During interphase of the cell cycle, a second chromatid containing a copy of the DNA molecule is assembled.
The process, called DNA replication, involves separating (unzipping) the DNA molecule into two strands, each of which serves as a template to assemble a new, complementary strand.
The result is two identical double-stranded molecules of DNA.
Because each of these double-stranded molecules of DNA consists of a single strand of old DNA (the template strand) and a single strand of new, replicated DNA (the complementary strand), the process is called semi-conservative replication.
During DNA replication, the enzyme helicase unwinds the DNA helix, forming a Y-shaped replication fork.
Single-strand binding proteins attach to each strand of the uncoiled DNA to keep them separate.
As helicase unwinds the DNA, it forces the double-helix in front of it to twist.
A group of enzymes, called topoisomerases, break and rejoin the double helix, allowing the twists to unravel and preventing the formation of knots.
Since a DNA double-helix molecule consists of two opposing DNA strands, the uncoiled DNA consists of a 3'-5' template strand and a 5'-3' template strand.
The enzyme that assembles the new DNA strand, DNA polymerase, moves in the 3'-5' direction along each template strand.
A new (complement) strand grows in the antiparallel, 5'-3' direction.
For the 3'-5' template strand, replication occurs continuously as the DNA polymerase follows the replication fork, assembling a 5'-3' complementary strand.
This complementary strand is called the leading strand.
For the 5'-3' template strand, however, the DNA polymerase moves away from the uncoiling replication fork.
This is because it can assemble nucleotides only as it travels in the 3'-5' direction.
As the helix is uncoiled, DNA polymerase assembles short segments of nucleotides along the template strand in the direction away from the replication fork.
After each complement segment is assembled, the DNA polymerase must return back to the replication fork to begin assembling the next segment.
These short segments of complementary DNA are called Okazaki segments.
The Okazaki segments are connected by DNA ligase, producing a single complement strand.
Because this complementary strand requires more time to assemble than the leading strand, it is called the lagging strand.
DNA polymerase is able to attach nucleotides only to an already existing complementary strand.
Therefore, to initiate a new complementary strand, another enzyme, primase, begins replication with a short segment of RNA (not DNA) nucleotides, called an RNA Primer.
The leading strand and every Okazaki segment on the lagging strand must begin with an RNA primer.
When the primer is in place, DNA polymerase can attach succeeding DNA nucleotides to the primer.
The RNA nucleotides of the RNA primer are later replaced with DNA nucleotides by DNA polymerase.
The steps of DNA replication are summarized here:

1. Helicase unwinds the DNA, producing a replication fork.
   Single-strand binding proteins prevent the single strands of DNA from recombining.
   Topoisomerase removes twists and knots that form in the doublestranded template as a result of the unwinding induced by helicase.
   
2. Primase initiates DNA replication at special nucleotide sequences (called origins of replication) with short segments of RNA nucleotides, called RNA primers.
   
3. DNA polymerase attaches to the RNA primers and begins elongation, the adding of DNA nucleotides to the complement strand.
   
4. The leading complementary strand is assembled continuously as the double-helix DNA uncoils.
   
5. The lagging complementary strand is assembled in short Okazaki fragments.
   
6. The Okazaki fragments are joined by DNA ligase.
   
7. The RNA primers are replaced by DNA nucleotides.
   

Energy for elongation is provided by two additional phosphates that are attached to each new nucleotide (making a total of three phosphates attached to the nitrogen base).
Breaking the bonds holding the two extra phosphates provides the chemical energy for the process.