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Transcript
Muma
Bio 6
Name:________________
Lab Section:_______________
DNA REPLICATION, PROTEIN SYNTHESIS AND MUTATIONS
Deoxyribonucleic acid (DNA) controls the cell's activities by directing the
formation of specific kinds of proteins in specific quantities. Proteins function as
enzymes, as transport molecules in membranes, as regulatory proteins, as receptor
molecules, etc. In fact, it is proteins that are responsible for the activities of cells
and give cells their distinctive features. Any section of the DNA molecule that
codes for a specific protein is called a gene.
The first step in making a protein is called transcription. During transcription,
enzymes copy pieces of DNA to produce molecules of RNA called messenger
RNA (mRNA). mRNA contains information, coded in the base sequences, for
making proteins.
The second step in protein synthesis is called translation. During translation the
information carried by the mRNA molecules is deciphered by the ribosome. The
ribosome helps to translate this message into a sequence of amino acids.
Sequences of amino acids are called proteins.
The DNA is contained within the nucleus, but the cellular machinery (ribosomes)
and raw materials (amino acids) needed to create proteins from this information
are located in the cytoplasm. How does the information from DNA get into the
cytoplasm? This laboratory will help you understand this process.
Activity 1: DNA REPLICATION
1) Construct the complementary strand of DNA (strand II) that would pair
with the DNA strand I sequence below. Remember that nucleotides are
always added in the 5 prime to three prime direction. The base pairing rules
are:
DNA
DNA
adenine (A) always bonds with thymine (T)
thymine (T) always bonds with adenine (A)
guanine (G) always bonds with cytosine (C)
cytosine (C) always bonds with guanine (G).
STRAND I: 3’ T A C T A T
STRAND II: 5’
AGT CTG TCT CCC ACT
5’
3’
2) Check to make sure you have constructed the proper DNA complementary
strand. In the living cell an enzyme known as DNA polymerase has the
ability to "proofread" just as you are doing. (enzymes names always end in
"ase").
3). Now you have successfully replicated a segment of DNA! Answer the
following questions below:
a. When does DNA replication occur? And why?
b. What are the functions of the following enzymes in DNA replication?
Helicase
Primase
DNA polymerase III
DNA polymerase I
Ligase
2
PROTEIN SYNTHESIS
ACTIVITY 2: TRANSCRIPTION
When a cell is carrying out its daily functions it must produce certain proteins to
function. DNA serves as the blueprint for determining the order in which amino
acids are placed within a protein. To do this DNA must first be transcribed into
mRNA. This process is called transcription.
1)
Beginning at the TAC end of DNA strand I, construct the mRNA according
to the DNA-RNA base pairing rules. DNA strand I is called the template
strand. RNA polymerase transcribes the message from one strand of the
DNA molecule into a single stranded mRNA molecule. The rules for
forming mRNA are the same as for DNA except that uracil (U) is used in
place of thymine (T). Therefore:
DNA
RNA
adenine (A) always bonds with uracil (U)
thymine (T) always bonds with adenine (A)
guanine (G) always bonds with cytosine (C)
cytosine (C) always bonds with guanine (G).
Transcribe you DNA strand I from the first page below. Remember that the
mRNA nucleotides are added in the 5’ to 3’ direction:
DNA strand I: 3’ T A C T A T
AGT CTG TCT CCC ACT
5’
mRNA strand:
2)
Check the sequence of bases on the mRNA molecule against the template
strand of the DNA. The mRNA molecule is now ready to move from the
nucleus into the cytoplasm where its message will be translated. Before
you do this, answer the following questions:
a) Why do you think mRNA can get out of the nucleus and DNA
cannot?
3
b) Where does transcription occur in the cell?
Activity 3:
1)
TRANSLATION
Every three nucleotides in your mRNA strand is a codon that codes for one
amino acid in your protein that you are making. Below please write down
the codons for your mRNA strand that you transcribed in Activity 2 above.
I have gotten you started by doing your first codon for you, now you
continue with the other codons.
Another type of RNA molecule is responsible for transferring amino acids
to the ribosome. This transfer RNA (tRNA) has an anticodon end which
base pairs with the mRNA codon. Determine the anticodon sequence for
the transfer RNA (tRNA) that would pair with each mRNA codon and write
it below. Your tRNA molecules bind to each codon so that the correct
amino acid is delivered in the proper order to make your protein. Use the
genetic code chart on the last page of this handout to determine which
amino acid is coded for by each mRNA codon and carried on your tRNA.
Note: You always use the codon to determine the amino acid not the
anticodon!
amino acids:
Methionine
tRNA anticodons:
UAC
mRNA codons:
AUG
4
2)
The amino acid sequence you constructed above is your final protein that
was encoded in your original DNA molecule! Now answer the following
questions:
a) What role does the ribosome play in protein synthesis?
b) What role does tRNA play in protein synthesis?
c) What role does mRNA play in protein synthesis?
d) Where does translation occur in the cell?
e) Write a short summary, in your own words, of the processes of
transcription and translation in protein synthesis.
5
Activity 4:
MUTATIONS
The goal of the following activity is to understand what might happen if a small
change occurred in the DNA base sequence. A mutation is any change in the
base sequence of a DNA molecule. Fill in the table below by starting with the
complete DNA base sequence you started with in Activity 1. See the first row
which I have filled in for you and follow it as an example. In the proceeding rows
make any changes you wish to the original sequence of DNA in the first box
(point, insertion and deletion mutation). Then figure out the mRNA sequence and
how that would change your final protein’s amino acid sequence. Compare the
resulting protein sequences and answer the questions that follow.
DNA base sequence
TAC TAT AGT CTG
mRNA base sequence
AUG AUA UCA GAC
Amino acid sequence
Met- Iso-Ser-Asp
__________________________________________________________________
Point mutation:
__________________________________________________________________
Insertion:
__________________________________________________________________
Deletion:
a) Did all of the final proteins change? How could you tell?
b)
What is a silent mutation?
c) What is meant by the "redundancy" of the genetic code?
6
How to read this chart: you are going to move from the center out. The big
central letters is the first nucleotide in your codon. Then move out from that first
letter to the second ring and find the letter of the second nucleotide in your codon,
and then to the third outer ring for the third nucleotide in your codon. For example
if my codon was GCA, find G in the center, C in the second ring from there, and
then A in my third ring, the amino acid would be Alanine.
7