Gene Action
Protein Synthesis
Overview of Protein Synthesis …
the short version
DNA contained in genes provides
instructions for making protein
Information from a specific section of DNA
is first transcribed to produce a specific
molecule of RNA
RNA attaches to a ribosome where the
information is translated into a
corresponding sequence of amino acids
Protein Synthesis Vocab
Nucleotide – building block of nucleic acid:
sugar, phosphate, base
Protein Synthesis Vocab
Base Triplet – a sequence of three
successive DNA nucleotides
Codon – a sequence of three successive
mRNA nucleotides that correspond with a
base triplet
TRANSCRIPTION
DNA base triplets  mRNA codons
Just copying a message in the same language!
– Nucleotide  nucleotide
Occurs in the Nucleus
Catalyzed by the enzyme RNA polymerase
– Promoter – a special sequence of DNA nucleotides
near the beginning of a gene that tell the RNA
polymerase where to begin reading
TRANSCRIPTION
Nucleotides pair complementarily:
DNA
C G A T -
RNA
G
C
U
A
TRANSCRIPTION
Terminator – a specific sequence of
DNA nucleotides that signals the end
of the gene.
RNA polymerase detaches from the
RNA molecule and the DNA strand.
The RNA leaves the nucleus through a
nuclear pore
Translation
The process in which mRNA associates
with ribosomes and directs the synthesis
of proteins by converting the sequence on
nucleotides in RNA to a sequence of
amino acids…HUH?
Changing Languages Now…
– Nucleotide  Amino Acids
Translation
http://staff.jccc.net/PDECELL/proteinsynth
esis/translation/steps.html
Translation
1. An mRNA molecule binds to the small
ribosomal subunit and a special tRNA
molecules (the initiator) binds to the start
codon (AUG), where translation begins
Translation
2. The large ribosomal subunit attaches to
the small subunit, creating a functional
ribosome
– The initiator tRNA binds to the start codon
– One end of the tRNA carries a specific amino
acid, the other consists of a triplet of bases
called an anticodon.
– The anticodon pairs with the complementatry
codon on mRNA
Translation
3. The anticodon of another tRNA with its
amino acid attaches to the next section of
mRNA
4. A peptide bond is formed between the
amino acids carried by the 2 tRNA’s
Translation
5. After the peptide bond forms, the tRNA
detaches from the ribosome and the
ribosome shifts the mRNA strand by 1
codon. A new tRNA with amino acid binds
to the exposed codon.
Steps 3-5 repeat as the polypeptide
lengthens
Protein synthesis ends when the ribosome
reaches a stop codon.
– The new protein detaches, the tRNA detaches
and the ribosome splits into its sub units
How fast can this possibly happen?
In the body translation occurs at a rate of
about 15 amino acids per second!
Before 1 ribosome finishes moving down
the mRNA another can attach to that same
strand allowing large amounts of protein to
be built.
tRNA