QUIZ - DEC 3/4
- describe the structure of DNA
- Describe the three steps of DNA semi conservative replication
- Identify the purpose and site of DNA replication
HOMEWORK
Homework: Please select a SHORT article or story about genetic engineering, gene therapy or a genetic disorder involving proteins (ex. Cystic Fibrosis). And e-mail it to my e-mail address by December 4th.
Check out Scientific American, Discover, BBC, CBC, etc...
Hannah.myles@sd41.bc.ca
Protein Synthesis Research Notes
The
connection between genes and proteins.
It
was believed since the early 1900s that genes determine the way an organism
looks
through
enzymes that catalyze specific chemical reactions in the cell. In other words,
some
diseases
are caused by missing or defective enzymes. In the 1930s George Beadle and
Edward Tatum were able to definitively establish the link between genes and
enzymes in their exploration of the metabolism of a bread mold. They bombarded
the mold with X-rays and screened the survivors for mutants that differed in
their nutritional needs.
The
normal mold could grow on agar containing very little nutrients. Beadle and Tatum
identified mutants that could not survive on this minimal medium, because they
were unable to synthesize certain essential molecules from the minimal ingredients.
However, most of these nutritional mutants were able to survive on a complete
growth medium that includes all 20 amino acids and a few other nutrients.
They
suggested that the mutants were unable to survive because they lacked an enzyme
necessary to make the particular nutrient missing from the medium. By providing
the missing nutrient, the mutants were able to survive.
Their
results provided strong evidence for the one
gene–one polypeptide hypothesis.
Proteins
carry information in their amino acid sequence and nucleic acids carry
information
in
their nucleotide sequence.
To
get from DNA (in nucleic acid language) to protein (in amino acid language)
requires
two
steps:
During
transcription, a DNA strand provides a template for the synthesis of a
complementary
RNA strand. This RNA molecule is called mRNA.
The
use of mRNA provides protection for the genetic information contained in DNA.
Also,
more protein can be made simultaneously because many mRNA copies of a gene can
be made. Lastly, each mRNA can be translated many times.
During
translation, the instructions are converted from nucleic acid language to amino
acid language.
The genetic code:
There
are only 4 bases but 20 amino acids so it is not sufficient for one nucleotide
to
represent
one amino acid.
The
code must be a series of triplets (three bases) which indicate a particular amino
acid. The genetic instructions for a polypeptide chain are written in DNA as a
series of non overlapping three-nucleotide words called codons.
Sixty-one
of the 64 triplets code for amino acids.
The
codon AUG not only codes for the amino acid methionine, but also indicates the “start”
of translation.
Three
codons do not indicate amino acids but are “stop” signals marking the termination
of translation.
Some
amino acids are coded for by two or more codons but a given codon ALWAYS codes
for only one amino acid. i.e.
there is redundancy but no ambiguity. e.g., GAA and
GAG both mean glutamic acid, but never
mean any other amino acid.
The
process of transcription is much like DNA replication except that DNA is acting
as a template for the construction of mRNA rather than new DNA
Initiation
RNA polymerase separates
the DNA strands at a particular sequence called the promoter and bonds the RNA
nucleotides as they base-pair along the DNA template.
Like
DNA polymerase, RNA polymerase can only assemble a polynucleotide in its 5’ to
3’ direction.
Unlike
DNA polymerase, RNA polymerase is able to start a chain from scratch; it does
not need a primer.
Elongation
As
RNA polymerase moves along the DNA, it untwists the double helix, 10 to 20 bases
at time, adding bases by base pairing rules to form mRNA.
Remember
that in RNA, U rather than T is paired with A.
Behind
the point of RNA synthesis, the double helix re-forms and the mRNA molecule
peels away.
If
there is high demand for a protein, the cell can have several RNA polymerases transcribing
the same gene simultaneously to produce several mRNAs.
Termination
Transcription
proceeds until RNA polymerase transcribes a terminator sequence
(AATAAA).
The
mRNA is then released from the RNA polymerase and sent to the cytosol.
A
transcription unit is the sequence between the start and stop sequences –
generally speaking, one gene.
Translation
In
the process of translation, a cell interprets a series of codons along an mRNA
molecule
and
builds a polypeptide. That is, mRNA is translated from nucleic acid language to
amino
acid
language.
The
interpreter is transfer RNA (tRNA), which carries amino acids to a ribosome. The
ribosome
then adds each amino acid carried by tRNA to the growing end of the polypeptide
chain.
Each
tRNA carries a specific amino acid attached to the amino acid attachment site.
At
the other end of the tRNA is a group of three nucleotides called the anticodon that
binds by complementary base pairing to the nucleotides of a codon.
E.g., if
the codon on mRNA is UUU, a tRNA with an AAA anticodon and carrying phenylalanine
will bind to it.
The
tRNA molecule is a translator, because it can read a nucleic acid word (the mRNA
codon) and translate it to a protein word (the amino acid).
Initiation
Each
ribosome has a binding site for mRNA and three binding sites for tRNA molecules.
(1)
The P site holds the tRNA carrying the growing polypeptide chain.
(2)
The A site carries the tRNA with the next amino acid to be added to the chain.
(3)
A tRNA that has dropped off its amino acid leaves the ribosome at the E (exit) site.
A
ribosome binds to mRNA and begins looking for the start codon (AUG)
To
extract the message from the genetic code requires specifying the correct starting
point.
This
establishes the reading frame; subsequent codons are read in groups of three nucleotides.
When
it is found, it is displayed in the P site so tRNA molecules can attempt to recognize
it by complementary base pairing with their anticodon. When this occurs, the
two ribosomal subunits come together to form the functional ribosome.
The
tRNA with the anticodon complementary to AUG always carries methionine (met) so
it is always the first amino acid in every protein. The tRNA carrying methionine
enters the P site.
The
ribosome now displays the next codon in the A site and waits for the tRNA with
the complementary anticodon to recognize it.
If
the cellular demand for a protein is high, several ribosomes can translate the
same
mRNA
simultaneously
Elongation (~ 60
ms per peptide bond)
The
tRNA with an anticodon complementary to the next codon enters the A site
The
growing polypeptide chain on the tRNA at the P site, now one amino acid longer,
is transferred to the tRNA at the A site. The ribosome forms a new peptide bond
by transferring the amino acid from tRNA in the P site to the amino acid on the
tRNA in the A site
The
ribosome moves over one codon on the mRNA so that the next codon is now displayed
in the A site.
The
tRNA (now empty) that had been in the P site is moved to the E site and then leaves
the ribosome.
the
appropriate tRNA moves in and the ribosome attaches the amino acids (now 2 of
them) from the tRNA in the P site to the amino acid on the tRNA in the A site. The
chain is now three amino acids in length
These
steps of elongation continue to add amino acids codon by codon until the polypeptide
chain is completed.
Termination
The
elongation process continues until one of the three stop codons is reached displayed
in the A site.
There
is no tRNA which recognizes any of the stop codons but a release factor binds
to the stop codon and hydrolyzes the bond between the polypeptide and its tRNA
in the P site.
This
frees the polypeptide, so it is released from the ribosome.
A
ribosome requires less than a minute to translate an average-sized mRNA into a polypeptide.
During
and after synthesis, a polypeptide coils and folds to its three-dimensional
shape spontaneously.
VIDEO - https://www.khanacademy.org/partner-content/crash-course1/crash-course-biology/v/crash-course-biology-111
Protein Synthesis Animations & quizzes
Gene Expression
Stages of Transcription
How Translation Works
Protein Synthesis
Vocabulary
Flashcards
