PROTEIN SYNTHESIS
• DNA is the genetic code for all life. DNA literally
holds the instructions that make all life possible.
Even so, DNA does not directly “do” anything.
• Therefore, the processes of 1) transcription and 2)
translation allow a cell to carry out the process of
taking the code of DNA to mRNA and eventually
from mRNA to protein.
• In other words, the flow of information in a cell goes
from:
DNA
mRNA
Protein
TRANSCRIPTION – DNA IS
“TRANSCRIBED” INTO MRNA
• What? DNA coding mRNA
• Where? Nucleus
• Why? DNA is double stranded and too large to get
out of the nucleus through the nuclear pores.
(mRNA is single stranded and can escape the
nucleus).
TRANSCRIPTION: KEY TERMS
• mRNA(messenger RNA)- takes DNA message to
ribosomes
• Promoter DNA – a nucleotide sequence on DNA
that signals for transcription to begin at this area
• This is the site for RNA Polymerase binding and determines
which of the two strands of DNA is to be transcribed
• Terminator DNA – sequence of DNA that signals the
end of transcription and the end of the gene
TRANSCRIPTION: KEY TERMS
TRANSCRIPTION: KEY TERMS
(CONTINUED)
• Helicase – transcription enzyme that breaks the
Hydrogen bonds between DNA bases so that
transcription can begin
• RNA (or DNA) Polymerase – transcription enzyme
that adds RNA nucleotides to the DNA template by
helping to form Hydrogen bonds between the
bases of DNA and mRNA
STEPS OF TRANSCRIPTION
• 1) Initiation – RNA polymerase binds to promoter
DNA
• 2) RNA elongation – RNA polymerase “slides” down
DNA template creating mRNA as it goes by adding
RNA nucleotides by correct base pairing rules (A to
U and C to G)
• As RNA synthesis continues, the RNA strand peels away from
its DNA template and the two DNA strands come back
together
• 3) Termination - RNA polymerase reaches
terminator DNA and the polymerase detaches from
the RNA and the gene (DNA)
STEPS OF TRANSCRIPTION
OTHER TYPES OF RNA PRODUCED BY
TRANSCRIPTION
• tRNA (transfer RNA) – transfers amino acids from
cytoplasm to ribosomes
• Has a site on top for amino acid attachment
• The bottom of the tRNA is known as an anticodon
• Acts as the “interpreter” when translating “nucleic acid
language” to protein “language”
• rRNA (ribosomal RNA) – a type of RNA that, along
with proteins, make up the 2 subunits of ribosomes
OTHER TYPES OF RNA PRODUCED BY
TRANSCRIPTION CONTINUED
TRANSLATION – MRNA IS
“TRANSLATED” INTO PROTEINS
• What? mRNA is read by
ribosomes and proteins are built
from these instructions
• Where? Ribosomes in the
cytoplasm
• Why? To create proteins to carry
out basically every function in
the body
TRANSLATION: KEY TERMS
• Codon – mRNA is read by the ribosome in groups of 3
bases. Each codon (3 mRNA bases) codes for 1 amino
acid
• Amino acid – monomer (building block) of protein
• Anticodon – 3 bases on the bottom of tRNA that are
complementary (opposite) to the codons on mRNA.
• Anticodons on the bottom of tRNA ensure that each codon
codes for only 1 amino acid
• Ribosome – Reads mRNA codons and sends out signal to
tRNA to bring in appropriate amino acid (by matching
codon of mRNA to anticodon of tRNA)
STEPS OF TRANSLATION
• 1) Initiation – binding of mRNA to ribosome
• mRNA binds to small ribosomal subunit
• tRNA then binds to the start codon (which is AUG) to bring
in first amino acid – MET
• Large ribosomal subunit binds to the small one, creating a
functional ribosome
• Ribosome now has 2 binding sites
• P site = holds tRNA with growing polypeptide
• A site = vacant site where next amino-acid bearing tRNA will
bind
STEPS OF TRANSLATION CONTINUED
• 2) Elongation – Amino acids are added one by one to
first amino acid. Occurs in 3 step process.
• Codon recognition – Anticodon of incoming tRNA molecule,
carrying its amino acid, pairs with mRNA codon in A site
• Peptide bond formation - Polypeptide separates from tRNA in P
site and attaches by a peptide bond to amino acid carried by
tRNA in A site
• Translocation - P site tRNA now leaves the ribosome, and
ribosome translocates (moves) the tRNA in the A site, with its
attached polypeptide, to the P site. The codon and anticodon
remain bonded so tRNA and mRNA move as a unit. This opens
the A site for the next amino acid to be brought in by a tRNA
STEPS OF TRANSLATION CONTINUED
STEPS OF TRANSLATION CONTINUED
• 3) Termination – Elongation continues until a stop
codon reaches the A site
• Ribosome then breaks apart and finished polypeptide is
released from tRNA where it was growing