Molecular Genetics and Gene Expression
Grade 12 · Science · Study Guide
Discover how DNA codes for proteins through transcription and translation, and how gene expression is regulated in living cells.
Molecular genetics explores how genetic information stored in DNA is expressed as functional proteins. The central dogma of molecular biology describes the flow of genetic information: DNA → RNA → Protein. In transcription, an enzyme called RNA polymerase reads the DNA template strand and synthesizes a complementary messenger RNA (mRNA) molecule. This occurs in the nucleus of eukaryotic cells. The mRNA is then processed — introns (non-coding sequences) are removed and exons (coding sequences) are spliced together — before leaving the nucleus.
In translation, ribosomes read the mRNA sequence in triplets called codons, each specifying a particular amino acid. Transfer RNA (tRNA) molecules carry amino acids to the ribosome, where they are joined by peptide bonds to form a growing polypeptide chain. There are 64 possible codons (4³) encoding 20 amino acids, making the code redundant (multiple codons can code for the same amino acid). Three codons (UAA, UAG, UGA) are stop codons that terminate translation.
Gene expression is tightly regulated. In prokaryotes, operons control transcription of related genes together. In eukaryotes, transcription factors, enhancers, and promoters control when and how much a gene is expressed. Post-transcriptional regulation includes RNA interference (RNAi), where small RNA molecules called microRNAs silence gene expression. Epigenetic modifications — changes in gene expression without changing the DNA sequence — include DNA methylation and histone modification.
| Feature | Transcription | Translation |
|---|---|---|
| Location (eukaryotes) | Nucleus | Cytoplasm (ribosomes) |
| Template | DNA | mRNA |
| Product | mRNA | Polypeptide (protein) |
| Key enzyme | RNA polymerase | Ribosome (peptidyl transferase) |
| Building blocks | Ribonucleotides | Amino acids |
Key terms
- Transcription
- The process of copying a DNA sequence into mRNA using RNA polymerase.
- Translation
- The process of reading mRNA codons and assembling a polypeptide chain at a ribosome.
- Codon
- A three-nucleotide sequence on mRNA that codes for a specific amino acid.
- Intron
- A non-coding DNA/RNA sequence that is removed during mRNA processing.
- Exon
- A coding sequence in DNA/mRNA that is retained after splicing and translated into protein.
- RNA polymerase
- The enzyme that synthesizes RNA from a DNA template during transcription.
- Epigenetics
- Changes in gene expression caused by mechanisms other than changes in DNA sequence, such as methylation.
Flashcards
- Central dogma
- DNA → RNA → Protein; describes the flow of genetic information in living cells
- mRNA
- Messenger RNA; carries genetic information from DNA in nucleus to ribosomes in cytoplasm
- tRNA
- Transfer RNA; carries specific amino acids to the ribosome; anticodon pairs with mRNA codon
- Stop codons
- UAA, UAG, UGA — signal end of translation; no tRNA carries amino acids for these
- Operon
- A cluster of prokaryotic genes with shared promoter/operator, regulated together (e.g., lac operon)
- Epigenetics
- Heritable changes in gene expression not caused by DNA sequence changes; includes methylation, histone modification
Practice questions
What is the process of copying DNA into mRNA called?
- Replication
- Translation
- Transcription
- Transduction
Show answer
Answer: Transcription
Transcription converts the DNA sequence into a complementary mRNA sequence. RNA polymerase unwinds the DNA and synthesizes the mRNA from the template strand using complementary base pairing.
Introns are the coding sequences that remain in the mature mRNA after processing.
Show answer
Answer: False
Exons are the coding sequences retained in mature mRNA. Introns are non-coding sequences that are spliced OUT during mRNA processing.
The three-nucleotide sequence on mRNA that codes for an amino acid is called a ___.
Show answer
Answer: codon
Each codon is a triplet of nucleotides. There are 64 possible codons (4³): 61 code for amino acids and 3 are stop codons.
Where does translation occur in eukaryotic cells?
- Nucleus
- Mitochondria
- Ribosomes in the cytoplasm
- Endoplasmic reticulum only
Show answer
Answer: Ribosomes in the cytoplasm
Translation occurs at ribosomes, which are found free in the cytoplasm or on the rough endoplasmic reticulum. mRNA must exit the nucleus first.
The genetic code is universal — the same codons specify the same amino acids in nearly all living organisms.
Show answer
Answer: True
The genetic code is nearly universal across all life forms — a codon like AUG codes for methionine in bacteria, plants, and humans alike. This is strong evidence for common ancestry.
Fun facts
- The human genome contains about 20,000-25,000 protein-coding genes, but only about 1.5% of total DNA actually codes for proteins.
- A single ribosome can synthesize about 20 amino acids per second — a typical protein of 300 amino acids takes just 15 seconds to make.
- CRISPR-Cas9, derived from a bacterial immune system, can edit specific genes with unprecedented precision, revolutionizing medicine and agriculture.
Get a personalized course on Molecular Genetics and Gene Expression built around your child.