QCE Biology - Unit 4 - Genetics and heredity

DNA, Chromosomes and Replication | QCE Biology

Learn DNA structure, genes, chromosomes, histones, plasmids and DNA replication with helicase, DNA polymerase and Okazaki fragments.

Updated 2026-05-18 - 6 min read

DNA, chromosomes and replication is part of the way QCE Biology turns living systems into evidence students can describe, analyse and evaluate. The safest way to study it is to connect each term to a data pattern, a biological mechanism and a limitation.

Core explanation

DNA structure

DNA is a double helix made of nucleotides. Each nucleotide has a phosphate, deoxyribose sugar and nitrogenous base. Complementary base pairing means adenine pairs with thymine and cytosine pairs with guanine.

Genes and chromosomes

A gene is a DNA sequence that contributes to a functional product. Eukaryotic DNA is packaged with histones into chromosomes, while prokaryotes usually have circular chromosomes and may also carry plasmids.

Chromosome vocabulary

Homologous chromosomes carry the same genes at the same loci, although alleles may differ. Sister chromatids are identical copies joined at the centromere after replication.

Replication

Helicase unwinds and separates DNA strands. DNA polymerase builds complementary strands. Because polymerase works in one direction, the lagging strand forms Okazaki fragments that must be joined.

Replication detail and enzyme roles

DNA replication is semi-conservative: each daughter DNA molecule contains one original template strand and one newly synthesised strand. This explains how genetic information can be copied accurately while still allowing rare mutations.

| Component | Role in replication | | --- | --- | | Helicase | Unwinds the double helix and separates the template strands | | Primase | Adds short RNA primers so DNA polymerase has a starting point | | DNA polymerase | Adds complementary DNA nucleotides to the 3' end of a growing strand | | Leading strand | Synthesised continuously toward the replication fork | | Lagging strand | Synthesised discontinuously away from the fork as Okazaki fragments | | DNA ligase | Joins Okazaki fragments into one continuous strand |

DNA strands are antiparallel. One strand runs 5' to 3' while the complementary strand runs 3' to 5'. DNA polymerase can only add nucleotides to the 3' end, which is why leading and lagging strands are copied differently.

Chromosomes also include non-coding DNA. Some non-coding regions regulate gene expression, such as promoters and enhancers, while other regions help chromosome structure. In eukaryotes, many genes include exons that remain in mature mRNA and introns that are removed during RNA processing.

DNA packaging and replication accuracy

DNA is packaged differently in prokaryotes and eukaryotes. Prokaryotes usually have one circular chromosome in the cytoplasm and may carry plasmids with extra genes such as antibiotic resistance. Eukaryotes have linear chromosomes in the nucleus, wrapped around histone proteins to form chromatin. Tightly packed chromatin is harder to transcribe, so packaging is linked to gene regulation as well as storage.

Replication accuracy is high because base pairing is specific and DNA polymerase can proofread many errors. Errors that remain after replication become mutations if they are not repaired before the next cell division. This matters because replication is both a copying process and a possible source of new genetic variation.

The replication fork is the Y-shaped region where DNA is being copied. Multiple replication origins allow eukaryotic chromosomes to be copied faster because replication can proceed from many sites at once. Prokaryotic circular chromosomes usually have fewer origins.

When drawing replication, label the template strands, the new strands, the 5' and 3' directions, helicase, DNA polymerase, the leading strand, the lagging strand, Okazaki fragments and DNA ligase. Those labels show that you understand the mechanism rather than just the final product.

DNA nucleotides within the same strand are joined by phosphodiester bonds between the phosphate group of one nucleotide and the sugar of the next. Complementary bases on opposite strands are held by hydrogen bonds. Adenine and thymine form two hydrogen bonds, while cytosine and guanine form three, which helps explain why GC-rich regions can require more energy to separate.

The sugar-phosphate backbone gives DNA structural stability, while the base sequence stores genetic information. This distinction is useful in diagrams: the backbone shows strand direction and continuity, while the bases show the code that is copied, transcribed or mutated.

How to use this in data questions

Start by identifying what has been measured. In Biology, a graph or table is rarely just asking for a trend; it is asking whether you can connect the trend to a process. Quote enough data to show the pattern, then use the concept language from the syllabus. If the evidence is limited, name the limitation precisely: sample size, sampling method, uncontrolled variables, measurement precision, population choice or the time scale of the data.

A useful study habit is to turn each heading into a data prompt. Ask what you would expect to happen if the relevant variable increased, decreased or was removed. For ecology topics, think about abundance, distribution, biodiversity, biomass and carrying capacity. For genetics topics, think about genotype, phenotype, gene expression, allele frequency and inheritance pattern. For evolution topics, think about variation, selection pressure, gene flow, isolation and relatedness.

When a question asks you to evaluate, do not just list problems with the experiment. Link the limitation to the confidence of the conclusion. For example, a small sample size matters because a few unusual individuals can distort the pattern. An uncontrolled abiotic factor matters because it gives another possible explanation for the same biological trend. This is the difference between naming a limitation and using it scientifically.

Worked example

Common exam traps

Other traps to watch for:

• using a general word when a syllabus term is available
• ignoring units, sample size or time scale
• treating a model as a perfect copy of the real ecosystem or cell
• writing a memorised paragraph that does not use the given data

Quick check

Sources

• QCAA Biology subject page
• QCAA Biology 2025 syllabus
• OpenStax Biology 2e: DNA Structure and Sequencing
• OpenStax Biology 2e: Basics of DNA Replication
• Khan Academy: DNA structure and replication review