Organisation of DNA
DNA is present in the cells of every living thing. However, the DNA is organised differently in different types of organism.
We can divide cells into two groups based on how they organise their DNA – eukaryotes and prokaryotes.
Prokaryotes
Bacteria are prokaryotes. They do not have a membrane-bound nucleus and their DNA is free in the The living substance inside a cell (not including the nucleus)..
Bacteria have a single circular chromosome in the centre of the cell that holds all the genes needed for that bacterium. Bacteria also have extra circles of DNA called plasmids.
These plasmids contain additional genes, such as for antibiotic resistance, which may increase a bacterium’s chance of survival.
Eukaryotes
Animals, plants and fungi are eukaryotes. They have a membrane-bound nucleus and their chromosomes are linear rather than circular.
The DNA found in the linear chromosomes within the nucleus is tightly coiled and packaged around special proteins called histones - as shown below.
Interestingly, circular chromosomes are also found in Structures in the cytoplasm of all cells where aerobic respiration takes place (singular is mitochondrion). and chloroplasts. They both use their own DNA to make some proteins needed for their function. This gives evidence for the theory that mitochondria and chloroplasts originated from prokaryotic cells that were engulfed by a larger cell.
