Download The Genetic Code

A
B
Use the image above to answer these questions.
1. Does the process shown above use ATP?
2. The process shown above moves molecules
[up, down] the concentration gradient.
3. A molecule of glucose would use which
protein to enter the cell?
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
Recall that DNA is
used to create new
proteins and
determine your
physical
characteristics.
The Genetic Code
determines the
exact proteins that
will be created.

Creating new proteins is a three step
process:
› First, DNA is transcribed within the nucleus.
This process forms RNA.
› Second, the RNA is exported from the
nucleus to the cytoplasm.
› Third, the RNA attached to ribosomes, and is
translated to create a chain of amino acids
– also known as a protein!
Transcription is the process of creating a
strand of RNA that is complimentary to a
given strand of DNA.
 The RNA is designed to be a temporary
copy, and will be quickly chewed by
enzymes in the cytoplasm.
 Think of DNA as a master copy, and RNA
as the photocopies!
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RNA stands for
ribonucleic acid.
RNA is unique in that
it is (usually!) single
stranded.
Also, RNA has four
bases, but they are
slightly different from
those of DNA.

Similarities


Differences
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Both are types of nucleic acid.
Both are composed of nucleotides.
Both have four nitrogen bases.
DNA is double-stranded, RNA is singlestranded.
DNA has the bases A, C, G and T. RNA
has the bases A, C, G and U.
› The U stands for uracil. It takes the place of
thymine in RNA, and base pairs with adenine.

DNA is permanent, and RNA is temporary.

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If the DNA base is…
…then the correct RNA base is…
A (adenine)
U (uracil)
C (cytosine)
G (guanine)
G (guanine)
C (cytosine)
T (thymine)
A (adenine)
Transcription is the process of making a
complimentary strand of RNA.
You WILL need to use base pairing rules on
some questions. Remember, RNA does NOT
have thymine!
1.
2.
3.
4.
Double-stranded DNA opens up,
forming a replication bubble.
RNA polymerase binds to promoters
sequences, and begins to transcribe the
DNA into messenger RNA (mRNA).
The polymerase detaches at a stop
sequence, and the mRNA breaks away.
The mRNA strand is processed (in
eukaryotes!) and moves to the
cytoplasm.
1
2a
2b
3
4
Often, the mRNA produced in
transcription needs to be processed
further. In eukaryotes, we add a 5’ cap
and a 3’ poly-A tail.
 Sometimes, we also need to remove
sequences from the mRNA.

› Sequences REMOVED: introns
› Sequences KEPT: exons
1.
Put these steps of transcription in order:
a. mRNA detaches from DNA
b. mRNA is created through base-pairing
c. mRNA is modified and exported.
d. DNA unwinds, and polymerase attaches
2.
Base pair the correct sequence of RNA
with this sequence of DNA:
CTAAGATCGATC

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In translation, the
mRNA we produced
in transcription will
be used to make a
protein.
But, how do we
know what proteins
will be made?


The cell groups each
sequence of three
nucleotides as a
codon.
Each codon
matches up with an
amino acid. This is
how we create
proteins!
Why three nucleotides in a codon?
 We need to be able to code for all
twenty amino acids. Using probability,
sequences of three letters are the
shortest that can code for all twenty.

› One base: 4 possibilities (A, C, G, U)
› Two bases: 16 possibilities (4x4)
› Three bases: 64 possibilities (4x4x4)
However, we now have a new problem:
we have too many sequences for all
twenty amino acids!
 As a result, many amino acids are
represented by multiple sequences. For
instance, alanine has four sequences,
and arginine has six!

› Note: the third letter of a codon is the LEAST
important. We’ll come back to this later.

1.
2.
3.
The steps of translation:
A ribosome is constructed at a start
codon. This will ALWAYS be AUG!
A transfer RNA (tRNA) binds to the
codon, and drops off the appropriate
amino acid.
This process repeats until the ribosomes
hits a stop sequence. This will be the first
instance of either UAA, UAG or UGA.
2
1
2
3
Use this sequence for these questions:
TGCAGCAATGAC
1. Transcribe this sequence to mRNA.
2. Translate the mRNA sequence you
made to a sequence of amino acids.
3. How many ATP molecules are made by
a bacteria in cellular respiration? Is this
an example of aerobic or anaerobic
respiration?
A gene is a portion of the DNA that is
used to code for protein. Genes are
quite rare; only about 1% of the DNA in
your cells is used to make protein!
 Genes are regulated, such that they are
NOT making protein all of the time. Cells
only want to activate genes when
needed.
 Genes can be repressible or inducible.
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A repressible gene is USUALLY ACTIVE, but can
be shut down by a repressor.
The catch is, the repressor is only active in the
presence of molecules created by the gene’s
enzyme!
This way, the gene turns off after it has been
used enough times.
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An inducible gene is USUALLY INACTIVE, but can
be activated by an inducer.
The catch is, the inducer that removes the
repressor is the substrate for the enzyme for this
gene!
This way, the gene turns is only on when the
enzymes can be used.