DNA in Prokaryotes vs Eukaryotes

Compare means similarities and differences on named criteria, both cell types in the same sentence. The dot point asks for this comparison to be constructed as diagrams, so make sure every point you make is one that could be drawn.

6 marks. One sentence of similarities, then four criteria: shape, location, packaging, and quantity or additional DNA.

In both cell types the genetic material is DNA, built from the same four nucleotides, arranged as a double helix held by complementary base pairing, and in both it carries the instructions for making the cell’s proteins and is replicated before the cell divides.

Prokaryotic DNA exists as a single circular chromosome, a closed loop with no free ends, whereas eukaryotic DNA is divided among several linear chromosomes, each with two ends — forty-six in a human somatic cell, arranged as twenty-three pairs.

The prokaryotic chromosome lies free in the cytoplasm, concentrated in a region called the nucleoid which is not bounded by any membrane, whereas eukaryotic chromosomes are enclosed within a nucleus bounded by a double nuclear membrane that separates them from the cytoplasm.

Prokaryotic DNA is generally not associated with histone proteins and is instead supercoiled to make it compact, whereas eukaryotic DNA is wound tightly around histone proteins to form nucleosomes, which coil further into chromatin and condense into visible chromosomes before cell division.

A prokaryote carries far less DNA — a few million base pairs against several billion in a typical eukaryote — and commonly also carries plasmids, small separate circles of non-essential DNA that can be replicated and transferred independently. Eukaryotes do not carry plasmids, but do have additional small circular DNA molecules within their mitochondria and, in plants, their chloroplasts, which resemble prokaryotic DNA in form.

Explain means give the cause. The question contrasts two cell types, so the answer needs the reason eukaryotes require packaging and the reason prokaryotes can manage without it — the second half is where most answers stop short.

4 marks. Establish the quantity problem, show how histones solve it, then explain why the problem is smaller in a prokaryote, and add what packaging enables beyond storage.

A eukaryotic cell contains an enormous quantity of DNA relative to the space available: the DNA in a single human nucleus totals around two metres in length, yet must fit within a nucleus roughly six micrometres across. Without organisation, a molecule of that length would be impossible to contain, and would tangle and break as the cell moved and divided.

DNA carries a negative charge along its phosphate backbone, and histones are positively charged proteins, so the DNA binds tightly around them by electrostatic attraction. Each histone core with its wound DNA forms a nucleosome, nucleosomes coil into a chromatin fibre, and chromatin coils further still, achieving a compaction of many thousandfold and allowing the whole genome to fit within the nucleus in an orderly, untangled arrangement.

A prokaryote faces a far smaller problem: its genome is typically a few million base pairs against several billion, so its single circular chromosome is perhaps a thousand times shorter, and it has no nucleus confining it to a smaller compartment than the cell itself. Supercoiling — twisting the closed loop on itself, assisted by proteins such as gyrase — compacts it sufficiently, so the additional machinery of histone packaging is not required.

Histone packaging also does more than store DNA compactly. It allows chromosomes to condense into discrete, separable structures before mitosis, so that many linear chromosomes can be sorted accurately between daughter cells, which a tangled mass could not be. It also regulates gene expression, since tightly wound DNA is inaccessible to the enzymes of transcription while loosely wound regions can be read — a form of control a prokaryote, with a small genome and most genes in use, has far less need of.