Biological Structures and Life Cycles of Bacteria, Viruses, and HIV

Bacteria: Structure, Morphology, and Function

Bacteria are microorganisms that can be organized into two main groups: Archaebacteria (often found in extreme conditions) and Eubacteria (more frequent). According to morphology, they can be classified as:

• Bacilli (rod-shaped)
• Cocci (spherical)
• Vibrios and Spirilla (curved or spiral)

Bacterial Components

Key parts of a bacterium include:

• Capsule: Provides a protective function.
• Cell Wall:
  • Gram-Positive (+): Characterized by a lipid bilayer and various layers of peptidoglycan. Stains purple/blue.
  • Gram-Negative (-): Characterized by two lipid bilayers and a thinner layer of peptidoglycan. Stains pink/red.
• Plasma Membrane: Contains invaginations called mesosomes, which are involved in metabolic reactions and DNA replication.
• Cytoplasm: Contains ribosomes, inclusions, vesicles, and gaseous structures.
• Genetic Material: Double-stranded circular DNA. The region where the single chromosome is located is called the nucleoid (lacking a true nucleus). Plasmids are also present (extrachromosomal circular DNA).
• Appendages: Fimbriae or pili (for adhesion and conjugation) and flagella (for movement).

Bacterial Metabolism and Function

Bacteria lack a nucleus, nucleoli, and membrane-bound organelles. They utilize both aerobic and anaerobic metabolism.

Movement and Survival

Bacteria move using flagella. In response to adverse stimuli, they can form resistance spores (endospores).

Nutrition

1. Autotrophic: Photosynthetic (e.g., green and purple sulfur bacteria) and chemosynthetic.
2. Heterotrophic: Used to form organic substances from other organic substances.

Reproduction and Genetic Exchange

Reproduction occurs by bipartition (binary fission), not mitosis. The DNA doubles, and the cell divides into two. Bacteria also utilize parasexual mechanisms to exchange genetic material with other bacteria:

1. Conjugation: Bacteria transmit DNA via pili.
2. Transduction: A virus passes DNA from one bacterium to another.
3. Transformation: Following the lysis (breakdown) of a bacterium, its DNA can enter a live bacterium.

Viruses: Structure and Obligate Parasitism

Viruses consist only of nucleic acid molecules (RNA or DNA, never both) coated by a capsid. They lack cellular organization and are obligate intracellular parasites.

Viral Structure

A typical virus structure includes:

1. Head: Contains the internal membrane envelope, capsid, enzymes, and the internal nucleic acid (DNA or RNA).
2. Tail (in bacteriophages): Includes the sheath, basal plate, and flow fibers.

Viruses can infect bacteria (known as bacteriophages), plant cells, and animal cells.

The Viral Life Cycle

The viral cycle involves several stages:

1. Stage-Setting (Adsorption and Penetration): The virus fixes its capsule onto a bacterium or cell, followed by penetration. The virus enters the cytoplasm and is uncoated (desencapsidation), freeing the nucleic acid.
2. Replication Phase: From this point, the cycle can proceed in two ways:

A) Lysogenic Cycle

If the virus has DNA, its DNA integrates into the host cell's DNA. If the virus has RNA (as in the case of the AIDS virus), the enzyme reverse transcriptase transforms the RNA into DNA, which then integrates into the host DNA. The viral DNA (now called a prophage or attenuated virus) can remain dormant without expressing genes, replicating later along with the host cell. The time spent in this state is called the latency period.

B) Lytic Cycle

The viral DNA or RNA is used to form new viral RNA and proteins. The structures of the virus are assembled, giving rise to several new viruses that exit the cell (lysis) and proceed to infect other cells.

HIV (Human Immunodeficiency Virus)

The HIV virus causes AIDS. Infection occurs primarily via blood, targeting CD4 lymphocytes. HIV is a retrovirus, meaning it possesses the enzyme reverse transcriptase, which converts its RNA genome into DNA so that it can integrate into the host cell's DNA.

HIV Life Cycle and Pathogenesis

The virus cycle involves:

1. Binding to CD4 receptors.
2. Entry into the cell.
3. Uncoating (desencapsidation).
4. Conversion of RNA into DNA (via reverse transcriptase).
5. Integration of the DNA into the lymphocyte's DNA.

The integrated DNA can remain dormant for a long time (lysogenic pathway). Alternatively, the DNA can be transformed back into RNA, synthesizing proteins and assembling the virus (lytic cycle). This multiplication allows the virus to infect other lymphocytes.

Phases of AIDS Disease

The disease progresses through distinct phases:

1. Primary Infection Phase: Often presents with flu-like symptoms (influenza-like illness) that may go unnoticed. The individual can spend a long time in this phase while still infecting others.
2. Symptomatic Phase (AIDS): The virus actively destroys CD4 cells, significantly reducing their number. This decrease in immune defenses leads to infections caused by fungi and other opportunistic pathogens. This phase is what causes AIDS (Acquired Immunodeficiency Syndrome).