18. Microbial genetics: Transcription and Translation

Question 1

what is prokaryotic transcription? **

Answer 1

• single gene may encode single mRNA transcript
  – translated to single protein
• suit of adjacent genes transcribed to sing mRNA species (operon)
  – translated into number of proteins

Question 2

what si eukaryotic transcription?

Answer 2

• single gene encodes single mRNA transcript
  – translated to single protein
• gene composed of exons and introns transcribed to RNA
  – by RNA polymerase
• processing involves ribozymes and proteins in nucleus to remove intron-derived RNA
  – splice together exon-derived RNA into mRNA
• after modification, mature mRNA travels to cytoplasm
  – directs protein synthesis

Question 3

describe eukaryotic RNA processing?

Answer 3

• capping
  – cap structure added to 5’ of newly transcribed mRNA precursor in nucleus prior to processing and subsequent transport of mRNA to cytoplams
• splicing
  – step by step removal of pre mRNA and joining of remaining exons, using spliceosomes
• addition of poly A tail (polyadenalation)
  – synthesis of poly A tail involves cleavage of 3’ end and then addition of 40-200 adenine residues to form poly A tail

Question 4

what is the process of translation?

Answer 4

• conversion of information within mRNA to protein

Question 5

what is unique about bacteria with transcription and translation?

Answer 5

• tightly coupled
• almost immediate release of transcribed mRNA strand
  – several RNA copies can be simultaneously transcribed and translated
• up to 15 ribosomes on 1.5 kb gene
• lifespan of mRNA in cytoplams determines amount of protein produced

Question 6

what is the genetic code?

Answer 6

• DNA base sequence corresponding to amino acid sequence of polypeptide specified by particular gene
• proteins made up of 20 amino acids specified by codon
  – nucleotide triplet
  – 64 combinations

Question 7

what is the discrete DNA structure?

Answer 7

• promoter
  – RNA polymerase recognition site
  – RNA polymerase binding site
• anti-leader
• coding region
  – sense strand
  – antisense strand
• terminator

Question 8

what is the whole process of protein synthesis?

Answer 8

• template strand created
  – 3’ to 5’ from 5’ to 3’ DNA
• transcription
  – mRNA formed
  – shine-dalgarno sequence
  – start from 5’ to 3’
• translation
  – start after AUG (which is after shine-dalgarno sequence

Question 9

how is genetic code organised?

Answer 9

• code degeneracy
  – 4 different codons for given amino acid
  – eg.) Gly = GGU,GGC,GGA,GGG
• only 61 codons
  – sense codons direct amino acids incorperation into protein
  – Met (AUG) called start codon
• 3 remaining codons (UGA, UAG, UAA)
  – involed in termination of translation
  –stop codons

Question 10

what is the difference in the number of amino acid codons and tRNAs?

Answer 10

• 5’ nucleotide anticodon can vary, generally if nucleotides in the first 2 positions compliment first 2 bases of mRNA codon
  – aminoacyl-tRNA with correct amin acid will bind to mRNA-ribosome complex

Question 11

what is important about the wobble effect?

Answer 11

• relieves cells of need to synthesis too many tRNA’s
• decreases ill effects of DNA mutations

Question 12

what initiates translation?

Answer 12

• doesn’t begin until ribosomal subunit reaches AUG codon in mRNA
• subunit complex forms 70S ribosome
• complex drives translations

Question 13

what is the process of translation?

Answer 13

• components needed to begin come together
• on assembled ribosome, tRNA carrying first amino acid is paired with start codon on mRNA
  – tRNA carrying second amino acid approaches
• where first tRNA sits called P sire
• A site is next
  – second codon of mRNA pairs with a tRNA carrying second amino acid
• first amino acid joins to second by peptide bond
  – first tRNA released
• ribosome moves along mRNA until second tRNA is on P site
  – process continues
• ribosome continues along mRNA, new amino acid added to polypeptide
• reaches stop codon
  – polypeptide released
• last tRNA released
  – ribosome comes apart
  – released polypeptide forms new protein

Question 14

what are the properties of proteins?

Answer 14

• depend on composition and sequence of amino acids
  – and post-translational modifications
• inital product of translation is polypeptide
  – not functional protein
• functional protein has unique, stable 3-D shape

Question 15

explain transcription and translation within eukaryotes **

Answer 15

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Question 16

what is the use of regulating gene expression?

Answer 16

• it takes alot of metabolic energy to synthesis RNA and protein
• some genes active all the time
  – their products are in constant demand (constitutive)
• others turned off mostly
  – only switched on when products needed (induced)