Lecture 13: Translation Machinery & Genetic Code

Question 1

In prokaryotes, transcription and translation occur ________

Answer 1

simultaneously

Question 2

Function of mRNA

Answer 2

Carry genetic information transcribed from DNA in form of codons

Question 3

What are codons?

Answer 3

3-nucleotide sequences, each of which specifies a particular amino acid

Question 4

Direction that ribosome reads mRNA and moves in

Answer 4

5’ to 3’

Question 5

Direction of polypeptide synthesis

Answer 5

N to C

Question 6

How long is one tRNA?

Answer 6

74-93 nucleotides (small RNA chain)

Question 7

Function of tRNA

Answer 7

Read codons in the mRNA through base pairing with their own anticodon and bring the corresponding amino acid

Question 8

Function of rRNA

Answer 8

Associate with set of proteins to form ribosomes

Question 9

What do ribosomes do?

Answer 9

Bind tRNAs associated with amino acids and move along the mRNA, catalyze get assembly of amino acids into polypeptide chains

Question 10

What does large subunit of ribosome (rRNA) do?

Answer 10

Catalyze reaction to form peptide bonds between AAs

Question 11

What does small subunit of ribosome (rRNA) do?

Answer 11

Recognize where translation starts (specific sequence)

Question 12

What does it mean that the DNA code is degenerate?

Answer 12

More than one triplet code for each AA (4 bases, 3 letter code, so 4^3 means 64 possibilities, which is more than enough for 20 AAs)

Question 13

Stop codons

Answer 13

UAA, UAG, UGA

Question 14

Start codon

Answer 14

AUG (also the AA codon for methionine)

Question 15

In a codon, which bases are the most important?

Answer 15

The first two

Question 16

Positive correlation between _____ of codons and ______ of the amino acid (except ______)

Answer 16

Number, frequency, Arg

Question 17

What are the exceptions to the AUG start codon and where do they occur?

Answer 17

GUG and UUG in prokaryotes, CUG in eukaryotes

Question 18

What is the open reading frame?

Answer 18

Sequence of codons that runs from a specific start codon to a stop codon; must be long enough to make a functional polypeptide

Question 19

Each nucleotide is part of only one codon, which means the genetic code is?

Answer 19

Non-overlapping

Question 20

How was it determined that the genetic code is non-overlapping?

Answer 20

In mutant proteins, there is only ONE amino acid substitution; if it were overlapping more than one AA would change

Question 21

Frameshift mutation

Answer 21

Insertion or deletion of bases not in multiples of 3, shifting the entire AA sequence (shift in the ORF)

Question 22

Components of tRNA

Answer 22

3 loops (inc. anticodon loop), acceptor stem

Question 23

Structure of tRNA’s anticodon loop

Answer 23

Contains anticodon region, which is complementary to a codon in mRNA

Question 24

Where does an AA attach to tRNA and how?

Answer 24

AA covalently attaches to acceptor stem at 3’ end (at CCA, the sequence at the 3’ tail)

Question 25

How does tRNA recognize and attach to the correct AA?

Answer 25

Enzymes called aminoacyl-tRNA synthetases covalently attach each AA to the correct tRNA (occurs before reaching ribosome) - most cells have 20 ARS's, one per AA)

Question 26

How are aminoacyl-tRNA synthetases named? Give example.

Answer 26

seryl-tRNA synthetases for serine

Question 27

How many ARS's (aminoacyl-tRNA synthetases) does each AA have?

Answer 27

One: it recognizes all the codon(s) for that AA

Question 28

How else does an ARS recognize the right tRNA?

Answer 28

Segments on the acceptor end, and bases elsewhere in the molecule, like nucleotide 73 (discriminator base)

Question 29

How and where are AAs linked to tRNAs by ARS's?

Answer 29

Covalently linked to tRNAs at the 2' or 3' OH - this requires ATP (ATP to AMP)

Question 30

What molecule is responsible for proofreading in the attachment of tRNA to AA?

Answer 30

ARS (ribosome, AA, and tRNA don't know if a mistake has been made); can remove an incorrect AA

Question 31

How can 61 different codons be read when cells have less than 61 different tRNAs?

Answer 31

One tRNA anticodon can recognize more than one codon corresponding to one AA

Question 32

How can one tRNA recognize more than one codon?

Answer 32

"Wobble" position: 3rd base (3') in mRNA codon = 1st base (5') in tRNA anticodon is more flexible

Question 33

How many H bonds between each base pairing?

Answer 33

2 between A and U/T, 3 between C and G

Question 34

What is inosine?

Answer 34

Rare base found in tRNA, often in the anticodon

Question 35

What can inosine base pair with, and where?`

Answer 35

Can base pair with A, C, or U in wobble position of mRNA codon (3rd position)

Question 36

Basic process of translation

Answer 36

mRNA used as template to make proteins