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Gene Expression
DNA in cells controls all sorts of things such as the color of your eyes, the color of your hair, and
whether or not you can digest milk. These characteristics are called traits. DNA also controls your
responses to stimuli in the environment to keep you alive. For example, when you are frightened, the
DNA tells your adrenal cells to produce adrenaline which will help you respond to the situation.
DNA does this by using its blueprints to make
different proteins at different times. A section of
DNA that contains the instructions for making one
specific type of protein is called a gene. There
are about 30,000 genes in each human cell!
Proteins are made up of amino acid molecules. There are 20 different amino acids molecules with
names like isoleucine, valine, and phenylalanine. These amino acids can be put together in any
combination based on the code in the DNA molecule. Proteins are usually 200-250 amino acids
long!!!! There are thousands of different kinds of proteins and each has its own special job based on
its structure.
1.
DNA controls a person’s __________________________ and ________________________.
2.
Gene =
3.
How many genes do humans have in each cell?
4.
What are the building blocks of proteins?
5.
How many amino acids are there?
Let’s take a look at the process of making a protein from
the directions in DNA (called gene expression). The DNA
code works in threes. This sequence of 3 bases in DNA is
called a codon.
This means that every three bases in a
gene has the directions for adding one amino acid to the
protein molecule being built.
If you arranged the 4 bases (A, T, C, G) in groups of three in every possible combination, you would
get a total of 64 possible combinations. So, this means that there are 64 codons in the genetic code.
Realize that there are only 20 amino acids, so some of the 64 codons code for the same amino acid.
Scientists have created a chart called a codon chart which shows all 64 codons and the amino acid
that each adds to the protein being built.
U
C
A
G
U
Phenylalanine
Phynylalanine
Leucine
Leucine
Leucine
Leucine
Leucine
Leucine
Isoleucine
Isoleucine
Isoleucine
Methionine
Valine
Valine
Valine
Valine
(based on the code in RNA)
C
A
Serine
Tyrosine
Serine
Tyrosine
Serine
Stop Codon
Serine
Stop Codon
Proline
Histidine
Proline
Histidine
Proline
Glutamine
Proline
Glutamine
Threonine
Asparagine
Threonine
Asparagine
Threonine
Lysine
Threonine
Lysine
Alanine
Aspartic Acid
Alanine
Aspartic Acid
Alanine
Glutamic Acid
Alanine
Glutamic Acid
1. Define codon:
2. How many codons are there in the genetic code?
3. Write your hypothesis here:
G
Cysteine
Cysteine
Stop Codon
Tryptophan
Arginine
Arginine
Arginine
Arginine
Serine
Serine
Arginine
Arginine
Glycine
Glycine
Glycine
Glycine
U
C
A
G
U
C
A
G
U
C
A
G
U
C
A
G
The DNA molecule needs a smaller helper molecule to get its instructions out the ribosome. This
molecule is RNA (ribonucleic acid). The RNA molecule is similar to the DNA molecule. However,
RNA structure differs from DNA structure in three ways. Check out this graphic and see if you can
figure out the three differences. Write them here.
DNA
RNA
Be aware that there are three types of RNA, each having a particular function in the process of
building a protein. As we learn about the process, each of these three molecules will be discussed.
TRANSCRIPTION
The process of gene expression occurs in two main steps. The first step is called transcription.
During this step, the code in a gene is copied into RNA. Enzymes unwind the double helix and unzip
it to expose the gene to be copied. Other enzymes match up RNA complementary base pairs to the
exposed pairs creating an RNA molecule called messenger RNA (mRNA). The mRNA molecule then
leaves the nucleus. Other enzymes help the DNA molecule to seal back up and reform the double
helix.
4. What happens during transcription?
5. What type of RNA is made in this process?
6. Where in the cell does transcription occur?
7. This is a sequence of bases in a gene. Fill in the bases for a complementary segment of
RNA to show the process of transcription. Don’t forget that RNA has U instead of T!! So U
would match with A.
DNA
RNA
A
A
T
C
G
T
C
C
G
TRANSLATION
The second main step of gene expression is called translation. The code carried in the mRNA is used
to make a protein at the ribosome.
Translation involves two other types of RNA – transfer RNA (tRNA) and
ribosomal (rRNA). Each tRNA molecule has a sequence of 3 bases on
one end called an anticodon. tRNA anticodons match up with the mRNA
codons. This is an important part of code translation because the tRNA
molecule carries a certain amino acid based on its anticodon. For
example, a UAC anticodon will carry the amino acid named methionine.
By matching the tRNA anticodon to the messenger RNA codon, the correct
amino acid is put into place as the protein is built. The ribosomal RNA
(rRNA) makes up the ribosome and helps the codon and anticodon join.
So once mRNA arrives at the ribosome, tRNA anticodons join with each mRNA codons one at a time.
Each sequence of three bases (codon) codes for one amino acid to be added onto the protein being
made. The tRNA drops off its amino acid cargo at this point. Each new amino acid that is dropped
off is joined to the rest to form a chain (which is the protein). The tRNA than leaves and picks up
another amino acid based on the code in its anticodon.
8. What type of RNA acts as an enzyme and makes up the ribosome?
9. What type of RNA brings amino acids to the ribosome?
10. What happens during translation?
11. Where does translation occur in the cell?
12. Complete the online Gene Expression activities listed on the webpage.