MT 30 (LEC): Ribosomes

Summary of the Article on Ribosomes

• Cells and Organelles

  • Cells form tissues, which then form organs.

  • Cells contain organelles, which function like organs within them.

  • Ribosomes are essential for protein synthesis, which supports growth and metabolism.

• Function of Ribosomes

  • Assemble amino acids into proteins by reading RNA instructions.

  • Play a key role in translation, where genetic information is converted into proteins.

  • Without ribosomes, metabolism and growth would stop.

• Types of Ribosomes

  • Membrane-bound ribosomes:

    • Found on the rough endoplasmic reticulum (RER).

    • Produce enzymes (which speed up chemical reactions) and proteins for cell membranes.

  • Free ribosomes:

    • Float in the cytosol.

    • Produce proteins used within the cell’s cytoplasm.

• Importance of Proteins

  • Essential for growth, metabolism, and biological functions.

  • Include hormones, transport proteins, structural proteins (keratin, collagen), and immune system components.

  • Act as an energy source and help maintain fluid balance.

• How Ribosomes Are Made

  • Nucleolus (inside the nucleus) forms ribosomes using ribosomal RNA (rRNA) and proteins.

  • Ribosomal proteins travel from the cytoplasm to the nucleus to combine with rRNA.

  • Ribosomes consist of two subunits (large and small), which join only during protein synthesis.

• Genetic Code and Ribosomes

  • Protein production follows two steps:

    1. Transcription (in the nucleus)

       • DNA is copied into messenger RNA (mRNA).

       • mRNA carries genetic instructions to the ribosome.

    2. Translation (in the cytoplasm)

       • mRNA attaches to the ribosome, and transfer RNA (tRNA) brings the appropriate amino acids.

       • The ribosome assembles amino acids into a protein chain.

       • Translation stops when a stop codon (UAG, UAA, or UGA) is reached.

• Do Ribosomes Make DNA or Lipids?

  • No, ribosomes do not make DNA – DNA replication occurs in the nucleus using enzymes like DNA polymerase.

  • No, ribosomes do not make lipids – Lipid synthesis occurs in the smooth ER, which produces phospholipids and cholesterol.

• Ribosomes in Prokaryotes vs. Eukaryotes

  • Found in both prokaryotic and eukaryotic cells.

  • Prokaryotic ribosomes are smaller (70S), while eukaryotic ribosomes are larger (80S).

  • Eukaryotic ribosomes are made in the nucleolus, while prokaryotic ones form in the cytoplasm.

• Do Viruses Have Ribosomes?

  • No, viruses lack ribosomes and cannot make proteins.

  • They hijack a host cell’s ribosomes to produce viral proteins.

  • Viruses contain genetic material (DNA or RNA) but cannot function independently.