lecture 3 - (01/10/24) Flashcards
How many types of base in nucleic acids?
4
How many types of amino acids in proteins?
20
must be 3 nucleotides per amino acid
What does degenerate mean?
each combination might code for more than one amino acid
Is the code overlapping?
No
because single base mutations only ever affect one amino acid
Crick, Brenne, et al.’s experiment
model system: Bacteriophage T4 which infects E. coli
generated mutants min rII gene of bacteriophage using proflavin
frameshift mutation
proflavin
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
How many nucleotides is one codon?
3
1 codon = 1 AA
Who deciphered the genetic code?
Marshall Nirenberg and Har Gobind Khorana
tRNAs were described by Robert Holley
they shared a Nobel prize in 1968
What is a stop codon?
a codon that doesn’t code for an AA
What does it mean by the genetic code is universal?
genetic code is the same in all organisms
What is the shape of a tRNA molecule?
clover-leaf shaped
What is the genetic code translated by?
aminoacyl-tRNA synthases and tRNAs
What is charging?
each synthase couples a particular AA to its corresponding tRNAs
What is the structure of a ribosome?
large subunit and small subunit
~49 proteins + 3 RNA ~33 proteins + 1 RNA
total = more than 80 proteins + 4 RNA molecules
What is a ribozyme?
RNA molecules that can catalyse chemical reactions
Name the different sites in a ribosome (small subunit)
E-site = exit site
P-site = peptidyl-tRNA site (where protein synthesis occurs)
A-site = aminoacyl-tRNA site
Translation steps
Initiation
Elongation
Termination
Initiation
- translation initiation factor binds to E-site
- initiator tRNA binds to P-site
- AUG is always start of protein synthesis
- Small ribosomal subunit, with bound initiator
tRNA moves along mRNA searching for first AUG - when AUG found translation initiation factor
dissociates (no longer has a purpose) - large ribosomal unit binds
- charged tRNA binds to a site
- first peptide bond forms (Met always 1st AA)
Elongation
- peptide bond formed at P-site
newly bound charged tRNA at A-site
tRNA ejected at E-site
A –> P –> E - Large subunit translocates (ribosome moves
along mRNA, mRNA doesn’t move through
ribosome ) - small subunit translocates and tRNA ejected
Termination
- Stop codon in A-site
- binding of release factor to A-site
- addition of H2O causes hydrolysis of polypeptide
chain from ribosome - ribosome dissociates
mRNA can form multiple copies
