1.10 - The Genetic Code

Question 1

how many amino acids can the 4 RNA nucleotides make?

Answer 1

20

Question 2

what does tRNA structure allow them to do?

Answer 2

recognise codon and bind to amino acid

Question 3

tRNA sequence that recognises codons (complementary to codons)

Answer 3

anticodon

Question 4

purpose of anticodon third base “wobble”

Answer 4

to increase efficiency allowing it to bind to slightly different codon that recruits the same amino acid

Question 5

non-Watson-Crick base pairing

Answer 5

tRNA can bind to a nucleotide base that is no its normal partner (due to “wobble”)

Question 6

non-Watson-Crick base pairings tRNA can perform (2)

Answer 6

1. G can bind to C or U
2. U can bind to A or G

Question 7

reasons non-Watson-Crick base pairing/anticodon third base “wobble” can happen (2)

Answer 7

1. additional space at third position which allows for the bit of RNA to move away from complementary strand
2. there can be a sixth nucleotide that can be included (inosine)

Question 8

inosine

Answer 8

modified form of adenosine processed by a tRNA specific deaminase

Question 9

inosine base pairing ability

Answer 9

able to engage in many base pairings, whether Crick-Watson or non-Watson-Crick

Question 10

what base pairings can inosine make? (3)

Answer 10

1. adenine (A)
2. cytosine (C)
3. uracil (U)

Question 11

what % of tRNA bases are modified?

Answer 11

10% (estimate)

Question 12

reason most amino acids can be coded by more than one codon

Answer 12

high level of redundancy in system to account for any issues with tRNA availability

Question 13

amino acids only coded for by one codon (exceptions) (2)

Answer 13

1. methionine
2. tryptophan

Question 14

methionine

Answer 14

amino acid all proteins start with (start codon) (AUG)

Question 15

tryptophan

Answer 15

least abundant amino acid, rare in the proteome

Question 16

how is impact minimised if there is a change in amino acid sequence?

Answer 16

most amino acids with similar properties have similar coding

Question 17

how does the mitochondrial genetic code differ? (3)

Answer 17

1. UGA is not a stop codon but codes for tryptophan
2. internal methionine encoded by AUG and AUA
3. mammalian mitochondria - AGA and AGG are not arginine codons but stop codons

Question 18

how does genetic code differ in fruit fly mitochondria?

Answer 18

AGA and AGG are not arginine codons but serine codons

Question 19

different types of point mutations (3)

Answer 19

1. substitution - nucleotide replaced with another
2. insertion - nucleotide added to sequence
3. deletion - nucleotide removed from sequence

(can be up to 3 nucleotides)

Question 20

silent mutations

Answer 20

substitution point mutations depending on location within codon can be silent

Question 21

silent mutations (2)

Answer 21

1. do not result in change to amino acid
2. generally in 3rd nucleotide of codon, can also occur in 2nd, never 3rd

Question 22

how are silent mutations possible?

Answer 22

wobbles in anticodon of tRNA and high levels of redundancy that exist

Question 23

missense mutations (2)

Answer 23

1. change in nucleotide which then changes amino acid coded
2. often happens if mutation is in 1st nucleotide of codon, can happen anywhere in codon

Question 24

how can missense mutations be considered conservative/non-conservative? (2)

Answer 24

1. conservative - if amino acid is similar in properties
2. non-conservative - if amino acid is not similar in properties

Question 25

amino acid properties (4)

Answer 25

1. charge 2.hydrophilicity/ hydrophobicity 3. size 4. function groups

Question 26

nonsense mutations

Answer 26

introduce stop codon in the middle of the protein

Question 27

effect of nonsense mutation

Answer 27

protein is truncated, often means it will not function properly or at all

Question 28

frameshift mutation

Answer 28

where there has been an insertion or deletion of a nucleotide which means framing of the codons change

Question 29

frameshift mutation effect

Answer 29

will change amino acid that will be coded combined with location of stop codon changing can change protein structure dramatically (often non-functional)

Question 30

entire codon added or removed effect

Answer 30

can cause issues and change structure of protein

Question 31

how can silent mutations still be bad? (3)

Answer 31

1. not all codons equal as not equal amounts of tRNA with their anticodons 2. some anticodons more prevalent, no issue in amino acid availability in translation 3. however, if mutation changes codon to rare one, can impede translation making gene expression slower

Question 32

what is required for tRNA to fulfil their function?

Answer 32

amino acids must be activated and attached to them

Question 33

how are amino acids activated?

Answer 33

amino acid is bound to ATP via the enzyme aminoacyl-tRNA synthetase

Question 34

result of amino acid activation

Answer 34

aminoacyl adenylate intermediate

Question 35

charged amino acid addition to tRNA (aminoacyl adenylate) (3)

Answer 35

1. aminoacyl adenylate intermediate undergoes nucleophilic attack by uncharged tRNA 2. then joined by an ester bond, charging tRNA 3. tRNA now has amino acid attached ready for translation

Question 36

what enzyme facilitates tRNA charging?

Answer 36

aminoacyl-tRNA synthetase (same enzyme involved in amino acid activation

Question 37

how many aminoacyl-tRNA synthetases are there?

Answer 37

20, one for each amino acid

Question 38

how does aminoacyl-tRNA synthetase bind to tRNA?

Answer 38

recognises the anticodon of tRNA through complementary binding site within enzyme or through specific sequences in the stem

Question 39

role of aminoacyl-tRNA synthetase synthesis site

Answer 39

has specific affinity for each amino acid, is where activation and charging occur

Question 40

aminoacyl-tRNA synthetase editing funcion

Answer 40

can check and correct if the wrong amino acid has been added to tRNA (correct amino acid cannot enter editing site)

Question 41

what are ribosomes made up of? (2)

Answer 41

1. proteins 2. RNA

Question 42

what are ribosomes responsible for? (3)

Answer 42

1. moving along mRNA 2. reading the codons 3. capture matching the tRNA (allowing the synthesis of amino acid chain to make a protein)

Question 43

ribosomal small subunit role

Answer 43

provides scaffolding for tRNA anticodons to be matched to mRNA codons

Question 44

ribosomal large subunit role

Answer 44

catalyses formation of peptide bonds between amino acids during protein synthesis

Question 45

EPA sites in ribosomes (3)

Answer 45

(3 binding sites for tRNA) 1. A-site (aminoacyl) 2. P-site (peptidyl) 3. E-site (exit)

Question 46

ribosomal proteins role

Answer 46

cover periphery of ribosome and provide structural integrity

Question 47

why is the ribosome a ribonucleic acid enzyme (ribozyme)?

Answer 47

most ribosomal RNA, with the transcripts forming the catalytic core that enables peptide bond formation