**Structure:**
1. **Composition:** Ribosomes are composed of ribosomal RNA (rRNA) and proteins.
2. **Subunits:** They consist of a large subunit and a small subunit, each with distinct rRNA and protein components.
3. **Size:** Ribosomes vary in size between prokaryotes (70S) and eukaryotes (80S in cytoplasm, 70S in mitochondria and chloroplasts).
**Function:**
1. **Protein Synthesis:** Ribosomes are the cellular machinery responsible for protein synthesis (translation).
2. **Site of Action:** They bind messenger RNA (mRNA) and transfer RNA (tRNA) to synthesize proteins according to the genetic code.
3. **Location:** Found either free in the cytoplasm or bound to the endoplasmic reticulum (ER), known as rough ER, depending on the destination of the proteins being synthesized.
**Types:**
1. **Free Ribosomes:** Found floating freely in the cytoplasm, synthesizing proteins that function within the cytoplasm or other organelles.
2. **Bound Ribosomes:** Attached to the endoplasmic reticulum (ER), involved in synthesizing proteins destined for secretion or for insertion into membranes.
**Biogenesis:**
1. **Assembly:** Ribosomes are assembled in the nucleolus (in eukaryotes) or in the cytoplasm (in prokaryotes) from rRNA and protein components.
2. **Export:** Once assembled, ribosomes are transported to the cytoplasm or endoplasmic reticulum where they become functional.
**Regulation:**
1. **Gene Expression:** The number and activity of ribosomes can be regulated in response to cellular conditions, such as nutrient availability or stress.
2. **Drug Target:** Ribosomes are targets for antibiotics that inhibit bacterial protein synthesis, such as erythromycin and tetracycline.
**Evolutionary Significance:**
1. **Conservation:** Ribosomes are highly conserved across all domains of life, reflecting their essential role in cellular function.
2. **Endosymbiotic Theory:** The presence of ribosomes in mitochondria and chloroplasts supports the theory that these organelles evolved from endosymbiotic bacteria.
**Clinical Relevance:**
1. **Diseases:** Mutations affecting ribosome function can lead to genetic disorders known as ribosomopathies.
2. **Drug Development:** Understanding ribosome structure and function aids in the development of antibiotics and other drugs.
Understanding ribosomes is crucial for comprehending basic cellular processes, evolution, and their implications in health and disease.
