Bio Trancrption adn tranlation notes

🧬 IB Biology Study Guide — Unit 1.2: Protein Synthesis

🔑 Big Picture

Protein synthesis is how cells use the genetic code (DNA → RNA → Protein) to build proteins that perform most cellular functions. It involves two main stages:

  1. Transcription (nucleus): DNA is copied into messenger RNA (mRNA).

  2. Translation (cytoplasm/ribosome): mRNA is read by ribosomes to assemble amino acids into a polypeptide.



1. DNA and the Genetic Code

  • DNA: Double-stranded helix, bases = A, T, C, G.

  • Triplet code: Three DNA bases code for one amino acid.

  • Gene: A section of DNA that codes for a polypeptide.

IB Tip: Remember that one gene corresponds to one polypeptide, not necessarily a whole functional protein (some proteins have multiple polypeptide chains).



2. Transcription (in the nucleus)

Goal: Copy a gene from DNA into a complementary mRNA strand.

Steps:

  1. Initiation:

    • RNA polymerase binds to promoter region of the gene.

    • DNA unwinds and separates.

  2. Elongation:

    • RNA polymerase adds complementary RNA nucleotides (A → U, T → A, C → G, G → C).

    • Forms pre-mRNA (in eukaryotes).

  3. Termination:

    • RNA polymerase reaches a stop signal.

    • mRNA strand is released.

  4. Processing (in eukaryotes):

    • Introns removed, exons joined (splicing).

    • 5′ cap and poly-A tail added for stability.

Key Word: mRNA = messenger RNA (carries the code to ribosomes).



3. Translation (at ribosomes in cytoplasm or RER)

Goal: Build a polypeptide using mRNA instructions.

Players:

  • mRNA: Blueprint for the protein.

  • Ribosome: Molecular machine (large + small subunits).

  • tRNA: Transfer RNA; brings amino acids to ribosome. Each has an anticodon that pairs with an mRNA codon.

  • Amino acids: Building blocks of proteins.

Steps:

  1. Initiation:

    • Ribosome binds mRNA at start codon (AUG = methionine).

  2. Elongation:

    • tRNAs bring amino acids to ribosome.

    • Codon–anticodon base pairing ensures correct sequence.

    • Peptide bonds form between amino acids.

  3. Termination:

    • Ribosome reaches stop codon (UAA, UAG, UGA).

    • Polypeptide released.

IB Tip: Know the difference between free ribosomes (proteins used inside the cell) and RER ribosomes (proteins for secretion/lysosomes/membrane).



4. Protein Folding & Post-Processing

  • Polypeptide folds into a specific 3D shape (secondary, tertiary, sometimes quaternary structure).

  • Modifications may happen in the ER and Golgi (e.g., adding carbohydrates → glycoproteins).



5. Key IB Vocabulary

  • Codon: 3-base sequence on mRNA that codes for an amino acid.

  • Anticodon: 3-base sequence on tRNA complementary to the codon.

  • Polypeptide: A chain of amino acids.

  • Gene expression: Process of turning DNA into a protein.

  • Universality of genetic code: Same codons code for same amino acids in almost all organisms → basis for genetic engineering.



6. Diagrams You Should Know

📍 If drawing on an exam, be sure to label:

  • Transcription: DNA, RNA polymerase, mRNA, base-pairing.

  • Translation: Ribosome (large + small subunit), mRNA, tRNA, amino acid chain.

  • Central Dogma summary: DNA → mRNA → Protein.



7. Applications & IB Links

  • Sickle-cell anemia: Single base substitution in gene → changes polypeptide (hemoglobin).

  • mRNA vaccines (modern application): Deliver mRNA instructions → ribosomes make viral proteins → immune response.


🌀 Splicing (in eukaryotes only)

What it is:

  • After transcription, the initial product is pre-mRNA.

  • Pre-mRNA contains:

    • Exons = coding regions (kept).

    • Introns = non-coding regions (removed).

Steps of splicing:

  1. Introns are cut out by a complex called the spliceosome.

  2. Exons are joined together to form a continuous coding sequence.

  3. A 5′ cap and poly-A tail are added to stabilize the mature mRNA.

Why it matters:

  • Splicing ensures that only useful coding information is sent to ribosomes.

  • Alternative splicing allows a single gene to produce different proteins by joining exons in different combinations.



🧬 Template strand vs Coding (sense) strand

DNA has two strands in transcription, but they’re not used the same way.

  • Template strand (antisense strand):

    • The strand used by RNA polymerase to make mRNA.

    • Complementary to the mRNA sequence.

    • Runs 3′ → 5′ (because mRNA is made 5′ → 3′).

  • Coding strand (sense strand):

    • The strand not used by RNA polymerase.

    • Has the same sequence as mRNA, except that in RNA, T is replaced with U.

    • Runs 5′ → 3′.



🔎 How to tell which is which:

  1. Find the strand that runs 3′ → 5′ → that’s usually the template strand, since RNA must be built 5′ → 3′.

  2. The mRNA matches the coding strand (sense), but with U instead of T.

  3. Look for the promoter sequence (like TATA box) → that’s always on the coding strand, because it’s not transcribed but signals transcription start.