Ribosomes can be found as free structures in the cytoplasm or strung along mRNA molecules as they participate in protein synthesis. Ribosomes also stud the rough endoplasmic reticulum (RER), giving it a rough appearance and enabling protein synthesis within the ER. Here, let’s learn more about these protein factories.
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Ribosomes
Ribosomes are non-membrane-bound cell structures that serve as protein synthesis sites. They have nearly equal amounts of protein and a type of ribonucleic acid known as rRNA (ribosomal RNA). Some ribosomes associate with the endoplasmic reticulum, while others float freely in the cytoplasm. Whether free or attached, ribosomes typically cluster in groups connected by a strand of another type of ribonucleic acid called mRNA (messenger RNA). Polyribosomes and polysomes are the names given to these clusters.
Types of Ribosomes
The two types of ribosomes are:
- 70S ribosomes
- 80S ribosomes
70S Ribosomes
They are smaller in size, with a sedimentation coefficient of 70S. They are found in prokaryotic cells such as blue-green algae and bacteria, as well as mitochondria and chloroplasts of eukaryotic cells. The 70S ribosome is made up of 2 subunits – 50S and 30S. The 50S subunit is larger and the 30S subunit of the ribosome is smaller in size. The 30S appears as a cap above the 50S subunit.
80S Ribosomes
The sedimentation coefficient of 80S ribosomes is 80S and is mostly found in eukaryotic cells of plants and animals. The 80S ribosome is also made up of 2 subunits – 60S and 40S. The dome-shaped and larger 60S ribosomal subunit is attached to the membranes of the endoplasmic reticulum, nucleus, and other organelles in ribosomes. The 40S subunit of this ribosome is smaller and sits above the 60s subunit, forming a cap-like shape. A narrow cleft separates the two subunits.
Do Check: NEET MCQs on Protein Synthesis
Protein Synthesis
Protein synthesis takes place in ribosomes. This process is divided into two steps.
- Transcription is the process by which a molecule of messenger RNA (mRNA) is produced that is complementary to a gene in DNA.
- Translation is the process by which proteins are assembled at ribosomes using genetic information from transcribed mRNA.
Also Check: Protein Factories
Transcription
The genetic information in DNA is transcribed into mRNA before being translated into proteins. Numerous enzymes are involved in transcription, which unwinds a region of a DNA molecule, initiates and ends mRNA synthesis, and modifies the mRNA after transcription is complete. In contrast to DNA replication, only a few genes are exposed, and only one of the two DNA strands is transcribed.
- RNA polymerase is an important enzyme in this process. It recognises a specific sequence of DNA nucleotides after unravelling a section of DNA.
- RNA polymerase attaches and begins joining complementary ribose nucleotides to the 3′ end of the DNA strand. The complementary bases in RNA are paired in the same way that they are in DNA, except that in RNA, the base uracil replaces the base thymine as a complement to adenine.
- RNA transcripts in bacteria can immediately function as messenger RNAs (mRNAs).
- In eukaryotes, the transcript of a protein-coding gene is known as a pre-mRNA and it must go through additional processing before it can direct translation.
- The ends of eukaryotic pre-mRNAs must be modified by the addition of a 3′ poly-A tail (at the end) and a 5′ cap (at the start).
- Some of these newly transcribed mRNA base sequences do not code for proteins. RNA splicing is the process of removing non coding regions from mRNA so that the coding region can be read continuously at the ribosome.
Translation
Another type of RNA, known as transfer RNA (tRNA), is required for translation. It aligns the different amino acids coded for by the mRNA so that a polypeptide can be formed. Also, several sites on the ribosome serve as mRNA and tRNA binding sites.
- mRNA binds to a small, distinct ribosomal subunit at the start of translation. The ribosome forms a ring around the mRNA and the first tRNA. This tRNA mostly carries methionine (amino acid), which matches with the AUG start codon. This setup is termed the initiation complex. It is required for the translation to begin.
- Then comes the elongation process by which the amino acid chain grows longer. Each codon in the mRNA is read one at a time, and the specific amino acid that corresponds to that codon is then added to the protein chain, which is expanding.
- The stage at which the completed polypeptide chain is set loose is known as termination. When a stop codon (UAA, UGA, or UAG) enters the ribosome, it sets off a chain of events that allows the chain to separate from its tRNA and moves out of the ribosome.
- After the termination process, the polypeptide may still need some folding to form the correct 3D shape. It also goes through processing and should be transported to the correct location within the cell before it can function as a fully functional protein.
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