D1.2 Protein synthesis

Transcription is the synthesis of an RNA strand using one DNA strand as the template.

RNA polymerase binds the selected DNA region, separates a short section, matches RNA nucleotides to exposed template bases and joins the nucleotides into RNA.

DNA template exposed → complementary RNA nucleotides pair → RNA polymerase forms the RNA backbone → RNA separates and DNA re-forms its double helix.

For a protein-coding gene, RNA polymerase produces an mRNA transcript whose base sequence can later be translated.

Transcription synthesizes RNA, not DNA or polypeptide. Promoter/transcription-factor detail belongs to the later HL initiation objective.

Temporary hydrogen bonds between complementary bases allow an RNA sequence to be specified from a DNA template during transcription.

DNA hydrogen bonds separate locally while the covalent sugar–phosphate backbones remain intact. RNA nucleotides form complementary hydrogen bonds to exposed template bases before RNA polymerase joins them.

DNA template A pairs with RNA U; DNA template T pairs with RNA A; DNA C pairs with RNA G; DNA G pairs with RNA C.

A DNA template segment 3′-TACG-5′ specifies RNA 5′-AUGC-3′: template adenine is represented by uracil in RNA, not thymine.

Complementary does not mean identical. Hydrogen bonds guide base choice; phosphodiester bonds form the stable RNA backbone.

A single DNA strand can act repeatedly as a transcription template without its base sequence changing.

RNA polymerase reads the template and joins separate RNA nucleotides; it does not consume or replace the DNA bases. After the transcription bubble passes, the DNA strands pair again.

DNA opens locally → RNA copy is synthesized → RNA leaves → original DNA sequence remains conserved and available for later transcription.

A non-dividing neuron may transcribe the same essential gene many times while conserving that gene's DNA sequence throughout the life of the cell.

Stable does not mean inaccessible: local opening permits transcription. The RNA transcript can change or degrade while the DNA template sequence remains unchanged.

Transcription is the first stage of gene expression and a key point at which expression can be switched on or off.

Cells contain many genes but do not express all of them at the same time. If a gene is not transcribed, its mRNA is unavailable for translation; starting transcription makes expression possible.

Regulatory state → transcription on/off → mRNA absent/present → translation possible/not possible. Different cell types express different subsets of the same genome.

A cell can switch on transcription of an enzyme gene when the enzyme is needed while other genes remain untranscribed.

Transcriptional control is a key switch, not the only possible control. More mRNA does not guarantee proportionally more protein if later steps are limiting.

Translation uses a ribosome to read mRNA codons and join amino acids in the encoded order.

tRNA molecules pair anticodons with codons and deliver specific amino acids. The ribosome forms peptide bonds as it moves from start toward stop.

Trace: start codon; tRNA matching; peptide bond; codon movement; stop codon.

A ribosome reading three codons adds three specified amino acids before a stop signal ends the chain.

Translation reads RNA; it does not copy mRNA back into DNA. The ribosome assembles the chain; later folding and modification determine whether the polypeptide becomes functional.

During translation, mRNA supplies codon order, tRNAs deliver specific amino acids and the ribosome positions the molecules and catalyses peptide-bond formation.

mRNA first binds to the small ribosomal subunit. The large subunit forms the catalytic complex and can hold two tRNAs simultaneously so the amino acid on one can be linked to the growing chain on the other.

mRNA: ordered codons. tRNA: anticodon plus specific amino acid. Small subunit: binds/positions mRNA. Large subunit: binds two tRNAs and supports peptide-bond formation.

One tRNA holds the growing chain while a second matching tRNA places the next amino acid beside it; a peptide bond extends the polypeptide.

No component determines the protein alone: mRNA sets sequence, tRNAs match codons, and the ribosome coordinates assembly.

A three-base tRNA anticodon forms complementary base pairs with a three-base mRNA codon during translation.

Correct codon–anticodon pairing positions the tRNA carrying the amino acid specified by that codon, transferring mRNA sequence information into polypeptide sequence.

Read the mRNA codon 5′→3′; write the antiparallel complementary anticodon; pair A–U and C–G; identify the amino acid from the mRNA codon, not the anticodon.

mRNA codon 5′-AUG-3′ pairs with tRNA anticodon 3′-UAC-5′, positioning the tRNA carrying methionine.

Codon and anticodon are complementary and antiparallel, not identical. DNA–RNA pairing during transcription is a different Objective.

The genetic code is a triplet, degenerate and nearly universal mapping from mRNA codons to amino acids or stop signals.

Four bases taken two at a time give only 16 combinations, fewer than the 20 amino acids; triplets give 4³ = 64 codons, enough for all amino acids plus stop signals.

Degenerate means more than one codon can specify the same amino acid. Universal means the same codon usually specifies the same amino acid across organisms, with limited exceptions.

UUU and UUC both code for phenylalanine, showing degeneracy; UUU has one defined meaning, so degeneracy is not ambiguity.

Read codons in a fixed triplet frame. Universality is a broad biological pattern, not a claim that no exceptions exist.

A genetic-code table is used by reading mRNA codons 5' to 3' and matching each to its amino acid.

The first, second and third bases select positions in the table. Reading-frame errors shift every later codon, so strand labels and grouping matter.

Use: locate start; split triplets; read each codon; stop at a stop codon; report amino-acid order.

For 5'-AUG-GCU-UAA-3', AUG starts, GCU adds alanine and UAA terminates. If one base is omitted before AUG, every later triplet may be regrouped, so the table must be used from the correct start frame.

Do not use the DNA template directly as if it were mRNA; transcribe first.

During elongation, a matching tRNA enters, a peptide bond forms and the ribosome moves to the next codon.

Repeating this cycle extends the chain in a fixed direction while mRNA advances through the ribosome. Energy from charged tRNAs and factors supports the process.

Track each cycle: codon recognition; chain transfer; translocation; empty tRNA exit.

After one cycle the chain has one additional amino acid and the next codon is exposed.

Elongation is not random amino-acid addition; codon recognition controls the order. The cycle stops at a termination signal rather than continuing until the physical end of the mRNA.

A point mutation can change one mRNA codon, replace one amino acid and alter how a polypeptide folds or functions.

The effect depends on code degeneracy, mutation position and the chemical properties of the original and replacement amino acids. Some substitutions are synonymous; others alter interactions stabilizing protein structure.

In sickle-cell disease, a point substitution changes a β-globin codon from GAG to GUG in mRNA, replacing glutamic acid with valine. The hydrophobic replacement promotes abnormal haemoglobin association and changes red-cell shape under low oxygen.

DNA base substitution → changed mRNA codon → possible amino-acid replacement → altered side-chain interactions → altered protein structure/function.

A mutation is not automatically harmful or structure-changing: degeneracy can make it synonymous, and some amino-acid replacements have little effect.