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Lesson Overview
12.3 DNA Replication
THINK ABOUT IT
Before a cell divides, its DNA must first be copied.
How might the double-helix structure of DNA make that possible?
Copying the Code
Base pairing in the double helix explained how DNA could be copied, or replicated, because each base on one strand pairs with only one base on the opposite strand. Each strand of the double helix has all the information needed to reconstruct the other half by the mechanism of base pairing.
Because each strand can be used to make the other strand, the strands are said to be complementary.
The Replication Process
Before a cell divides, it duplicates its DNA in a copying process called replication. This process ensures that each resulting cell has the same complete set of DNA molecules.
The Replication Process
During replication, the DNA molecule separates into two strands and then produces two new complementary strands following the rules of base pairing.
Each strand of the double helix of DNA serves as a template, or model, for the new strand.
The enzyme helicase attaches to the DNA at the origin of replication and separates or “unzips” the 2 strands allowing two replication forks to form.
The Replication Process
The Replication Process
DNA polymerase adds new bases to both strands following the rules of base pairing - A & T, C & G - in opposite directions
The strand being added in the same direction the DNA unzips is the leading strand and is added continuously The strand being added in the opposite direction is the l agging strand and is added in fragments called okazaki fragments
The Replication Process
The enzyme DNA ligase joins the okazaki fragments together on the lagging strands to complete the new strand of DNA
The Replication Process
The result of replication is two DNA molecules identical to each other and to the original molecule. Each DNA molecule resulting from replication has one original strand and one new strand.
The Replication Process
The Role of Enzymes
helicase - unzips DNA DNA polymerase - adds/combines new nucleotides DNA ligase - combines fragments
replication fork - where DNA splits leading strand - side where DNA is built smoothly lagging strand - side where DNA is built in pieces okazaki fragments - unconnected pieces of DNA on lagging strand
Telomeres
The tips of chromosomes are known as telomeres. The ends of DNA molecules, located at the telomeres, are particularly difficult to copy. Over time, DNA may actually be lost from telomeres each time a chromosome is replicated. An enzyme called telomerase compensates for this problem by adding short, repeated DNA sequences to telomeres, lengthening the chromosomes slightly and making it less likely that important gene sequences will be lost from the telomeres during replication.
