Transcription & Translation

• DNA

  Deoxyribonucleic acid - Bearer of genetic information

  
• RNA

  Ribonucleic acid
• Transcription

  1. Process where ribonucleic acid (RNA) is transcribed/ synthesised from DNA

  2. Only DNA that encodes for a gene is transcribed into RNA

  3. Gene is a sequence of nucleotides that encodes for a specific protein or functional RNA molecule, influencing an organism's traits and functions

  
• Products of DNA transcription

  • Messenger RNA (mRNA)
  • Ribosomal RNA (rRNA)
  • Transfer RNA (tRNA)
• Promoter region

  Specific sequence upstream of the gene that determines which DNA strand will act as the template
• Transcription: Initiation (RNA polymerase)

  • Enzyme that catalyses the synthesis of RNA from DNA
  • Unwinds and rewinds DNA
  • Reads DNA in 3' to 5' direction
  • Synthesises RNA in 5' to 3' direction
  • Requires nucleoside triphosphates (NTPs) - ATP, GTP, CTP, UTP
  • Forms phosphodiester bonds between nucleotides

  
• RNA polymerase binding

  RNA polymerase binds to the promotor region to start transcription

  • it determines which DNA strand will act as the template
• Transcription: Elongation

  1. Chain elongation as RNA polymerase moves along the DNA molecule

  2. Elongated in 5' to 3' direction (Nucleotides added to the 3' growing end)

  • RNA polymerase moves along the DNA template strand, synthesising a pre-mRNA molecule by adding RNA nucleotides that are complementary to the DNA template strand.
• Transcription: Termination

  1. At the end of the gene, RNA polymerase receives a termination signal

  2. Formation of a hairpin structure due to GC-rich sequence that folds itself into a hairpin loop

  3. RNA polymerase dissociates from DNA
• Template strand

  • Only one of the two DNA strands serves as a template for mRNA formation
  • applies to a gene but not the whole DNA molecule
• Coding strand

  The non-template DNA strand, not usually used for transcription
• Transcription and translation are coupled in prokaryotes, but not in eukaryotes
• What are promoters in transcription?

  Specific DNA sequences that recruit RNA polymerase to initiate transcription
• What triggers transcription initiation?

  The binding of transcription factors to promoters
• What is the function of transcription factors?

  To regulate gene expression by controlling transcription
• What are transcription factors?

  Proteins that regulate gene expression by binding to specific DNA sequences
• RNA Polymerase

  The enzyme responsible for transcribing DNA into RNA.
• Transcription

  The process by which the information encoded in a gene's DNA is copied into a complementary RNA molecule.
• Initiation

  The stage of transcription where RNA polymerase binds to the DNA at a specific region called a promoter.
• Promoter

  A region in DNA that serves as a binding site for RNA polymerase, initiating transcription.
• Templates

  The DNA strands that provide the template for RNA synthesis during transcription.
• Post-Transcriptional Modification

  Process of splicing, editing, and modifying the RNA molecule
• Transcription vs. Translation

  Transcription: DNA -> mRNA, Translation: mRNA -> Protein
• RNA Polymerase

  An enzyme that binds to the promoter region of a gene, unwinds the double helix DNA, and adds nucleotides to the growing RNA chain.
• RNA Synthesis Direction

  From 5' to 3', meaning that RNA polymerase adds nucleotides to the growing RNA chain in a 5' to 3' direction.
• 5' to 3' Direction Importance

  The direction in which nucleotides are added to the RNA chain, affecting the synthesis of the RNA molecule, secondary and tertiary structures, and interactions with other molecules.
• Translation

  Process where ribonucleic acid (mRNA) is translated into an amino acid sequence with a protein code (called genetic code)

  • Polypeptides produced from mRNA via translation, and processed further to become functional proteins

  
• Genetic code

  Decodes mRNA into proteins, consists of 20 amino acids, each amino acid is a triplet of bases (codon)

  • Always read from 5 to 3 direction
  • The START codon is AUG, which encodes a.a. MET
  • 3 STOP codon - UGA, UAA, UAG

  
  

  • When reading from DNA (5 to 3 ) to RNA (3 to 5) REMEMBER to start from the same number and start encoding to figure the protein and mRNA
• Properties of the genetic code

  • Non-overlapping
  • Degenerate (most amino acids coded for ≥ one codon)
  • Not uniform (six codons encoding for amino acids Ser & Leu, two codons encoding for His & Tyr, one codon encoding for Met & Trp)
• Start codon

  Mostly the codon AUG, encoding for amino acid methionine (MET)
• Stop codons

  UGA, UAA, UAG
• How to translate mRNA

  1. Find the start codon (AUG)

  2. Translate the mRNA using the genetic code until you reach the stop codon

  
• Ribosomes

  • Site of protein synthesis, made up of rRNAs and proteins, consist of two subunits (small and large)
• Ribosome binding sites

  mRNA binding site, A (aminoacyl) site, P (peptidyl) site, E (exit) site
• Translation: Initiation

  Binding of mRNA and aminoacyl-tRNA to ribosome

  • the small ribosomal subunit binds to the mRNA at the start codon (AUG) which is recognised by the initiator tRNA.
  • tRNA being positioned in the P site of the ribosome
• Translation: Elongation

  Polypeptide chain elongation requires peptide bond formation betweeen a.a., catalysed by peptidyl transferase

  
  

  1. Charged tRNA enters the A site.
  2. The amino acid from the tRNA in the P site is transferred to the amino acid on the tRNA in the A site, forming a peptide bond.
  3. The ribosome moves one codon down the mRNA in the 5' to 3' direction.
  4. This shift moves the tRNA with the growing polypeptide chain from the A site to the P site.
  5. The now-empty tRNA in the P site moves to the E site and is eventually released from the ribosome.
• Translation: Termination

  Polypeptide chain release from ribosome

  • When a stop codon (UAA, UAG, UGA) is reached, elongation stops, and the process of termination begins, leading to the release of the newly synthesized polypeptide chain.
• mRNAs are read by multiple ribosomes simultaneously

  • allows cell to make multiple copies of same polypeptides quickly
• Post-translational modification

  • refer to the chemical changes that a protein undergoes after its synthesis (translation) on a ribosome.
  • Occurs in endoplasmic reticulum and Golgi apparatus, includes removal of initiating Met, formation of disulfide bonds, glycosylation, amino acid cleavage
• Central dogma of molecular biology

  Flow of genetic information from DNA → RNA → Proteins