DNA Replication
Copying the Code
● Main Idea: Cells copy DNA so each new cell has the complete genetic code. DNA
replication makes two DNA molecules, each with one old strand and one new strand
(this is called semi-conservative replication) .
● Helicase (an enzyme) unwinds and “unzips” the DNA double helix by breaking the bonds
between base pairs, creating a Y-shaped replication fork .
● After the strands separate, each single DNA strand serves as a template for building a
new complementary strand (each base on the template pairs with its partner on the new
strand) .
The Replication Process
● Main Idea: DNA replication is a multi-step process involving unwinding the DNA, adding
RNA primers, synthesizing new strands, and sealing the DNA molecules .
● First, helicase unwinds the DNA double helix and separates the strands, forming
replication forks .
● Next, the enzyme primase adds short RNA primers to the DNA strands; then DNA
polymerase uses these primers to start building new DNA by adding matching
nucleotides (A with T, C with G) .
● DNA polymerase makes the new strand continuously on the “leading” template strand,
but on the other (“lagging”) strand it makes short segments called Okazaki fragments .
● Finally, DNA ligase seals the gaps between Okazaki fragments on the lagging strand,
joining them into one continuous DNA strand .
The Role of Enzymes
● Main Idea: Different enzymes carry out the key tasks in DNA replication, each handling a
specific step .
● Helicase: Unwinds and separates the DNA strands by breaking the hydrogen bonds
between bases .
● Primase: Builds short RNA primers and attaches them to the DNA; these primers mark
where DNA polymerase should begin adding new nucleotides .
● DNA Polymerase: Binds to the RNA primer and adds new DNA nucleotides one by one,
matching each base on the template (A with T, C with G) to synthesize the daughter
strand .
● DNA Ligase: Seals the remaining gaps in the sugar-phosphate backbone by joining the
Okazaki fragments on the lagging strand into a continuous strand .
Telomeres
● Main Idea: Telomeres are special repeating DNA sequences at the ends of eukaryotic
chromosomes that protect genes during replication .
● Telomeres act like caps at the chromosome tips, so that important genetic information
isn’t lost each time a cell copies its DNA .
● Each time DNA is replicated, telomeres get a little shorter; when they become too short,
the cell can no longer divide normally, which is linked to aging .
● Some cells (like stem cells) have the enzyme telomerase, which rebuilds and extends
telomeres, allowing more rounds of cell division .
Replication of Living Cells
● Main Idea: Every living cell must replicate its DNA before it divides, so each daughter
cell inherits a full copy of the genetic code .
● DNA replication happens during the cell cycle (in the S phase) just before cell division,
ensuring the new cells receive the same DNA as the parent .
● This copying allows organisms to grow and repair tissues; enzymes like DNA
polymerase also proofread the new DNA to correct errors and prevent mutations .
Prokaryotic DNA Replication
● Main Idea: Prokaryotic cells (bacteria) usually have one circular DNA molecule, and
replication starts at a single origin on that circle .
● Enzymes recognize the origin of replication, where helicase unwinds the DNA and two
replication forks form; each fork copies DNA in opposite directions around the circle .
● DNA polymerases then move along each fork, adding new nucleotides very quickly
(around 1,000 bases per second) to build new strands .
● Because prokaryotes have simpler DNA (no nucleus, less packaging), their replication is
faster and involves fewer steps than in eukaryotic cells .
Eukaryotic DNA Replication
● Main Idea: Eukaryotic cells have multiple linear chromosomes, and each chromosome is
copied starting at many origins of replication .
● Each chromosome can have thousands of origins, which speeds up copying the large
genome; however, overall replication is still slower (about 50–100 bases per second)
than in prokaryotes .
● In eukaryotes, DNA is wrapped around histone proteins (forming chromatin), so these
proteins must be moved aside and reassembled as DNA is copied; this extra step makes
replication more complex .
● The ends of linear chromosomes have telomeres (see above) to protect the DNA during
replication of the chromosome tips .
