Genetics Exam 1 (Translation)

Lectures

What is the goal of translation

Make a polypeptide according to mRNA sequence

What is a polypeptide

polymer of amino acid monomers connected by peptide bonds

What is open-reading frame

• the codons of an mRNA that are read sequentially to specify amino acids in the resulting polypeptide (includes start and stop codons)

  • mRNA read 5’ to 3’

Triplet code

• each mRNA codon consists of 3 nucleotides 

Continuous

• the mRNA is read 3 nts at a time without skipping any 

Nonoverlapping

the mRNA is read in successive groups of 3 nts

Universal

all known organisms (even E.T.) have the same genetic language

• Ex: Round-up resistant plants, human insulin production in bacteria

Redundant/degenerate

• more than one codon occurs for each amino acid

• Third nt is often different

• These codons are synonymous

Unambiguous

• One codon specifies only one amino acid

Genes have a start and stop signal – Call for the beginning and end of translation

• AUG = methionine (met)

• UAG, UAA, UGA = stop = nonsense (versus a sense codon)

Codon usage bias

•  the tendency for an organism to use a certain codon for an amino acid more than others, advantageous for fast growing organisms, why?

Wobble

• Third base pairing between tRNA anticodon and codon doesn’t follow complementary base pairing rules, more than one codon can be recognized by the same tRNA. Advantageous. Why?

The sequence of amino acids dictates a protein’s structure (STRUCTURE=FUNCTION) (T/F)

True

The _____ along the polypeptide determine how it will fold and thus function

R groups

Polypeptide versus protein

• A protein can consist of one polypeptide

  • Ex: insulin consists of one polypeptide 

• A protein can consist of two or more polypeptides 

  • Ex: hemoglobin protein consists of four polypeptide subunits

• A protein can consist of polypeptide and a functional RNAs

  • Called a ribonucleoprotein

  • Ex: telomerase consists of polypeptide and snRNA

tRNA

• Folded into a cloverleaf structure

  • Fits nicely into the ribosome sites

• Each has an anticodon 

  • 3 nt sequence that complementary base pairs with the mRNA codon

A tRNA becomes ‘charged’ when a high energy acyl bond connects it to its cognate amino acid 

• Amino acid added to the 3’ end of the tRNA

  • Uncharged tRNA example: tRNA^phe

  • Charged tRNA example: phe-tRNA^phe

• Charging is also called aminoacylation

• Aminoacylation is catalyzed by an enzyme called tRNA synthetase

  • Each tRNA synthetase recognizes its cognate amino acid and tRNA

  • Example: leucyl-tRNA synthetase binds to leucine and tRNA^leu but not other amino acids and tRNAs – those each have their own synthetases

Polysomes

• One mRNA is being translated by many ribosomes at any given time

• Both prokaryotes and eukaryotes

An overview of the mechanism

• A (aminoacyl), P (peptidyl), E (exit), sites – tRNA binds here during different stages of polypeptide synthesis

• mRNA passes through the small subunit 

• Ribosome translocates along the mRNA 5’ to 3’