D1.1 Flashcards
DNA Replication
Outline the purposes of DNA replication
Required for reproduction and for growth and tissue replacement in multicellular organisms
Describe the meaning of semi conservative in relation to DNA replication.
Each strand in the DNA double helix serves as a template for the synthesis of a new, complementary strand and the new double helix contains one strand of conserved DNA and one strand of new DNA
What types of molecules are important for the process of DNA replication?
- enzymes needed for replication
- free nucleotides
Explain the role of complementary base pairing in DNA replication
allows the base pairs to be arranged in a more energetically favourable way and when the double helix is “unzipped” each strand can be used as a template for another double-stranded DNA molecule
Why must DNA strands be separated prior to replication?
This separation allows the hydrogen-bond donor on each DNA base to become exposed for base-pairing with the appropriate incoming free nucleotide
Outline the role of helicase
Breaks hydrogen bonds between anti parallel strands of DNA, and uncoils DNA
Outline the role of DNA polymerase
Adds nucleotides in a 5’ to 3’ direction
Why are there separate DNA polymerases for each strand of template DNA
Because of the antiparallel orientation of the two DNA strands in the DNA double helix this would require one daughter strand to go in the 5′-to-3′ direction and the other in the 3′-to-5′ direction.
Outline the key features of DNA polymerases:
- Always requires a template
- Require a short stretch of nucleotides (primer)
- Proofread and remove wrong nucleotides
Where does the energy for adding nucleotides come from?
From the nucleotides themselves, which have three phosphates attached. When the phosphate bond is broken, the energy released is used to form a bond between the incoming nucleotides and the growing chain.
Describe the formation of the covalent bond between the adjacent nucleotides during DNA replication.
Adjacent nucleotides are linked by a phosphodiester bond.
Describe the formation of a phosphodiester bond in DNA
forms between a (-OH) group on the 3’ carbon of deoxyribose and the (-PO4) group attached to the 5’ carbon of deoxyribose
State that DNA polymerases can only add free nucleotides to an existing DNA strand
DNA polymerases can only add nucleotides to the 3’ end of an existing DNA strand. (They use the free -OH group found at the 3’ end as a “hook,” adding a nucleotide to this group in the polymerization reaction.)
Explain the difference between replication on the leading versus lagging strands of DNA
The formation of the lagging strand involves fragments and is discontinuous.
The leading strand is assembled continuously toward the progressing replication fork.
The lagging strand is assembled by the production of fragments moving away from the progressing replication fork.
The fragments of the lagging strand are called Okazaki fragments.
Explain the need for RNA primers in DNA replication
The synthesis of a primer is necessary because the enzymes that synthesize DNA, which are called DNA polymerases, can only attach new DNA nucleotides to an existing strand of nucleotides.
Compare the number of RNA primers on the leading and lagging strands
The leading strand can be extended from one primer alone, whereas the lagging strand needs a new primer for each of the short Okazaki fragments.
Outline the function of DNA primase
Synthesises RNA primer
Outline the function of RNA primer
Marks the beginning of the replication process
Outline the function of DNA polymerase I
Removes RNA primer at end of replication and replaces it with the DNA nucleotides
Outline the function of DNA polymerase III
Synthesises the new strand by catalysing the addition of nucleotides in a 5’ to 3’ direction
Outline the function of DNA ligase
Joins the ends of DNA segments and Okazaki fragments
Explain why there are gaps between adjacent Okazaki fragments on the lagging strand
The purpose of Okazaki fragments is to allow DNA polymerase to synthesize the lagging strand in segments since it is not oriented correctly for continuous synthesis.
Describe what happens during termination
Primers are removed and replaced with new DNA nucleotides and the backbone is sealed by an enzyme called DNA ligase
What is the role of SSB (single-stranded binding proteins)
After binding single-stranded DNA, SSB destabilizes helical duplexes, thereby allowing DNA polymerases to access their substrate more easily.
Describe the function of topoisomerase
to unwind the chromosomes and DNA double-helix by creating small, reversible cuts in the DNA.
