HBG 22 ( Transcription)

• Transcription is the process of making an RNA copy from DNA
• Transcription starts at a specific DNA sequence (TATAA) that serves as an attachment site for transcription molecules
• RNA polymerase synthesizes RNA from DNA template and binds to recognition sites in the promoter region
• RNA polymerase synthesizes RNA from 5' - 3' direction and does not need a dNTP template to start
• RNA polymerase continues to untwist the DNA while extending the RNA molecule
• Terminator sequence signals that the RNA transcript is complete and the RNA transcript is released from the RNA polymerase
• At the end of transcription, mRNA is not ready for translation but is ready for translation into protein after post-transcriptional processing
• Toxins can interfere with transcription by binding to RNA polymerase II and inhibiting its action in cells
• Post-transcriptional processing involves converting pre-mRNA transcript into mature mRNA
• Forms of modification in post-transcriptional processing include 5' capping, 3' polyadenylation, and splicing
• Addition of a modified guanine nucleotide at the 5' end of mRNA protects the transcript and assists in ribosome attachment during translation
• Polyadenylation stabilizes the transcript, helps in transcription termination, and regulates nuclear transport
• Introns are non-coding areas in mRNA that are removed during splicing, leaving only exons that encode for proteins
• Splicing of introns is done by the spliceosome, which also glues the exons together to form a mature mRNA
• Translation is the process by which ribosomes use the information in mRNA to synthesize proteins
• tRNA transfers specific amino acids to a growing polypeptide chain at the ribosomal site of protein synthesis
• Ribosomes, composed of rRNA and ribosomal proteins, translate mRNA into proteins by reading the codons and adding amino acids
• The translation process continues until a stop codon enters the A site, then release factor enzymes add a water molecule to the last amino acid of the chain
• Gene regulation controls and regulates the expression of genes
• Factors that regulate gene expression include external signals, internal signals, and protein signals from other cells
• There are five levels of gene expression control: Genomic Level Control, Transcriptional Level Control, RNA Processing & Nuclear Transport Control, Translational Level Control, and Post-Translational Processing
• Epigenetics refers to changes in phenotype or gene expression caused by mechanisms other than changes in the DNA sequence, and these changes can be inherited
• In Medicine, gene expression and regulation play a crucial role in diseases like COVID-19 and Alzheimer's
• COVID-19 vaccine development involves transcription and translation of the COVID-19 virus spike gene
• Alzheimer's disease is linked to epigenetic changes like hypomethylation of the promoter region of Amyloid Precursor Protein (APP) and Tau protein gene
• Cancer involves the loss of epigenetic control, such as hypomethylation of proto-oncogenes and hypermethylation of tumor suppressor genes (TSGs)
• Mechanisms controlling DNA methylation can be imperfect, leading to cancer progression by the loss of tumor suppressor genes through promoter methylation
• Epigenetic drugs are used in cancer treatment to reverse aberrant methylation of genes
• Cell Cycle

  1. G1 phase - Cell growth

  2. S phase (synthesis) - DNA synthesis occur

  3. G2 phase - Growth and Mitosis preparation
• For Meiosis, the process is doubled compared to Mitosis
• DNA Replication Characteristics


  • Act like photocopy machine lah tbh
  • Occur before the cells divide
  • Each strand of DNA molecule is template to produce parent-identical complementary strand
  • Known as semi-conservative replication
  • Able to copy 50 base pairs/second
• DNA Replication Process

  1. Replication starts from 5’ to 3’ direction

  2. Starts at replication origin

  3. Unwinds the helix by DNA helicase (helped by ATP)

  4. Multiples of opening; replication fork (Y-shaped part) formed

  5. SSB prevents/remains the strand from being coiled/folded again

  6. Primase catalyzes synthesis of the short RNA primer that is complementary to the template itself

  7. DNA Polymerase III extends the chains
• Enzymes involved in DNA Replication

  • DNA Helicase - unzipping enzymes
  • Primase - the starter/indicator
  • DNA Polymerase - the builder
  • DNA Ligase - the gluer
• Limitation during DNA Replication: Need assistance of primer to solve limitations
• First limitation: Synthesizing new DNA strand from 5’ to 3’ direction
• Second limitation: DNA polymerase will not synthesize DNA from one of the single strands (lagging strand) as it starts from 5’ end first
• RNA primer = filler
• Type of DNA strands

  • Leading strand
  • Lagging strand (Okazaki fragments)
• DNA Ligase

  Glue; connecting one Okazaki fragment to another
• Topoisomerases remove helical twists by cutting the strand and resealing the cut