Gene expression and Regulation
Genetic Expression
An organism contains many types of cells, with distinct shapes and functions. However, all cells have the
same genome. The genes in a genome do not affect cellular functions until they are "expressed". Different
cell types express distinct sets of genes, thereby exhibiting diverse shapes and functions.
Genetic expression means the production of a protein or a functional RNA from its gene. Several steps
are required:
1. Transcription: Transcription is a process in which one DNA strand is used as a template to synthesize
a complementary RNA (primary transcript)
I. Binding of polymerases to the initiation site, called promoter. Prokaryotic polymerases recognize the
promoter and bind to it directly, but eukaryotic polymerases have to rely on other proteins called
transcription factors.
II. Unwinding (melting) of the DNA double helix by helicase. Prokaryotic polymerases have helicase
activity. Unwinding of eukaryotic DNA is carried out by a specific transcription factor.
III. Synthesis of RNA based on the sequence of the DNA template strand. RNA polymerases use
nucleoside triphosphates (NTPs) to construct an RNA strand.
IV. Termination of synthesis. Prokaryotes and eukaryotes use different signals to terminate transcription.
Feature Prokaryotes (Bacteria) Eukaryotes (Humans, Plants, etc.)
Location Cytoplasm Nucleus
RNA Polymerase Only 1 type 3 types (I, II, and III)
Timing Happens simultaneously with Transcription must finish before translation
translation
Initiation Requires a Sigma Requires Transcription Factors
factor
Post-Transcriptional Mods Little to none Extensive (Splicing, Capping, Tailing)
DNA Structure "Naked" DNA DNA wrapped in Histones (Chromatin)
1
, 2. RNA processing: RNA processing is to generate a mature mRNA (for protein genes) or a functional
tRNA or rRNA from the primary transcript. Processing of pre-mRNA involves the following steps:
1. End modification (5′ capping and 3′ polyadenylation)
This happens at both ends of the pre-RNA.
(a) At the 5′ end, a cap made of modified guanine is added.
This cap:
I. Protects RNA from degradation
II. It stabilizes mRNA by protecting it from 5'
exonuclease.
III. Helps the ribosome recognize the RNA
IV. Assists RNA export from the nucleus
(b) At the 3′ end, a poly(A) tail—a long stretch of adenine nucleotides—is added.
This tail:
I. Increases RNA stability
II. Helps control how long the RNA survives
III. Improves translation efficiency
2. Splicing (removal of introns)
Pre-RNA contains:
• Exons → useful sequences
• Introns → non-coding interruptions
During splicing, introns are cut out, and exons are joined together. This is done by a molecular machine called the
spliceosome.
3. Cutting (RNA cleavage)
Some RNAs are produced as long precursors that must be cut into smaller, functional pieces.
Examples:
❖ rRNA and tRNA are synthesized as long chains
❖ Specific enzymes cut them into correct lengths
This step ensures:
I. Correct RNA size
II. Proper structure for function
III. Removal of unnecessary spacer sequences
4. Chemical modification
Here, new chemical groups are added to specific nucleotides in the RNA.
Examples include:
2
Genetic Expression
An organism contains many types of cells, with distinct shapes and functions. However, all cells have the
same genome. The genes in a genome do not affect cellular functions until they are "expressed". Different
cell types express distinct sets of genes, thereby exhibiting diverse shapes and functions.
Genetic expression means the production of a protein or a functional RNA from its gene. Several steps
are required:
1. Transcription: Transcription is a process in which one DNA strand is used as a template to synthesize
a complementary RNA (primary transcript)
I. Binding of polymerases to the initiation site, called promoter. Prokaryotic polymerases recognize the
promoter and bind to it directly, but eukaryotic polymerases have to rely on other proteins called
transcription factors.
II. Unwinding (melting) of the DNA double helix by helicase. Prokaryotic polymerases have helicase
activity. Unwinding of eukaryotic DNA is carried out by a specific transcription factor.
III. Synthesis of RNA based on the sequence of the DNA template strand. RNA polymerases use
nucleoside triphosphates (NTPs) to construct an RNA strand.
IV. Termination of synthesis. Prokaryotes and eukaryotes use different signals to terminate transcription.
Feature Prokaryotes (Bacteria) Eukaryotes (Humans, Plants, etc.)
Location Cytoplasm Nucleus
RNA Polymerase Only 1 type 3 types (I, II, and III)
Timing Happens simultaneously with Transcription must finish before translation
translation
Initiation Requires a Sigma Requires Transcription Factors
factor
Post-Transcriptional Mods Little to none Extensive (Splicing, Capping, Tailing)
DNA Structure "Naked" DNA DNA wrapped in Histones (Chromatin)
1
, 2. RNA processing: RNA processing is to generate a mature mRNA (for protein genes) or a functional
tRNA or rRNA from the primary transcript. Processing of pre-mRNA involves the following steps:
1. End modification (5′ capping and 3′ polyadenylation)
This happens at both ends of the pre-RNA.
(a) At the 5′ end, a cap made of modified guanine is added.
This cap:
I. Protects RNA from degradation
II. It stabilizes mRNA by protecting it from 5'
exonuclease.
III. Helps the ribosome recognize the RNA
IV. Assists RNA export from the nucleus
(b) At the 3′ end, a poly(A) tail—a long stretch of adenine nucleotides—is added.
This tail:
I. Increases RNA stability
II. Helps control how long the RNA survives
III. Improves translation efficiency
2. Splicing (removal of introns)
Pre-RNA contains:
• Exons → useful sequences
• Introns → non-coding interruptions
During splicing, introns are cut out, and exons are joined together. This is done by a molecular machine called the
spliceosome.
3. Cutting (RNA cleavage)
Some RNAs are produced as long precursors that must be cut into smaller, functional pieces.
Examples:
❖ rRNA and tRNA are synthesized as long chains
❖ Specific enzymes cut them into correct lengths
This step ensures:
I. Correct RNA size
II. Proper structure for function
III. Removal of unnecessary spacer sequences
4. Chemical modification
Here, new chemical groups are added to specific nucleotides in the RNA.
Examples include:
2
