Question

The start and stop sites for mRNA synthesis (on the DNA) are different from the start and stop sites for protein synthesis (on the mRNA). Explain. (Sections \(4.7\) and 4.11)

Short answer

mRNA synthesis starts at a DNA promoter and stops at a terminator, while protein synthesis starts at an AUG codon and stops at a UAA, UAG, or UGA codon.

Step-by-step solution

Identify start and stop sites for mRNA synthesis on DNA

mRNA synthesis starts at a DNA promoter region and ends at a terminator sequence. The promoter is recognized by RNA polymerase, which starts synthesizing the mRNA, and terminates when it reaches a terminator sequence.

Understand the start and stop sites for protein synthesis on mRNA

Protein synthesis starts at the start codon (usually AUG) on the mRNA and ends at a stop codon (UAA, UAG, or UGA). Ribosomes recognize these codons to begin and terminate translation into a protein.

Compare the differences

The start site for mRNA synthesis on DNA (the promoter) is not the start codon for protein synthesis on mRNA. Similarly, the stop site for mRNA synthesis (the terminator on DNA) differs from the stop codon for protein synthesis (UAA, UAG, or UGA on mRNA). The nucleotide sequences defining these sites are different, reflecting their distinct roles in the central dogma of molecular biology.

Key concepts

DNA Promoter

The DNA promoter is a specific region of DNA that acts as a starting point for mRNA synthesis.
Think of it as a signal that tells RNA polymerase, an essential enzyme, where to start copying the DNA into mRNA.
RNA polymerase binds to this promoter region, unwinding the DNA to access the gene that needs to be transcribed.
Promoters are crucial because they help ensure that genes are expressed at the right time and in the right place.
Without them, the RNA polymerase wouldn't know where to begin, leading to errors in gene expression.

mRNA Start Codon

The mRNA start codon is the sequence where protein synthesis begins.
After mRNA is created, ribosomes read this mRNA and start translating it into protein at the start codon.
The most common start codon is AUG, which codes for the amino acid methionine.
Think of the start codon as the 'green light' for protein synthesis, signaling ribosomes to begin the translation process.
Without it, the ribosomes wouldn't know where to start building the protein.

mRNA Stop Codon

The mRNA stop codon is where protein synthesis ends.
During translation, ribosomes read the mRNA and halt the protein-building process upon reaching a stop codon.
There are three stop codons: UAA, UAG, and UGA.
These codons do not code for any amino acids, signaling to the ribosome that the protein synthesis is complete.
Without stop codons, protein synthesis would continue indefinitely, leading to nonfunctional proteins.

RNA Polymerase

RNA polymerase is an enzyme that plays a critical role in mRNA synthesis.
It binds to the DNA promoter region, unwinds the DNA strand, and starts transcribing the DNA sequence into mRNA.
Think of RNA polymerase as a molecular machine that reads the DNA blueprint and converts it into a readable mRNA format.
Without RNA polymerase, cells wouldn't be able to produce mRNA, leading to a halt in protein production.

Ribosomes

Ribosomes are complex molecular machines responsible for protein synthesis.
Once mRNA is synthesized, ribosomes read the mRNA sequence and start assembling amino acids into proteins.
They recognize the mRNA start codon to begin translation and the stop codon to end it.
Think of ribosomes as construction workers who read the mRNA blueprint and build the protein accordingly.
Without ribosomes, the information in mRNA would never be translated into functional proteins.

Terminator Sequence

The terminator sequence is a specific region on DNA that signals the end of mRNA synthesis.
When RNA polymerase reaches this sequence, it stops transcribing and releases the newly made mRNA.
Think of the terminator sequence as a 'stop sign' that tells RNA polymerase to finish its job.
Without a terminator sequence, RNA polymerase would keep transcribing indefinitely, leading to incomplete and dysfunctional mRNA.
This mechanism ensures that mRNA molecules are the correct length and contain the right genetic information for protein synthesis.