IB Biology HL D1.2 Protein synthesis Notes

Transcription is the copying step that makes mRNA from a gene. RNA polymerase uses the DNA template strand to synthesize a complementary RNA strand, so the gene information becomes a mobile mRNA copy that can be translated at a ribosome.

mRNA is the mobile copy of gene information.

During transcription, free RNA nucleotides align with the DNA template by complementary base pairing and hydrogen bonding. The exam trap is uracil: DNA adenine pairs with RNA uracil, while cytosine pairs with guanine.

RNA uses uracil during transcription.

The DNA template is read during transcription, but its base sequence is not altered. Temporary base pairing guides RNA synthesis, while the DNA sugar-phosphate backbone and base pairing preserve the stored genetic information.

The DNA template is read, not rewritten.

Transcription is the first stage of gene expression because a gene must be transcribed before its protein can be made. Cells regulate which genes are transcribed according to tissue type, developmental stage, and signals, so different cells can make different proteins from the same genome.

Gene expression starts with transcription choice.

Translation is the decoding step at the ribosome. The ribosome reads mRNA codons, and the order of codons determines the order of amino acids in the growing polypeptide.

mRNA codon order determines polypeptide order.

Translation needs three jobs working together: mRNA provides codons, tRNA carries activated amino acids and has anticodons, and the ribosome holds mRNA and tRNAs in position so peptide bonds can form.

Each translation component has a different role.

Correct translation depends on two matching steps. A tRNA anticodon pairs with a complementary mRNA codon by hydrogen bonding, and that tRNA is attached to a specific amino acid, so the correct amino acid is added to the growing polypeptide.

Anticodon pairing positions the correct amino acid.

The genetic code describes how mRNA codons are read. It is triplet because three bases make one codon, degenerate because several codons can specify the same amino acid, and almost universal because most organisms use the same code; codons can specify amino acids, a start signal, or stop signals.

The genetic code is a rulebook for mRNA codons.

Genetic code tables use mRNA codons, not DNA triplets. If a question gives template DNA, first transcribe it into complementary mRNA, then read the mRNA codons 5' to 3' to find amino acids.

The code table reads mRNA.

During elongation, the ribosome moves along mRNA one codon at a time from start to stop. tRNAs bring amino acids, peptide bonds join them, and multiple ribosomes can translate the same mRNA at once as a polysome.

Elongation adds amino acids codon by codon.

A mutation can change a DNA base sequence, which can change an mRNA codon and therefore the amino acid sequence. A changed primary structure may alter protein folding and function; sickle-cell haemoglobin is the key example of an amino acid change affecting haemoglobin structure.

Protein effects depend on sequence and folding.

RNA polymerase reads the template DNA strand 3' to 5' while synthesizing RNA 5' to 3'. Ribosomes also translate mRNA codons in the 5' to 3' direction, so directionality controls which sequence is read and how codons are ordered.

Direction tells you which sequence is read.

A promoter is a DNA sequence that marks where transcription should start and which direction it should proceed. In eukaryotes, transcription factors help RNA polymerase bind to the promoter and initiate transcription of the correct gene region.

Promoters help start transcription in the right place and direction.

Non-coding DNA does not code for polypeptide amino acid sequences, but it can still be functional. Examples include introns, regulatory sequences, telomeres, rRNA genes, and tRNA genes.

Non-coding DNA can still have functions.

In eukaryotes, the first RNA product is pre-mRNA, not immediately a finished message. Before export and translation, processing adds a 5' cap, adds a poly-A tail, and removes introns by splicing to produce mature mRNA.

Processing prepares eukaryotic mRNA.

Alternative splicing means different exon combinations can be joined from the same pre-mRNA. This lets one gene produce multiple protein variants in different cells or developmental stages.

Different exon combinations can produce different proteins.

Translation initiation starts at the AUG start codon. Ribosomal subunits assemble there, the initiator tRNA enters the P site, and the A, P, and E sites organize tRNA entry, peptide holding/transfer, and exit.

Initiation sets the reading frame.

A newly made polypeptide is not always the final active protein. Polypeptides may be folded, cleaved, or chemically modified; preproinsulin processing into active insulin is the key example.

Post-translational processing can activate proteins.

Proteasomes degrade proteins that are tagged, damaged, or no longer needed. This releases amino acids for new protein synthesis and helps maintain proteome quality control.

Proteasomes connect protein breakdown to new synthesis.

Transcription makes mRNA as a mobile copy of gene information; RNA polymerase synthesizes RNA complementary to the DNA template strand. Free RNA nucleotides align by complementary base pairing and hydrogen bonding; DNA adenine pairs with RNA uracil and cytosine pairs with guanine. DNA template strands are transcribed without altering the base sequence; the sugar-phosphate backbone and base pairing preserve genetic information. Transcription is the first stage of gene expression; cells regulate which genes are transcribed according to tissue, stage, and signals. Translation decodes mRNA at ribosomes to synthesize polypeptides; mRNA codon order determines amino acid sequence. mRNA provides codons, tRNA carries activated amino acids with anticodons, and ribosomes hold mRNA and tRNAs so peptide bonds can form. tRNA anticodons pair with complementary mRNA codons by hydrogen bonding; specific tRNA-amino acid attachment helps ensure correct amino acid addition. The genetic code is triplet, degenerate, and almost universal; codons specify amino acids, a start signal, or stop signals. Genetic code tables use mRNA codons, not DNA triplets; convert template DNA to mRNA first, then read codons 5' to 3'. Ribosomes move along mRNA one codon at a time from start to stop; peptide bonds join amino acids and multiple ribosomes can form a polysome. Mutations can change codons and therefore amino acid sequence; changed primary structure may alter folding and function, such as sickle-cell haemoglobin.

Use this for SL/core questions about transcription, RNA base pairing, gene expression, translation, tRNA/ribosome roles, genetic code table use, elongation, and mutation effects.

RNA polymerase reads template DNA 3' to 5' and synthesizes RNA 5' to 3'; ribosomes translate mRNA codons in the 5' to 3' direction. Promoters mark transcription start regions and orientation; transcription factors help RNA polymerase bind and initiate in eukaryotes. Non-coding DNA does not code for polypeptide amino acid sequences and includes introns, regulatory sequences, telomeres, rRNA genes, and tRNA genes. Eukaryotic pre-mRNA is modified before export and translation by adding a 5' cap and poly-A tail and removing introns by splicing. Alternative splicing joins different exon combinations from one pre-mRNA, so one gene can produce multiple protein variants in different cells or stages. Translation initiation assembles ribosomal subunits at the start codon AUG; initiator tRNA enters the P site and A, P, and E sites organize tRNA movement. Newly made polypeptides may be folded, cleaved, or chemically modified; preproinsulin processing to active insulin is a key example. Proteasomes degrade tagged, damaged, or unneeded proteins; amino acid recycling supports new protein synthesis and proteome quality control.

Use this for HL questions about transcription/translation directionality, promoters, transcription factors, non-coding DNA, post-transcriptional modification, alternative splicing, translation initiation, polypeptide modification, and proteasomes.