• All proteins are unbranched linear polymers built from the same 20 amino acids.
• Every amino acid has the same basic structure a central (α) carbon bonded to

a carboxyl group (–COOH),

an amino group (–NH₂),

a hydrogen atom, and

• a fourth group R that differs from one amino acid to another.
• (The exception is glycine, whose R is simply a second hydrogen atom; the other 19 have a distinct R group.)

• With the help of suitable enzymes, amino acids are joined by dehydration (condensation) reactions into peptide bonds, forming a polypeptide.
• Proteins differ because of the number, kind and sequence of amino acids, the number of polypeptide chains in the protein, and the weak hydrogen bonds that fold the molecule into its special shape.
• Making a protein is a complicated process involving many kinds of molecules – nucleic acids, ribosomes, enzymes and amino acids.

1. The enzyme RNA polymerase binds to a special nucleotide sequence on DNA called the promoter, which sits at the start of a gene. The orientation of the promoter indicates which strand is to be transcribed.
2. The two DNA strands separate locally, and one strand (the template strand) is used as a mould.
3. RNA polymerase moves along the template, reading it in the 3′→5′ direction and joining complementary RNA nucleotides one by one, so the RNA grows in the 5′→3′ direction. (A pairs with U, T with A, G with C, C with G.)
4. Only one strand of any section is transcribed, even though either could in principle give an RNA.

• Polymerases in prokaryotes, one RNA polymerase transcribes rRNA, mRNA and tRNA; in eukaryotes, there are three different RNA polymerases.
• Structure of mRNA:

• At the start is a ribosome-binding site that positions the ribosome so that the first codon, AUG (start codon), is correctly placed for translation.
• At the end of the coding region is a stop codon – UAA, UAG or UGA.
• At the other end is a poly-A tail, a chain of up to ~200 adenine nucleotides, which protects the mRNA from breakdown by cytoplasmic enzymes. The tail is not translated into amino acids.

• Prokaryote vs eukaryote In prokaryotes, mRNA is ready for translation immediately, and ribosomes can attach to the front end and begin translation while the other end is still being transcribed – transcription and translation occur at the same time because there is no nuclear envelope.
• In eukaryotes, the primary transcript must first be processed in the nucleus before the mature mRNA moves to the cytoplasm.

• Template DNA strand 3′‑TAC GGA TTT‑5′ → mRNA 5′‑AUG CCU AAA‑3′.
• The mRNA is complementary to the template, so it has the same sequence as the other DNA strand, with U for T.

• Number of nucleotides in the coding region of mRNA = 3 × (number of amino acids) (+3 for the stop codon, if it is counted).
• Number of base pairs in the gene’s coding region = 3 × number of amino acids (as the book’s simple exercises assume).
• Number of different codons = 4³ = 64.

• Original mRNA codon GAA (Glu).
• A substitution at the third base to GAG still gives Glu (silent, because of degeneracy).
• A substitution at the first base to AAA gives Lys (changes the protein).
• A change to UAA gives a stop codon, truncating the protein.

• As the beginning of an mRNA leaves the ribosome, it may bind another small subunit and begin a new round of translation.
• Typically each mRNA carries from several to over 100 ribosomes at once.
• One mRNA with many ribosomes is a polyribosome (polysome).
• It lets the cell make many copies of the same protein quickly from one mRNA.

• 📖

🔍 In-Depth Explanations, Deductions & Multi-Book Insights:

• DABOUR HUB ACADEMY • EGYPTIAN HIGH SCHOOL (THANAWEYA AMMA)
• EXPLANATION Lesson 1: Types of RNA, Genetic Code and Protein Translation
• Course Biology (Senior 3) | Chapter 6 – Molecular Biology – RNA, Protein Synthesis and Genetic Engineering | Official Ministerial Syllabus 2026/2027
• ―――――――――――――――――――――――――――――――――――――――――――――――――――――――

1. Academic Lesson Overview & Pedagogical Objectives

2. Hierarchy Classification Tree (Cognitive Concept Map)

• Below is the hierarchical structural tree organizing the conceptual core of this lesson:

Protein Biosynthesis Pathway

├── 1. Types of Cellular RNA

3. The Master Teacher's Blackboard & Exam Rules