Fiche de révision : Genetic Translation and Protein Synthesis

📋 Course Outline

1. Genes, DNA and RNA
2. Transcription and mRNA synthesis
3. Genetic code and codons
4. Translation machinery
5. Translation stages and protein folding

📖 1. Genes, DNA and RNA

🔑 Key Concepts & Definitions

• DNA : DNA is the nuclear support of inherited traits made of nucleotide sequences.
• Gene : A gene is a specific sequence of nucleotides located on the DNA molecule.
• RNA : RNA is an acid made of ribonucleotides, with ribose as its sugar.
• Uracil : Uracil is a base that characterizes RNA molecules among the nucleic acids.

📝 Essential Points

• DNA is located in the nucleus.
• Genetic traits are encoded in genes carried by the DNA molecule.
• A ribonucleotide contains ribose, a five-carbon sugar.
• RNA is a short single-stranded molecule with ribonucleotides (A, G, C, and U).

💡 Memory Hook

Think DNA stays in the nucleus; RNA is the single-strand message builder using U instead of T.

📖 2. Transcription and mRNA synthesis

🔑 Key Concepts & Definitions

• Transcription : Transcription is the nuclear step that biosynthesizes mRNA from a DNA template strand.
• mRNA : mRNA is the messenger RNA that transfers genetic information from the nucleus to ribosome sites.
• RNA polymerase : RNA polymerase is the enzyme that synthesizes mRNA during transcription.

📝 Essential Points

• Genetic information in DNA is expressed in two stages: transcription then translation.
• Transcription occurs in the nucleus using RNA polymerase and template-guided nucleotide complementarity.
• mRNA synthesis uses one DNA strand (the template strand) to produce the mRNA sequence.

💡 Memory Hook

Transcription = nucleus + RNA polymerase + DNA template → mRNA.

📖 3. Genetic code and codons

🔑 Key Concepts & Definitions

• Genetic code : The genetic code is the special code that maps mRNA information into amino acid sequences.
• Codon : A codon is a triplet of bases on mRNA that specifies an amino acid or a stop signal.

📝 Essential Points

• The unit of the genetic code is a triplet of bases.
• Most amino acids can be specified by several codons.
• Stop codons are UAA, UAG, and UGA and they do not code any amino acid.
• AUG codes methionine and acts as the translation start codon.
• UGG codes tryptophan.

💡 Memory Hook

Triplets: AUG starts with Met; UAA/UAG/UGA stop the reading.

📖 4. Translation machinery

🔑 Key Concepts & Definitions

• Ribosome : A ribosome is an organelle made of proteins and tRNA that contains distinct functional subunits.
• tRNA : tRNA is a specialized RNA that fixes, transports, and presents the amino acids matching mRNA codons.
• Polyribosome : A polyribosome is a distinct complex of ribosomes translating the same mRNA.

📝 Essential Points

• Each ribosome has a small subunit for mRNA reading and a large subunit with two catalytic sites.
• tRNA recognizes the matching codon through a complementary anticodon made of 3 nucleotides.
• Translation requires amino acid activation by enzymes using ATP energy.
• Polyribosomes allow synchronized reading of the same mRNA to increase the amount of protein produced.

💡 Memory Hook

Ribosome = reader (small subunit) + two catalytic sites; polyribosome = many readers on one mRNA.

📖 5. Translation stages and protein folding

🔑 Key Concepts & Definitions

• Initiation : Initiation is the start of translation when ribosomes begin at the start codon on mRNA.
• Elongation : Elongation is the phase where codon-by-codon peptide chains grow via peptide bond formation.
• Termination : Termination is the translation end when the ribosome reaches a stop codon and the peptide is released.

📝 Essential Points

• Translation always starts at AUG, where the methionine carried by the corresponding tRNA is placed on the ribosome.
• During elongation, the ribosome moves codon to codon and peptide bonds form between the amino acid in the reading site and the amino acid in the catalytic site.
• Translation ends when the ribosome reaches a stop codon, the last tRNA dissociates, and the polypeptide becomes free.
• The newly formed polypeptide acquires its three-dimensional structure automatically to produce a functional protein.

💡 Memory Hook

Met at AUG → elongation builds bonds while ribosome moves → stop codon frees polypeptide, which folds itself.

⚠️ Common Pitfalls & Confusions

1. Students may confuse DNA location with mRNA location by forgetting that DNA is in the nucleus while translation happens in the cytosol.
2. Students may mistakenly think RNA is double-stranded or long; the source states mRNA is short and single-stranded.
3. Students may swap uracil rules, wrongly using thymine in RNA when the base specific to RNA is uracil.
4. Students may forget the exact stop codons (UAA, UAG, UGA) and incorrectly assign them as coding amino acids.
5. Students may mix up AUG as a generic start signal rather than specifically coding methionine in the initiation step.
6. Students may confuse ribosome subunit functions, incorrectly attributing the reading site to the large subunit instead of the small one.
7. Students may miss that polyribosomes increase protein output by having multiple ribosomes read the same mRNA simultaneously.

✅ Exam Checklist

1. Identify where DNA is located in the cell and what role it plays as a support of inherited traits.
2. Define a gene as a specific sequence of nucleotides on DNA.
3. Describe the structural features of mRNA: short, single-stranded, and made of ribonucleotides with U present.
4. Explain why the genetic information in DNA is expressed in two stages: transcription then translation.
5. State the key conditions of transcription: nuclear location, RNA polymerase, and mRNA produced from a DNA template strand with complementary nucleotide matching.
6. Define the genetic code as the mapping that expresses mRNA information into amino acid sequences.
7. Use the rules of the codon triplet: identify which codons do not code amino acids and are stop codons.
8. Recognize AUG as the start codon and methionine as the amino acid it codes.
9. Recognize UGG as the codon for tryptophan.
10. Describe ribosome structure in terms of small subunit (mRNA reading site) and large subunit (two catalytic sites).
11. Explain how tRNA matches codons using an anticodon of 3 complementary nucleotides.
12. State the role of ATP and amino acid activation enzymes required for translation.
13. Sequence the translation stages: initiation at AUG with methionine tRNA, elongation by peptide bond formation as the ribosome moves, then termination at a stop codon with peptide release.
14. Explain what happens after termination: the polypeptide folds automatically into a three-dimensional functional protein.