DNA replication is a biochemical process that makes 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.

The phrase “exact copies” is doing real work here. Cells aren’t aiming for a near match to the old DNA; they need the same genetic information, in the same order, so daughter cells or offspring inherit the instructions needed to function.

DNA replication must happen before cell division. During reproduction, genetic information has to pass from parent to offspring. In multicellular organisms, replication is also needed for growth and tissue replacement: before a body cell divides, it copies its DNA so that each daughter cell receives a complete set of genetic information.

This links to one of the big continuity ideas in biology. Living things change over generations through mutation, recombination and selection, but ordinary cell division relies on a highly reliable copying system.

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 is not kept whole or 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 acts as a template for building 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 the 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 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. Remember the distinction: helicase unzips the two strands from each other; it doesn’t cut the strands into pieces.

DNA polymerase synthesizes a DNA strand by joining DNA nucleotides to a growing strand, using 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 move into position 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 turns that information into 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 have very little DNA in them. 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 matter 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 allows primers to 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 carry a negative charge because of their phosphate groups, so they move toward the positive electrode.

The gel works like a molecular sieve. Smaller DNA fragments pass through the gel matrix more easily and travel further in a set time; larger fragments move more slowly and stay closer to the wells. Gel electrophoresis therefore separates DNA fragments mainly according to 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 paired. PCR makes more copies of selected DNA, and gel electrophoresis separates the amplified fragments so the pattern can be analysed.

DNA profiling

DNA profiling is a way to identify or compare individuals by looking at variation in selected regions of DNA. Many profiles use repeated DNA sequences, where the number of repeats differs between people. More repeats make a longer PCR product; fewer repeats make a shorter one. 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 could not 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 from a database. The logic is the same: as more markers match, the chance that the match happened by chance becomes lower.

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 is less probable. One matching marker gives weak evidence. Many matching markers, considered together, give much stronger evidence.

This is the Nature of Science point here: reliability improves when the number of measurements or observations increases. In DNA profiling, each marker adds 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 general 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′ end is the end of a nucleotide strand where a phosphate group attached to the fifth carbon of the sugar is available at the terminus. The 3′ end is the end of a nucleotide strand where a hydroxyl group on the third carbon of the sugar is available at the terminus.

Those numbers come from the carbon atoms in deoxyribose sugar. Don’t treat 5′ and 3′ as diagram labels; they show which chemical end DNA polymerase can extend.

DNA polymerases attach the 5′ phosphate of a free DNA nucleotide to the 3′ end of the growing strand. So new DNA is synthesized in the 5′ to 3′ direction. The next nucleotide is not added to the 5′ end of the growing strand.

In a DNA double helix, the two strands are antiparallel: they run in opposite directions alongside each other, with one strand oriented 5′ to 3′ and the other 3′ to 5′. This directionality is one of the biological mechanisms that makes replication asymmetric. At a replication fork, the two template strands cannot be copied in exactly the same way.

DNA polymerase also needs an existing 3′ end to extend. It can add nucleotides to a strand, but it cannot begin a completely new DNA strand from nothing. That point matters when we look at primers and the lagging strand.

DNA polymerase can synthesize only 5′ to 3′, so the two new strands don't get built 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.

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

The lagging strand is the newly synthesized DNA strand made discontinuously away from the replication fork in short sections. DNA polymerase can only extend from a 3′ end and can only synthesize 5′ to 3′, so this strand has to be restarted again and again.

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

So the essential comparison is:

It’s a neat example of directionality shaping a biological mechanism. DNA polymerase has directional chemistry, so the replication fork has to work around that constraint.

This statement applies only to the prokaryotic system. Several enzymes work at the replication fork in prokaryotes, and because their names are easy to mix up, it helps to keep each job separate.

DNA primase is an RNA polymerase enzyme that synthesizes a short RNA primer on a DNA template during replication. The primer gives DNA polymerase the free 3′ end it needs. The leading strand needs one primer, while the lagging strand needs many.

DNA polymerase III is the main prokaryotic replication enzyme. It 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. After DNA polymerase I has finished, a sugar-phosphate bond is still missing 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 the RNA primers have been replaced.

So the lagging-strand sequence is: primase lays down 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.