Download Decoding the information in DNA

Have you ever wondered how computers think? Computers understand a language called binary
code. It is made up of only 2 options—zero and one. 000110010010100011110101 is an example of
a computer code. From simple games to web browsers, complex software is built by stringing
together zeros and ones into long computer codes. Think about this as you read this section.
Decoding the information in DNA
List 3 ways that
RNA differs from
DNA:
1.
2.
3.
Traits, such as eye color, are determined by proteins that are built
according to instructions coded in DNA. Recall that proteins have many
functions, including acting as enzymes and cell membrane channels. Proteins,
however, are not built directly from DNA. Ribonucleic (rie boh noo KLAY
ihk) acid is also involved.
Like DNA, Ribonucleic acid (RNA) is a nucleic acid—a molecule made of
nucleotides linked together. RNA differs from DNA in three ways. First,
RNA consists of a single strand of nucleotides instead of the two strands
found in DNA, as shown in the diagram below Second, RNA nucleotides
contain the five-carbon sugar ribose (RIE bohs) rather than the sugar
deoxyribose, which is found in DNA nucleotides. Ribose contains one more
oxygen atom than deoxyribose contains. And third, in addition to the A, G,
and C nitrogen bases found in DNA, RNA nucleotides can have a nitrogen
base called (YUR uh sihl)—abbreviated as U. No thymine (T) bases are found
in RNA. Like thymine, uracil is complementary to adenine whenever RNA
base-pairs with another nucleic acid.
The instructions for building a
protein are found in the DNA
of a gene and are “rewritten”
to a molecule of RNA during
transcription. The RNA is
then “deciphered” during
translation
What are the types of RNA?
RNA, like DNA, is a nucleic acid. RNA nucleotides contain the sugar ribose instead of deoxyribose and the
base uracil instead of thymine. RNA usually exists in
single strands. Cells contain three main types of
RNA—messenger RNA, ribosomal RNA, and transfer
RNA. Messenger RNA (mRNA) molecules are long
strands of RNA nucleotides that direct ribosomes to
make proteins. They travel from the nucleus to the
ribosome. Ribosomal RNA (rRNA) molecules make up
part of the ribosomes of the cell in the cytoplasm.
Transfer RNA (tRNA) molecules transport amino acids
to the ribosomes.
List the three types of RNA
1.
2.
3.
The reading tells us that “Engineers tell
workers how to make the cars” What
organelle performs the same function
for the cell?
What is the role of RNA in the cell?
Think of a car factory and an assembly line. A car is a
complicated piece of machinery. But cars are build by
following simple steps. Engineers tell workers how to
make the cars, and workers follow the directions to
build the cars on an assembly line. Suppliers bring
parts to the assembly line so that they can be used to
build the cars. This is similar to the role of DNA and
RNA in a cell. DNA gives the instructions to make the
proteins. Workers build proteins. Other workers
bring the parts, in this case amino acids, to the
assembly line. In our example the workers are the
RNA.
In the reading, the “workers” are
compared to what cellular structure?
mRNA transports the information for
making proteins from the:
_____________________
to the
_____________________
Transcription
DNA is not able to leave the nucleus and therefore needs a “messenger” to take the instructions
from the nucleus to the ribosomes where the proteins are assembled. In order to get the information to the ribosome, messenger RNA (mRNA) must be made. In this processes called Transcription, an RNA copy is made from the DNA
As shown in the diagram, the DNA is unzipped
in the nucleus. RNA polymerase (an enzyme)
binds to the DNA and begins to add
nucleotides to the new strand of mRNA. Uracil
is added to the mRNA strand instead of thymine, making a complement to the DNA strand.
The mRNA strand moves out of the nucleus
through nuclear pores into the cytoplasm.
The Code
The only way DNA varies among organisms is the sequence of bases. There are 20 amino
acids that are used to make proteins, so DNA must provide at least 20 codes. In the 1960s,
scientists discovered that the DNA code is a three-base code called a codon. Each codon is
transcribed into the mRNA code.
The genetic code for all 64 codons is
shown in the table on the right. Three
codons—UAA, UAG, and UGA—do not
code for an amino acid. They are called
stop codons because they stop the
transcription process. AUG codes for
methionine and is the start codon.
What is a codon?
Apply Determine the amino acid chain
that would result from the sequence
AUG-CCC-GGA-UUA-UAG.
_____________
_____________
_____________
_____________
_____________
Translating the Information in DNA
The process of taking the information in DNA and making a protein with it is a two step process. We have already looked at the first step (remember transcription where we made a
mRNA copy of DNA). This information must now be translated into the language of protein
synthesis (remember synthesis means creation). This happens in a process surprisingly
enough called translation. But what happens during translation?
After the mRNA is made, it leaves the nucleus and
moves to the cytoplasm. There the mRNA connects
to a ribosome. The ribosome is where the code is
read and translated to make proteins. During
translation, tRNA (transfer RNA) molecules act as
interpreters of the mRNA. Each tRNA molecule is
folded into a cloverleaf shape. At the middle of
the folded strand, there is a three-base coding
sequence called an anticodon. Each anticodon is
complementary (opposite) to a codon on the mRNA.
How is a protein made?
You can think of the ribosome as a factory for making
What determines the order of amino
acids in a protein?
proteins. We know that proteins are a polymer of smaller
subunits called amino acids. So, lets think of a protein as
a chain and the amino acids are the links that hold the
chain together.
As the mRNA attaches to a ribosome, the ribosome
What is the signal for the ribosome to
begin making a protein?
begins to look for a START codon (AUG) that tells it to
start making a protein.
A tRNA molecule with the
anticodon CAU (remember it is a complement—the
opposite of the codon) carrying a methionine will move in
and bind to the mRNA at that site.
Now that the ribosome knows to start, the process can
To what does the reading compare a
protein?
continue. A second tRNA, that is complementary to the
second mRNA codon now moves in. The ribosome bonds
the new amino acid to the chain. The process continues
as the ribosome then moves along the mRNA molecule,
What forms the links in the chain?
adding new amino acids (links) to the growing protein
chain in the sequence determined by the mRNA.
The ribosome continues moving along the mRNA, linking
amino acids together, until it reaches a stop codon ending
protein synthesis.
How does a ribosome know that it is
finished making a protein?
Page 294 of your textbook has a good example of how
this process works.
Where in the cell does transcription
take place? (page 2)
Look back at the Codon Chart on page 3. What
would happen if the DNA sequence was changed
from:
AGAUCGAGU
Where does translation take place?
to
ACAUCGAGU