Genetics - Lecture 10 (After Test)

• Transcription in eukaryotes differs from prokaryotic transcription in several ways

  • Transcription in eukaryotes occurs in the nucleus and isn’t coupled to translation
  • Eukaryotic transcription requires chromatin to become uncoiled, making the DNA accessible to RNA polymerase and other regulator proteins. This transition is referred to as chromatin remodeling
• Eukaryotes have DNA that is MUCH more coiled than prokaryotes, and are covered in lots of histones
• Eukaryotic RNA polymerases rely on transcription factors (TFs) to scan and bind to DNA
• In addition to promoters, enhancers and silencers also control transcription regulation
• Enhancers are small sequences of nucleotides on the gene that speed up the process of mRNA synthesis
• Silencers slow down/stop mRNA synthesis

  Promoters cause mRNA polymerase to bind to molecule

  Such things don’t exist in prokaryotic cells
• Eukaryotic mRNAs require processing to produce mature mRNAs

  1. Addition of a 5’ caps
  2. Addition of a 3’ tail
  3. Excision of introns
• Eukaryotes possess 3 forms of RNA polymerase which transcribes different types of genes
• RNA polymerase I - makes rRNA in the nucleolus
• RNA polymerase II - makes mRNA and snRNA in the nucleoplasm
• RNA polymerase III - makes 5S rRNA and tRNA in the nucleoplasm
• In bacterial cells, there is 1 polymerase that does everything
• RNA polymerase II (RNP II) is responsible for a wide range of genes in eukaryotes
• RNP II promoters have a core promoter element and promoter that determine where RNP II binds to the DNA and where it begins copying the DNA into RNA
• The other 3 regulatory DNA sequences, proximal-promoter elements, enhancers, and silencers, influence the efficiency or rate of transcription initiation
• The TATA box is a core promoter element that binds the TATA-binding protein (TBP) of transcription factor TFIID and determines the start site of transcription
• Enhancers and silencers

  • Can be upstream, within, or downstream of the gene
  • Can modulate transcription from a distance
  • Act to increase or decrease transcription in response to cell’s requirement for a gene product or at a particular time during development or place within an organism
• There are 2 broad categories of transcription factors that facilitate RNP II binding and initiation of transcription

  • General transcription factors are absolutely required for all RNP II-mediated transcription
  • Transcription activators and repressors influence the efficiency or the rate of RNP II transcription initiation
• RNA polymerase opens up and separates (denatures) the 2 strands so that the template strand may pass through its active site during RNA synthesis
• As transcription proceeds, the enzyme moves along the DNA until the termination is encountered
• Heterogenous nuclear RNA (hnRNA) is  post-transcriptionally processed before it can leave the nucleus

  • Addition of a 5’ cap (7-methylguanosine residue) that protects from nuclease attack and may be involved in the transport of the transcript across the nucleus
  • poly-A tail (poly-a-polymerization) added to aid transport to cytoplasm
  • Introns are removed by splicing
• The coding regions of eukaryotic genes are interrupted by intervening sequences
• Introns (intervening sequences) are regions of the initial RNA transcript that aren’t expressed in the amino acid sequence of the protein
• Introns are removed by splicing and the exons are joined together in the mature mRNA
• The size of the mature mRNA is usually much smaller than that of the initial RNA
• Pre-mRNA introns are spliced out by the spliceosome in a reaction involving the formation of a lariat structure
• The snRNAs (small nuclear RNAs) that make up the major spliceosome are named U1 U2 U4 U5 and U6, and participate in several RNA-RNA and RNA-protein interactions. The RNA component of the small nuclear ribonucleic protein or snRNP small nuclear ribonucleoproteins is rich in uridine (the nucleoside analog of the uracil nucleotide)
• RNA editing may modify the final transcript
• The are 2 main types of RNA editing prior to translation

  Substitution editing: the identities of individual nucleotide bases are altered; prevalent in mitochondria and chloroplast RNA derived in plants

  Insertion/deletion editing: nucleotides added/deleted from the total number of bases