Viruses containing both DNA and RNA:
Viruses that contain both DNA and RNA genomes are known as retroviruses. Retroviruses are unique in the virus world as they possess genetic material in the form of both DNA and RNA. One well-known example of a retrovirus is the Human Immunodeficiency Virus (HIV).

Structure of retroviruses:
- Retroviruses have a single-stranded RNA genome, which is reverse transcribed into DNA by the enzyme reverse transcriptase.
- The DNA copy of the viral genome integrates into the host cell's DNA, allowing the virus to replicate along with the host cell.
- The integrated DNA is transcribed into RNA, which serves as both messenger RNA for viral protein synthesis and genomic RNA for new virus particles.

Life cycle of retroviruses:
- Attachment: Retroviruses attach to specific receptors on the host cell membrane.
- Entry: The viral RNA is released into the host cell, and reverse transcription occurs to produce a DNA copy of the viral genome.
- Integration: The viral DNA integrates into the host cell genome.
- Transcription: The integrated DNA is transcribed into viral RNA.
- Assembly: New virus particles are assembled from viral proteins and RNA.
- Release: The new virus particles are released from the host cell to infect other cells.

Role of retroviruses in disease:
Retroviruses like HIV are responsible for causing diseases in humans by infecting immune cells and weakening the immune system. Understanding the structure and life cycle of retroviruses is crucial for developing effective treatments and vaccines against these pathogens.