30 Questions
What is the name of the enzyme that joins the Okazaki fragments?
DNA ligase
What is the problem with replicating the lagging strand at the end of a linear chromosome?
DNA polymerase cannot add to the end without a primer
What is the term for the region at the end of a eukaryotic chromosome?
Telomere
How are the leading and lagging strands replicated in relation to each other?
The leading and lagging strands are replicated coordinately by a replisome
What is the role of DNA polymerase I in lagging strand synthesis?
It removes the RNA primers and replaces them with DNA
What is the purpose of the RNA primer in lagging strand synthesis?
To initiate Okazaki fragment synthesis
What is the direction of DNA synthesis catalyzed by DNA polymerase?
5′ → 3′
What provides the energy input for polymerizing DNA?
dNTPs
How many replication bubbles are formed in bacterial cells during DNA replication?
One, at the origin of replication
What is the direction of movement of the replication forks during DNA replication?
Away from the origin of replication
How do eukaryotic cells differ from bacterial cells in terms of replication bubbles?
Eukaryotic cells have multiple replication bubbles, one on each chromosome
What is the role of DNA polymerase in DNA synthesis?
To catalyze the synthesis of DNA
What is the primary function of DNA helicase during DNA replication?
To separate the two strands of DNA
What is the role of single-strand DNA-binding proteins (SSBPs) during DNA replication?
To attach to separated DNA strands and prevent them from closing
What is the direction of synthesis for DNA polymerase?
5' → 3'
What is the limitation of DNA polymerase in terms of starting synthesis?
It can only extend from the 3' end of an existing strand
What is the result of 5' → 3' synthesis on the two strands of DNA?
The opposite direction of synthesis on the two strands
What is the purpose of Topoisomerase during DNA replication?
To relieve tension on the DNA molecule ahead of the fork
What is the consequence of single-stranded DNA being left at the end of the lagging strand during replication?
It would shorten the chromosome by 50 to 100 nucleotides each time replication occurs.
What is the primary function of telomerase?
To replicate telomeres, using an RNA template.
What is the purpose of the single-stranded 'overhang' formed during telomere replication?
It provides a binding site for telomerase.
What is the consequence of telomere shortening during replication?
Linear chromosomes would eventually vanish.
Where is telomerase primarily found?
In gametes and stem cells.
What is the end replication problem?
The shortening of chromosomes during replication.
What is the role of primase in DNA synthesis?
To synthesize RNA primers
In which direction is the leading strand synthesized?
5' → 3'
What is the name of the process by which the lagging strand is synthesized?
Discontinuous replication hypothesis
What is the function of DNA polymerase in leading strand synthesis?
To add dNTPs to the primer
What is the problem faced during telomere replication?
End replication problem
What is the replisome responsible for?
Coordinating leading and lagging strand synthesis
Study Notes
The End Replication Problem
- Single-stranded DNA is left at the end of the lagging strand during replication
- This single-stranded DNA is eventually degraded, which would shorten the chromosome by 50 to 100 nucleotides each time replication occurs
- Over time, linear chromosomes would vanish if not for the solution of telomeres
Telomeres and Telomerase
- Telomeres are regions at the end of eukaryotic chromosomes that do not contain genes
- Consist of short, repeating stretches of bases
- Telomerase solves the end replication problem by replicating telomeres using an RNA template
- Telomerase adds short DNA sequences to the end of the parent strand, preventing the shortening of telomeres
Effect of Telomere Length on Cell Division
- Telomerase is primarily found in gametes and stem cells, as well as in cancer cells
- Telomere length affects cell division, with shorter telomeres limiting the number of cell divisions
Lagging Strand Synthesis
- The lagging strand is synthesized as short, discontinuous fragments called Okazaki fragments
- DNA polymerase I removes the RNA primers and replaces them with DNA
- The enzyme DNA ligase joins the Okazaki fragments, making the lagging strand intact
DNA Replication
- DNA replication occurs in a 5' → 3' direction
- The leading strand is synthesized continuously in the 5' → 3' direction
- The lagging strand is synthesized discontinuously in the 5' → 3' direction
- Replication bubbles form at specific sequences, known as origins of replication, and grow in two directions
- The replisome is a large grouping of enzymes that coordinates DNA replication
Enzymes Involved in DNA Replication
- Helicase: separates the two strands of DNA
- Single-Strand Binding Proteins: holds separated single strands apart
- Topoisomerase: relieves tension on the DNA molecule ahead of the fork
- RNA primase: synthesizes a short piece of RNA (the primer)
- DNA polymerase III: adds nucleotides to the primer
- DNA polymerase I: a repair enzyme that removes the RNA primer
- DNA ligase: repairs the sugar-phosphate backbone
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