Biology 123 SI- Dr. Raut’s Class
Session Eighteen-04/01/2015
1. Briefly explain the process by which prokaryotic and eukaryotic cells perform DNA
replication. (not the details)
Prokaryotic cells have circular DNA. The process of replication begins at the origin of
replication which is a small specific sequence of nucleotides. From there replication extends out
forming a replication bubble and a replication fork which is the area where the two strands of
DNA are being separated so replication can occur. After all of the DNA has been replicated two
circular DNAs exist both having one parent strand and one new strand. Eukaryotic DNA
replication occurs on linear DNA which is much longer; therefore, there will be several origins
of replication, forming several replication bubbles. The replication bubbles eventually fuse
together forming two new DNA each having one parent strand and one new strand.
2. What are all of the enzymes used in DNA replication? What are their functions?
Helicase: separates the strands of DNA so replication can occur
Single-strand binding proteins- keeps the DNA strands from reconnecting after they are
separated
Topiosomerase- insures that the DNA that is not currently being replicated is not underwound or
overwound- makes sure the rest of the DNA is not affected by the separation at the replication
fork
RNA Primase- attaches a small segment of RNA between five and ten nucleotides long in the 5’
to 3’ direction so that DNA polymerase can add new DNA
Sliding clamp protein-assists the primer and polymerase III and ensures that everything stays in
place properly
DNA polymerase III: adds on new nucleotides
DNA ligase: seals together DNA segments
3. Explain in detail how the leading strand is synthesized.
RNA primase puts down a single RNA primer and then DNA polymerase III adds nucleotides on
to the 3’ end of the growing strand. *Only one RNA primer is needed to start the leading strand.
4. Explain in detail how the lagging strand is synthesized.
The main problem with the lagging strand is the fact that DNA polymerase can only add
nucleotides onto the 3’ end of the growing strand! Therefore, since the new strand needs to be
made in the 3’ to 5’ direction, it must be made in small segments. RNA primase will go up the
strand a ways and lay down an RNA primer. DNA polymerase III will come in and add DNA
until it reaches the next RNA primer. The RNA primer is removed and then DNA polymerase
adds on more nucleotides to replace the primer. DNA ligase then seals together the Okazaki
fragments.
5. What form are new nucleotides in when they first come into DNA replication?
New nucleotides come in the form of DNTP (Deoxyribose nucleotide triphosphate). AKA: ATP,
GTP, CTP, or TTP. Only one phosphate is needed in the strand, so two phosphates will be
removed before the nucleotide is added to the growing DNA strand.
*Think about it. This uses up a bunch of energy.
6. How are mistakes corrected in DNA replication?
Often polymerase is able to correct mistakes as they occur but not always. If a problem does
occur, mismatch repair enzymes will come in to fix the problem. Nuclease cuts out the faulty
sequence. Then, DNA polymerase lays down the correct sequence. Then, DNA ligase seals the
segments together.
7. What are telomeres, and what is their function?
Telomeres are ends of DNA which do not code for genes, but instead function to ensure that the
important information is not lost during DNA replication. Since DNA polymerase can only add
to the 3’ end of the growing strand, there will always be a very small piece at the very end of the
3’ end of the template strand that cannot be replicated. Telomeres come in the form of the
repeating sequence TTAGGG.
8. How is DNA packaged in the cell?
DNA is very big and therefore has to be packaged very neatly in order to fit in the nucleus. DNA
is wrapped around proteins called histones. There are four different histones. Two of each of the
histones come together to make a nucleosome. DNA wraps twice around the nucleosome and
then moves to another nucleosome. The DNA between nucleosomes is called linker DNA. The
DNA will then further coil forming looped domains which will eventually form chromatin.
9. What is the Central Dogma of genetics?
DNA
Transcription
RNA
Translation
Proteins
10. How does protein synthesis differ between prokaryotic and eukaryotic cells?
In prokaryotic cells, transcription occurs in the cytoplasm and immediately produces mRNA
which can be sent to ribosomes immediately. In eukaryotic cells, transcription occurs in the
nucleus and forms pre-mRNA which contains both introns and exons. The pre-mRNA then has a
5’cap and a poly A tail added on to the ends and the introns cut out.
11. What is the function of mRNA and tRNA?
mRNA codes for proteins. It will go into a ribosome where its codons will match up with
anticodons that code for specific amino acids. tRNA brings in the amino acids during protein
synthesis. The tRNA has the anticodon.
12. How many codons are there? How many nucleotides per codon? There are four codons you
should know; what are they, and what do they mean? Why are there so many codons and only 20
amino acids?
There are 64 codons. There are three nucleotides per codon. The four codons you should
remember are AUG which is a start codon and codes for methionine and UAA, UGA, and UAG
which are stop codons. In a codon the first two nucleotides are the most important. The last
nucleotide is usually, but not always, flexible. For example, AGG and AGA code for the same
amino acid.
13. Codons run complementary to the _______Template_______ strand.
14. What are the three steps of transcription?
Initiation, elongation and termination
15. What are the promotor and the transcription factors? What enzyme transcribes DNA to
mRNA?
The promotor is the TATA box and is where RNA polymerase II will bind. Transcription factors
are special proteins that help RNA polymerase bind easier. RNA polymerase II. RNA
polymerase I and III exist, but they code for other types of RNA.