Before mitosis occurs, the cell’s entire genome—its complete set of DNA—must be copied with precision. This fundamental biological prerequisite ensures that when a single cell divides into two daughter cells, each new cell receives an identical copy of the genetic instructions necessary for life. Without this faithful replication, the resulting cells would lack critical genes, leading to malfunction, developmental disorders, or cell death. The process of copying DNA takes place during a specific phase of the cell cycle known as the Synthesis (S) phase, which precedes the actual mitotic division (M phase).
Quick note before moving on.
The Cell Cycle Context: Interphase Preparation
To understand what must be copied, one must first understand when it happens. The cell cycle is broadly divided into Interphase and the Mitotic (M) phase. Interphase itself consists of three stages: Gap 1 (G1), Synthesis (S), and Gap 2 (G2). Mitosis cannot begin until Interphase is complete The details matter here. Practical, not theoretical..
During G1 phase, the cell grows, performs its normal metabolic functions, and synthesizes proteins necessary for DNA replication. It is a checkpoint period where the cell assesses its size, nutrient availability, and DNA integrity. Consider this: only when conditions are favorable does the cell commit to entering the S phase. This is the exclusive window for DNA replication. Once the S phase concludes, the cell enters G2 phase, where it continues to grow and prepares the machinery—such as microtubules and centrosomes—required for the physical separation of chromosomes during mitosis.
Which means, the direct answer to "what must be copied" is the DNA (Deoxyribonucleic Acid) contained within the nucleus. Even so, the implications of this copying extend far beyond the double helix itself.
The Molecular Machinery of DNA Replication
The copying of DNA is not a simple photocopying process; it is a highly orchestrated biochemical event involving dozens of enzymes and proteins. The goal is semi-conservative replication, meaning each new DNA molecule consists of one original (parental) strand and one newly synthesized strand Which is the point..
1. Unwinding the Double Helix
The process begins at specific locations called origins of replication. In eukaryotes (humans, animals, plants), there are thousands of these origins per chromosome to ensure the massive genome is copied quickly. The enzyme helicase unwinds the DNA double helix, breaking the hydrogen bonds between complementary base pairs (A-T and C-G). This creates a Y-shaped structure known as the replication fork. Single-strand binding proteins (SSBs) immediately coat the separated strands to prevent them from snapping back together or degrading.
2. Priming and Synthesis
DNA polymerase, the primary enzyme that builds the new strand, cannot start synthesis from scratch; it requires a free 3' hydroxyl (-OH) group. An enzyme called primase synthesizes a short stretch of RNA nucleotides (an RNA primer) complementary to the DNA template. This provides the necessary starting block.
Once the primer is in place, DNA polymerase III (in prokaryotes) or DNA polymerase δ and ε (in eukaryotes) adds deoxyribonucleotides (dNTPs) to the 3' end of the primer. Synthesis proceeds in the 5' to 3' direction only. Because the two template strands are antiparallel, replication occurs differently on each:
- Leading Strand: Synthesized continuously in the same direction as the replication fork movement.
- Lagging Strand: Synthesized discontinuously in short fragments called Okazaki fragments, moving away from the fork.
3. Proofreading and Repair
Accuracy is essential. DNA polymerases possess 3' to 5' exonuclease activity (proofreading). If an incorrect base is inserted, the enzyme recognizes the structural distortion, removes the mismatched nucleotide, and tries again. This reduces the error rate from roughly 1 in 10^5 to 1 in 10^7 Worth knowing..
4. Primer Removal and Ligation
After the bulk of synthesis, the RNA primers must be removed. RNase H or DNA polymerase I (in prokaryotes) / FEN1 (in eukaryotes) excises the RNA primers. DNA polymerase then fills the resulting gaps with DNA nucleotides. Finally, DNA ligase seals the nicks in the sugar-phosphate backbone, creating a continuous, intact double helix Simple, but easy to overlook..
Beyond the Nucleotide Sequence: Chromatin and Epigenetics
Copying the sequence of bases (A, T, C, G) is only half the battle. Practically speaking, eukaryotic DNA is packaged into chromatin—DNA wrapped around histone proteins to form nucleosomes. For the daughter cells to function identically to the parent, the chromatin structure and epigenetic marks must also be copied.
Histone Deposition
During replication, parental histones (carrying epigenetic modifications like methylation or acetylation) are distributed randomly to the two daughter strands. New histones, synthesized during S phase, are assembled onto the remaining gaps. This ensures that the "memory" of gene expression patterns—whether a gene is active or silenced—is inherited Surprisingly effective..
Centromere and Kinetochore Identity
A specific region of the chromosome, the centromere, must be faithfully replicated. The centromere is defined not just by DNA sequence but by a specialized histone variant, CENP-A. The propagation of CENP-A nucleosomes is essential for the assembly of the kinetochore, the protein structure where spindle microtubules attach during mitosis. If centromere identity is lost, chromosomes cannot segregate properly.
Telomere Maintenance
At the ends of linear chromosomes lie telomeres, repetitive DNA sequences (TTAGGG in humans) that protect genes from erosion. Because DNA polymerase cannot replicate the very 5' end of the lagging strand (the "end replication problem"), telomeres shorten with every division. In stem cells and germ cells, the enzyme telomerase adds these repeats back using an RNA template. In most somatic cells, telomerase is inactive, limiting the number of times a cell can divide—a mechanism linked to aging and cancer prevention That alone is useful..
The Centrosome: Copying the Division Machinery
While the nucleus replicates its DNA, the cytoplasm replicates the centrosome (in animal cells), the primary Microtubule Organizing Center (MTOC). The centrosome consists of a pair of centrioles surrounded by pericentriolar material.
During S phase, each centriole duplicates once, forming a "daughter" centriole perpendicular to the "mother." By G2, the cell has two centrosomes. That said, these will migrate to opposite poles of the cell during prophase/prometaphase to form the mitotic spindle. If the centrosome fails to duplicate, the cell may form a monopolar spindle, leading to catastrophic segregation failure. If it over-duplicates, multipolar spindles form, causing aneuploidy (abnormal chromosome numbers), a hallmark of cancer.
Organelle Duplication: Preparing the Cytoplasm
Mitosis divides the nucleus (karyokinesis) and the cytoplasm (cytokinesis). Worth adding: for the daughter cells to be viable, they need a full complement of organelles. While not "copied" in the same templated manner as DNA, organelles must duplicate or proliferate during Interphase.
- Mitochondria: These possess their own circular DNA (mtDNA) and replicate independently of the nuclear cycle via binary fission, though their division is coordinated with the cell cycle. They must be distributed to both daughters to provide ATP.
- Endoplasmic Reticulum and Golgi Apparatus: These membrane systems fragment and disperse during mitosis but expand their membrane surface area during Interphase.
- Ribosomes: Massive production of ribosomal RNA (rRNA) and ribosomal proteins occurs in the nucleolus during G1 and S phase to ensure protein synthesis capacity in daughter cells.
Checkpoints: The Quality Control System
The cell does not blindly rush into mitosis after copying. Checkpoints act as surveillance mechanisms.
The G1/S Checkpoint (Restriction Point)
Before entering S phase, the cell checks
