Download File

After Transcription…
 The mRNA must exit the nucleus
 Ribosomes bind to the mRNA because they recognize
the 5’ cap
 This binding is the initiation of translation
The Ribosome
 The ribosome consists of
two subunits
 Small subunit (40s)
 Large subunit (60s)
 The S refers to the rates at which
various components sediment
when centrifuged
 The two subunits bind to
the mRNA, clamping the
mRNA between them
 The ribosome moves
along the mRNA in the 5’
to 3’ direction
 One amino acid is added
to the growing
polypeptide chain for
each codon read
Reading Frame
 The phase in which the mRNA is read is known as the
reading frame
 The reading frame can differ depending on the base
pair from which the ribosome starts reading
Examples of Reading Frames
tRNA (Transfer RNA)
 tRNA delivers the correct amino acid to the ribosome
so it can be added to the polypeptide chain
 tRNA is a small, single stranded nucleic acid whose
structure resembles a cloverleaf
 On one side of the cloverleaf is a sequences of three
bases called the anticodon
 This anticodon recognizes and matches up with the
complimentary codon on mRNA
 The opposite side of the tRNA carries the amino acid
Amino Acid attached here
(at the 3’ end)
Anticodon here
 Every tRNA carries only one amino acid
 There are between 20 and 64 different types of tRNA
molecules available depending on the organism
 An amino acid is attached to a tRNA by the enzyme
aminoacyl-tRNA synthetases
 A tRNA molecule with an amino acid attached at the 3’
site is called an aminoacyl-tRNA
 RECALL: more than one codon can code for the same amino acid (UAU
and UAC both code for tyrosine). This minimizes errors in the
polypeptide chain
Elongation
 The first codon that is recognized by the ribosome is
the start codon AUG
 AUG ensures that the correct reading frame is used by
the ribosome
 The AUG codon codes for methionine which means
that every protein initially starts with the amino acid
methionine
 The ribosome has two
sites
 The A (acceptor) site is
where the tRNA carrying
the appropriate amino
acid enters
 The P (peptide) site is
the site where peptide
bonds are formed
between adjoining
amino acids in the
growing polypeptide
chain
The first tRNA that is brought into the P site
carries methionine because the start codon is
AUG. The second tRNA enters the A site
A peptide bond forms between methionine and
alanine. The ribosome translocates one codon
over and the next tRNA brings in the appropriate
amino acid into the A site. The ribosome moves
along the mRNA and another amino acid is added
to the chain
The process is repeated until the ribosome reaches a
stop codon for which no tRNA exists. A release factor
protein aids the dismantling of the ribosome-mRNA
complex, releasing the polypeptide chain
 The tRNAs that are released from the ribosome are
recycled by aminoacyl-tRNA synthetase adding a new
(correct) amino acid to them
 The three stop codons are UGA, UAG and UAA
 These codons do not code for an amino acid and
therefore there are no tRNAs which can enter the
ribosome
 The release factor protein recognizes that the
translation has stopped and releases the polypeptide
from the ribosome
 Termination is finalized when the two ribosome sub
units detach from the mRNA transcript
 In some cases, sugars (glycosylation) or phosphates
(phosphorylation) are added to the newly formed
polypeptide
 In other cases, enzymes may cleave the polypeptide at
specific places