Gene Expression and Regulation
Understand how genetic information flows from DNA to protein through transcription and translation, how gene expression is regulated, and how epigenetic mechanisms influence phenotype.
Transcription: DNA to mRNA
Transcription is the first step of gene expression, where the information in a gene's DNA is copied into messenger RNA (mRNA). It occurs in the nucleus of eukaryotic cells and is catalysed by RNA polymerase.
Steps of Transcription
1. Initiation
RNA polymerase binds to the promoter region upstream of the gene
2. Elongation
RNA polymerase reads the template strand 3'→5' and synthesises mRNA 5'→3' using complementary base pairing (A-U, T-A, G-C, C-G)
3. Termination
RNA polymerase reaches a terminator sequence and releases the mRNA
4. RNA Processing (eukaryotes)
5' cap and poly-A tail added; introns spliced out, exons joined
Note: In RNA, uracil (U) replaces thymine (T). So if the DNA template strand reads 3'-TACGGA-5', the mRNA reads 5'-AUGCCU-3'.
Translation: mRNA to Protein
Translation occurs at the ribosome (in the cytoplasm for eukaryotes). The mRNA sequence is read in sets of three bases called codons, each specifying an amino acid. Transfer RNA (tRNA) molecules carry amino acids and match their anticodons to the mRNA codons.
Initiation
Ribosome assembles on mRNA at the start codon AUG (methionine). First tRNA binds.
Elongation
tRNAs deliver amino acids. Peptide bonds form between adjacent amino acids. Ribosome moves along mRNA codon by codon.
Termination
Ribosome reaches a stop codon (UAA, UAG, or UGA). Polypeptide is released and folds into a functional protein.
The Central Dogma
Gene Regulation and Epigenetics
Not all genes are active at all times. Gene regulation controls when, where, and how much of a gene product is made. This is essential for cell differentiation -- all cells have the same DNA, but different genes are expressed in different cell types.
Transcription Factors
Proteins that bind to specific DNA sequences (promoters, enhancers) to activate or repress transcription. They determine which genes are "switched on" in a given cell type.
Epigenetic Modifications
Chemical changes that alter gene expression without changing the DNA sequence. Two major types:
- DNA methylation: Adding CH3 groups to cytosines -- typically silences genes.
- Histone modification: Acetylation loosens chromatin (activates); deacetylation tightens it (silences).
Why Epigenetics Matters
Epigenetic changes can be influenced by environment (diet, stress, toxins) and can sometimes be inherited across generations. They explain how identical twins with the same DNA can develop different traits or disease susceptibilities over time. Epigenetics is a rapidly growing field connecting genetics to environmental factors.
Key Vocabulary
Transcription
The process of copying a gene's DNA sequence into a complementary mRNA molecule, catalysed by RNA polymerase in the nucleus.
Translation
The process of decoding mRNA into a polypeptide chain (protein) at the ribosome, using tRNA to match codons to amino acids.
Codon
A sequence of three mRNA bases that codes for a specific amino acid (or a stop signal). There are 64 codons coding for 20 amino acids plus stop signals.
Epigenetics
Heritable changes in gene expression that do not involve changes to the DNA sequence itself. Includes DNA methylation and histone modification.
Worked Examples
Given the DNA template strand 3'-TACCCGAATGCC-5', determine the mRNA sequence and identify the amino acids coded (use AUG=Met, CCC=Pro, UUA=Leu, CGG=Arg).
Step 1: Write the complementary mRNA (5'→3'): 5'-AUGGGCUUACGG-3'
Step 2: Split into codons: AUG | GGC | UUA | CGG
Answer: Met - Gly - Leu - Arg (AUG is also the start codon)
Explain why a nerve cell and a skin cell have different functions despite containing identical DNA.
Step 1: Both cells contain the complete genome, but different genes are expressed in each.
Step 2: Gene regulation (transcription factors, epigenetic modifications) ensures nerve-specific genes are active in nerve cells and skin-specific genes are active in skin cells.
Answer: Differential gene expression, controlled by transcription factors and epigenetic mechanisms, produces different proteins in each cell type, giving them different structures and functions.
Describe how DNA methylation can silence a gene.
Step 1: Methyl groups (CH3) are added to cytosine bases in the promoter region of a gene by DNA methyltransferase enzymes.
Step 2: Methylation prevents transcription factors and RNA polymerase from binding to the promoter.
Answer: Without transcription factor binding, transcription cannot be initiated, so the gene is effectively "switched off" without any change to the DNA sequence itself.
Knowledge Check
Select the correct answer for each question. Click "Check Answer" to see if you are right.
Question 1
During transcription, RNA polymerase reads the DNA template strand in which direction?
Question 2
The start codon AUG codes for which amino acid?
Question 3
DNA methylation typically:
Question 4
In eukaryotic mRNA processing, introns are:
Question 5
Which of the following is NOT a stop codon?
Key Concepts Summary
- ●Transcription copies DNA to mRNA in the nucleus. RNA polymerase reads template strand 3'→5' and synthesises mRNA 5'→3'.
- ●Translation decodes mRNA into protein at ribosomes. Codons (3 bases) specify amino acids; tRNA delivers them.
- ●In eukaryotes, pre-mRNA is processed: 5' cap, poly-A tail, and intron splicing produce mature mRNA.
- ●Gene regulation via transcription factors and epigenetic modifications determines which genes are expressed in each cell type.
- ●Epigenetics (DNA methylation, histone modification) alters gene expression without changing the DNA sequence and can be influenced by the environment.