Translation and the Genetic Code

BIOL 2004 Genetics Kamran Virginia Tech

central dogma

DNA → RNA → proteins

genes encode ________

proteins

What determines the phenotype/traits?

proteins

One-gene One-enzyme Hypothesis (old)

each gene encodes only one enzyme

One-gene One-enzyme Hypothesis (updated)

• each gene encodes only one POLYPEPTIDE

• not every encoded protein is an enzyme

amino acid basic structure

• central carbon

• amine group

• carboxyl group

• a hydrogen

• R group (determines identity)

What type of bonds form between amino acids?

peptide bonds

Where is the peptide bond formed between amino acids?

between the carboxyl group of one amino acid and the amine group of the other

primary protein structure

sequence of amino acids

How is the primary protein structure determined?

directly by the DNA

secondary protein structure

folding patterns of ɑ-helix and β-sheets

How is the secondary protein structure stabilized?

hydrogen bonds

tertiary protein structure

3D shape of one polypeptide

How is the tertiary protein structure formed?

interactions between R-groups

quaternary protein structure

multiple polypeptides chained together

codon

3-base mRNA sequence

1 codon codes for…

1 amino acid

What does it mean for the genetic code to be degenerate?

multiple codons can build the same amino acid

What does it mean for the genetic code to be unambiguous?

each codon only codes for ONE amino acid

How many total codons are there?

64

How many stop codons are there?

3

What are the 3 stop codons?

UAG, UGA, UAA

What is the start codon?

AUG

What does the start codon do?

• codes for methionine

• sets the reading frame

What does the reading frame do?

determines how codons are read

What happens if you shift the reading frame?

you get a completely different protein

Wobble Hypothesis

• the 3rd base of a codon is flexible

• one tRNA can match with multiple codons

• ↳ don’t need 61 different tRNAs

What does tRNA do?

matches each codon with its amino acid

Charging process of tRNA

• amino acid and ATP bind

• aminoacyl-tRNA synthetase attaches amino acid to tRNA (ATP consumed)

What enzyme is used to charge tRNA?

aminoacyl-tRNA synthetase

What sites exist on a ribosome?

A site, P site, E site

What is the function of the A site?

where new tRNA enters

What does the “A” in A site stand for?

aminoacyl

What is the function of the P site?

holds the growing chain

What does the “P” in P site stand for?

peptidyl

What is the function of the E site?

where tRNA leaves

What does the “E” in E site stand for?

exit

What are the steps of translation?

• initiation

• elongation

• termination

What happens during initiation of translation in bacteria?

• subunit binds shine delgarno sequence

• start codon (AUG) is positioned

• first tRNA carries fMet (modified methionine)

What happens during initiation of translation in eukaryotes?

• subunit binds to 5’ cap

• scans to find AUG

• first amino acid is methionine

What happens during elongation?

• tRNA enters A site

• peptide bond forms between A and P site amino acids

• ribosome shifts

  • A site shifts to P site

  • P site shifts to E site

• repeats until stop codon is reached

What happens during termination?

• stop codon enters A site

• release factor binds to A site

• polypeptide is released

How is energy used in translation?

• ATP charges tRNA

• GTP used for initiation, elongation, and translocation

What are polysomes, and what is their effect?

• multiple ribosomes translating one mRNA at the same time

• protein production is more efficient

How is mRNA quality controlled in bacteria?

tmRNA system —> rescues stalled ribosomes

How is mRNA quality controlled in eukaryotes

nonsense-mediated decay —> mRNA removed with early stop codons