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Cells work quickly
Every day, your body makes billions of new cells. Each time a cell divides, it must first copy all its DNA—the genetic information contained within its nucleus. In a human cell, that’s 6.2 billion nucleotides.
The process cells use to copy DNA is called DNA replication. Using this activity, copying one human cell’s worth of DNA would take more than 95 years.* Yet the molecular machines in your cells accomplish this feat in 6 to 8 hours.
This speed comes from two factors. One, cellular machinery is fast. It adds about 50 nucleotides per second!** Two, DNA replication begins at multiple places along each chromosome. Molecular machines pull the two DNA strands apart and copy them in both directions at once.
Watch a real-time 3d animation of molecular machines copying DNA. DNA replication is a tightly coordinated process that relies on multiple proteins working together.
Complementary base pairing
A DNA molecule consists of two strands. Each strand is made up of a string of tiny building blocks called nucleotides. There are four types of nucleotides in DNA, represented by the letters A, T, C, and G. The nucleotides in opposite strands are joined together through complementary base pairing: A pairs with T, and C pairs with G.
Because of complementary base pairing, each DNA strand contains within it the information needed for building its partner. During DNA replication, the two DNA strands are pulled apart, and each original serves as a template for building a new partner strand. This process ensures that DNA is copied accurately, with just a few errors (out of 6.2 billion letters!) for each round of replication.
Structure of a nucleotide
The models on this page are based on the molecular structure of real nucleotides. The grey and white circles on the models represent partial positive and partial negative charges that form hydrogen bonds between complementary bases. These bonds work kind of like tiny magnets to hold the two DNA strands together. Forces between neighboring nucleotides stack the bases on top of one another and twist the DNA strands into a double helix.
*Assuming a rate of 2 base pairs per second x the 6 billion base pairs you inherit from your parents.
**DNA replication in one direction is straight-forward. But replication in the other direction happens a little
differently. For an explanation, see the the animations below.
Genetic Science Learning Center. (2016, March 1) Build a DNA Molecule.
Retrieved September 22, 2023, from https://learn.genetics.utah.edu/content/basics/builddna/
Build a DNA Molecule [Internet]. Salt Lake City (UT): Genetic Science Learning Center; 2016
[cited 2023 Sep 22] Available from https://learn.genetics.utah.edu/content/basics/builddna/
Genetic Science Learning Center. "Build a DNA Molecule." Learn.Genetics.
March 1, 2016. Accessed September 22, 2023. https://learn.genetics.utah.edu/content/basics/builddna/.