DNA replication
55 flashcards covering DNA replication for the MCAT Biology & Biochemistry section.
DNA replication is the biological process by which a cell copies its DNA molecule to produce an identical version, ensuring that genetic information is passed accurately to new cells. Think of DNA as a blueprint for all living things; replication acts like making a perfect photocopy before a cell divides, which is essential for growth, repair, and reproduction. This process involves unwinding the DNA double helix and using each strand as a template to build a new complementary strand, with specific enzymes coordinating the steps to maintain accuracy.
On the MCAT, DNA replication appears in biology and biochemistry questions, often as multiple-choice items testing mechanisms, enzymes like DNA polymerase and helicase, or the semi-conservative model. Common traps include mistaking the leading strand for the lagging strand or overlooking error correction, which can lead to questions about mutations. Focus on key details like initiation, elongation, and termination, as well as how replication ties into genetics and cell cycles.
Practice drawing the replication fork to visualize the process.
Terms (55)
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DNA Replication
DNA replication is the biological process in which a DNA molecule is copied to produce two identical DNA molecules, occurring during the S phase of the cell cycle to ensure genetic information is passed to daughter cells.
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Semiconservative Replication
Semiconservative replication is the mechanism of DNA copying where each new DNA molecule consists of one original strand and one newly synthesized strand, as demonstrated by Meselson and Stahl's experiments.
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Origin of Replication
The origin of replication is a specific sequence on a DNA molecule where replication begins, serving as the starting point for unwinding the double helix and initiating DNA synthesis.
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Replication Fork
A replication fork is the Y-shaped region on a DNA molecule where the double strands have separated and new strands are being synthesized, formed by the action of helicase.
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Leading Strand
The leading strand is the DNA strand synthesized continuously in the 5' to 3' direction towards the replication fork, allowing for uninterrupted addition of nucleotides.
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Lagging Strand
The lagging strand is the DNA strand synthesized discontinuously in short segments called Okazaki fragments in the 5' to 3' direction, away from the replication fork.
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DNA Polymerase
DNA polymerase is the enzyme that synthesizes new DNA strands by adding nucleotides to the 3' end of a growing chain, requiring a primer and ensuring fidelity through proofreading.
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Helicase
Helicase is an enzyme that unwinds the double-stranded DNA at the replication fork by breaking hydrogen bonds between base pairs, using ATP as an energy source.
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Primase
Primase is the enzyme that synthesizes a short RNA primer on the DNA template, providing a starting point for DNA polymerase to begin adding nucleotides.
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RNA Primer
An RNA primer is a short segment of RNA synthesized by primase that serves as the initial binding site for DNA polymerase during DNA replication.
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Okazaki Fragments
Okazaki fragments are short segments of DNA synthesized on the lagging strand, later joined together by DNA ligase to form a continuous strand.
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DNA Ligase
DNA ligase is the enzyme that joins the sugar-phosphate backbones of adjacent DNA fragments, such as Okazaki fragments, by forming phosphodiester bonds.
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Topoisomerase
Topoisomerase is an enzyme that relieves supercoiling ahead of the replication fork by cutting and rejoining DNA strands, preventing tangling during unwinding.
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Single-Strand Binding Proteins
Single-strand binding proteins stabilize the unwound single strands of DNA at the replication fork, preventing them from reannealing until replication is complete.
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Initiation of DNA Replication
Initiation of DNA replication involves the recognition of the origin of replication by proteins like the origin recognition complex, leading to the assembly of the pre-replication complex.
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Elongation in DNA Replication
Elongation is the phase of DNA replication where nucleotides are added to the growing strands by DNA polymerase, occurring at the replication fork in a 5' to 3' direction.
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Termination of DNA Replication
Termination of DNA replication occurs when the replication forks meet at the end of the DNA molecule, followed by the separation of the two daughter molecules.
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Proofreading in DNA Replication
Proofreading is the error-checking mechanism of DNA polymerase that removes incorrectly paired nucleotides by its 3' to 5' exonuclease activity, maintaining replication fidelity.
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' to 3' Directionality
DNA synthesis occurs in the 5' to 3' direction, meaning nucleotides are added to the 3' end of the growing strand, due to the chemical structure of DNA polymerase.
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Antiparallel Strands
Antiparallel strands refer to the opposite orientations of the two DNA strands, one running 5' to 3' and the other 3' to 5', which affects how replication proceeds.
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Replication Bubble
A replication bubble is the region of unwound DNA formed at the origin of replication in eukaryotes, where bidirectional synthesis occurs from multiple origins.
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Eukaryotic DNA Replication
Eukaryotic DNA replication involves multiple origins per chromosome, occurs in the nucleus, and is regulated by cell cycle checkpoints to ensure accuracy.
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Prokaryotic DNA Replication
Prokaryotic DNA replication features a single origin per circular chromosome, is faster than in eukaryotes, and lacks histones, allowing for quicker cell division.
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Telomeres
Telomeres are repetitive sequences at the ends of linear chromosomes that protect them from degradation during replication, shortening with each cell division.
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Telomerase
Telomerase is an enzyme that adds telomeric repeats to chromosome ends in certain cells, counteracting telomere shortening and enabling continued replication.
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DNA Replication Fidelity
DNA replication fidelity is the accuracy of copying, achieved through base pairing rules, proofreading, and repair mechanisms, with an error rate of about one in a billion nucleotides.
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Pre-Replication Complex
The pre-replication complex is a group of proteins assembled at the origin of replication that prepares the DNA for unwinding and synthesis.
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Origin Recognition Complex
The origin recognition complex is a multi-protein structure in eukaryotes that binds to the origin of replication and recruits other initiation factors.
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DNA Unwinding
DNA unwinding is the process of separating the double helix into single strands at the replication fork, facilitated by helicase and ATP hydrolysis.
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Primer Removal
Primer removal is the step where RNA primers are excised by enzymes like DNA polymerase I in prokaryotes, and the gaps are filled with DNA nucleotides.
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Gap Filling
Gap filling occurs after primer removal, where DNA polymerase adds nucleotides to fill the single-nucleotide gaps before ligation seals the backbone.
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Ligation Process
Ligation is the enzymatic joining of DNA fragments by DNA ligase, using energy from ATP or NAD+ to form covalent bonds in the sugar-phosphate backbone.
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Energy Requirements for Replication
Replication requires energy from ATP for processes like helicase unwinding and nucleotide activation, ensuring the synthesis of new strands.
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Common Trap: Leading vs. Lagging
A common error is confusing the leading strand's continuous synthesis with the lagging strand's discontinuous fragments; remember, both are synthesized 5' to 3'.
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Strategy for Remembering Replication
To remember DNA replication steps, visualize the replication fork: helicase unwinds, primase adds primers, polymerase extends strands, and ligase joins fragments.
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Replication in Circular DNA
In circular DNA like bacterial chromosomes, replication starts at a single origin and proceeds bidirectionally until the forks meet, forming theta structures.
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Worked Example: Okazaki Fragments
In a worked example, if the lagging strand has a template from 5' to 3', primase adds a primer, polymerase synthesizes a 1000-nucleotide Okazaki fragment, then repeats for the next.
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Role of Nucleotides
Nucleotides are the building blocks of DNA, providing the bases, sugars, and phosphates needed for polymerase to extend the new strand during replication.
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Deoxyribonucleotides
Deoxyribonucleotides are the specific nucleotides used in DNA replication, lacking an oxygen on the 2' carbon of the sugar, distinguishing them from ribonucleotides.
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DNA Repair Mechanisms
DNA repair mechanisms, such as mismatch repair, correct errors from replication by identifying and replacing mismatched bases to maintain genetic integrity.
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Mismatch Repair
Mismatch repair is a post-replication process that detects and fixes incorrectly paired bases, using nearby sequences to identify the new strand.
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Base Excision Repair
Base excision repair removes and replaces damaged bases that could arise from replication errors or environmental factors, involving glycosylases and polymerases.
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Histone Role in Replication
Histones package DNA in eukaryotes and must be disassembled and reassembled during replication to allow access to the template strands.
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Checkpoint Controls
Checkpoint controls in the cell cycle halt replication if DNA damage is detected, ensuring that faulty DNA is not passed to daughter cells.
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Cdc6 and Cdt1
Cdc6 and Cdt1 are proteins that help load the MCM helicase complex onto DNA origins, regulating the initiation of replication in eukaryotes.
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Strand Separation
Strand separation is the initial step at the replication fork where helicase pulls apart the DNA double helix, exposing templates for synthesis.
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Inhibitors of DNA Replication
Inhibitors like antibiotics can block replication enzymes, such as DNA gyrase in bacteria, leading to halted cell growth and death.
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Polymerase Chain Reaction
Polymerase chain reaction is a lab technique that amplifies DNA segments through repeated cycles of denaturation, annealing, and extension, mimicking replication.
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Telomere Shortening Effects
Telomere shortening with each replication cycle can lead to cellular aging or senescence, as critically short telomeres trigger DNA damage responses.
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Bidirectional Replication
Bidirectional replication means synthesis proceeds from the origin in both directions, common in both prokaryotes and eukaryotes for efficiency.
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Error Rate in Replication
The natural error rate in DNA replication is low, around 1 in 10^9 to 10^10 bases, due to proofreading and repair, but mutations can still occur.
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Regulation of Replication
Regulation of replication ensures it happens once per cell cycle, controlled by licensing factors that prevent re-replication until the next cycle.
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DNA Gyrase
DNA gyrase is a type of topoisomerase in prokaryotes that introduces negative supercoils to relieve tension during rapid replication.
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Exonuclease Activity
Exonuclease activity in DNA polymerase removes mismatched nucleotides from the 3' end, a key part of the proofreading process during replication.
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Replication Licensing
Replication licensing involves proteins that mark origins for a single round of replication per cell cycle, preventing over-replication.