HSC Exam Practice

Sample response. The nucleoid is the region of a prokaryotic cell that contains the cell's main DNA molecule (usually one circular chromosome). It is not surrounded by a nuclear membrane.

Marking notes. 1 mark for identifying it as the location of the main bacterial DNA / circular chromosome; 1 mark for stating that it is not membrane-bound (i.e. not a nucleus). Award full marks only if both ideas are present.

Sample response. The main bacterial chromosome is a large circular DNA molecule located in the nucleoid and carries most of the cell's genes. A plasmid is a much smaller, separate circular DNA molecule that often carries only a few additional genes (e.g. antibiotic resistance) and can replicate independently of the main chromosome.

Marking notes. 1 mark for noting both are circular DNA but a plasmid is smaller / separate from the main chromosome; 1 mark for the difference in gene content (most of the cell's essential genes vs accessory / extra genes); 1 mark for independent replication or transferability of the plasmid.

Sample response. The main DNA in a human somatic cell is linear, arranged as 46 chromosomes (23 pairs), and is located inside the membrane-bound nucleus.

Marking notes. 1 mark for "linear"; 1 mark for "46 chromosomes" (accept "multiple linear chromosomes" if number not given but cell type identified); 1 mark for "nucleus" (must be the membrane-bound organelle, not "nucleoid").

Sample response. A nucleus is defined as a membrane-bound organelle containing DNA, but a prokaryote's DNA-containing region is not surrounded by any membrane. The term "nucleoid" is used to acknowledge that the DNA is regionally organised within the cell, without claiming there is a true nuclear envelope.

Marking notes. 1 mark for identifying the absence of a membrane / nuclear envelope around the DNA in prokaryotes; 1 mark for recognising that the nucleoid still represents a defined DNA region within the cell. Reject answers that say "prokaryotes have no DNA organisation".

Sample response. Chromatin and chromosomes are the same DNA in different states of condensation. Chromatin is eukaryotic DNA associated with proteins (such as histones) in a less condensed form, allowing genes to be accessed. During cell division this chromatin condenses further to form the highly compact visible chromosomes seen at metaphase.

Marking notes. 1 mark for defining chromatin as DNA + proteins in a less condensed state; 1 mark for defining chromosomes as the condensed form visible during cell division; 1 mark for explicitly linking the two as the same material in different states.

Sample response. Plasmids are small, separate, circular DNA molecules that can be extracted from bacteria, have foreign DNA inserted into them, and then be returned to a host bacterial cell where they replicate independently of the main chromosome. This makes them practical and reliable vectors for transferring genes of interest into bacteria.

Marking notes. 1 mark for identifying a structural feature (small / circular / separate / replicates independently); 1 mark for linking that feature to a biotechnology function (carrying inserted DNA into a host cell). A vague "they help in biotech" answer scores 0.

Sample response (a). The two prokaryotic species (Mycoplasma genitalium ~0.58 Mb and Escherichia coli ~4.6 Mb) have much smaller genomes than the two eukaryotic species (S. cerevisiae ~12 Mb and H. sapiens ~3 200 Mb). On the log axis the prokaryotes fall below 10 Mb and the eukaryotes range from ~10 Mb up to thousands of Mb — the human genome is roughly 700 times larger than E. coli's.

Sample response (b). Eukaryotes organise their DNA as multiple linear chromosomes inside a membrane-bound nucleus. This DNA is associated with proteins to form chromatin, which can be compacted enough to fit metres of DNA inside a nucleus only micrometres across, yet still be locally uncondensed so genes can be accessed. Splitting the genome across many separate chromosomes also allows each chromosome to be replicated, inherited and regulated more or less independently. Together, these structural features make it possible for a eukaryotic cell to manage a genome ~700× larger than a prokaryotic one.

Marking notes. Part (a) — 1 mark for stating the prokaryote vs eukaryote size difference; 1 mark for quoting at least one supporting figure from the chart. Part (b) — 1 mark for naming chromatin + role in packaging; 1 mark for noting multiple linear chromosomes / nucleus; 1 mark for linking these to the ability to manage a much larger genome.

Sample response. Plasmids are small, separate, circular DNA molecules found in many prokaryotes, distinct from the main bacterial chromosome. The statement is strongly supported: the very features that define plasmids in bacterial cells are also what make them indispensable in biotechnology. A plasmid like pBR322 is only ~4 361 bp long, circular, located in the cytoplasm, and replicates independently of the main E. coli chromosome. This combination allows scientists to extract the plasmid intact from bacteria, cut it at specific restriction sites, insert a gene of interest, and reintroduce it into bacterial cells, where it is faithfully copied along with the host. By contrast, a eukaryotic chromosome such as human chromosome 1 is ~249 million bp long, linear, packaged with histone proteins as chromatin, and confined to the nucleus where it replicates only once per S-phase. Its size, linear ends and tight nuclear regulation make it impractical as a routine vector. Crucially, plasmids in nature already transfer genes between bacterial cells (for example, antibiotic-resistance plasmids in E. coli), so using them in biotechnology extends an existing biological function rather than inventing a new one. The statement is therefore well supported. It is, however, the specific features of plasmids — small, circular, independently replicating, separable from the main chromosome — and not "prokaryotic DNA in general" that explain their biotech utility: bacterial main chromosomes are also prokaryotic and circular yet are not routinely used as vectors. Accordingly, plasmids are an excellent example of how prokaryotic DNA organisation supports biotechnology, provided this is justified using their structural features rather than as a sweeping prokaryote-versus-eukaryote claim.

Marking notes. 1 mark — defines plasmid (small, circular, separate from main chromosome). 1 mark — names at least one biotechnology use of plasmids as vectors. 1 mark — links at least two specific structural features (size, circular form, location, independent replication) to a biotech advantage. 1 mark — contrasts these with at least two features of eukaryotic chromosomes (size, linear, nucleus-bound, chromatin packaging, S-phase replication). 1 mark — uses a named example (e.g. pBR322, antibiotic-resistance plasmids in E. coli, insulin production in E. coli). 1 mark — reaches an evaluative judgement that supports the statement. 1 mark — qualifies the judgement so it cannot be reduced to "prokaryotic DNA is better than eukaryotic DNA" (acknowledges that it is plasmid structure, not prokaryotic DNA in general, that is the active reason). Uses precise lesson terminology throughout.