The size of a genome can vary greatly between different species, and it is determined by a variety of factors. Some genomes are very large, such as the human genome, which contains around 3 billion base pairs, while others are much smaller, such as the genome of a bacterium, which may contain only a few thousand base pairs. One factor that can influence the size of a genome is the complexity of the organism. Generally, organisms with more complex body plans and higher levels of cellular differentiation require more genes to code for the various structures and functions. For example, humans have many more genes than simple organisms like bacteria or yeast. Another factor is the amount of repetitive DNA in the genome. Repetitive sequences, such as transposable elements and tandem repeats, can make up a large proportion of a genome, and their presence can contribute to larger genome sizes. In some cases, however, repetitive DNA sequences may be deleted or lost from a genome over time, leading to a smaller genome size. Finally, the evolutionary history of a species can also play a role in determining its genome size. For example, some plant species have undergone multiple rounds of genome duplication, resulting in much larger genomes than their close relatives. Additionally, some organisms may have a high rate of mutation or recombination, leading to frequent changes in genome size over time.