Protein Synthesis
DNA is transcribed into
Messenger RNA. Messenger
RNA is translated into Protein
Goals
• Compare the structure of RNA with that
of DNA
• Summarize the process of transcription
• Relate the role of codons to the
sequence of amino acids after
translation
Are Genes and Enzymes Related?
• In 1909,Archibald Garrod “suggested”
that genes dictate phenotypes through
enzymes.Garrod's One Gene-One
Enzyme Hypothesis (click the link)
• In 1936 Beadle & Ephrussi supported
Archibald’s ideas with research
concerning fruit fly eye color.
http://www.genetics.org/cgi/reprint/22/1/
76.pdf
One Gene – One Enzyme
• 1941 George Beadle and Edward Tatum
experimented with the bread mold,
Neurospora crasa.
• They showed that a single gene
mutation could result in a single enzyme
deficiency.Genome News Network Timeline: 1941. (Click the Link to Learn
More.)
Current Hypothesis
• One gene directs the synthesis of a
single polypeptide.
ONE GENE = ONE POLYPEPTIDE
Central Dogma
Transcription
Transcription
• RNA molecules-Nucleic acids made of
linked nucleotides
• Different How?
• Single stranded, not double
• Ribose instead of Deoxyribose
• Contain Uracil instead of Thymine
Transcription
• Transcription-When the instructions for
making a protein are transferred from a
gene to an RNA molecule
• Translation-When instructions on an
RNA molecule are read and coded as
proteins
The Stages of Transcription
1) RNA polymerase binds to
the promoter and unwinds
the DNA and begins RNA
synthesis.
2) RNA polymerase adds RNA
nucleotides to the template
strand of DNA in a 5’ to 3’
direction.
3) RNA polymerase
transcribes the terminator
sequence which signals the
process to end. The RNA
is then released.
Transcription
• The DNA molecule unzips
• One side of the DNA acts as a template
for making RNA
• At the end, the RNA is unzipped from
the DNA
• Portions called introns are taken out.
• Portions called exons are spliced
together
• http://www.youtube.com/watch?v=ztPkv
7wc3yU&feature=related&safety_mode
=true&persist_safety_mode=1&safe=act
ive
Transcription to translation
• RNA is transcribed
on the DNA in the
nucleus
• mRNA is translated
in the cytoplasm
Ribosome
• The ribosome is made
of two subparts
• The ribosome becomes
attached to the mRNA
• After a tRNA has given
up its amino acid it
breaks off and goes
back into the cytoplasm
to find another amino
acid
Messenger RNA
• The long form of RNA is called
messenger RNA (mRNA)
• The mRNA leaves the nucleus and goes
into the cytoplasm
• The mRNA lies along an endoplasmic
reticulum
mRNA
• mRNA has thousands of nitrogenous
bases
• The bases are divided into 3 base
words called codons
• The mRNA becomes attached to a
ribosome
• The ribosome moves along the mRNA
and allows tRNA to join onto the codons
Translation
• Process by which the proteins in our
body are coded for
Translation
• Genetic Code-codes that give us the
proteins in our body, this is based on
three letters
• How many three letter words can be made
from the letters A, R, E, and T
• A tRNA molecule consists of a strand of
about 80 nucleotides that folds back on
itself to form a three-dimensional structure.
• It includes a loop containing the anticodon and
an attachment site at the 3’ end for an amino
acid.
Transfer RNA (tRNA)
• tRNA molecules find a specific amino
acid within the cytoplasm
• tRNA molecules have an anticodon on
their ends.
• tRNA takes its amino acid to a specific
codon on the mRNA
• Each ribosome has a binding site for mRNA
and three binding sites for tRNA molecules.
• The P site holds the tRNA carrying the growing
polypeptide chain.
• The A site carries the tRNA with the next
amino acid.
• Discharged tRNAs leave the ribosome at the E
site.
• Initiation brings together mRNA, a tRNA
with the first amino acid, and the two
ribosomal subunits.
• First, a small ribosomal subunit binds with
mRNA and a special initiator tRNA, which
carries methionine and attaches to the start
codon.
• Initiation factors bring in the large subunit such
that the initiator tRNA occupies the P site.
• The three steps of elongation continue
codon by codon to add amino acids until
the polypeptide chain is completed.
• Termination occurs when one of the three
stop codons reaches the A site.
Translation Steps….#1
• mRNA leaves the nucleus and enters
cytoplasm
• Methionine is always the first amino
acid
• Start codon is AUG
• tRNA molecule binds methionine to
AUG codon
Step #2
• Codon in A site of ribosome is ready to
receive the next tRNA
• tRNA with the complementary anticodon
arrives and binds to the codon with the
right amino acid
Step #3
• A and P site are holding tRNA
molecules
• Enzymes bind the adjacent amino acids
together
Step #4
• tRNA goes to the E site, where it
detaches to go get another amino acid
Step #5
• tRNA in the A site moves over and fills
the empty P site
• Because the A site is open another
tRNA brings an amino acid and bonds
to the growing protein
Step #6
• tRNA goes to E site and exits
Step #7
• Steps 2 through 6 are repeated until a
stop codon is reached
• Stop codons-UAG, UAA, UGA
• Newly made protein is released into the
cell
• http://www.youtube.com/watch?v=zb6r1MMTkc&NR=1&safety_mode=true
&persist_safety_mode=1&safe=active
• http://www.youtube.com/watch?v=41_N
e5mS2ls&safety_mode=true&persist_sa
fety_mode=1&safe=active
Gene Regulation and Structure
Goals
• Describe how the lac operon is turned
on or off
• Summarize the role of transcription
factors in regulation eukaryotic gene
expression
• Describe how eukaryotic genes are
organized
Regulation
• Both prokaryotic and eukaryotic cells
are able to control which genes are
expressed and which are not
Regulation
• Operator-The piece of DNA that
overlaps the promoter site and serves
as the on-off switch
Regulation
• In Bacteria
• Genes that code for enzymes involved in
the same function, their promoter site and
operator is called the OPERON
• Lac Operon-controls the metabolism of
lactose
Regulation
• Repressor-a protein that binds to an
operator and physically blocks RNA
polymerase from binding to a promoter
site