Ch 10 - Translation

proteins

the building blocks of all living organism

amino acids

the building blocks of all proteins

parts of an amino acid

• alpha carbon atom (center)

• H atom

• amino group (NH₃⁺)

• carboxyl group (COOH^-)

• radical (R) group

How many types of amino acids are found in humans?

20

peptide bonds

joins amino acids together

polypeptide

a “string” of amino acids, which becomes a protein

What are the levels of organization of a protein?

• primary structure (polypeptide chain)

• secondary structure (alpha helices and beta pleated sheets)

• tertiary structure (conformational shape)

• quaternary shape (where a protein gets its function)

What molecules help newly synthesized proteins fold into the correct shape?

molecular chaperones

How are most proteins modified?

post-translationally

translation

• a mature mRNA is read by ribosomes to synthesize proteins

• ribosomes covalently link amino acids with peptide bonds

• tRNAs are used to read the mRNA and transport the correct amino acid to the ribosome

What are mature ribosomes made of?

many proteins associated with 4 rRNAs, which are

• 18s

• 5.8s

• 28s

• 5s (transcribed from a different loci)

make up about 80% of cellular RNA

What is the precursor rRNA transcript for 18s, 5.8s, and 28s?

45s

What is the structure of ribosomes?

consists of 2 subunits: 1 large and 1 small

structure of tRNAs

• has a cloverleaf secondary structure

• has rare bases

• has an amino acid attachment site (always has the sequence CCA)

• has an anticoding arm (pairs with corresponding mRNA sequence)

How is the genetic code degenerate?

because more than one codon (triplet of bases) codes for the same amino acid

synonymous codons

codons that specify the same amino acid

anticodons

found on tRNA, complementary to the codons on mRNA

What is the start codon? Which amino acid does it code for?

AUG, codes for methionine

What are the stop codons? What do they code for?

UAG, UGA, UAA, they do not code for an amino acid

What allows for degeneration (multiple codons coding for the same amino acid) of the genetic code?

• the flexibility of the third base in a codon

• Watson-Crick pairing rules are relaxed for the 3rd base in a codon

Is the genetic code overlapping?

no

open reading frame (ORE)

goes from the start codon to the stop/nonsence codon

What is translation carried out by?

ribosomes

In what direction is polypeptide synthesis carried out in?

• 5’ → 3’ or from N-terminus (amino group) to the C-terminus (carboxyl group)

What are the three distinct sites of a ribosomes?

• aminoacyl (A) site

• peptidyl (P) site

• exit (E) site

aminoacyl (A) site

where a charged tRNA (with an amino acid) first attaches

peptidyl (P) site

where peptide bonds are formed

exit (E) site

where tRNA (without an amino acid) dissociates and leaves the ribosome

translation initiation

• small ribosomal subunit binds to the mRNA

• scans mRNA for a start codon surrounded by a Kozak sequence

• tRNA^fmet is added to the P site and the large ribosomal subunit binds

What is the attachment of the small subunit of the ribosome to the mRNA facilitated by?

by the poly-A tail and the 5’ methyl cap binding proteins

kozak sequence

5’-ACCAUGG-3’

translation elongation

• an elongation factor attaches to a GTP and adds new tRNA to the A site

• a polypeptide bond is formed between the amino acid in the P site and the amino acid in the A site

• the ribosome shifts forward

  • the P site tRNA shifts to the E site

  • the A site tRNA shifts to the P site

• the tRNA in the E site is released from the ribosome

What provides energy for translation elongation?

the conversion of GTP to GDP

translation termination

• the A site reaches a stop codon, causing a release factor 1 (RF1) to attach to it

• a release factor 3 (RF3) in complex with a GTP binds to the outside of the ribosome

• GTP is hydrolyzed to GDP and the mRNA, tRNA, and release factors are released from the ribosome