DNA Replication and Repair

DNA pairing rules help guide DNA repair. In DNA, bases pair in a specific way, and those matching pairs support accurate copying and fixing of the molecule.

When DNA is damaged or copied incorrectly, repair systems can use the normal base-pairing pattern as a reference. This helps cells find mismatched bases and restore the correct sequence. The process depends on the idea that each base has a preferred partner.

Understanding base-pairing is important because it explains how cells protect...

The DNA replication fork is the active region where DNA copies itself. It forms when the two strands of the double helix separate so each strand can serve as a template for a new partner strand.

At the fork, the copying process moves in a set direction. The two strands are built in different ways as replication continues, so the fork helps organize how the new DNA is made.

The replication fork is a key part of accurate DNA duplication. It shows how the cell opens the double helix, copies the...

Lagging strand synthesis builds new DNA in short pieces called Okazaki fragments. This happens during DNA replication because the lagging strand runs in the opposite direction from the direction the replication machinery moves.

As the DNA double helix opens, DNA polymerase adds nucleotides only in the 5' to 3' direction. On the lagging strand, this forces the cell to copy the template in segments instead of one continuous stretch. Each segment begins near the replication fork and extends away...

The replisome is the protein machine that copies DNA during cell division. It works at the replication fork, where the double helix opens so new strands can be made.

DNA polymerase is the core enzyme in the replisome. It adds nucleotides to the growing DNA strand. In bacteria, two DNA polymerases work together at the fork. These enzymes are linked to the helicase by a clamp-like protein so they stay attached to the DNA.

Helicase unwinds the DNA ahead of the fork. Single-stranded binding...

DNA polymerase uses proofreading to help correct errors during DNA replication. As it builds the daughter strand, the enzyme adds nucleotides that match the template DNA strand. This matching helps keep DNA copying accurate and supports replication fidelity.

DNA is made in the 5' to 3' direction, and several DNA polymerases help with this process. In nuclear DNA replication, DNA polymerase delta and DNA polymerase epsilon have proofreading ability. After a new base is added, these polymerases...

Prokaryotic DNA copies itself through a set of coordinated steps called initiation, elongation, and termination. In bacteria, replication starts when initiator proteins bind to a single origin of replication, or ori, on the circular chromosome. The DNA is then copied in both directions until two DNA molecules are made.

Several proteins help open and stabilize the DNA during replication. Topoisomerase cuts one side of the double-stranded phosphate-sugar backbone so the helix can unwind faster.

Eukaryotic DNA replication copies the genome before cell division. It happens in a tightly regulated way so each daughter cell gets a complete set of chromosomes in the nucleus. The process has three stages: initiation, elongation, and termination.

Eukaryotic cells use many proteins to manage replication because their chromosomes are linear and their genomes are larger than those of prokaryotes. Replication begins at multiple origins of replication along each chromosome. Initiator proteins...

Telomeres and telomerase help protect chromosomes during DNA replication. Telomeres are repeated DNA sequences at chromosome ends, and telomerase is the enzyme that can extend them. Together, they help keep important genetic information from being lost when cells divide.

Each time a chromosome is copied, the very end of the DNA strand can be difficult to replicate fully. Telomeres act like a buffer zone at the tips of chromosomes. Telomerase adds new repeats to these ends, which helps maintain...

DNA repair and genetic recombination help cells respond to damage in DNA. DNA can be harmed by normal cell activity or by outside factors. When damage is not fixed, it can affect how genetic information is copied and used.

Cells use repair pathways to detect and correct many kinds of DNA damage. Some repair steps remove damaged bases or replace missing pieces of DNA. Other pathways help keep the DNA sequence stable after damage has occurred.

Genetic recombination is also part of this process.

Base excision repair is a DNA repair pathway that fixes small base damage. It helps cells correct changes that can appear in DNA before they cause larger problems.

In this pathway, a damaged base is removed first. The cell then processes the site and replaces the missing part with the correct DNA building block. This repair step helps keep genetic information accurate.

Base excision repair is one of the main ways cells handle everyday DNA damage. It is an important part of maintaining DNA...

UV damage can bend DNA and block replication or transcription. Cells often face mutagens, which are outside factors that damage DNA and can create mutations. If these changes are not repaired, they can lead to disease or cancer, depending on which DNA sequences are affected.

Nucleotide excision repair, or NER, is the pathway that helps cells find and remove damaged DNA. In prokaryotes, three proteins carry out this work: UvrA, UvrB, and UvrC. UvrA and UvrB move along the DNA together and look...

DNA mismatch repair fixes copying errors that happen during DNA replication. It helps cells find the wrong nucleotide, remove it, and replace it with the correct one before the cell divides.

Human cells carry more than 3 billion base pairs of DNA, so replication must be very accurate. Even with DNA polymerase proofreading, a copying error can still occur about once every 1 million base pairs. One common error is a mismatch, such as A paired with G or T paired with C. The Mutator protein family...

DNA repair checkpoints help cells pause the cell cycle after DNA damage. This pause gives the cell time to check for breaks and repair them before division continues. Cells must do this at critical stages of the cycle.

The amount of DNA in the cell depends on when the pause happens. If the cycle stops before DNA replication, the cells will contain twice the normal amount of DNA. If cells stop after DNA replication but before mitosis, they will contain four times the normal amount.

Cells use...

Repairing DNA double-strand breaks is an important part of DNA maintenance. A double-strand break happens when both strands of the DNA molecule are cut. Cells must fix this damage to keep genetic information stable.

The repair process can happen by more than one pathway. Some breaks are repaired by direct joining of the broken ends. Other breaks are repaired by using a matching DNA sequence as a template. These pathways help cells respond to damage in different ways.

This topic is closely...

Homologous recombination is a DNA repair process that uses a matching DNA sequence as a template. It helps cells fix breaks in DNA and keep genetic information accurate.

The process begins when a broken DNA strand finds a similar, or homologous, piece of DNA. That matching sequence guides repair and allows the damaged DNA to be restored. Because the repair uses a copied sequence, the final DNA can closely match the original information.

Homologous recombination is also important in genetic...

Gene conversion is a DNA repair event linked to genetic recombination. It can change the sequence of one DNA region by using a related DNA strand as a template. This process is important for understanding how cells repair damage and reshuffle genetic information.

During recombination, gene conversion can copy information from one DNA molecule to another. The result is not a simple swap of DNA segments. Instead, one sequence is corrected or replaced while the related partner sequence remains...

DNA replication can stop before it is finished when the fork stalls. A stalled replication fork is a point where the DNA-copying machinery slows down or pauses during chromosome duplication.

Cells can restart these forks to keep copying DNA. This process helps the replication machinery move forward again after the pause. It is an important part of maintaining normal DNA replication when problems arise during copying.

The term stalled replication fork describes the stopped or delayed state of...