Glossary
5' to 3' direction
The direction in which DNA polymerase can synthesize new DNA strands, adding nucleotides only to the 3' end of the growing strand.
Example:
Because DNA polymerase can only work in the 5' to 3' direction, one strand is synthesized continuously while the other is made in fragments.
DNA Polymerase I
An enzyme that removes the RNA primers and replaces them with DNA nucleotides, and also performs proofreading to correct errors during replication.
Example:
After the new DNA is built, DNA polymerase I acts as a clean-up crew, removing the temporary RNA primers and filling in the gaps with DNA.
DNA Polymerase III
The primary enzyme responsible for synthesizing new DNA strands by adding complementary DNA nucleotides to the 3' end of a growing strand, working in the 5' to 3' direction.
Example:
The main builder of the new DNA strand is DNA polymerase III, which efficiently adds thousands of nucleotides per second.
DNA Replication
The biological process of producing two identical replicas of DNA from one original DNA molecule. It ensures that genetic information is accurately passed from a parent cell to daughter cells during cell division.
Example:
Before a skin cell divides to repair a cut, it undergoes DNA replication to ensure each new cell receives a complete set of chromosomes.
Eukaryotes
Organisms whose cells contain a nucleus and other membrane-bound organelles. In these organisms, DNA replication occurs within the nucleus.
Example:
Humans, as eukaryotes, perform DNA replication inside the nucleus, protecting the genetic material within a membrane.
Helicase
An enzyme that unwinds the DNA double helix by breaking the hydrogen bonds between complementary base pairs, separating the two strands to allow replication.
Example:
During DNA replication, helicase acts like a zipper, unzipping the DNA molecule to create a replication fork.
Lagging Strand
The DNA strand that is synthesized discontinuously in short fragments (Okazaki fragments) in the 5' to 3' direction, moving away from the replication fork.
Example:
Due to the directionality constraint, the lagging strand is built in small, disconnected segments, like building a road in short bursts.
Leading Strand
The DNA strand that is synthesized continuously in the 5' to 3' direction, moving towards the replication fork.
Example:
The leading strand is like a smooth highway, allowing DNA polymerase to synthesize DNA without interruption.
Ligase
An enzyme that catalyzes the formation of phosphodiester bonds to join DNA fragments, specifically sealing the nicks between Okazaki fragments on the lagging strand.
Example:
Think of ligase as the molecular glue that connects all the newly synthesized DNA pieces into a continuous strand.
Okazaki fragments
Short, newly synthesized DNA fragments that are formed on the lagging strand during DNA replication.
Example:
The Okazaki fragments on the lagging strand are later joined together by DNA ligase to form a continuous DNA molecule.
Prokaryotes
Single-celled organisms that lack a nucleus and other membrane-bound organelles. In these organisms, DNA replication occurs in the cytoplasm.
Example:
A bacterium, being a prokaryote, replicates its circular DNA directly in the cytoplasm, allowing for rapid cell division.
RNA Primase
An enzyme that synthesizes a short RNA primer, which provides a free 3'-hydroxyl group for DNA polymerase to begin adding DNA nucleotides.
Example:
Before DNA polymerase can start building, RNA primase lays down a small RNA segment, acting as a crucial starting flag.
Semiconservative Replication
The model of DNA replication where each new DNA molecule consists of one original (parental) strand and one newly synthesized strand.
Example:
When a cell divides, each daughter cell receives a DNA molecule that is semiconservative replication, meaning it's half old and half new.
Single-Strand Binding Proteins (SSBPs)
Proteins that bind to the separated DNA strands during replication, preventing them from re-annealing (coming back together) and protecting them from degradation.
Example:
After helicase unwinds the DNA, single-strand binding proteins cling to the individual strands, keeping them open and ready for new nucleotide addition.
Telomeres
Repetitive, non-coding nucleotide sequences located at the ends of eukaryotic chromosomes that protect the genetic information from being lost during replication.
Example:
Like the plastic tips on shoelaces, telomeres protect the ends of chromosomes from fraying and shortening with each cell division.
Topoisomerase
An enzyme that prevents the DNA double helix from supercoiling and tangling ahead of the replication fork by cutting, unwinding, and rejoining DNA strands.
Example:
Without topoisomerase, the DNA ahead of the replication fork would become too tightly wound, halting the replication process.