Cycle 7: Transcription and Translation

• Transcription and translation are the 


  Basis of the central dogma of genetics. The two fundamental steps in protein synthesis
• Proteins are the


  Functional units of the cell, used for every biological process
• All RNAs are encoded by

  Genes
• mRNA is the only RNA that is


  Translated into protein, but it is not the only one involved in protein synthesis. Some RNAs have very complex functions
• Noncoding RNA doesn't get


  Translated into a protein because they function as RNA
• Housekeeping = rRNA, tRNA and snRNA


  Always there/ active, involved in transcription and translation of all genes
• Regulatory = RNAi = miRNA and siRNA


  Regulates gene expression or other processes in the cell
• Non-template/ coding/ sense strand

  Sequence of transcript matches this sequence
• Template/ anti-coding/ antisense strand


  Sequence of transcript is complementary to this sequence
• mRNA is synthesized


  5' to 3'
• In translation, codons/ groups of 3 bases are read


  5' to 3', and each codon specifies an amino acid to be incorporated into the new polypeptide
• Codons are unique to

  mRNAs
• Non-coding RNAs are not read as

  Having codons
• Silent Mutation


  No change in amino acid encoded
• Nonsense Mutation


  Premature termination of protein in translation
• Missense Mutation


  Change in amino acid encoded
• Frameshift Mutation


  Insertions and deletions shift the reading frame
• Simultaneous transcription and translation only occurs in


  Prokaryotes
• Pre-mRNA must be processed into


  Mature mRNA before translation in eukaryotes
• RNA complementary base pairs with


  Itself, it has secondary structure
• rRNA


  = catalytic component of ribosome, which is made up of rRNA and protein
• snRNA


  = another type of housekeeping RNA, involved in snRNPs (protein + RNA, and is responsible for alternative splicing)
• Chloroplasts and mitochondria


  Have their own transcription and translation machinery. Prokaryote-like due to their origins. Genome is greatly reduced
• Promoter (structure of a transcriptional unit)


  Regulates where, when and to what level a gene is expressed. This is where RNA pol. binds
• Coding Sequence (structure of a transcriptional unit)


  Encodes the RNA product (tRNA, mRNA, rRNA, ...). Also called the open reading frame (ORF)
• Terminator (structure of a transcriptional unit)


  Sequence that must be transcribed into RNA before it can be interpreted. Usually an inverted repeat
• mRNA structure (structure of a transcriptional unit)


  5' | 5" UTR (has its own regulatory sequences) | (start codon, always AUG) Coding Sequence (stop codon, UGA, UAA, or UAG) | 3' UTR (has its own regulatory sequences) | Terminator
• -35 and -10 regions in prokaryotes are


  Recognition sites for RNA pol. binding, easier for RNA pol. to melt apart, consenses sequences (highly conserved)
• Prokaryotic Transcription Termination

  Terminator sequence is an inverted repeat. The terminator sequence is transcribed into RNA and produces a hairpin loop that is recognized by RNA polymerase as a sign to stop transcribing
• 3 modifications are made during mRNA processing


  Addition of a 5’ guanine cap, addition of a poly(A) tail, splicing out introns and joining together of exons
• In eukaryotes, instead of having -35 and -10 regions

  There is a TATA box within the promoter
• The TATA box is named


  Because it consists of thymines and adenines
• In prokaryotes (differences)


  RNA polymerase is able to directly recognize the DNA at the promoter and bind to the -35/-10 region before scanning for the +1 start site
• In eukaryotes (differences)


  Various transcription factors (proteins) recognize the promoter at the TATA box first. The transcription factors are what RNA polymerase is able to recognize and latch onto, and it is the transcription factors that mediate RNA pol’s attachment to DNA
• These more distant regulatory sites are often also genes


  They encode proteins called transcription factors that are able to bind to other promoters and essentially alter the way that RNA polymerase is able to bind
• If RNA polymerase binding is enhanced


  Then expression of the gene product is increased
• If RNA polymerase is blocked from binding

  Then expression of the gene product is decreased
• The difference between prokaryotes and eukaryotes (regulation of expression)


  Prokaryotes do not share this same kind of regulation. All regulation occurs right at the promoter of every gene
• Eukaryotic gene contains (eukaryotic transcription termination)


  Cleavage site in 3' UTR. This site is transcribed by RNA pol. Once transcribed, the cleavage site RNA is recognized by RNAses that cleave the RNA. This stops RNA polymerase
• Addition of 5’ cap and poly(A) tail does 2 things


  Improves stability of mRNA and protect it from degradation by RNases in the cytosol. Aids in export through the nuclear envelope. 5’ cap is additionally involved in initiating translation, since it can be recognized by the ribosome