Chapter 8: Gene Expression: The Flow of Information from DNA to RNA to Protein Learning Objectives

Question 1

o Relate tRNA’s structure to its function

Answer 1

 Transfer RNAs (tRNAs) are the adapters that link mRNA codons to amino acids at the ribosome. Amino acyl tRNA synthetases connect the correct amino acids to their corresponding tRNAs

Question 2

o List the steps in mRNA formation that occur in eukaryotes but not prokaryotes

Answer 2

 Transcription:
• Finding gene and opening up DNA molecule so that enzymes can get to work
• RNA polymerase continues opening DNA and synthesizing mRNA
• RNA polymerase encounters terminator, it transcribes terminator sequence then stops transcription; results in mature mRNA
 Translation:
• Ribosome recognizes mRNA and latches onto 5’ cap/ribosome slurps up mRNA and scrutinizes it looking for codons that form word of genetic code
• tRNA supply amino acids dictated by each codon/ribosome assembles polypeptide chain
• Ribosome continues to assemble polypeptide chain until it reaches stop codon; results in mature protein

Question 3

o Explain why most loss-of-function alleles are recessive to wild-type, but some are incompletely dominant or (rarely) dominant

Answer 3

 Loss of function: DNA mutation that reduces/abolishes activity of gene
 When phenotype varies continuously with levels of protein function, incomplete dominance results

Question 4

o Explain the reasoning establishing that a sequence of three nucleotide (a triplet codon) is the basic unit of the code relating DNA to protein

Answer 4

 Universal genetic code consists of 64 codons, each one composed of 3 nucleotides

Question 5

o Discuss mutations outside the coding sequence that could affect gene expression

Answer 5

 Mutations outside coding sequence can alter gene expression by altering amount, time, or place of protein production

Question 6

o Discuss the function of enhancer sequences in eukaryotic transcription

Answer 6

 Enhancer sequence far from promoter are needed for transcription initiation
 Control how often gene is transcribed

Question 7

o Describe the key steps of translation, indicating how each depends on the ribosome

Answer 7

 Translation initiation begins when a charged tRNAMet binds the initiator codon, AUG, at the ribosomal P site. During elongation, the amino acid connected to the tRNA at the P site forms a peptide bond with the amino acid connected to a tRNA at the A site. The ribosome then moves in the 5’-to-3’ direction along the mRNA to the next codon. Termination occurs when the ribosome encounters an in-frame stop codon in the mRNA.

Question 8

o Define reading frame and discuss its significance to the genetic code

Answer 8

 Series of nonoverlapping codons

 Start codon defines position of reading frame

Question 9

o Compare transcription initiation in prokaryotes and eukaryotes

Answer 9

 In eukaryotes: enhancer sequences far from promoter, transcription initiation requires promoters to be cleared of chromatin

Question 10

o Describe the three stages of transcription: initiation, elongation, and termination

Answer 10

 Initiation: finding gene and opening up DNA molecule so that enzymes can get to work
 Elongation: RNA polymerase continues opening DNA helix and synthesizing mRNA until whole thing is transcribed
 Termination: RNA polymerase encounters terminator, it transcribes terminator sequence then stops transcription

Question 11

o Explain how the nuclear membrane affects gene expression in eukaryotes

Answer 11

 In prokaryotes, transcription and translation occur simultaneously. The nuclear membrane of eukaryotes restricts transcription to the nucleus; mRNAs are then translated after transport into the cytoplasm

Question 12

o Give examples of mutations that can have global effects on gene expression

Answer 12

 Lethal mutations affecting machinery of gene expression

 Mutations in tRNA genes can suppress mutations in protein-coding genes

Question 13

o Describe the differences in translation initiation between prokaryotes and eukaryotes

Answer 13

 In eukaryotes: initiator tRNA carries methionine, mRNA have only one site and can direct synthesis of only one kind of polypeptide, small ribosomal subunit binds first to methylated cap of mature mRNA then scans mRNA to find ribosome binding site
 In prokaryotes: unique initiator tRNA carries formylmethionine, mRNAs have multiple ribosome binding sites and can direct synthesis of several polypeptides, small ribosomal subunits immediately binds to mRNA’s ribosome binding site

Question 14

o List three ways by which eukaryotes process mRNA after transcription

Answer 14

 Add 5’ methylated cap
 Add poly-A tail
 Excision of introns when exons are joined by splicing

Question 15

o List three categories of posttranslational processing and provide examples of each

Answer 15

 Posttranslational processing may cleave a polypeptide or add chemical constituents to it

Question 16

o Compare silent mutations, missense mutations, nonsense mutations, and frameshift mutations in terms of how they alter a gene product

Answer 16

 Silent mutation: don’t alter protein’s primary structure
 Missense mutation: replace one amino acid with another
 Nonsense mutation: shorten a polypeptide by replacing codon with a stop
 Frameshift mutation: results in change in reading frame downstream of addition/deletion