Chapter 12 - Gene expression at the molecular level

Question 1

Two gene functions

Answer 1

1. molecular function of the product protein
2. organism’s trait conferred by the gene
   - these are connected

Question 2

Archbold Garrod

Answer 2

• first proposed the relationship between genes and the production of enzymes
  
  • didn’t know that DNA was genetic material at the time
• notice a recessive pattern of inheritance
• defect in enzyme results in the inability to make the final product

Question 3

Beadle and Tatum

Answer 3

• looked at growth of Neurospora crassa with mutant strains of certain mutant genes and supplementing with different enzymes
• used results to (wrongfully) support the one gene, one enzyme hypothesis

Question 4

Modern understanding of “one gene, one enzyme”

Answer 4

• genes code for other proteins besides enzymes
• several polypeptides can work together for one function
• “one gene, one polypeptide”

Question 5

what is the central dogma?

Answer 5

• transcriptions produces and RNA copy of a gene and this mRNA specifies the amino acid sequence of a polypeptide

Question 6

what can genes code for besides polypeptides?

Answer 6

• structural or regulatory RNAs
• RNA can be the final, functional product

Question 7

what is a gene

Answer 7

an organized unit of base sequences that enables a segment of DNA to be transcribed into RNA and ultimately results in the formation of a functional product

Question 7

four parts of a protein-encoding gene

Answer 7

1. regulatory sequence - site for the binding of regulatory proteins, which influence the speed of transcription/inc or dec. the expression of a gene
2. promoter - signals the beginning of transcription
3. transcribed region - contains the information that specifies an amino acid sequence
4. terminator - signals the end of transcription

Question 8

initiation

Answer 8

• first step in transcription
• recognition step
• recognized by sigma factor in prokaryotes
• completed when DNA strands separate near promotor

Question 9

sigma factor

Answer 9

• protein in eukaryotes that recognizes the promoter region

Question 9

direction of transcription

Answer 9

• always synthesized 5’ to 3’
• direction and which strand is used varies
• they can be attached after since they were all made 5’ to 3’ and read in the same direction relative to the strand

Question 10

elongation

Answer 10

• step 2 in transcription
• RNA polymerase synthesizes RNA
• template strand of DNA is used
• Uracil substituted for thymine
• synthesized 5’ to 3’

Question 10

termination

Answer 10

• step 3 in transcription
• RNA polymerase reaches termination sequence
• causes both the polymerase and newly-made RNA transcript to dissociate from DNA

Question 11

differences between eukaryotic and prokaryotic transcription

Answer 11

• Eukaryotic has three types of RNA polymerase:
  
  • RNA polymerase II - transcribes mRNA
  • RNA polymerase I and III - transcribes nonstructural genes for rRNA and tRNA
• RNA polymerase II requires 5 general transcription factors to initiate transcription
• prokaryotes need signma factor to recognize promoter

Question 12

preinitiation complex

Answer 12

• not in bacteria
• group of 5 transcription factors required to initiate transcription

Question 13

RNA modification in eukaryotes

Answer 13

(bacteria mRNA can be immediately translated)
- introns are transcribed but removed before translation
- addition of caps and tails

Question 14

capping

Answer 14

• Modified guanosine attached to 5’ end
• needed for mRNA to exit nucleus and bind to ribosome

Question 15

Poly A tail

Answer 15

• 100 to 200 adenine nucleotides added to 3’ end
• increases stability and lifespan in cytosol

Question 16

RNA splicing

Answer 16

• spliceosome removes introns
  
  • composed of snRNPS
• alternative splicing produces different products
  
  • allows for variation in genes

Question 17

ribozymes

Answer 17

• RNAs that can catalyze reactions
• ex: rRNA and tRNA are self splicing

Question 18

genetic code

Answer 18

sequence of bases in an mRNA molecule

Question 19

codons

Answer 19

• sequence of 3 amino acids
• most code for specific amino acids
• some are start or stop codons
• degenerate code
• AUG is typical start codon
  
  • tRNA would match with UAC

Question 20

how does an addition or removal of one nucleotide affect translation?

Answer 20

• throws off reading frame
• can affect reading of the stop codon
• depends on how early or late the mutation occurs
• insertions or deletions of 3 don’t affect reading frame

Question 21

tRNA

Answer 21

• anticodons to the codons on mRNA
• cloverleaf structure
• acceptor stem for amino acid binding at 3’ single-stranded region
• if mRNA is 3’-CAG-5’, tRNA has 5’-GUC-3’

Question 22

four main components of translation

Answer 22

1. mRNA
   2.tRNA
2. ribosomes
3. translation factors

Question 23

aminoacyl-tRNA synthase

Answer 23

• catalyzes attachment of amino acids to tRNA
• one for each of 20 different amino acids
• results in charged tRNA with amino acid attached
• dephosphorylation of ATP changes conformation, allowing tRNA to get in and get the amino acid

Question 24

ribosome structure

Answer 24

• prokaryotes have one kind of ribosome, while eukaryotes have many
• large (50s) and small (30s) subunits
  3 sites:
  1. aminoacyl site
  
  • where the charged tRNA enters
    2. peptide site
  • where the tRNA attaches its amino acid to the growing chain
    
    3. exit site
  • uncharged tRNA leaves

Question 25

start and stop of transcription vs translation

Answer 25

• transcription goes from promoter to terminator
• translation goes from start codon to stop codon
• translation initiation requires ribosomal initiation factors