Download Lecture 4, Exam III Worksheet Answers

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Lecture 4, Exam III
Worksheet
1. What is the purpose of telomeres? What type of cells are they most important within? What
types of cells are they least important in? What enzyme creates telomeres and how? What is
special about the enzyme that allows it to carry out its function?
There is a portion of a cell’s complementary DNA that DNA polymerases can’t
replicate. DNA polymerase can only add nucleotides to 3’ ends, and even when there is an
Okazaki fragment and it is replaced on the lagging strand, there is no way to replicate the 3’
tip of the original DNA strand, (because when the primer is made for it and removed there is
no 3’ end for the DNA to attach to). So whenever the primer is removed, there is an empty
area that cannot be filled; therefore, as replication continues, and the DNA continues to get
shorter. There is no way to complete the 5’ ends of daughter strands. Telomeres are
repeated sequences of DNA that do not encode for anything; therefore, the genes in the DNA
that encode for proteins are protected. These are most important within gametic cells and
stem cells. They are least important in skin cells and cells that are replicated very often.
Telomerase catalyzes the lengthening of telomeres in these important cells. It is a form of
DNA polymerase and has its own built in primer.
2. What type of mutation does the sun cause? What type of bond is found within the mutation?
What enzyme is able to repair this? What can the enzyme cleave? What other enzymes may
have to be involved in this process?
Thymine dimers within DNA. Two thymines beside each other are bonded
together by a covalent bond. Nuclease. It is an enzyme that cleaves nucleic acids. Other
enzymes that may be needed in this process are the enzymes needed to create DNA and link
it together: primase, both DNA polymerases, and ligase.
3. How many times does DNA wrap around histones and how many of these histones make up
a nucleosome? What is the next step? How are “scaffolds” involved? Then what happens to
the scaffolds?
Two times; seven histones with DNA wrapped around them make up a
nucleosome. Nucleosomes then are wound around H1, a histone, which is like a scaffold. The
scaffolds are then wound up on top of each other .
4. Describe the structure of chromatin/DNA mentioned in #3 at each part of the cell cycle
below:
Part of Cell Cycle:
How DNA is arranged:
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When DNA is expressed
Interphase
Prophase
Prometaphase; metaphase
Unraveled from histones.
Nucleosomes, (like beads on a string), are
twisted up to 30 nanometers.
Nucleosomes are looped on
H1(scaffolds).
Scaffolds are looped together
5. How does transcription/translation differ between eukaryotes and prokaryotes?
6. Put the order of events in correct order within eukaryotes:
1 a. DNA is transcribed into pre-mRNA
3b. mRNA leaves the nucleus through the nuclear pore within the nuclear envelope.
4 c. the mRNA is translated into a polypeptide within the cytoplasm.
2 d. pre-mRNA is processed into mRNA by adding a poly-A tail. splicing the introns
out, and adding the 5’ cap.
5 e. The polypeptide encodes for physiological reactions within the body.
7. What enzyme transcribes DNA into RNA? How does it differ from DNA polymerase? What
does it NOT need, such as enzymes? What does it need in terms of knowing where to
start/stop transcribing? What direction does it work in?
RNA polymerase; larger than DNA polymerase, can take two nucleotides and
add them together. Can make the beginning of a nucleotide without needing primase to
make a primer; doesn’t need topoisomerase; doesn’t need helicase, because it has its own
helicase activity; doesn’t need ssb proteins; and doesn’t need ligase.
8. What do transcription factors do? How many different kinds are there?
In the promoter, which is the area of DNA that the RNA polymerase attaches
to, there are specific sequences, including the TATA Box. The transcription factors recognize
this sequence and bind to it bringing in more transcription factors and the RNA polymerase.
There are transcription factors for every type of specific gene.
9. What is the 3’ UTR and the 5’ UTR? What are the purposes of them?
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10. Describe how pre-mRNA is processed.