Biology of the Cell Lab (BIOL 1021)
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Background
“Central Dogma”, a phrase coined by Francis Crick of
the famed Watson and Crick duo, is used to describe
the flow of molecular information through a trio of
macromolecules, DNA, RNA and protein. A gene is
a nucleotide sequence in DNA to which a specific
genetic function can be assigned such as the
production of the pigment for eye color. Genes
provide the “blueprint” for the production of proteins.
Proteins are macromolecules that consist of one or
more polypeptides that function as enzymes,
hormones, antibodies or structural components of
cells.
So how is information in nucleus-bound DNA
expressed as proteins found throughout the cells?
Information is passed on through the processes of
transcription and translation. The nucleic acid
sequence of a gene on the coding strand of DNA is
used to create a complementary nucleic acid
sequence messenger RNA (mRNA). Messenger
RNA as the name implies, takes its genetic
information or message out of the nucleus and
into the cytoplasm of the cell. This transfer of
information from DNA to RNA is the process of
transcription. Transcription generates a single
stranded mRNA complementary in sequence to the
coding or sense strand of DNA. Two other RNA
intermediates important to translation and protein
synthesis are similarly transcribed from specific gene
sequences, ribosomal RNA (rRNA) and transfer RNA
(tRNA).
Once outside the nucleus, mRNA takes its genetic
message from the nucleus to the ribosome, the site
of translation and protein synthesis.
Translation converts the nucleotide sequence of the
mRNA into a specific sequence of amino acids to
produce a specific protein. Translation of mRNA is
accomplished by translating a series of three (triplet)
nucleotides, called a codon, into the corresponding
amino acid as specified in the Genetic Code. (See
Chart) The genetic code consists of the three letter
codons present in the nucleotide sequence of mRNA,
as read in the 5’-3’ direction down the strand. For
example the first triplet found in all mRNA as
transcribed from DNA is composed of the bases
adenine, uracil and guanine (AUG) and translates to
the amino acid methionine (Met), the translation
initiator codon or start codon. Without the AUG
sequence, translation will not take place.
The Genetic Code:
Proteins contain 20 different amino acids. The order
of the base pairs in the DNA determines the order of
amino acids making up the protein. DNA contains a
Transcription/Translation Simulation
combination of 4 bases: adenine (A), thymine (T),
cytosine (C) and guanine (G). When DNA is
transcribed into mRNA, T is replaced by uracil
(U). Because DNA remains in the nucleus of the cell,
the strand of mRNA goes out of the nucleus into the
cytoplasm where it is met by the small subunit of the
ribosome. mRNA has a three basepair codon that
codes for the amino acid. tRNA reads the codon with
its anticodon and delivers a specific amino acid that it
carriers to the ribosome. The docking site located on
a small subunit of the ribosome is called the platform,
or P site. The P site is where all the protein
building is initiated. At the P site a special initiator
tRNA aligns with the start codon of the mRNA.
Finally, a larger ribosome subunit binds around the
smaller subunit and forms the protein building
complex. Translation of the codon begins. As the
codon is being translated, the tRNA delivers the
amino acid coded for. Once the amino acid is
delivered and attached with a peptide bond, the
tRNA is released from the P site. The next amino
acid is delivered, attached and the tRNA is released
until the end codon is signaled for. There are three
end codons that signal the end of protein called stop
codons. Their codons are UAA, UGA and UAG.
Objective:
Students are “cellular spies”. In this “case” the
students are given one important “clue”, a coding
strand of DNA, and are to deduce the amino acid
sequence found in the protein that this segment of
DNA codes for. Working alone, the “spies” must
transcribe nucleotide information in their coding
strand to synthesize the complementary strand of
mRNA. Remember, the base uracil in RNA is
complementary to adenine and synonymous with
thymine in DNA. Messenger RNA triplets are then
translated into the corresponding amino acids.
Students will then assemble their polypeptide in the
order indicated by their transcribed mRNA. Students
will submit a polypeptide strand, ie. beads,
constructed from information contained in their
coding strand of DNA to the teacher.
Biology of the Cell Lab (BIOL 1021)
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Transcription/Translation Simulation
Please tape your DNA clue below.
Record the transcription of your “clue” (mRNA).
Record the translation of your “clue” (protein sequence)?
Answer the following questions.
1) How many amino acids long is your protein?
2) Did your “clue” include a stop codon?
3) Did your “clue” start with an initiator codon?
4) What would you expect to happen if your “clue” did not have an initiator codon?
5) What would happen if you made an error in transcribing your “clue” and you substituted a
different nucleotide for the 10th nucleotide?