Molecular Genetics - Translation

Question 1

What is translation and where does it occur in the cell?

Answer 1

• The process by which info coded in an mRNA molecule is interpreted, linking amino acids to make a polypeptide
• Occurs in the cytosol

Question 2

What components are needed for translation to occur?

Answer 2

1. Mature mRNA transcript
2. A ribosome
3. tRNA

Question 3

What are the 3 stages in translation?

Answer 3

1. Initiation
2. Elongation
3. Termination
   (same names as transcription)

Question 4

What are ribosomes made up of?

Answer 4

• rRNA molecules combined with ribosomal proteins, forming 2 ribosomal subunits
• Small subunit (40s) and large subunit (60s), do not combine until they have mRNA to translate
• mRNA fits between the 2 subunits
• Eukaryotes: 4 rRNA and 82 protein molecules in complete ribosome
• Prokaryotes: smaller ribosomes

Question 5

How is tRNA used in translation?

Answer 5

• tRNA delivers amino acids to ribosome
  mRNA “read” by ribosome in 3-nucleotide groups called codons
• Each codon specific specific amino acid to be incorporated to the forming amino acid chain
• A tRNA anticodon (complementary to codon) specifies the amino acid the tRNA is carrying

Question 6

What occurs during the initiation stage of translation?

Answer 6

• Large and small subunits bind to an mRNA
• Ribosome must recognize start codon AUG at 5’ end of mRNA
• AUG starts translation by “opening the reading frame” (the order in which codons are recoded by ribosome)
• A tRNA w/ anticodon UAC + “charged” w/ methionine enters ribosome in P site, codon/anticodon base pairing occurs

Question 7

What are the 3 sites within the ribosome?

Answer 7

A = aminoacyl site
P = peptidyl site
E = exit site
- Sites move relative to mRNA sequence

Question 8

What occurs in the elongation stage of translation?

Answer 8

• Individual amino acids are sequentially added to the growing polypeptide
• A second charged tRNA (with complementary anticodon to next codon on mRNA) enters ribosome in A site and base pairs
• Peptide bond forms between the two amino acids
• Methionine is released from its tRNA + new dipeptide stays attached to second tRNA
• Ribosome then “slides” along mRNA (5’ to 3’) to enclose third codon
• First codon now in E site, tRNA detaches from mRNA and can bond a new amino acid
• Process continues until all codons on mRNA have been “read”

Question 9

What occurs in the termination stage of translation?

Answer 9

• Ribosome eventually reaches STOP codon
• No tRNAs w/ anticodons complementary to stop codon, causes ribosome to “stall” on mRNA
• This stimulates a “release factor” protein to release polypeptide from ribosome
• Ribosome then “falls off” mRNA by separating into subunits

Question 10

What is wobble pairing and how does it allow for the redundancy in our genetic code?

Answer 10

• Sometimes tRNA with incorrect anticodon is “allowed” to pair with codon on mRNA
• Only occurs in situations where tRNA is carrying correct amino acid
• Redundancy: since amino acids may be carried by several different tRNA molecules + be coded for by several codons, “imperfect” codon-anticodon binding is allowed

Question 11

What is a reading frame and why is it important for proper translation?

Answer 11

• The order in which codons are decoded by ribosome
• Depending on where you start “reading” mRNA sequence, the codons created will be different
• In order to produce polypeptide w/ correct amino acid sequence, the ribosome must recognize where to start reading the codons
• The start of the reading frame is dictated by the start codon