HIV Replication Cycle and Bacterial Gene Transfer Mechanisms

HIV Replication Cycle in T Cells

When the Human Immunodeficiency Virus (HIV) enters the body, it reaches the bloodstream and is distributed throughout the organism. The mechanism of action within T lymphocytes occurs in the following steps:

1. Attachment: Viral GP120 proteins bind to the CD4 receptors on the surface of the T lymphocyte.
2. Fusion and Entry: The virus fuses with the cell membrane, allowing the viral nucleocapsid to enter the cell's cytoplasm.
3. Uncoating: The nucleocapsid breaks down, releasing the viral RNA genome and essential enzymes like reverse transcriptase and integrase.
4. Reverse Transcription: Reverse transcriptase synthesizes a single DNA strand using the viral RNA as a template.
5. DNA Synthesis: The original RNA strand is degraded, and a complementary DNA strand is synthesized, forming a double-stranded viral DNA molecule.
6. Nuclear Entry: The double-stranded viral DNA is transported into the host cell's nucleus.
7. Integration: The enzyme integrase incorporates the viral DNA into the host cell's chromosomal DNA, forming a provirus.
8. Transcription: The host cell's machinery transcribes the proviral DNA into messenger RNA (mRNA) and new viral genomic RNA.
9. Cytoplasmic Migration: Viral mRNA and genomic RNA migrate from the nucleus into the cytoplasm.
10. Protein Synthesis: Host cell ribosomes translate the viral mRNA into viral proteins (structural proteins, enzymes, etc.).
11. Assembly: New viral components (genomic RNA, proteins, enzymes) assemble near the cell membrane.
12. Budding: Immature virus particles push outwards, enveloping themselves in a portion of the host cell membrane, forming buds.
13. Maturation and Release: The buds pinch off from the host cell. The enzyme protease cleaves long viral proteins into their functional forms, resulting in mature, infectious virions that are released to infect other cells.

Bacterial Genetic Exchange Mechanisms

While bacteria primarily reproduce asexually via binary fission (resulting in genetically identical cells), they also possess parasexual mechanisms for genetic exchange. These processes allow for the transfer of genetic material, sometimes even between different species, contributing to genetic diversity and adaptation.

Conjugation

This is a process where one donor bacterium transfers DNA directly to a recipient bacterium, typically through a specialized structure called a sex pilus.

• Donor Types: Donor bacteria possess a fertility factor (F factor), which is a plasmid. They can be F+ (F factor exists as a separate plasmid) or Hfr (High frequency of recombination, where the F factor is integrated into the bacterial chromosome). Recipient bacteria lack the F factor and are designated F-.
• F+ Transfer: When an F+ cell conjugates with an F- cell, a copy of the F factor plasmid is usually transferred through the pilus. The recipient F- cell becomes F+, and typically no chromosomal DNA is transferred.
• Hfr Transfer: When an Hfr cell conjugates with an F- cell, replication of the integrated F factor begins, and part of the Hfr cell's chromosome is transferred along with a portion of the F factor. The connection often breaks before the entire chromosome (including the full F factor) is transferred. The transferred chromosomal DNA can recombine with the recipient's DNA, potentially introducing new genes. The recipient usually remains F- because it rarely receives the complete F factor.

Transduction

This mechanism involves the transfer of bacterial DNA from one bacterium to another via a bacteriophage (a virus that infects bacteria). During phage replication within a host bacterium, fragments of the host's DNA can be mistakenly packaged into new phage particles. When these phages infect a new bacterium, they inject the previous host's DNA instead of (or along with) viral DNA, allowing for potential recombination.

Transformation

Transformation is the process by which a bacterium takes up naked DNA fragments directly from its environment. This free DNA often comes from other bacteria that have died and lysed (broken open), releasing their cellular contents, including DNA. If the recipient bacterium is competent (able to take up external DNA), it can incorporate these fragments into its own genome through recombination.