BIOL1003 Lecture Notes - Lecture 4: Stop Codon, Start Codon, Transfer Rna
BIOL1003: Module 3 – Molecular Genetics
Lecture 4 – The Genetic Code, Protein Synthesis
• The rules used to convert RNA information's into protein information
o Combinations of 4 bases must specify 20 amino acids
o 3 bases per words (codon) is how the body codes for amino acids
o There are some codons which do not create a specific amino acids
• AUG - start codon: codes for amino acid methionine (all proteins contain/start
with methionine)
• UAA - stop codon (no amino acid)
• UAG - stop codon (no amino acid)
• UGA - stop codon (no amino acid)
• Features of the genetic code
o Degenerate and redundant, but unambiguous (there is no three letter code that codes
for 2 separate amino acids)
o Start and top codons
o Non-overlapping between codes (three, then next tree, then three again…etc.)
o Universal (almost) - all living things use the same system of three letter code, and no
different nucleotide's
o Three possible reading frames depending on where you start from:
•
• Reading of the genetic code
o Conversation of a base sequence to an amino acid sequence requires an adapter
molecule
• Bases and amino acids are not complimentary
o The adapter molecule is a small RNA molecule called a transfer RNA (tRNA)
o A tRNA molecule connects codons and amino acids
• Hydrogen bonding between complimentary base pairs makes the interaction
between a codon and tRNA anticodon specific
• NOTE: anticodon = the reverse codon that is complimentary to the codon read to
create the RNA
o Protein synthesis is catalysed by the ribosome
• The ribosome has 2 subunits
• Each subunits consists of the proteins and RNA molecules (called ribosomal RNA,
or rRNA)
• Ribosome structure depends on rRNA (protein polymerase - makes protein)
• There are different domains - some hairpin shaped, some lollipop shape, but they
are all linked in and cannot be unwound (due to h-bonds between domains)
o mRNA: encodes a protein
o tRNA: adapter molecules that converts a sequence of codons to a sequence od amino
acids
o rRNA: a compliment of the ribosome
o Functional proteins = enzymes
o Structural proteins = create the phenotype
o Regulatory proteins = hormones
o Formation of a peptide bonds: joining amino acids by covalent bonds
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