DNA Transcription

• Transcription - the synthesis of RNA based on the nucleotide sequence on a portion of DNA

  -takes place in the nucleus

  -uses RNA Polymerase
• 1. Template strand (3'-5')-the transcribed strand, RNA molecules that are complementary and antiparallel to one of the two nucleotide strand.

  2. Non-template strand(5'-3')- the other strand which is not ordinarily transcribed
• Transcription Unit:

  1. promoter- DNA sequence the transcription apparatus recognizes and binds. Determines DNA strand to be read as the template and direction of transcription.

  2. RNA-coding region- sequence of DNA nucleotides that is copied into an RNA molecule

  3. Terminator- sequence of nucleotides that signals where transcription is to end. Transcription stops only after terminator has been copied into RNA
• Accessory proteins:

  1. Basa transcription factors (BFT)- general transcription factors + RNA polymerase

  -assembles near the start site and initiate minimal levels of transcription

  2. Transcriptional activator proteins (TAP)- bind to specific DNA sequences and bring higher levels of transcription by stimulating assembly of basal transcription apparatus at the start site
• Activators- proteins bind to genes at sites known as enhancers and speed the rate of transcription

  2. Repressors- proteins bind at selected sets of genes at sites known as silencers, slowing transcription

  3. Coactivators- "adapter" molecules integrate signals from activators and repressors

  4. Basal transcription factors- these factors position RNA polymerase t the start of transcription site
• Transcription termination in Prokaryotes:

  1. Rho-dependent termination- Requires a protein called Rho factor. Rho binds to specific sequences on the RNA transcript called rut sites (Rho utilization sites). Binding of Rho disrupts the RNA polymerase complex, causing it to detach from the DNA and release the RNA.
• Transcription termination in Prokaryotes

  2. Rho-independent termination- Relies on the formation of a specific structure in the RNA transcript itself. This structure consists of a hairpin loop (GC-rich inverted repeats followed by 4 A residues) followed by a U-rich tail. The hairpin loop pauses the RNA polymerase, and the U-rich tail facilitates the dissociation of the RNA transcript from the DNA and polymerase.
• RNA Polymerases in Eukaryotes:

  1. RNA polymerase I- 8S, 18S & 28S rRNA genes

  2. RNA polymerase II- all protein-coding genes

  3. RNA polymerase III- genes for small RNAs, tRNA genes
• What happens to the RNA transcripts?

  -in prokaryotes, it creates polyribosomes which made up of RNA transcripts with many ribosomes attached, this allows immediate translation.

  -in eukaryotes, the pre-mRNA undergoes splicing to be mature mRNA and released to the cytoplasm for translation
• Eukaryotic pre-mRNAs are modified by addition of 5' cap and 3' poly(A) tail, why?

  -at 5' cap, the GTP (guanosine triphosphate) is added and occurs at the start of transcription to use as recognition signal for ribosomes to bind to mRNA.

  -at 3' poly(A) tail, 150 or more adenine nucleotides is added, this is to stabilize the mRNA.
• Why do splicing (pre-mRNA processing) happens?

  -eukaryotic pre-mRNAs has introns, which does not a coding genes only the exons, thus introns are looped out and cut away from exons by snRPS (small nuclear ribonucleicproteins) to form spliceosome which will excised the introns. Thus, a mature mRNA with coding genes exit the nucleus to be translated in cytoplasm
• Alternative Splicing- this process allows for different combinations of exons to be included or excluded from the final mRNA transcript derived from a single pre-mRNA molecule. Thus, creating more diverse proteins from a single RNA strand
• Types of RNA:

  1. Messenger RNA (mRNA)- carries information from DNA to ribosomes, the site of protein synthesis; coding RNA

  2. Transfer RNA (tRNA)- has 3 loops, with the middle loop bearing the anticodon at its tip, responsible for carrying amino acids to the ribosome during protein synthesis

  3. Ribosomal RNA (rRNA)- forms the structure of the ribosome, responsible for “reading” the amino acid that is carried by the tRNA to the ribosomes or for translating the information in mRNA
• prokaryotic ribosomes- size: 70S; large subunits: 50S, small subunits: 30S

  eukaryotic ribosomes- size: 80S; large subunits: 60S, small subunits: 40S

  -S is the rate at which the rRNA molecules sediment during centrifugation (sedimentation rate)
• Roles of diff kinds of RNA

  
• Roles of diff kinds of RNA