Before any type of cell division, the cell must double the amount of DNA, so that each new cell has a copy of all the genetic instructions. DNA replication is the molecular basis for growth and renewal in the body, such as in bones, skin and hair.

Why this is important…We could not make more cells without copying our DNA. Without new cells, we cannot maintain a healthy body.

The process of copying DNA also has applications for genetic testing, such as paternity testing or identifying genetic material from a crime scene.

Foundations

DNA replication makes the sister chromatids needed for cells to divide. Imagining the molecular process is a challenge, but the basic concepts are fairly simple. DNA is a double-stranded molecule and each strand codes for the other. One main enzyme – DNA polymerase – is responsible for reading the genetic code from one strand of DNA and constructing the new strand. Several other enzymes help the process by unwinding DNA and stitching together the loose bits.

Like the other biomolecules, DNA is made of repeating units – the building blocks. These are a lot like the bricks to build a house or letters to build a sentence. Each building block is similar, but has small differences. In the case of DNA, there are 4 types of building blocks that are linked into long strands. The sequence of the building blocks is the genetic code that instructs the cell.

Key terms

Monomer – single building block of a biomolecule (Greek: mono = one, mer = part)

Polymer – many building blocks of a biomolecule assembled into a chain (Greek: poly = many, mer = part)

DNA – Deoxyribonucleic acid; information storage molecule composed of long chains of nucleotides

Nucleotide – individual building block for a nucleic acid; these come in 4 varieties for DNA – adenine (A), thymine (T), cytosine (C) and guanine (G)

Complimentary base pair – a pair of nucleotides that are found on opposite strands of the DNA double helix and held together by hydrogen bonding

Semi-conservative replication – process of making copies of DNA resulting in two copies, each with one new and one original strand of DNA

DNA polymerase – enzyme that constructs new DNA strands

Helicase – enzyme that unwinds and unzips the DNA double helix

DNA ligase – enzyme that stitches together segments of DNA that were constructed separately

Leading strand –DNA strand that is formed continuously, without interruption

Lagging strand – DNA strand that is formed in pieces or fragments; these need to be stitched together by DNA ligase

Details

Nucleic acids store information. The DNA in the center of the cell stores information about how to build and operate a cell. These instructions are recipes for building proteins. Each recipe is what we call a gene.

Nucleotides are the building blocks for DNA. There are 4 different nucleotides that make up DNA (Figure 3-8). Each nucleotide has the same basic parts – a phosphate, deoxyribose sugar, and a nitrogen base.

Chemical diagrams of DNA and structure of nucleotides.

FIGURE 3-8. DNA nucleotides. The nitrogen base is the only part that varies and comes in four types – adenine, thymine, cytosine and guanine. The rest is formed by a sugar (deoxyribose) and a phosphate. (“Nucleotides” by OpenStax College is licensed under CC BY 3.0)

Nucleotides are linked together to form nucleic acids – DNA or RNA. Nucleic acids (the polymers) are formed by linking many nucleotide building blocks (monomers). Covalent bonds form between their sugars and phosphates, forming a “backbone” holding the nucleic acid together (Figure 3-9). Nitrogen bases are hold onto another DNA strand creating the double-stranded DNA.

DNA forms a double helix in cells. What we usually refer to as “DNA” or a “DNA molecule” is really a complex of two DNA molecules (Figure 3-9). Both of these long strands are held together by weak attractions (hydrogen bonding, which will be discussed more later) between the two DNA molecules. These two long strands are arranged parallel to one another, but they are pointed in opposite directions. Study Figure 3-9 to see how the nucleotides on the left point in the opposite direction as those on the right. This arrangement is called antiparallel, and is similar to how a two-lane road is arranged, with both lanes parallel, but traffic moves in opposite directions. Unlike a road, however, the strands are twisted around one another in a spiral (or helix), forming what we call the double helix of DNA.

Three representations of DNA as a double helix. First is artistic spiral ladder. Second is chemical structures. Third is abbreviated chemical structures.

FIGURE 3-9. DNA structure as a double helix. Several representations are shown. Look for the common elements of each. ("DNA Nucleotides" by OpenStax College is licensed under CC BY 3.0 || "Genetik -DNA-Basen -stark vereinfach " by A.Spielhoff is licensed under CC BY 4.0)

Complimentary base pairs of nucleotides provide a template for making copies of DNA. The nucleotides on each strand of the double helix are always arranged in specific pairs (Figure 3-10). We call them complimentary base pairs (or “base pairs” for short). The nitrogen bases facing one another are not identical, but complement one another. The complimentary base pairs are held together by 2 or 3 weak attractive forces (hydrogen bonding), which is what holds the double helix together. These pairing rules are also the key to making new DNA.

Stop & Think #5: What would normally happen in a cell after DNA replication?

Chemical structure of DNA. Four pairs of nucleotides shown with backbone of sugar and phosphate. Nitrogen bases are held together by hydrogen bonding.

FIGURE 3-10. Complimentary base pairing. Nitrogen bases on adjacent strands pair up – A with T and G with C. Attractive forces (hydrogen bonding) hold these together. ("DNA chemical structure" by Madeleine Price Ball is in the Public Domain, CC0)

Before reading about DNA replication, watch the following videos that provide an overview of how it works:

Video 3d: DNA Replication basics

The following animation is also helpful: DNA replication animation (by interact Medical

The basic process of DNA replication is surprisingly straightforward. The two halves of the double helix are split apart, then enzymes form new strands complimentary to the old strands. This results in two new double helices, each having one original strand and one newly made strand (Figure 3-11). This is referred to as semi-conservative replication because each new DNA complex keeps (or conserves) an original strand, but only half (semi-) of it is the original. The details are more complicated, but are easier to understand if we separate them into 3 major stages.

Starting the process (initiation phase) requires splitting apart the two strands of DNA and adding RNA primers. The enzyme helicase needs a little energy to split apart the hydrogen bonding between the two strands. It also unwinds the DNA. A different enzyme (primase) adds a short segment of RNA nucleotides that are complimentary to the DNA nucleotides at the starting point. This allows the next enzyme, DNA polymerase, to recognize the starting point.

DNA polymerase then creates a new DNA strand which keeps getting longer (elongation phase).
DNA polymerase moves along each of the DNA strands adding new nucleotides that are complimentary to the original strands. It has a very specific process to follow which requires it to move along the original (template) strand in a specific direction – 3’ to 5’ (pronounced: “3-prime to 5-prime”). It builds the new strand in the opposite direction (5’ to 3’). Since DNA is arranged antiparallel, each strand has to be created differently. See Video 3d for more details on leading and lagging strand construction.

When DNA polymerase reaches a stopping point (termination phase) it falls away from the DNA strand. This happens when it reaches either (a) the end of the chromosome or (b) another segment of DNA that has already been replicated. Another enzyme, DNA ligase, stitches the pieces together into one strand.

Stop & Think #6: What would the sequence of complimentary bases be for the following DNA molecule? 3’—ATTACGGATGATAATC—5’

Diagram of DNA split into a fork. Each strand has enzymes adding new nucleotide bases in complimentary fashion.

FIGURE 3-11. DNA replication. Enzymes unwind DNA, match complimentary bases, and bind together fragments. The lower strand is the leading strand, while the upper strand is the lagging strand, replicated in fragments. ("DNA replication in prokaryotes" by Eunice Laurent is licensed under CC BY-SA 4.0)

New DNA is constructed at many sites at once, which speeds up the work tremendously. In humans, it would take hundreds of hours if there was just one DNA polymerase working. Since there are thousands, each working on different parts of every chromosome, the process only takes 8 to 10 hours. As DNA polymerase adds new bases, it also proofreads its work and corrects errors. This is important because any mistakes in copying DNA could cause serious damage to all future cells. DNA polymerase works with amazing accuracy – it only makes a wrong match once every billion base pairs.

Video 3e: DNA Replication details

I recommend watching animations like these to help bring the process to life.

DNA Replication- 3D by Yourgenome. This is a simple model and a good place to start.
DNA Replication Updated by Amoeba Sisters

After watching these videos, you should be able to:

describe the direction that DNA polymerase reads and writes
describe the difference between leading and lagging strands
explain why Okazaki fragments are made on the lagging strand
explain why RNA primers are needed
describe the role of DNA ligase

Once DNA replication is complete, there are two identical copies of each chromosome. These copies are held together at their center – the centromere - forming sister chromatids. The cell is now ready to divide.

Stop & Think #7: Stem cells that divide frequently need a constant supply of new DNA to make new chromosomes. Where do we get the raw material to make new DNA?

DNA polymerase cannot replicate to the very end of the chromosome. Each time DNA is replicated, the chromosomes get a little shorter. Fortunately, each chromosome adds extra DNA to each end (these parts are called telomeres). Even so, a chromosome can only be copied about 50 times before it starts deleting important genetic information. Most cells stop dividing at this point, but stem cells, like the ones are able to keep dividing to make new cells like skin or blood. This is possible because they keep adding DNA to the ends of their chromosomes.

So that we don’t neglect bacteria, we should note that DNA replication occurs differently in prokaryotes than eukaryotes. In prokaryotes (like bacteria) the process is simpler since there is one circular piece of DNA, which eliminates the problem of eroding ends of chromosomes. And since there is less DNA in bacteria, replication in bacteria takes about 40 minutes, instead of the hours it takes in eukaryotes.

Learning objectives

Identify nucleic acids from structural diagrams.
Describe the structure of the DNA double helix
Explain the major functions of nucleic acids.
Describe the basic process of DNA replication
Describe how complimentary base pairing works and why it is important
Explain the purpose of DNA replication and the context in which it occurs in the body

Stop & Think Questions Explained

5. After DNA replication, a cell would normally divide by mitosis, binary fission, or meiosis.

6. Most of our nucleotides come from the food we eat, since it is made of cells with DNA. Our cells can also make nucleotides from the organic molecules of our food.

7. 5’ —TAATGCCTACTATTAG —3’