Difficulty: Introductory | Prerequisites: DNA structure and replication (see Part 1 study notes). You should understand base pairing, the 5' to 3' convention, and the role of RNA primers before starting here.
Protein synthesis is how a cell converts the information stored in DNA into a functional protein. The process follows the central dogma of molecular biology: DNA is transcribed into mRNA, which is then translated into a polypeptide chain that folds into a protein. This topic also covers what happens when things go wrong during copying or reading, producing mutations that range from harmless to disease-causing. If you are comfortable with DNA structure and replication, this is the natural next step. Gene control (Part 3) builds directly on the transcription machinery introduced here.
Cells build proteins in two stages: transcription copies a gene's DNA into messenger RNA inside the nucleus, and translation reads that mRNA at a ribosome to assemble amino acids into a polypeptide. Errors in this process produce mutations, some silent and some with serious consequences, such as the single base-pair substitution behind sickle cell anaemia.
Central dogma
The flow of genetic information in a cell: DNA is transcribed to RNA, which is translated to protein. In shorthand: DNA > RNA > Protein.
Transcription
The process of copying a gene's DNA sequence into a complementary mRNA molecule. Occurs in the nucleus.
Translation
The process of reading an mRNA sequence at a ribosome and assembling a chain of amino acids (a polypeptide). Occurs on ribosomes in the cytoplasm.
mRNA (messenger RNA)
The RNA molecule that carries the copied genetic instructions from the nucleus to the ribosome. Think of it as the order slip sent from the office to the factory floor.
tRNA (transfer RNA)
A small RNA molecule that delivers the correct amino acid to the ribosome during translation. Each tRNA has an anticodon that matches a specific mRNA codon.
rRNA (ribosomal RNA)
The RNA component of ribosomes. It provides the structural framework and catalytic activity for translation.
Codon
A three-nucleotide sequence on mRNA that codes for one amino acid (or a stop signal). There are 64 possible codons.
Anticodon
A three-nucleotide sequence on tRNA that is complementary to an mRNA codon. This pairing ensures the right amino acid is added.
Start codon
AUG, the codon that signals the beginning of translation. It codes for the amino acid methionine (Met).
Stop codons
UAA, UAG, and UGA. These codons signal the ribosome to stop translating and release the polypeptide.
RNA polymerase
The enzyme that reads the DNA template strand and synthesises a complementary mRNA strand during transcription. It binds to the promoter to begin.
Promoter
A DNA sequence upstream of a gene where RNA polymerase binds to begin transcription. In operons, it comes before the operator.
Intron
A non-coding segment of pre-mRNA that is removed (spliced out) before the mRNA leaves the nucleus. Sometimes called "junk DNA," though introns can help catch mutations.
Exon
A coding segment of mRNA that remains after introns are removed. Exons are spliced together to form the mature mRNA.
5' cap
A modified guanine nucleotide added to the 5' end of mRNA after transcription. It protects the mRNA from degradation and helps the ribosome recognise it.
Poly-A tail
A string of adenine nucleotides added to the 3' end of mRNA after transcription. It protects the mRNA from enzymatic breakdown.
Peptide bond
The covalent bond that links amino acids together in a polypeptide chain, formed during translation at the ribosome.
Polyribosome (polysome)
A cluster of ribosomes translating the same mRNA molecule simultaneously, producing multiple copies of the same polypeptide.
Substitution mutation
A mutation in which a single nucleotide is replaced by another. Only the codon containing that nucleotide is affected.
Frameshift mutation
A mutation caused by the insertion or deletion of nucleotides (not in multiples of three), which shifts the reading frame of every codon downstream. Generally more damaging than a substitution.
Sickle cell anaemia
A genetic disorder caused by a single base-pair substitution in the haemoglobin gene, producing abnormal haemoglobin that causes red blood cells to deform into a sickle shape.
Transcription is the first stage of protein synthesis. The cell copies one gene's information from DNA into a messenger RNA molecule.
Where: inside the nucleus.
Enzyme: RNA polymerase binds to the promoter sequence upstream of the gene.
Direction: RNA polymerase reads the template DNA strand 3' to 5' and builds the mRNA strand 5' to 3'. RNA nucleotides are added only to the 3' end.
Base pairing: the same complementary rules apply, except that uracil (U) pairs with adenine (A) in place of thymine.
Example: if the DNA template strand reads 3'-TAG AGG ATC-5', the resulting mRNA reads 5'-AUG UCC UAG-3'.
The initial transcript (pre-mRNA) must be processed before it can leave the nucleus and be translated.
Introns (non-coding regions) are cut out.
Exons (coding regions) are spliced together to form the continuous coding sequence of the mature mRNA.
Introns may serve as a buffer: mutations landing in introns do not alter the protein product.
A 5' cap (a modified guanine, often noted as a G cap) is added to the 5' end of the mRNA. It protects the molecule from degradation and helps the ribosome latch on.
A poly-A tail (a string of adenine nucleotides) is added to the 3' end. It also protects against enzymatic breakdown.
After processing, the mature mRNA looks like this: 5'-cap + AUG UCC UAG + AAAAA (poly-A tail)-3'. It then exits the nucleus through nuclear pores and travels to a ribosome in the cytoplasm.
Translation takes place on ribosomes (composed of rRNA and proteins) in the cytoplasm. The mRNA is read in three-letter chunks called codons, and tRNA molecules deliver the matching amino acids.
The mRNA binds to the small ribosomal subunit.
The ribosome scans until it finds the start codon, AUG.
A tRNA carrying methionine (Met) binds to the start codon via its anticodon (UAC).
The large ribosomal subunit joins, forming a complete ribosome.
A new tRNA, carrying the amino acid specified by the next codon, enters the ribosome.
A peptide bond forms between the growing chain and the new amino acid.
The empty tRNA is released, the ribosome shifts one codon along the mRNA, and the next tRNA enters.
This cycle repeats, adding amino acids one at a time.
A stop codon (UAA, UAG, or UGA) is reached.
No tRNA recognises a stop codon. Instead, a release factor binds.
The polypeptide is released, and the ribosome disassembles from the mRNA.
The polypeptide will fold into its functional three-dimensional shape to become a working protein.
A polyribosome is a cluster of ribosomes translating the same mRNA at the same time, which allows the cell to produce many copies of the same protein quickly.
A mutation is a permanent change in the DNA sequence. Mutations are not always harmful, and they must be heritable (passed to the next generation) to have lasting evolutionary significance. DNA polymerase proofreads during replication and catches most errors, but some slip through. External factors such as radiation, toxins, and carcinogens increase mutation rates.
A single nucleotide is swapped for another.
Only the codon containing that nucleotide is affected.
The result may be silent (same amino acid), missense (different amino acid), or nonsense (premature stop codon).
Caused by the insertion or deletion of nucleotides (not in multiples of three).
Shifts the reading frame of every codon downstream of the mutation.
Almost always produces a non-functional protein, making frameshifts generally worse than substitutions.
Caused by a single base-pair substitution in the haemoglobin gene.
The altered haemoglobin causes red blood cells to deform into a sickle shape, leading to blood clots and other complications.
Inheritance pattern:
aa = sickle cell disease
AA = normal haemoglobin
Aa = carrier (sickle cell trait), with partial resistance to malaria
The heterozygous advantage (malaria resistance) is why the sickle cell allele persists at higher frequencies in populations historically exposed to malaria.
Understanding transcription and translation is essential for modern medicine. Antibiotics such as tetracycline work by blocking bacterial ribosomes during translation, stopping the bacteria from making proteins without harming human cells. mRNA vaccines (like those developed for COVID-19) deliver synthetic mRNA into cells so the ribosome can translate it into a viral protein, training the immune system to recognise the real virus.
Students often say "RNA to DNA" when describing translation. Translation is RNA to protein, not RNA to DNA. The correct direction is: DNA > RNA (transcription) > Protein (translation).
Students often think introns are useless. While they do not code for protein, introns can serve as a mutational buffer and play roles in gene regulation and alternative splicing.
Students often assume that any mutation will cause a disease. Many substitution mutations are silent because the genetic code is redundant (multiple codons can specify the same amino acid).
Students often forget that the start codon AUG always codes for methionine. Every protein initially begins with methionine, though it may be removed later.
Be able to walk through the full path from DNA to protein, naming each stage and where it occurs (transcription in the nucleus, translation on ribosomes).
Know the three types of RNA (mRNA, tRNA, rRNA) and the specific role each plays.
Know what happens during post-transcriptional modification: intron removal, exon splicing, 5' cap, poly-A tail.
Be able to distinguish initiation, elongation, and termination during translation.
Know the start codon (AUG) and all three stop codons (UAA, UAG, UGA).
Understand the difference between substitution and frameshift mutations and why frameshifts are typically more damaging.
Be ready to explain sickle cell anaemia as a case study of a substitution mutation, including the heterozygous advantage.
True or False: Transcription occurs on ribosomes in the cytoplasm. (False, transcription occurs in the nucleus.)
Fill in the blank: The three-nucleotide sequence on tRNA that pairs with an mRNA codon is called an ________. (Anticodon.)
True or False: A frameshift mutation affects only a single codon. (False, it shifts the reading frame of every codon downstream.)
Fill in the blank: The start codon ________ codes for the amino acid methionine. (AUG.)
True or False: The poly-A tail is added to the 5' end of mRNA. (False, it is added to the 3' end.)
Q: Describe the central dogma of molecular biology in your own words.
A: Genetic information flows from DNA to RNA through transcription, then from RNA to protein through translation. DNA serves as the permanent archive, mRNA serves as the temporary working copy, and the ribosome reads that copy to build a chain of amino acids.
Q: Why must pre-mRNA be modified before leaving the nucleus?
A: Pre-mRNA contains introns (non-coding sequences) that would produce a faulty protein if translated. The introns must be removed and the exons spliced together. Additionally, a 5' cap and poly-A tail are added to protect the mRNA from degradation and to help the ribosome recognise it.
Q: Compare and contrast substitution and frameshift mutations. Which is generally more damaging, and why?
A: A substitution swaps one nucleotide for another, affecting only the codon at that position; it may be silent if the new codon specifies the same amino acid. A frameshift mutation inserts or deletes nucleotides, shifting the reading frame of every subsequent codon and almost always producing a non-functional protein. Frameshifts are generally more damaging.
Q: Explain how sickle cell anaemia is both a genetic disease and a survival advantage.
A: Sickle cell anaemia results from a single base-pair substitution that alters haemoglobin, causing red blood cells to deform. Individuals homozygous for the mutation (aa) suffer from the disease. However, heterozygous carriers (Aa) have enough normal haemoglobin to avoid severe symptoms while gaining partial resistance to malaria, giving them a survival advantage in regions where malaria is endemic.
Q: What are the three stages of translation, and what happens in each?
A: Initiation: the mRNA binds to the ribosome, and the start codon (AUG) is recognised by a tRNA carrying methionine. Elongation: tRNAs bring amino acids to the ribosome one at a time, peptide bonds form between them, and the chain grows. Termination: a stop codon is reached, the polypeptide is released, and the ribosome disassembles.
Protein synthesis depends directly on the DNA replication machinery covered in Part 1: both processes share the logic of complementary base pairing and the 5'-to-3' directionality rule. The transcription factors and regulatory elements introduced here are the bridge to gene control (Part 3), where you will see how cells decide which genes to transcribe and when. Mutations connect to evolution: they are the source of genetic variation on which natural selection acts.
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