13.4 Gene Expression | Chromosome And DNA | Biology 11

Gene expression is the process by which the information encoded in a gene is used to produce a functional product — most commonly a protein. It is the fundamental mechanism by which genotype gives rise to phenotype.

The Central Dogma of Molecular Biology

Proposed by Francis Crick (1958), the Central Dogma describes the directional flow of genetic information:

$D N A \to R N A \to P r o t e in$

Step	Process	Enzyme	Location
$D N A \to m R N A$	Transcription	RNA Polymerase	Nucleus (eukaryotes)
$m R N A \to P r o t e in$	Translation	Ribosomes	Cytoplasm

Note: Reverse transcriptase (found in retroviruses) can also copy RNA back into DNA, which is an exception to the classical Central Dogma.

Stage 1: Transcription

Transcription is the synthesis of mRNA from a DNA template.

Key Components

Template strand (antisense strand): The DNA strand read by RNA polymerase (3' → 5' direction)
Coding strand (sense strand): The non-template strand with the same sequence as the mRNA (except T is replaced by U)
RNA Polymerase: The enzyme that catalyzes mRNA synthesis (5' → 3' direction)

Stages of Transcription

Initiation: RNA polymerase binds to the promoter region and unwinds the DNA double helix
Elongation: RNA polymerase moves along the template strand, adding complementary ribonucleotides
Termination: Transcription stops at a terminator sequence; the mRNA is released

The Promoter Region

The promoter is a specific DNA sequence upstream (5') of a gene where RNA polymerase binds to initiate transcription. In eukaryotes, the TATA box (a conserved AT-rich sequence ~25–30 bp upstream of the start site) is a key promoter element recognized by transcription factors.

RNA Processing in Eukaryotes (Pre-mRNA → Mature mRNA)

In eukaryotes, the initial transcript is called pre-mRNA and must be processed before leaving the nucleus:

1. 5' Capping

A modified guanosine nucleotide (7-methylguanosine cap) is added to the 5' end. It protects mRNA from degradation and assists ribosome binding.

2. 3' Poly-A Tail

A string of ~200 adenine nucleotides is added to the 3' end, protecting mRNA from degradation and aiding export from the nucleus.

3. RNA Splicing — Introns and Exons

Feature	Exons	Introns
Meaning	Expressed sequences	Intervening sequences
Fate	Retained in mature mRNA	Removed (spliced out)
Translation	Yes — translated into protein	No

Alternative splicing allows different combinations of exons to be joined, producing multiple different proteins from a single gene — greatly expanding the proteome.

Stage 2: Translation

Translation is the synthesis of a polypeptide chain using the mRNA sequence as a template, occurring on ribosomes in the cytoplasm.

The Genetic Code

mRNA is read in triplets called codons (e.g., AUG, UUU)
Each codon specifies one amino acid
AUG = start codon (methionine)
UAA, UAG, UGA = stop codons
The code is degenerate (multiple codons can specify the same amino acid) and universal (same in almost all organisms)

Roles of the Three Types of RNA

RNA Type	Role in Translation
mRNA (messenger RNA)	Carries the genetic code from DNA to ribosome
tRNA (transfer RNA)	Carries specific amino acids; anticodon pairs with mRNA codon
rRNA (ribosomal RNA)	Structural and catalytic component of ribosomes

Stages of Translation

Initiation: Ribosome assembles at the start codon (AUG) on mRNA
Elongation: tRNA molecules bring amino acids; peptide bonds form between successive amino acids
Termination: Ribosome reaches a stop codon; polypeptide is released

Gene Expression and Cellular Differentiation

Differential Gene Expression

All somatic cells in a multicellular organism contain the same genome (identical DNA). However, different cell types (e.g., liver cells, neurons, muscle cells) look and function very differently because they express different subsets of genes.

This selective activation and silencing of genes is called differential gene expression and is the basis of:

Cell differentiation — development of specialized cell types
Tissue formation — organization of cells into tissues and organs
Development — coordinated changes during embryonic development

Example: A liver cell expresses genes for albumin and clotting factors; a neuron expresses genes for neurotransmitter synthesis. Both cells have both sets of genes, but only the relevant ones are expressed.

Summary: Levels of Gene Expression Control

Gene expression can be regulated at multiple levels:

Transcriptional control — most common; controls whether a gene is transcribed (promoters, transcription factors)
RNA processing control — alternative splicing, capping, polyadenylation
mRNA stability control — degradation rate of mRNA
Translational control — whether mRNA is translated
Post-translational control — protein modification, folding, degradation

The Central Dogma of Molecular Biology

Proposed by Francis Crick (1958), the Central Dogma describes the directional flow of genetic information:

$D N A \to R N A \to P r o t e in$

Step	Process	Enzyme	Location
$D N A \to m R N A$	Transcription	RNA Polymerase	Nucleus (eukaryotes)
$m R N A \to P r o t e in$	Translation	Ribosomes	Cytoplasm

Note: Reverse transcriptase (found in retroviruses) can also copy RNA back into DNA, which is an exception to the classical Central Dogma.

Stage 1: Transcription

Transcription is the synthesis of mRNA from a DNA template.

Key Components

Template strand (antisense strand): The DNA strand read by RNA polymerase (3' → 5' direction)
Coding strand (sense strand): The non-template strand with the same sequence as the mRNA (except T is replaced by U)
RNA Polymerase: The enzyme that catalyzes mRNA synthesis (5' → 3' direction)

Stages of Transcription

Initiation: RNA polymerase binds to the promoter region and unwinds the DNA double helix
Elongation: RNA polymerase moves along the template strand, adding complementary ribonucleotides
Termination: Transcription stops at a terminator sequence; the mRNA is released

The Promoter Region

RNA Processing in Eukaryotes (Pre-mRNA → Mature mRNA)

In eukaryotes, the initial transcript is called pre-mRNA and must be processed before leaving the nucleus:

1. 5' Capping

A modified guanosine nucleotide (7-methylguanosine cap) is added to the 5' end. It protects mRNA from degradation and assists ribosome binding.

2. 3' Poly-A Tail

A string of ~200 adenine nucleotides is added to the 3' end, protecting mRNA from degradation and aiding export from the nucleus.

3. RNA Splicing — Introns and Exons

Feature	Exons	Introns
Meaning	Expressed sequences	Intervening sequences
Fate	Retained in mature mRNA	Removed (spliced out)
Translation	Yes — translated into protein	No

Alternative splicing allows different combinations of exons to be joined, producing multiple different proteins from a single gene — greatly expanding the proteome.

Stage 2: Translation

Translation is the synthesis of a polypeptide chain using the mRNA sequence as a template, occurring on ribosomes in the cytoplasm.

The Genetic Code

mRNA is read in triplets called codons (e.g., AUG, UUU)
Each codon specifies one amino acid
AUG = start codon (methionine)
UAA, UAG, UGA = stop codons
The code is degenerate (multiple codons can specify the same amino acid) and universal (same in almost all organisms)

Roles of the Three Types of RNA

RNA Type	Role in Translation
mRNA (messenger RNA)	Carries the genetic code from DNA to ribosome
tRNA (transfer RNA)	Carries specific amino acids; anticodon pairs with mRNA codon
rRNA (ribosomal RNA)	Structural and catalytic component of ribosomes

Stages of Translation

Initiation: Ribosome assembles at the start codon (AUG) on mRNA
Elongation: tRNA molecules bring amino acids; peptide bonds form between successive amino acids
Termination: Ribosome reaches a stop codon; polypeptide is released

Gene Expression and Cellular Differentiation

Differential Gene Expression

This selective activation and silencing of genes is called differential gene expression and is the basis of:

Cell differentiation — development of specialized cell types
Tissue formation — organization of cells into tissues and organs
Development — coordinated changes during embryonic development

Summary: Levels of Gene Expression Control

Gene expression can be regulated at multiple levels:

Transcriptional control — most common; controls whether a gene is transcribed (promoters, transcription factors)
RNA processing control — alternative splicing, capping, polyadenylation
mRNA stability control — degradation rate of mRNA
Translational control — whether mRNA is translated
Post-translational control — protein modification, folding, degradation