What are the small circles of DNA that can replicate independently in some bacteria called?

The small circles of DNA that can replicate independently in some bacteria are known as plasmids. Plasmids are extrachromosomal DNA molecules that exist independently of the chromosomal DNA within bacterial cells. They often carry genes that provide bacteria with genetic advantages, such as antibiotic resistance or the ability to metabolize certain substances.

Plasmids can replicate independently because they have their own origin of replication, which allows them to duplicate during cell division without being integrated into the bacterial chromosome. This distinctive characteristic makes them important tools in genetic engineering and biotechnology, where they are frequently used as vectors to introduce foreign genes into host cells.

The other options refer to different biological structures. Chromosomes are the larger DNA molecules that contain most of an organism's genetic material. Ribosomes are cellular structures that synthesize proteins but do not contain DNA. Vectors generally refer to the vehicles used to transfer genetic material into a host organism, which can include plasmids, but this term itself is broader and does not specifically denote the independent replicating DNA structures found in bacteria.