Florida International University CHM 4220 EXAM 2 Chapter 12 Q&A
Fall 2023/2024.
1. What are the two major conditions for genetic information being accurately copied every time a cell divides?
1. it has to be extremely accurate. 2. it has to take place at a high speed.
2. What were 3 models of DNA replication proposed and which one was confirmed valid?
1. conservative replication model 2. dispersive replication model 3. semiconservative replication model
semiconservative replication model was confirmed valid.
. How Meselson and Stahl proved correct DNA replication model?
they used two nitrogen isotopes, grew E. coli in 15N media first, then moved to 14N media. Cultured E.coli were
subjected to equilibrium density gradient centrifugation
3. What is replicon and the major characteristics of 3 modes of DNA replication?
Replicon is a segment of DNA that undergoes replication. Each replicon contains an origin of replication.
Theta Replication, Rolling Circle Replication and Linear Eukaryotic Replication
Theta: Double-stranded DNA unwinds at the replication origin producing single-starnded templates for the synthesis
of new DNA. A replication bubble forms, usually with a replication fork at each end. The forks proceed around the
circle. Eventually 2 circular DNA molecules are produced.
Rolling Circle: (virus/ F factor) Replication is initiated by a break in one of the nucleotides strands. DNA synthesis
begins at the 3” end of the broken strand, the inner strand is used as a template. The 5” end of the broken strand is
displaced. Cleavage releases a single-stranded linear DNA and a double-stranded circular DNA. The linear DNA may
circularize and serve as a template for synthesis of a complementary strand. The product is one circular molecule and
one linear molecule that may circulize.
Linear Eukaryotic: Each chromosome contains numerous origins. At each origin the DNA unwind, producing a
replication bubble. DNA synthesis takes place on both strands at each end of the bubble as the replication forks
proceed outward. Eventually, the forks of adjacent bubbles run into each other and the segments of DNA fuse,
producing two identical linear DNA molecules.
. How would you define the major difference between theta and rolling circle replication?
Theta creates a bubble; it can be either bi or unidirectional. Rolling circle is only unidirectional and is replicated by
peeling off a strand of DNA during its replication.
. Is linear replication bidirectional or unidirectional? Explain.
Bidirectional because it has multiple replicons, multiple origins of replication.
4) What are the requirements for DNA replication?
A template strand, Raw material: nucleotides, Enzymes and other proteins.
. What serve as template?
DNA is a double helix. So, during replication it unwinds and separates into 2 strands. 1 strand will be called the non-
coding and the other will be called the coding strand. The non-coding strand will be the template 3’ to 5’for the new
one.
, . What are substrates?
2'-deoxynucleoside triphosphates (dNTPs)
. Which enzymes are required?
DNA polymerases, DNA primase, DNA helicase, DNA ligase, and topoisomerase.
5) How DNA replication proceeds?
The opening of the double helix and separation of the DNA strands, the priming of the template strand, and the
assembly of the new DNA segment.
. Which direction it proceeds?
5’ to 3’
. What is leading and lagging strand?
Leading strand synthesized continuously in the 5’ to 3’ direction but is exposed in the 3’ to 5’. The lagging strand
synthesized discontinuously opposite to the unwinding in the 5’ to 3’ direction but is exposed in the 5’ to 3’.
. What are Okazaki fragments?
continuously synthesized short DNA fragments forming the lagging strand.
6) Steps in bacterial DNA replication
. What initiate DNA replication?
an initiator protein DnaA in E.coli, 245 bp in the oriC (single origin replicon)
. Which components are necessary for DNA unwinding?
•Initiator protein • DNA helicase • Single-strand-binding proteins (SSBs) • DNA gyrase (topoisomerase)
. What are the primers and enzyme that adds the primers?
Primers are existing group of RNA nucleotides with a 3’-OH group to which a new nucleotide can be added; they are
usually 10–12 nucleotides long, they are synthesized at the beginning of every Okazaki fragment. The enzyme that
adds the primers are Primase.
. Which enzyme is involved in elongation, primer removal, and DNA ligation?
Elongation is carried out by DNA polymerase III, Removing RNA primer by DNA polymerase I and DNA ligation is
by DNA ligase.
. What is proofreading capability and which enzymes possess it?
DNA polymerase I: 3’ to 5’. exonuclease activity removes the incorrectly paired nucleotide.
Fall 2023/2024.
1. What are the two major conditions for genetic information being accurately copied every time a cell divides?
1. it has to be extremely accurate. 2. it has to take place at a high speed.
2. What were 3 models of DNA replication proposed and which one was confirmed valid?
1. conservative replication model 2. dispersive replication model 3. semiconservative replication model
semiconservative replication model was confirmed valid.
. How Meselson and Stahl proved correct DNA replication model?
they used two nitrogen isotopes, grew E. coli in 15N media first, then moved to 14N media. Cultured E.coli were
subjected to equilibrium density gradient centrifugation
3. What is replicon and the major characteristics of 3 modes of DNA replication?
Replicon is a segment of DNA that undergoes replication. Each replicon contains an origin of replication.
Theta Replication, Rolling Circle Replication and Linear Eukaryotic Replication
Theta: Double-stranded DNA unwinds at the replication origin producing single-starnded templates for the synthesis
of new DNA. A replication bubble forms, usually with a replication fork at each end. The forks proceed around the
circle. Eventually 2 circular DNA molecules are produced.
Rolling Circle: (virus/ F factor) Replication is initiated by a break in one of the nucleotides strands. DNA synthesis
begins at the 3” end of the broken strand, the inner strand is used as a template. The 5” end of the broken strand is
displaced. Cleavage releases a single-stranded linear DNA and a double-stranded circular DNA. The linear DNA may
circularize and serve as a template for synthesis of a complementary strand. The product is one circular molecule and
one linear molecule that may circulize.
Linear Eukaryotic: Each chromosome contains numerous origins. At each origin the DNA unwind, producing a
replication bubble. DNA synthesis takes place on both strands at each end of the bubble as the replication forks
proceed outward. Eventually, the forks of adjacent bubbles run into each other and the segments of DNA fuse,
producing two identical linear DNA molecules.
. How would you define the major difference between theta and rolling circle replication?
Theta creates a bubble; it can be either bi or unidirectional. Rolling circle is only unidirectional and is replicated by
peeling off a strand of DNA during its replication.
. Is linear replication bidirectional or unidirectional? Explain.
Bidirectional because it has multiple replicons, multiple origins of replication.
4) What are the requirements for DNA replication?
A template strand, Raw material: nucleotides, Enzymes and other proteins.
. What serve as template?
DNA is a double helix. So, during replication it unwinds and separates into 2 strands. 1 strand will be called the non-
coding and the other will be called the coding strand. The non-coding strand will be the template 3’ to 5’for the new
one.
, . What are substrates?
2'-deoxynucleoside triphosphates (dNTPs)
. Which enzymes are required?
DNA polymerases, DNA primase, DNA helicase, DNA ligase, and topoisomerase.
5) How DNA replication proceeds?
The opening of the double helix and separation of the DNA strands, the priming of the template strand, and the
assembly of the new DNA segment.
. Which direction it proceeds?
5’ to 3’
. What is leading and lagging strand?
Leading strand synthesized continuously in the 5’ to 3’ direction but is exposed in the 3’ to 5’. The lagging strand
synthesized discontinuously opposite to the unwinding in the 5’ to 3’ direction but is exposed in the 5’ to 3’.
. What are Okazaki fragments?
continuously synthesized short DNA fragments forming the lagging strand.
6) Steps in bacterial DNA replication
. What initiate DNA replication?
an initiator protein DnaA in E.coli, 245 bp in the oriC (single origin replicon)
. Which components are necessary for DNA unwinding?
•Initiator protein • DNA helicase • Single-strand-binding proteins (SSBs) • DNA gyrase (topoisomerase)
. What are the primers and enzyme that adds the primers?
Primers are existing group of RNA nucleotides with a 3’-OH group to which a new nucleotide can be added; they are
usually 10–12 nucleotides long, they are synthesized at the beginning of every Okazaki fragment. The enzyme that
adds the primers are Primase.
. Which enzyme is involved in elongation, primer removal, and DNA ligation?
Elongation is carried out by DNA polymerase III, Removing RNA primer by DNA polymerase I and DNA ligation is
by DNA ligase.
. What is proofreading capability and which enzymes possess it?
DNA polymerase I: 3’ to 5’. exonuclease activity removes the incorrectly paired nucleotide.
