D1.1 DNA replication

Question 1

D1.1.1 DNA replication as production of exact copies of DNA with identical base sequences

Answer 1

What DNA replication is –
DNA replication is the process by which a DNA double helix is copied exactly, producing two new DNA molecules with identical base sequences to the original. Each strand of the original helix serves as a template for building a complementary strand, so the result is two identical double helices.

Why it is required –
- For reproduction: before a cell divides (by mitosis or meiosis), its DNA must be copied once, so each daughter cell gets a full, identical set of DNA.
- For growth and tissue replacement in multicellular organisms: as an organism grows and cells are lost or damaged, new cells must be produced by cell division. Each time a cell divides, its DNA is replicated so that all new cells have the same genetic information.

DNA replication is the production of exact copies of DNA with identical base sequences and is required for reproduction, growth, and tissue replacement in multicellular organisms.

Question 2

D1.1.2 Semi-conservative nature of DNA replication and role of complementary base pairing

Answer 2

Semi‑conservative replication:
“Semi‑conservative” means that each new DNA double helix is made up of:
- One original (parental) strand and
- One newly made (daughter) strand.
This ensures that the original sequence is preserved in one strand and used as a precise template for the new strand.

Role of complementary base pairing:
During replication, bases on the new strand pair specifically with bases on the old template strand:
A pairs with T, G pairs with C.
This strict base‑pairing rule means that the new strand is a faithful complementary copy of the template strand.
If the wrong base is added, repair systems can often detect and correct the mismatch.

Why this allows high accuracy –
The combination of: using one original strand as a template (semi‑conservative), and complementary base pairing (A–T, G–C), means that DNA polymerase can copy the original sequence very precisely, so the daughter molecules are almost always identical to the parent molecule.

The semi‑conservative nature of DNA replication, in which each new DNA molecule contains one original strand and one new strand, combined with complementary base pairing, allows a high degree of accuracy in copying base sequences

Question 3

D1.1.3 Role of helicase and DNA polymerase in DNA replication

Answer 3

two key enzymes—helicase and DNA polymerase—work together to make DNA replication possible.

Helicase – helicase unwinds the double helix at the replication fork, separating the two DNA strands. It does this by breaking the hydrogen bonds between complementary bases (A–T and G–C), so the two strands can act as templates.

DNA polymerase – DNA polymerase is the main enzyme that builds the new DNA strand.
It: Moves along the template strand, Adds nucleotides one at a time in the correct order (A–T, G–C), Joins them into a phosphodiester‑bonded sugar–phosphate backbone.
It always adds nucleotides in the 5′ → 3′ direction, so replication is continuous on one strand and discontinuous (via Okazaki fragments) on the other.

Helicase unwinds the DNA double helix by breaking the hydrogen bonds between strands at the replication fork, and DNA polymerase synthesises the new DNA strand by adding nucleotides in a sequence complementary to the template strand.

Question 4

D1.1.4 Polymerase chain reaction and gel electrophoresis as tools for amplifying and separating DNA

Answer 4

lab tools used to work with DNA: polymerase chain reaction (PCR) to amplify (copy many times) a specific DNA region, and gel electrophoresis to separate DNA fragments by size.

Polymerase chain reaction (PCR) –
PCR makes millions of copies of a target DNA sequence using a few key components:
- Primers: Short, synthetic pieces of DNA designed to bind (anneal) to the ends of the region you want to copy. Each primer is complementary to the 3′ end of each strand of the target sequence.
- Temperature changes (cycles): Denaturation (~95 °C): DNA is heated so the double helix splits into single strands.
- Annealing (~50–65 °C): Primers bind to their complementary sequences on the single‑stranded DNA.
- Extension (~72 °C): New DNA is synthesized between the primers.
These steps repeat 25–40 times, giving exponential amplification of the target region.

Taq polymerase: A thermostable DNA polymerase (from a hot‑spring bacterium) that can survive repeated heating to 95 °C.
It extends the primers, adding nucleotides to build new DNA strands in the 5′ → 3′ direction, greatly increasing the amount of DNA each cycle.

Gel electrophoresis of DNA –
This technique separates DNA fragments by size in a gel matrix:
DNA samples are loaded into wells at the negative end of an agarose gel. An electric current is applied; because DNA has a uniform negative charge on its phosphate backbone, it migrates toward the positive electrode (anode).
The gel acts like a molecular sieve: smaller DNA fragments move more easily through the pores and travel further. Larger fragments move more slowly and stay closer to the wells. After running the gel and staining it, DNA appears as bands; the pattern lets you estimate the size of fragments and compare samples.

The polymerase chain reaction (PCR) uses primers, cycles of temperature changes, and Taq polymerase to amplify specific DNA sequences, while gel electrophoresis separates DNA fragments by size based on their movement through a charged gel matrix.

Question 5

D1.1.5 Applications of polymerase chain reaction and gel electrophoresis

Answer 5

PCR and gel electrophoresis are used in real‑life situations, especially DNA profiling for paternity and forensics.

Main applications of PCR and gel electrophoresis –
- DNA profiling (DNA fingerprinting): PCR is used to amplify short tandem repeat (STR) regions from a DNA sample. Gel electrophoresis (or capillary electrophoresis) then separates the PCR‑amplified fragments by size, producing a pattern of bands unique to each individual.
This profile is used for: Paternity testing – comparing child, mother, and alleged father to see if the child’s STR bands match the father’s; forensic investigations – matching DNA from crime‑scene evidence (blood, hair, saliva) to a suspect or excluding innocent people.
- Medical and genetic applications:
Detecting genetic diseases (e.g., cystic fibrosis, sickle‑cell disease) by amplifying and analysing specific gene regions.
Diagnosing infectious diseases (e.g., viral infections like HIV or SARS‑CoV‑2) by detecting pathogen DNA/RNA sequences.
Gene expression analysis and research in molecular biology (e.g., checking gene presence, mutation, or cloning success).

NOS point: increasing reliability –
In DNA profiling, increasing the number of STR markers –STR markers are short tandem repeat markers, a specific type of DNA marker used in DNA profiling– (different loci) tested reduces the probability of a false match. The more markers you use, the less likely it is that two unrelated people by chance have the same pattern. In general terms, this illustrates the idea that reliability in scientific tests is enhanced by taking more measurements (more markers, more samples, repeated trials).