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DNA structure and replication review

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Key terms

DNA (deoxyribonucleic acid)Nucleic acid that transmits genetic information from parent to offspring and codes for the production of proteins
NucleotideBuilding block of nucleic acids
Double helixStructure of two strands, intertwining around an axis like a twisted ladder
DNA replicationProcess during which a double-stranded DNA molecule is copied to produce two identical DNA molecules
Base pairingPrinciple in which the nitrogenous bases of the DNA molecules bond with one another

DNA structure

DNA is a nucleic acid, one of the four major groups of biological macromolecules.


All nucleic acids are made up of nucleotides. In DNA, each nucleotide is made up of three parts: a 5-carbon sugar called deoxyribose, a phosphate group, and a nitrogenous base.
DNA uses four kinds of nitrogenous bases: adenine (A), guanine (G) cytosine (C), and thymine (T).
RNA nucleotides may also contain adenine, guanine and cytosine bases, but instead of thymine they have another base called uracil (U).

Chargaff's rules

In the 1950s, a biochemist named Erwin Chargaff discovered that the amounts of the nitrogenous bases (A, T, C, and G) were not found in equal quantities. However, the amount of A always equalled the amount of T, and the amount of C always equalled the amount of G.
These findings turned out to be crucial to uncovering the model of the DNA double helix.

Double helix

The discovery of the double helix structure of DNA was made thanks to a number of scientists in the 1950s.
Image of a DNA double helix, illustrating its right-handed structure. The major groove is a wider gap that spirals up the length of the molecule, while the minor groove is a smaller gap that runs in parallel to the major groove. The base pairs are found in the center of the helix, while the sugar-phosphate backbones run along the outside.
DNA double helix. Image modified from OpenStax, CC BY 3.0.
DNA molecules have an antiparallel structure - that is, the two strands of the helix run in opposite directions of one another. Each strand has a 5' end and a 3' end.
Solving the structure of DNA was one of the great scientific achievements of the century.
Knowing the structure of DNA unlocked the door to understanding many aspects of DNA's function, such as how it is copied and how the information it carries can be used to produce proteins.

DNA replication

Semi-conservative replication produces two helices that contain one old and one new DNA strand.
Semi-conservative replication. Image modified from OpenStax, CC BY 3.0.
DNA replication is semi-conservative. This means that each of the two strands in double-stranded DNA acts as a template to produce two new strands.
Replication relies on complementary base pairing, that is the principle explained by Chargaff's rules: adenine (A) always bonds with thymine (T) and cytosine (C) always bonds with guanine (G).

The replication process

Schematic of Watson and Crick's basic model of DNA replication.
  1. DNA double helix.
  2. Hydrogen bonds break and helix opens.
  3. Each strand of DNA acts as a template for synthesis of a new, complementary strand.
  4. Replication produces two identical DNA double helices, each with one new and one old strand.
DNA replication occurs through the help of several enzymes. These enzymes "unzip" DNA molecules by breaking the hydrogen bonds that hold the two strands together.
Each strand then serves as a template for a new complementary strand to be created. Complementary bases attach to one another (A-T and C-G).
DNA template strand and the creation of its complementary strand
The primary enzyme involved in this is DNA polymerase which joins nucleotides to synthesize the new complementary strand. DNA polymerase also proofreads each new DNA strand to make sure that there are no errors.

Leading and lagging strands

DNA is made differently on the two strands at a replication fork.
One new strand, the leading strand, runs 5' to 3' towards the fork and is made continuously.
The other, the lagging strand, runs 5' to 3' away from the fork and is made in small pieces called Okazaki fragments.
Diagram of leading and lagging replication strands

Example: Determining a complementary strand

DNA is only synthesized in the 5' to 3' direction. You can determine the sequence of a complementary strand if you are given the sequence of the template strand.
For instance, if you know that the sequence of one strand is 5’-AATTGGCC-3’, the complementary strand must have the sequence 3’-TTAACCGG-5’. This allows each base to match up with its partner:
These two strands are complementary, with each base in one sticking to its partner on the other. The A-T pairs are connected by two hydrogen bonds, while the G-C pairs are connected by three hydrogen bonds.

Common mistakes and misconceptions

  • DNA replication is not the same as cell division. Replication occurs before cell division, during the S phase of the cell cycle. However, replication only concerns the production of new DNA strands, not of new cells.
  • Some people think that in the leading strand, DNA is synthesized in the 5’ to 3’ direction, while in lagging strand, DNA is synthesized in the 3’ to 5’ direction. This is not the case. DNA polymerase only synthesizes DNA in the 5’ to 3’ direction only. The difference between the leading and lagging strands is that the leading strand is formed towards replication fork, while the lagging strand is formed away from replication fork.

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