lecture 3 - (01/10/24)

Question 1

How many types of base in nucleic acids?

Answer 1

4

Question 2

How many types of amino acids in proteins?

Answer 2

20
must be 3 nucleotides per amino acid

Question 3

What does degenerate mean?

Answer 3

each combination might code for more than one amino acid

Question 4

Is the code overlapping?

Answer 4

No
because single base mutations only ever affect one amino acid

Question 5

Crick, Brenne, et al.’s experiment

Answer 5

model system: Bacteriophage T4 which infects E. coli
generated mutants min rII gene of bacteriophage using proflavin
frameshift mutation

Question 6

proflavin

Answer 6

planar molecule
antiseptic
anti viral
intercalates between base pairs in DNA and can cause:
- insertion of a single extra base
- deletion of a single base

Question 7

How many nucleotides is one codon?

Answer 7

3
1 codon = 1 AA

Question 8

Who deciphered the genetic code?

Answer 8

Marshall Nirenberg and Har Gobind Khorana
tRNAs were described by Robert Holley
they shared a Nobel prize in 1968

Question 9

What is a stop codon?

Answer 9

a codon that doesn’t code for an AA

Question 10

What does it mean by the genetic code is universal?

Answer 10

genetic code is the same in all organisms

Question 11

What is the shape of a tRNA molecule?

Answer 11

clover-leaf shaped

Question 12

What is the genetic code translated by?

Answer 12

aminoacyl-tRNA synthases and tRNAs

Question 13

What is charging?

Answer 13

each synthase couples a particular AA to its corresponding tRNAs

Question 14

What is the structure of a ribosome?

Answer 14

large subunit and small subunit
~49 proteins + 3 RNA ~33 proteins + 1 RNA

total = more than 80 proteins + 4 RNA molecules

Question 15

What is a ribozyme?

Answer 15

RNA molecules that can catalyse chemical reactions

Question 16

Name the different sites in a ribosome (small subunit)

Answer 16

E-site = exit site
P-site = peptidyl-tRNA site (where protein synthesis occurs)
A-site = aminoacyl-tRNA site

Question 17

Translation steps

Answer 17

Initiation
Elongation
Termination

Question 18

Initiation

Answer 18

1. translation initiation factor binds to E-site
2. initiator tRNA binds to P-site
3. AUG is always start of protein synthesis
4. Small ribosomal subunit, with bound initiator
   tRNA moves along mRNA searching for first AUG
5. when AUG found translation initiation factor
   dissociates (no longer has a purpose)
6. large ribosomal unit binds
7. charged tRNA binds to a site
8. first peptide bond forms (Met always 1st AA)

Question 19

Elongation

Answer 19

1. peptide bond formed at P-site
   newly bound charged tRNA at A-site
   tRNA ejected at E-site
   A –> P –> E
2. Large subunit translocates (ribosome moves
   along mRNA, mRNA doesn’t move through
   ribosome )
3. small subunit translocates and tRNA ejected

Question 20

Termination

Answer 20

1. Stop codon in A-site
2. binding of release factor to A-site
3. addition of H2O causes hydrolysis of polypeptide
   chain from ribosome
4. ribosome dissociates

mRNA can form multiple copies