What Do Ribosomes Do in an Animal Cell?

Ribosomes play a crucial role in protein synthesis within animal cells. They are small, complex structures found either free-floating in the cytoplasm or attached to the endoplasmic reticulum (ER). The main function of ribosomes is to translate the genetic information encoded in messenger RNA (mRNA) into proteins.

The Role of Ribosomes in Protein Synthesis

  • Essential Cellular Structures: Ribosomes are tiny cellular organelles found in all organisms, including animal cells. They play a crucial role in protein synthesis, making them vital for cell activity and overall organism function.
  • Protein Production: Ribosomes synthesize proteins required for structural support, chemical reactions, cell membrane regulation, disease protection, and the formation of hair, nails, and tissues in animal cells.
  • Transcription: This process involves copying genetic information from DNA into RNA (messenger RNA or mRNA) within the cell nucleus, providing instructions for protein synthesis.
  • Translation: During translation, mRNA carrying genetic information exits the nucleus to bind with ribosomes in the cytoplasm. Ribosomes then read the genetic code and assemble amino acids to form proteins.
  • Two-Subunit Structure: Ribosomes consist of a large subunit and a small subunit, both containing ribosomal RNA (rRNA) and proteins.
  • rRNA Synthesis: rRNA molecules are synthesized within the nucleolus in the cell nucleus, combined with proteins, and transformed into ribosomal subunits for protein synthesis.
  • Protein Synthesis Sites: Ribosomes can be found freely in the cytoplasm, attached to the outer nuclear envelope, or bound to the endoplasmic reticulum, providing multiple locations for protein synthesis in animal cells.

Transcription: From DNA to RNA

  • Initiation: Transcription begins when RNA polymerase, an enzyme, binds to the DNA strand’s promoter region. This specific DNA sequence signals the start of the gene to be transcribed.
  • Unwinding: RNA polymerase unwinds the DNA double helix, exposing the DNA template strand to be transcribed into RNA.
  • RNA Synthesis: RNA polymerase adds complementary RNA nucleotides to the growing RNA strand, following the base-pairing rules; adenine (A) pairs with uracil (U), and cytosine (C) pairs with guanine (G).
  • Elongation: As RNA polymerase moves along the DNA template, the RNA strand continues to grow, forming a new RNA molecule that matches the DNA sequence.
  • Termination: Transcription ends when RNA polymerase reaches a specific DNA sequence called the terminator. The enzyme detaches from the DNA, and the newly-formed RNA molecule is released.
  • RNA Processing: Before the RNA molecule can be used for protein synthesis, it undergoes further modifications, such as the removal of non-coding sequences (introns) and the addition of protective caps and tails.

Translation: From RNA to Proteins

  • Decoding the Genetic Code: Ribosomes read the information carried in an mRNA molecule – the genetic code – and convert this information into protein sequences. This process is called translation and involves groups of three consecutive nucleotides in RNA called codons.
  • Ribosome Composition: Ribosomes are composed of two major components, the small and large ribosomal subunits. Each subunit contains one or more RNA molecules and many ribosomal proteins. Together, these components form the protein synthesis machinery known as the ribosome.
  • mRNA Sequence Reading: During translation, ribosomes read the mRNA sequence in sets of three nucleotides, known as codons. Each codon corresponds to an amino acid, which is the building block of proteins.
  • Amino Acid Selection: Transfer RNA (tRNA) molecules carry specific amino acids and bind to the codon on mRNA sequences in the ribosome via an anticodon. This ensures the correct sequence of amino acids is used to build a protein.
  • Protein Synthesis Phases: Ribosomes convert mRNA to proteins via four phases – initiation, elongation, termination, and recycling. Translation begins with the start codon (AUG) and ends with one of the three stop codons (UAA, UAG, UGA).
  • Ribosome Evolution: Ribosomes in bacteria and eukaryotic cells have a high degree of structural similarity, indicating a common origin. Structural differences allow some antibiotics to target bacterial ribosomes without affecting human ribosomes.
  • Ribosomal Organization: Ribosomes are often found associated with the endoplasmic reticulum, an intracellular membrane involved in protein synthesis and processing. They can also function in groups, synthesizing multiple protein molecules simultaneously from a single mRNA chain.

Ribosomal Subunits and their Functions

endoplasmic reticulum smooth

In an animal cell, ribosomes play a crucial role in protein synthesis, and their efficiency is achieved through the presence of two distinct subunits – the small ribosomal subunit and the large ribosomal subunit. Both subunits consist of ribosomal RNA (rRNA) molecules and proteins, which come together to form the ribosome’s functional structure. The small subunit is responsible for decoding the messenger RNA (mRNA), which carries genetic information from DNA in the cell nucleus.

On the other hand, the large subunit facilitates the formation of the polypeptide chain, linking amino acids together during protein synthesis. The combined efforts of these subunits ensure that the ribosomes effectively carry out their primary function of producing proteins vital for various cellular processes and overall organismal functioning.

Ribosome Biogenesis

  • Composition: Ribosomes are complex molecular machines comprised of RNA and proteins. They consist of two subunits – smaller and larger – that work together during protein synthesis. These subunits contain both protein and ribonucleic acid components, which enable them to carry out their vital function.
  • Location: Ribosomes are found in both prokaryotic and eukaryotic cells. In animal cells, they are located within the cytosol and bound to the endoplasmic reticulum.
  • Protein Synthesis: Ribosomes facilitate the process of protein synthesis by decoding the information carried by messenger RNA (mRNA) and linking together amino acids to form proteins. This crucial cellular function ensures that cells maintain proper structure and function.
  • Differences between Prokaryotic and Eukaryotic Ribosomes: Prokaryotic ribosomes are 70S and contain three individual rRNA molecules. Meanwhile, eukaryotic ribosomes are 80S and contain four individual rRNA molecules. This difference in composition allows for specific targeting of antibiotics in prokaryotic cells.
  • Ribosome Assembly: Both ribosomal subunits are assembled in the nucleolus from ribosomal proteins and ribosomal RNA (rRNA). These molecules are exported to the cytoplasm, where they form functional ribosomes to carry out protein synthesis.

Free Ribosomes vs. Bound Ribosomes

  • Location: Free ribosomes are not attached to any structure and are found floating freely in the cytoplasm. Bound ribosomes are attached to the endoplasmic reticulum, forming the rough endoplasmic reticulum.
  • Mobility: Free ribosomes can move throughout the cell, while bound ribosomes stay fixed in their location as they are attached to the endoplasmic reticulum.
  • Protein Synthesis: Both free and bound ribosomes are responsible for protein synthesis. However, they produce different types of proteins.
  • Protein Usage: Free ribosomes synthesize proteins for internal cellular activities, such as enzymes for metabolic processes, mitochondria, and cytoskeleton structure. Bound ribosomes produce proteins that are exported from the cell, including hormones, cell surface receptors, and signaling molecules.
  • Role in the Cell: Free ribosomes play a crucial role in maintaining the cell’s internal functions, while bound ribosomes aid in the transportation of proteins to other parts of the organism.

Ribosomes and Cellular Regulation

Ribosomes play a crucial role in cellular regulation, ensuring the proper functioning of an animal cell. These tiny organelles, composed of ribosomal RNA (rRNA) and proteins, are responsible for synthesizing proteins, which are vital for a wide variety of cellular processes. Protein synthesis occurs in ribosomes, which can be found freely floating in the cytoplasm or attached to the rough endoplasmic reticulum. Proteins produced by ribosomes provide structural support, catalyze chemical reactions, regulate the passage of substances across cell membranes, protect against disease, and form essential components of hair, nails, bones, and tissues.

Ribosomes not only contribute to cellular regulation by synthesizing proteins as required but also play a significant role in gene expression and the determination of cellular health. Overall, ribosomes are indispensable structures in animal cells, driving proper functioning and regulatory processes essential for life.

Evolutionary Perspective on Ribosomes

  • Ancient Origins: Ribosomes are evolutionarily ancient, dating back more than 3.5 billion years. They are believed to have originated in the “RNA world,” a hypothetical stage in the Earth’s development where RNA molecules played crucial roles.
  • Universal Presence: Ribosomes are found in all living organisms, including animals, plants, and prokaryotic cells, underlining their essential role in protein synthesis across different species.
  • Shared Components: Despite variations in size and structure, ribosomes across species share a core structure made of ribonucleic acid (rRNA) and proteins, highlighting their evolutionary conservation.
  • Adaptation to Environments: Ribosomes have evolved to adapt to environmental pressures, such as temperature, resulting in species-specific variations in their structural and functional aspects.
  • Antibiotic Resistance: The evolutionary arms race between bacteria and antibiotics has led to the development of antibiotic-resistant ribosomes. This exemplifies how evolution influences ribosomes in responding to changing environmental conditions.


In conclusion, ribosomes are essential organelles in animal cells that play a crucial role in protein synthesis. As microscopic protein-making factories, they are responsible for translating encoded genetic information from messenger RNA (mRNA) and linking amino acids together in a specific order.




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