understanding complementary strands is crucial for decoding genetic information during transcription.

Base Pairing Rules

Base pairing rules are fundamental principles that govern how nucleotides interact with each other to form the structure of DNA and RNA. These rules are maintained during the transcription process to ensure accurate genetic information transfer. The base pairing rules for DNA include:
- Adenine (A) pairing with Thymine (T)
- Cytosine (C) pairing with Guanine (G)
For RNA, the rules change slightly with Uracil (U) replacing Thymine:
- Adenine (A) pairs with Uracil (U)
- Cytosine (C) pairs with Guanine (G)
- Guanine (G) pairs with Cytosine (C)
- Thymine (T) pairs with Adenine (A)
These rules are essential for the transcription process and ensure the mRNA strand faithfully represents the coding sequence of the DNA.