Difficulty: Intermediate | Prerequisites: DNA structure, basic transcription and translation
This topic compares how prokaryotes and eukaryotes handle transcription and gene expression. The differences are substantial and show up frequently on exams, so understanding them clearly is worth the effort. You should already know what RNA polymerase does and what mRNA is before reading these notes.
Prokaryotes and eukaryotes both transcribe DNA into RNA, but they do it differently. Prokaryotes use a single RNA polymerase, group related genes into operons, and transcribe and translate simultaneously in the cytoplasm. Eukaryotes use three RNA polymerases, transcribe one gene at a time in the nucleus, splice out introns, add protective caps and tails to mRNA, and translate it separately in the cytoplasm.
RNA polymerase
The enzyme responsible for transcription. It reads the template strand of DNA and assembles a complementary mRNA strand. Prokaryotes have one type; eukaryotes have three (RNA Pol I, II, and III), each handling different classes of RNA.
Intron
A non-coding segment of a eukaryotic gene that is transcribed into pre-mRNA but then spliced out before translation. Think of introns as the parts of the script that get cut in the editing room.
Exon
A coding segment of a eukaryotic gene that remains in the mature mRNA after splicing and is translated into protein. In simple terms, exons are the parts that "exit" the nucleus.
Polycistronic mRNA
An mRNA molecule that encodes multiple proteins, produced from an operon. This is characteristic of prokaryotes, where several related genes are transcribed as a single continuous message.
Monocistronic mRNA
An mRNA molecule that encodes a single protein. This is the standard in eukaryotes, where each gene has its own promoter and produces its own transcript.
TATA box
A conserved DNA sequence (typically TATAAA) found in eukaryotic promoters, roughly 25 to 30 base pairs upstream of the transcription start site. It helps position RNA polymerase II correctly. Think of it as the "landing pad" for the transcription machinery.
Transcription initiation factors
Proteins that help RNA polymerase bind to the promoter and begin transcription. In eukaryotes, these factors assemble with RNA polymerase into an initiation complex at the TATA box. Prokaryotes use a sigma factor instead and do not form the same kind of complex.
Post-transcriptional modification
Processing steps applied to eukaryotic pre-mRNA before it leaves the nucleus. These include adding a 5' cap (a modified guanine), a 3' poly-A tail (a string of adenine nucleotides), and splicing out introns. Prokaryotes skip all of this.
Histone
A protein around which eukaryotic DNA is wound to form chromatin. Histones help package DNA into the nucleus and play a role in regulating access to genes. Prokaryotic DNA is not permanently condensed around histones in the same way.
70S ribosome
The ribosome found in prokaryotes, composed of a 30S small subunit and a 50S large subunit. The "S" stands for Svedberg units, a measure of sedimentation rate.
80S ribosome
The ribosome found in eukaryotes, composed of a 40S small subunit and a 60S large subunit. It is larger than the prokaryotic ribosome, reflecting the greater complexity of eukaryotic translation.
RNA polymerase is the central enzyme of transcription. It binds to a promoter on the DNA, unwinds a short stretch of the double helix, and reads the template strand 3' to 5', building a complementary mRNA strand 5' to 3'.
The resulting mRNA carries the genetic instructions from the DNA to the ribosome, where translation occurs.
Genes are organised into operons: clusters of functionally related genes under the control of a single promoter. A single transcription event produces a polycistronic mRNA encoding multiple proteins.
Prokaryotic genes have no introns. The entire transcribed sequence codes for protein.
Transcription and translation happen simultaneously in the cytoplasm. Ribosomes begin translating the mRNA while RNA polymerase is still transcribing it.
Prokaryotic DNA is not permanently condensed around histones. It sits in the nucleoid region, loosely associated with proteins.
Prokaryotes have one type of RNA polymerase that handles all transcription.
Promoter elements include sequences at -10 and -35 nucleotides upstream of the start site.
Prokaryotes use 70S ribosomes (30S + 50S subunits).
Regulation occurs primarily at the transcriptional level (e.g., operons).
No post-transcriptional modifications occur: no 5' cap, no poly-A tail, no splicing.
Genes are transcribed individually, each with its own promoter. The resulting mRNA is monocistronic, encoding a single protein.
Eukaryotic genes contain introns that interrupt the coding sequence. After transcription, introns are spliced out of the pre-mRNA, leaving only exons in the mature mRNA.
Transcription occurs in the nucleus; translation occurs in the cytoplasm. The two processes are temporally and spatially separated.
Eukaryotic DNA forms a stable, condensed complex with histones, creating chromatin. This packaging plays a role in gene regulation (tightly packed chromatin silences genes; loosely packed chromatin allows transcription).
Eukaryotes have three RNA polymerases: RNA Pol I (ribosomal RNA), RNA Pol II (mRNA and some small RNAs), and RNA Pol III (tRNA and other small RNAs).
Promoters include the TATA box, and transcription initiation factors assemble with RNA Pol II to form the initiation complex.
Eukaryotes use 80S ribosomes (40S + 60S subunits).
Post-transcriptional modifications:
5' cap: a modified guanine added to the front of the mRNA, protecting it from degradation and helping ribosome recognition.
3' poly-A tail: a string of adenine nucleotides added to the end, stabilising the mRNA.
Splicing: removal of introns and joining of exons by the spliceosome.
Regulation can occur at multiple levels: transcriptional, post-transcriptional, translational, and post-translational.
The difference in ribosome size between prokaryotes (70S) and eukaryotes (80S) is the basis for many antibiotics. Drugs like tetracycline and erythromycin target the 70S ribosome, blocking bacterial protein synthesis without affecting human cells.
Alternative splicing of introns allows eukaryotic cells to produce multiple different proteins from a single gene. This is one reason the human genome can encode far more proteins than it has genes, and it underlies the complexity of eukaryotic organisms.
Students often think prokaryotes have no gene regulation because they lack a nucleus. Prokaryotes regulate gene expression extensively, primarily at the transcriptional level through operons.
"Polycistronic" does not mean the mRNA is longer or more important. It simply means the mRNA encodes more than one protein, a direct consequence of the operon structure.
Students sometimes assume introns are "junk." While introns do not code for protein, they play roles in gene regulation and allow alternative splicing, which increases protein diversity.
The 5' cap and poly-A tail are sometimes confused with each other. The cap is at the front (5' end) and is a modified guanine. The tail is at the back (3' end) and is a run of adenines. Both protect the mRNA, but they sit at opposite ends.
The prokaryotic vs eukaryotic comparison table is a near-certainty on the exam. Know every row: location of transcription, mRNA type, introns, RNA polymerase count, ribosome size, post-transcriptional modifications, and where regulation occurs.
Be ready to explain why prokaryotes can transcribe and translate simultaneously (no nuclear membrane), while eukaryotes cannot.
The three post-transcriptional modifications (5' cap, poly-A tail, splicing) are high-frequency exam targets. Know what each one does and when it happens.
If the exam asks about ribosome sizes, remember: 70S is prokaryotic, 80S is eukaryotic. The subunits do not add up arithmetically (30S + 50S = 70S, not 80S) because Svedberg units measure sedimentation rate, not mass.
True or False: Prokaryotes have three types of RNA polymerase.
False. Prokaryotes have one type. Eukaryotes have three.
Fill in the blank: Prokaryotic mRNA that encodes multiple proteins from a single transcript is called ________ mRNA.
Polycistronic.
True or False: In eukaryotes, transcription and translation occur at the same time and place.
False. Transcription occurs in the nucleus and translation in the cytoplasm; they are temporally and spatially separated.
Fill in the blank: The three post-transcriptional modifications to eukaryotic mRNA are a 5' ________, a 3' ________, and ________.
Cap, poly-A tail, and splicing (removal of introns).
True or False: Eukaryotic DNA is permanently condensed around proteins called histones.
True. (Though the degree of condensation varies and affects gene expression.)
Q: Explain why prokaryotes can carry out transcription and translation simultaneously, while eukaryotes cannot.
A: Prokaryotes have no nuclear membrane. Their DNA, mRNA, and ribosomes all occupy the same compartment (the cytoplasm), so ribosomes can latch onto the mRNA and begin translating it while RNA polymerase is still transcribing. In eukaryotes, transcription happens inside the nucleus and the mRNA must be processed (capped, tailed, spliced) and exported to the cytoplasm before ribosomes can translate it.
Q: A eukaryotic gene has five exons and four introns. After transcription and processing, how many of these segments remain in the mature mRNA?
A: Five. The four introns are spliced out, leaving only the five exons joined together in the mature mRNA.
Q: Why do antibiotics like tetracycline kill bacteria without harming human cells?
A: Tetracycline targets the 70S ribosome found in prokaryotes. Human cells use 80S ribosomes, which have a different structure, so the drug does not bind to them effectively.
Q: Compare the promoter elements used by prokaryotes and eukaryotes. What key difference does this create for transcription initiation?
A: Prokaryotes use simple promoter sequences at -10 and -35 positions, and their single RNA polymerase binds with help from a sigma factor but without forming a large initiation complex. Eukaryotes use the TATA box (and other elements), and multiple transcription initiation factors must assemble with RNA Pol II into an initiation complex before transcription can begin. This makes eukaryotic transcription initiation more complex and more tightly regulated.
This topic connects to gene regulation (operons are a prokaryotic regulatory strategy covered in the previous set of notes) and to genetic engineering (knowing whether a host is prokaryotic or eukaryotic determines how you design your expression system, since eukaryotic genes with introns cannot be expressed directly in bacteria). The post-transcriptional modifications covered here also connect to mRNA stability and translation efficiency, which appear in more advanced molecular biology courses.
Transcription, RNA polymerase, RNA Pol I, RNA Pol II, RNA Pol III, mRNA, pre-mRNA, intron, exon, splicing, spliceosome, polycistronic, monocistronic, operon, TATA box, transcription initiation factors, initiation complex, sigma factor, 5' cap, 3' poly-A tail, post-transcriptional modification, histone, chromatin, nucleosome, 70S ribosome, 80S ribosome, prokaryotic gene expression, eukaryotic gene expression, prokaryote vs eukaryote, gene structure, BIO 311C