1.10 - The Genetic Code Flashcards
how many amino acids can the 4 RNA nucleotides make?
20
what does tRNA structure allow them to do?
recognise codon and bind to amino acid
when are amino acids attached to tRNA?
during protein synthesis
tRNA sequence that recognises codons (complementary to codons)
anticodon
purpose of anticodon third base “wobble”
to increase efficiency allowing it to bind to slightly different codon that recruits the same amino acid
non-Watson-Crick base pairing
tRNA can bind to a nucleotide base that is no its normal partner (due to “wobble”)
non-Watson-Crick base pairings tRNA can perform (2)
- G can bind to C or U
- U can bind to A or G
reasons non-Watson-Crick base pairing/anticodon third base “wobble” can happen (2)
- additional space at third position which allows for the bit of RNA to move away from complementary strand
- there can be a sixth nucleotide that can be included (inosine)
inosine
modified form of adenosine processed by a tRNA specific deaminase
inosine base pairing ability
able to engage in many base pairings, whether Crick-Watson or non-Watson-Crick
what base pairings can inosine make? (3)
- adenine (A)
- cytosine (C)
- uracil (U)
effect of inosine allowing “wobble” to occur
allows for less different types of tRNA to exist
what % of tRNA bases are modified?
10% (estimate)
reason most amino acids can be coded by more than one codon
high level of redundancy in system to account for any issues with tRNA availability
amino acids only coded for by one codon (exceptions) (2)
- methionine
- tryptophan
methionine
amino acid all proteins start with (start codon) (AUG)
tryptophan
least abundant amino acid, rare in the proteome
how is impact minimised if there is a change in amino acid sequence?
most amino acids with similar properties have similar coding
how does the mitochondrial genetic code differ? (3)
- UGA is not a stop codon but codes for tryptophan
- internal methionine encoded by AUG and AUA
- mammalian mitochondria - AGA and AGG are not arginine codons but stop codons
how does genetic code differ in fruit fly mitochondria?
AGA and AGG are not arginine codons but serine codons
different types of point mutations (3)
- substitution - nucleotide replaced with another
- insertion - nucleotide added to sequence
- deletion - nucleotide removed from sequence
(can be up to 3 nucleotides)
how point mutations be silent?
substitution point mutations depending on location within nucleotide can be silent
silent mutations (2)
- do not result in change to amino acid
- generally in 3rd nucleotide of codon, can also occur in 2nd, never 3rd
how are silent mutations possible?
wobbles in anticodon of tRNA and high levels of redundancy that exist
missense mutations (2)
- change in nucleotide which then changes amino acid coded
- often happens if mutation is in 1st nucleotide of codon, can happen anywhere in codon
how can missense mutations be considered conservative/non-conservative? (2)
- conservative - if amino acid is similar in properties
- non-conservative - if amino acid is not similar in properties
amino acid properties (4)
- charge
2.hydrophilicity/ hydrophobicity - size
- function groups
nonsense mutations
introduce stop codon in the middle of the protein
effect of nonsense mutation
protein is truncated, often means it will not function properly or at all
frameshift mutation
where there has been an insertion or deletion of a nucleotide which means framing of the codons change
frameshift mutation effect
will change amino acid that will be coded combined with location of stop codon changing can change protein structure dramatically (often non-functional)
entire codon added or removed effect
can cause issues and change structure of protein
how can silent mutations still be bad? (3)
- not all codons equal as not equal amounts of tRNA with their anticodons
- some anticodons more prevalent, no issue in amino acid availability in translation
- however, if mutation changes codon to rare one, can impede translation making gene expression slower
what is required for tRNA to fulfil their function?
amino acids must be activated and attached to them
how are amino acids activated?
amino acid is bound to ATP via the enzyme aminoacyl-tRNA synthetase
result of amino acid activation
aminoacyl adenylate intermediate
tRNA charging (aminoacyl intermediate) (3)
- aminoacyl adenylate intermediate undergoes nucleophilic attack by uncharged tRNA
- then joined by an ester bond, charging tRNA
- tRNA now has amino acid attached ready for translation
what enzyme facilitates tRNA charging?
aminoacyl-tRNA synthetase (same enzyme involved in amino acid activation
how many aminoacyl-tRNA synthetases are there?
20, one for each amino acid
how does aminoacyl-tRNA synthetase bind to tRNA?
recognises the anticodon of tRNA through complementary binding site within enzyme or through specific sequences in the stem
role of aminoacyl-tRNA synthetase synthesis site
has specific affinity for each amino acid, is where activation and charging occur
aminoacyl-tRNA synthetase editing funcion
can check and correct if the wrong amino acid has been added to tRNA (correct amino acid cannot enter editing site)
what are ribosomes made up of? (2)
- proteins
- RNA
what are ribosomes responsible for? (3)
- moving along mRNA
- reading the codons
- capture matching the tRNA
(allowing the synthesis of amino acid chain to make a protein)
ribosomal structure and components (3)
- one large and small protein subunit
- 2/3 ribosomal RNA transcripts associated with large protein subunit
- one ribosomal RNA transcript with small protein subunit
ribosomal small subunit role
provides scaffolding for tRNA anticodons to be matched to mRNA codons
ribosomal large subunit role
catalyses formation of peptide bonds between amino acids during protein synthesis
EPA sites in ribosomes (3)
(3 binding sites for tRNA)
- A-site (aminoacyl)
- P-site (peptidyl)
- E-site (exit)
ribosomal proteins role
cover periphery of ribosome and provide structural integrity
why is the ribosome a ribonucleic acid enzyme (ribozyme)?
most ribosomal RNA, with the transcripts forming the catalytic core that enables peptide bond formation
