SACE Biology: RNA & Protein Synthesis Notes

DNA, RNA & Protein Synthesis Overview

• Central Dogma: DNA Replication → Transcription (DNA to RNA) → Translation (RNA to Protein).

• Purpose: Cells use DNA information to synthesize specific proteins.

RNA Types and Roles

• mRNA (messenger RNA): Carries genetic code from DNA to ribosomes for protein synthesis.

• tRNA (transfer RNA): Carries specific amino acids to the ribosome during translation.

• rRNA (ribosomal RNA): Forms part of the ribosome, essential for translation.

Transcription

• Definition: The process where a gene (section of DNA) is copied into an RNA molecule (pre-mRNA).

• Location: Cell nucleus.

• Enzyme: RNA polymerase.

• Steps:

  1. Initiation: RNA polymerase binds to a DNA promoter site.

  2. Elongation: RNA strand is synthesized by adding nucleotides in a $5'$ to $3'$ direction.

  3. Termination: RNA polymerase transcribes a DNA terminator sequence, ending synthesis.

DNA vs. RNA

• DNA: Double stranded, contains Thymine (T), Deoxyribose sugar, very long.

• RNA: Single stranded, contains Uracil (U), Ribose sugar, relatively short.

RNA Splicing

• Process: Pre-mRNA is edited to remove non-coding regions.

• Exons: Coding sections that are expressed to make protein.

• Introns: Non-coding sections that are removed.

• Result: Mature mRNA, ready for translation.

Genetic Code & Codons

• Codon: A sequence of $3$ bases on mRNA that codes for a specific amino acid.

• Start Codon: AUG (codes for Methionine and initiates translation).

• Stop Codons: UAG, UAA, UGA (signal termination of translation, do not code for amino acids).

• Degeneracy: $4$ bases allow for $4^3 = 64$ possible codons, sufficient to code for $20$ different amino acids.

Translation

• Definition: The process where mRNA sequence is read to build a polypeptide (protein).

• Location: Ribosomes in the cytoplasm.

• Components: mRNA, ribosomes (rRNA), tRNA (carrying amino acids).

• Steps:

  1. Initiation: Small ribosomal subunit binds to mRNA. tRNA carrying Methionine binds to the start codon (AUG). Large ribosomal subunit attaches.

  2. Elongation: More tRNAs bring amino acids; peptide bonds form, linking amino acids into a growing chain.

  3. Termination: Ribosome encounters a stop codon, releasing the finished polypeptide.

Protein Folding

• Newly synthesized polypeptide chain folds into a specific three-dimensional (3D) shape to become a functional protein.