D1.1 DNA replication

DNA replication makes a matching copy of a DNA molecule so each daughter cell can receive the genetic information needed for its functions.

Complementary base pairing lets each original strand guide a new strand. The double helix is opened, nucleotides are added and the two resulting molecules carry the same sequence information, barring copying errors.

Trace: helix opens; each old strand templates a new strand; complementary nucleotides join; two DNA molecules result.

Before a cell divides, one DNA molecule becomes two molecules with matching base sequences so each daughter nucleus can inherit a copy.

Replication is copying DNA, not translating it into protein and not making two completely new strands without templates.

Semi-conservative replication produces DNA molecules in which each double helix contains one parental strand and one newly synthesized strand.

When the original strands separate, each acts as a template. Complementary base pairing preserves information while retaining one physical strand from the original molecule in each product.

Identify a product by checking: one old strand; one new strand; complementary pairing between them.

After one round, heavy parental DNA in a density experiment is replaced by two intermediate molecules, each containing one old and one new strand.

Semi-conservative does not mean half the bases are copied randomly; the strand pattern is the key prediction.

Helicase separates the DNA strands by breaking hydrogen bonds, while DNA polymerase adds complementary nucleotides to a growing strand.

Opening creates template strands with exposed bases. Polymerase matches each incoming nucleotide and forms phosphodiester bonds, but it can extend only from an existing 3′ end.

Separate the enzyme jobs: helicase opens; polymerase selects and joins; the template determines sequence.

At a replication fork, helicase exposes a short region and polymerase adds a nucleotide opposite each template base.

Helicase does not synthesize DNA, and polymerase does not unzip the whole helix by itself.

PCR makes many copies of a chosen DNA region, while gel electrophoresis separates DNA fragments mainly by length.

PCR cycles denaturation, primer binding and extension, doubling the target region repeatedly. In an electric field, negatively charged DNA moves through a gel; shorter fragments travel farther.

Use the workflow: choose primers; cycle amplification; load fragments; compare band positions with a size marker.

A PCR can amplify a short suspect-specific region, then electrophoresis shows a band at the expected length if that region is present.

PCR amplification and electrophoresis have separate roles: amount is increased first, then fragment sizes are compared.

DNA profiling identifies or compares biological samples by examining variation in selected DNA regions and the resulting fragment or allele pattern.

Different individuals usually differ at variable loci. PCR or restriction analysis reveals those regions, and a matching multi-locus pattern supports a common source while controls test contamination and interpretation.

Evaluate a profile by checking: locus choice; band or allele match; controls; probability of coincidental match.

If a sample and reference share the expected alleles at several independent loci, the evidence for a common source is stronger than a single matching band.

A matching profile supports identity; it does not by itself prove when or how DNA arrived at a scene.

DNA polymerase adds each new nucleotide to the free 3′ end, so the new strand is synthesized in the 5′ to 3′ direction.

The chemical reaction uses the 3′ hydroxyl group of the growing strand. Because the two template strands are antiparallel, their geometry forces continuous and discontinuous synthesis.

For a diagram, mark template and new-strand directions first, then place the growing 3′ end where the next nucleotide can be added.

A polymerase moving along a template in the 3′ to 5′ direction builds a complementary strand in the 5′ to 3′ direction.

The direction describes the new strand being built, not the direction the polymerase ‘reads’ in a vague sense.

The leading strand is synthesized continuously toward the replication fork, while the lagging strand is built away from the fork in short Okazaki fragments.

Polymerase must always extend 5′ to 3′. As the fork opens, one template permits continuous addition; the other requires repeated primers and fragments that are later joined.

Trace: fork direction; template orientation; primer placement; continuous or fragment synthesis; ligation.

On the lagging strand, several short fragments are extended 5′ to 3′ and DNA ligase seals the remaining nicks.

Leading and lagging refer to synthesis pattern, not which strand contains more genes or is biologically ‘better’.

DNA replication needs coordinated enzymes: helicase opens DNA, primase provides primers, polymerase extends strands, ligase joins fragments and topoisomerase relieves twisting.

Each enzyme removes a different bottleneck created by copying a long antiparallel molecule. Together they maintain speed, continuity and manageable DNA tension at the fork.

Explain a stalled step by naming the missing job: opening; priming; extension; joining; or tension relief.

Without ligase, Okazaki fragments exist but the lagging strand remains discontinuous even though polymerase has extended them.

Naming an enzyme is not enough; connect it to the physical problem it solves.

DNA polymerase proofreading detects some mismatched bases, removes the incorrect nucleotide and allows synthesis to continue with a corrected end.

A mismatch distorts the newly formed double helix. Polymerase can reverse, excise the error and reinsert a complementary base, lowering but not eliminating mutation rate.

Interpret an error-control claim by separating: mismatch detection; excision; correct replacement; remaining probability of error.

If a wrong base is inserted, proofreading can remove it before the strand is extended further, preserving the template sequence.

Proofreading is not perfect repair of every mutation; some errors escape and other repair pathways may act later.