If cells are capable of producing new cells, then they must have a way of producing new copies of their DNA to pass on to the new cells. The Watson and Crick model of DNA explains easily how the double-stranded molecule of DNA duplicates, or replicates itself.
Basically, the double stranded DNA molecule unzips and unwinds in order to allow each single strand to serve as a mold or template for the construction of new complementary strands.
Several basic enzymes are needed by a cell nucleus in order to complete DNA replication. One is called helicase - it is responsible for separating the complementary strands of DNA. Another is topoisomerase - it is responsible for untwisting the DNA molecule. The last and most important is DNA polymerase - it is responsible for putting together new complementary strands that match the separated strands. Note the symbols used to represent two of the enzymes.
In order for replication to take place, a cell has new nucleotides floating around in the nucleus. These are free nucleotides
(not bound to DNA). They are obtained from foods and diffuse into the nucleus of a cell.
Before we begin to demonstrate the entire process, remember, replication takes place in the NUCLEUS - that is where the DNA, free nucleotides, and enzymes are, for replication to take place.
The animations are Copyright © 1989, Steve Kuensting, All Rights Reserved.
Leading and Lagging Strands
You should have noticed that the direction of replication was opposite on the two strands of DNA after helicase unzipped the double-stranded DNA. Furthermore, the upper strand replicated in sections while the lower strand replicated smoothly. The replication on the upper strand is referred to as "lagging" and the replication on the lower strand is referred to as "leading". The lower strand only required DNA polymerase (paintbrush) while the upper strand required DNA polymerase and ligase (green icon), which bonds the disjointed small sections of lagging-strand DNA.
If you had checked the original DNA nucleotide sequences you could now see that the copy process is usually exact. Both new double stranded pieces of DNA are identical to each other.
Below are the original strand of DNA and the 2 copies. Check to see that they are identical.
You can see then that cells have a simple and relatively error-free method for copying their genetic code. Copy mistakes could be costly - they could easily result in the death of the cell. Also, note that DNA is actually a spiral helix, something like a "twisted" ladder, but this would have been too difficult to show on a simulation. The DNA polymerase enzymes can only work in specific directions on a DNA molecule. To explain this direction, biologists name the ends of each strand as 3' (sugar end) or 5' (phosphate end). Note that the two strands of a DNA molecule are antiparallel to each other. The DNA polymerase enzymes only move in the 3' to 5' direction on an existing DNA strand.
Each DNA nucleotide consists of a nitrogen base and a phosphate attached to a deoxyribose sugar. The carbons of the sugar are numbered from 1 to 5 and are referred to as 1' (1-prime), 2' (2-prime), 3' (3-prime), 4' (4-prime) and 5' (5-prime). The important carbons are: 1' for attaching the nitrogen base, 5' for attaching the phosphate, and 3' for attaching to the next nucleotide. The 2' carbon differs between DNA and RNA nucleotides: RNA has an -OH group attached there while DNA has only a -H group. Below are DNA nucleotides.
Okazaki Fragments and DNA Replication
In 1966, replication was shown to be both continuous and discontinuous. Two Japanese scientists, Kiwako Sakabe and Reiji Okazaki were working with Escherichia coli and discovered small DNA fragments being produced during DNA replication. They developed a theory that explained this finding. The small fragments are still referred to as "Okazaki fragments". The reason for their findings is in the enzyme DNA polymerase. DNA polymerase can only work in the 3' to 5' direction during replication. On one strand it works continuously, following helicase as it opens the DNA strands. That side is called the "leading side". On the other side, DNA polymerase works discontinuously, traveling away from helicase and producing the Okazaki fragments that Reiji Okazaki discovered. That side is called the "lagging side". Another enzyme - ligase - is needed to join the Okazaki fragments together on the discontinuous side.
Below is an animation of the process. Ligase is represented by:
. DNA polymerase is represented by either
(continuous side) or
Discontinuous DNA Replication - Top Strand
Continuous DNA Replication - Bottom Strand
The animations are Copyright © 2013, Steve Kuensting, All Rights Reserved.
Linear chromosomes are replicated at many regions simultaneously, forming "bubbles" that merge over time. Special enzymes bind to a chromosome and open up a bubble, which allows helicase and many polymerases to work simultaneously. Each bubble has two replication forks. Each fork has one region of continuous replication and one region of discontinuous replication. Okazaki fragments are found on the discontinuous/lagging side (in red). DNA polymerase ALWAYS slides on the existing strand in the 3' to 5' direction, putting together nucleotides in the opposite orientation (5' to 3'). The diagram below shows continuous/leading side replication in blue.
Whole Chromosomes Replication - Merging Bubbles
Each chromosome in a human cell has multiple replication origins. Each origin produces its own replication bubble with forks moving in opposite directions. As one fork merges with an opposing fork from a different bubble, the two bubbles fuse. Eventually as all bubbles fuse on a chromosome, replication is complete.