DNA replication is a biochemical process that produces new DNA molecules with the same base sequence as the original DNA molecule. A base sequence is the order of nitrogenous bases along a DNA strand, written using the letters A, T, C and G.

“Exact copies” is the key idea here. A cell isn’t aiming for DNA that is just roughly similar to the original molecule; it needs the same genetic information in the same order, so daughter cells or offspring receive the instructions they need to function.

Before a cell divides, DNA replication has to happen. In reproduction, genetic information passes from parent to offspring. Multicellular organisms also rely on replication for growth and tissue replacement: before a body cell divides, it copies its DNA so that each daughter cell gets a complete set of genetic information.

This links to one of biology’s big ideas about continuity. Living things can change over generations through mutation, recombination and selection, but normal cell division depends on a copying system that is highly reliable.

Semi-conservative copying

Semi-conservative replication is a mode of DNA replication in which each new DNA molecule contains one original parental strand and one newly synthesized strand. The original DNA double helix isn’t kept whole, and it isn’t chopped up randomly into both daughter molecules; instead, each original strand is conserved as a template.

A template strand is a nucleic acid strand whose base sequence determines the base sequence of a new complementary strand. During replication, the two DNA strands separate, and each one guides the building of a new strand.

The active Y-shaped region where DNA is being copied is a replication fork, a region of a DNA molecule where the parental strands are separated and new complementary strands are synthesized.

Complementary base pairing gives accuracy

Complementary base pairing is a molecular recognition rule in which adenine pairs with thymine, and cytosine pairs with guanine, because these pairings allow stable hydrogen bonding and correct fit in the DNA double helix. A hydrogen bond is a weak attraction between a slightly positive hydrogen atom in one molecule or group and a slightly negative atom in another.

So the template base selects the next nucleotide. If the template has A, the new strand should receive T; if the template has C, the new strand should receive G. A wrongly matched nucleotide is much less stable because it does not produce the correct hydrogen bonds and shape, so it is unlikely to stay in place long enough to be joined into the strand.

Semi-conservative replication helps ensure genetic continuity between generations in this way. Each old strand carries the original base sequence, complementary base pairing recreates the missing partner strand, and the two resulting DNA molecules have the same base sequence as the starting molecule. The system is not magical or perfect, but it is accurate enough that most cell divisions pass on essentially unchanged genetic information.

DNA replication happens when enzymes work together at the replication fork. Keep the enzyme roles simple here: helicase opens the DNA; DNA polymerase builds new DNA.

Helicase is an enzyme that unwinds the DNA double helix and separates the two strands by breaking hydrogen bonds between complementary bases. It does not break the covalent bonds in the sugar-phosphate backbone. That distinction is worth remembering: helicase unzips the two strands from each other, rather than cutting the strands into pieces.

DNA polymerase is an enzyme that synthesizes a DNA strand by joining DNA nucleotides to a growing strand according to complementary base pairing with a template strand. A nucleotide is a monomer of nucleic acids composed of a sugar, a phosphate group and a nitrogenous base.

DNA polymerase uses the exposed parental strands as templates. Free DNA nucleotides line up by base pairing with the template, then DNA polymerase links each accepted nucleotide into the sugar-phosphate backbone of the new strand. Helicase makes the template readable; DNA polymerase uses that information to make a new DNA strand.

PCR amplifies selected DNA

Polymerase chain reaction is a laboratory technique used to amplify a selected DNA sequence through repeated cycles of strand separation, primer binding and DNA synthesis. DNA amplification means producing many copies of a DNA sequence from a small starting amount.

PCR is useful because biological samples often contain only tiny amounts of DNA. Rather than starting with a large original sample, PCR makes the target sequence abundant enough to detect, compare or sequence.

A primer is a short single-stranded nucleic acid that binds to a complementary sequence and gives DNA polymerase a starting point. In PCR, primers are essential because they select the region copied: only the DNA between the two primers is amplified efficiently.

PCR relies on repeated temperature changes:

• Denaturation is a heating step that separates the two DNA strands by breaking hydrogen bonds between bases.
• Annealing is a cooling step that lets primers bind to complementary sequences on the single-stranded DNA.
• Extension is the DNA synthesis step in which DNA polymerase adds nucleotides to make new strands.

Taq polymerase is a heat-stable DNA polymerase from a thermophilic bacterium that stays functional after the high-temperature denaturation step in PCR. That heat stability is the clever part: ordinary enzymes would be denatured again and again, but Taq polymerase survives the cycling.

Gel electrophoresis separates DNA fragments

Gel electrophoresis is a laboratory technique that separates charged molecules as they move through a gel under an electric field. DNA fragments have a negative charge because of their phosphate groups, so they move toward the positive electrode.

Think of the gel as a molecular sieve. Smaller DNA fragments pass through the gel matrix more easily and travel further in a set time; larger fragments are slowed more and stay closer to the wells. Gel electrophoresis therefore separates DNA fragments mainly by length.

After the run, a stain or fluorescent dye makes the DNA bands visible. A DNA ladder is a mixture of DNA fragments of known lengths used as a reference to estimate the lengths of unknown fragments. In practice, you compare the position of a band in an unknown sample with the ladder bands in a neighbouring lane.

PCR and gel electrophoresis are often used together. PCR increases the amount of selected DNA, then gel electrophoresis separates the amplified fragments so the pattern can be analysed.

DNA profiling

DNA profiling identifies or compares individuals by analysing variation in selected regions of DNA. Many profiles use repeated DNA sequences, and the number of repeats differs between people. More repeats give a longer PCR product; fewer repeats give a shorter PCR product. Gel electrophoresis then separates these products by length.

In paternity testing, the child’s DNA profile is compared with the profiles of the mother and the possible father. A child inherits DNA from both biological parents, so the bands in the child’s profile should be accounted for by bands in the mother and the father. If the child has bands that cannot have come from the tested man, he is excluded as the biological father.

For forensic investigations, DNA from a crime scene can be compared with DNA from a suspect or a database. The logic is the same: as more markers match, it becomes less likely that the match happened by chance.

Reliability and number of markers

A genetic marker is a DNA region with detectable variation between individuals or populations. In DNA profiling, more independent markers make the result more reliable, because a false match becomes less probable. One matching marker is weak evidence. Many matching markers, taken together, give much stronger evidence.

This links to the Nature of Science idea here: reliability improves when the number of measurements or observations increases. In DNA profiling, each marker gives another comparison. Using more markers reduces the probability that two unrelated individuals happen to share the same overall profile.

PCR also has diagnostic uses. For example, a pathogen can be detected by amplifying a DNA sequence specific to that organism. If the target sequence is present, amplification produces a detectable signal or band; if it is absent, there is no amplification of that target. The key idea is specificity: primers can be designed to amplify one chosen sequence from a mixed sample.

A DNA strand has direction because its two ends are chemically different. The $5^\prime$ end is the end of a nucleotide strand with a phosphate group attached to the fifth carbon of the sugar available at the terminus. The $3^\prime$ end is the end of a nucleotide strand with a hydroxyl group on the third carbon of the sugar available at the terminus.

Those numbers refer to the carbon atoms in deoxyribose sugar. Don’t treat $5^\prime$ and $3^\prime$ as decoration on diagrams; they show which chemical end DNA polymerase can extend.

DNA polymerases add the $5^\prime$ phosphate of a free DNA nucleotide to the $3^\prime$ end of the growing strand. So, new DNA is synthesized in the $5^\prime$ to $3^\prime$ direction. The next nucleotide is not added to the $5^\prime$ end of the growing strand.

The two strands in a DNA double helix are antiparallel, meaning they run in opposite directions alongside each other, with one strand oriented $5^\prime$ to $3^\prime$ and the other $3^\prime$ to $5^\prime$. Because of this directionality, replication is asymmetric: the two template strands cannot be copied in exactly the same way at a replication fork.

DNA polymerase also needs an existing $3^\prime$ end to extend. It can add nucleotides to a strand, but it cannot begin a completely new DNA strand from nothing. That becomes crucial when primers and the lagging strand come up.

DNA polymerase can only synthesize $5'$ to $3'$, so the two new strands aren't made in the same way at a replication fork. Since the parental template strands are antiparallel, one template is copied toward the fork, while the other has to be copied away from it in short sections.

leading strand

The leading strand is the newly synthesized DNA strand made continuously toward the replication fork. It starts with an RNA primer just once. After that, DNA polymerase keeps extending the same growing strand as helicase opens more template.

lagging strand

The lagging strand is the newly synthesized DNA strand made discontinuously away from the replication fork in short sections. It has to restart again and again because DNA polymerase can only extend from a $3'$ end and can only synthesize $5'$ to $3'$.

Okazaki fragment

An Okazaki fragment is a short length of newly synthesized DNA made on the lagging strand between RNA primers. Each fragment starts from its own RNA primer. Later, the primers are removed, replaced with DNA and the fragments are joined into one continuous strand.

The key comparison is:

This is a neat example of directionality shaping a biological mechanism. DNA polymerase works in one direction, so the replication fork has to be organized around that constraint.

This statement applies only to the prokaryotic system. Prokaryotes use several enzymes at the replication fork, and the names can be annoyingly similar, so keep the jobs separate.

DNA primase is an RNA polymerase enzyme that synthesizes a short RNA primer on a DNA template during replication. The primer provides the free $3'$ end needed by DNA polymerase. One primer is needed on the leading strand, while many primers are needed on the lagging strand.

DNA polymerase III is the main prokaryotic replication enzyme that extends RNA primers by adding DNA nucleotides to the $3'$ end of a growing strand. It builds DNA in the $5'$ to $3'$ direction and uses complementary base pairing to select nucleotides. On the leading strand it can keep going for a long distance; on the lagging strand it extends each primer to form an Okazaki fragment.

DNA polymerase I is a prokaryotic enzyme that removes RNA primers and replaces them with DNA nucleotides. Once DNA polymerase I has done that, the strand still lacks a sugar-phosphate bond between adjacent DNA sections.

DNA ligase is an enzyme that joins adjacent DNA fragments by forming a phosphodiester bond in the sugar-phosphate backbone. On the lagging strand, it seals the remaining breaks between DNA fragments after RNA primers have been replaced.

So, on the lagging strand, the sequence is: primase lays an RNA primer, DNA polymerase III extends it to make an Okazaki fragment, DNA polymerase I removes the RNA primer and fills the space with DNA, and DNA ligase seals the final backbone nick. That order is the safest way to remember the prokaryotic enzyme roles.

DNA proofreading is an error-correction process during DNA replication in which DNA polymerase removes a newly added mismatched nucleotide and replaces it with the correctly paired nucleotide.

In the prokaryotic system required here, DNA polymerase III carries out proofreading. If DNA polymerase III adds a nucleotide whose base does not match the template base, the mismatch is picked up at the $3^\prime$ end of the growing strand.

DNA polymerase III then cuts out the mismatched nucleotide from the $3^\prime$ terminal of the strand. It adds a nucleotide with the correct complementary base, and replication continues.

Proofreading makes replication more faithful, which helps prevent mutations. It also helps explain how genetic continuity is maintained: semi-conservative replication provides the template system, complementary base pairing gives the copying rule, and proofreading fixes many of the mistakes that still slip through.