Ribosomes are the cell structures that build proteins, which are needed for growth, repair, signaling, movement, and almost every chemical process in living things. They read genetic instructions carried by mRNA and connect amino acids in the correct order. This makes ribosomes the main sites of protein synthesis in both prokaryotic and eukaryotic cells.
Understanding ribosomes helps explain how information in DNA becomes working cell machinery.
A ribosome has a large subunit and a small subunit, both made of rRNA and proteins. The small subunit helps hold and read the mRNA, while the large subunit helps form peptide bonds between amino acids. tRNA molecules bring specific amino acids by matching their anticodons to mRNA codons. Ribosomes may float freely in the cytoplasm to make proteins used inside the cell, or attach to rough endoplasmic reticulum to make proteins for membranes, secretion, or lysosomes.
Key Facts
- A ribosome is made of a large subunit and a small subunit that fit around an mRNA strand during translation.
- mRNA is read in codons, and each codon is a group of 3 nucleotides.
- Number of codons = number of mRNA nucleotides ÷ 3.
- Each tRNA carries one amino acid and has an anticodon that pairs with a codon on mRNA.
- Peptide bonds join amino acids to form a growing polypeptide chain.
- Free ribosomes make proteins mainly used in the cytoplasm, while bound ribosomes make proteins for secretion, membranes, and the endomembrane system.
Vocabulary
- Ribosome
- A cell structure made of rRNA and proteins that translates mRNA into a polypeptide.
- mRNA
- Messenger RNA is a copy of genetic information that carries codons from DNA to a ribosome.
- tRNA
- Transfer RNA is an RNA molecule that brings a specific amino acid to the ribosome during translation.
- Codon
- A codon is a three nucleotide sequence on mRNA that specifies an amino acid or a stop signal.
- Polypeptide
- A polypeptide is a chain of amino acids joined by peptide bonds that can fold into a protein.
Common Mistakes to Avoid
- Calling ribosomes membrane bound organelles is wrong because ribosomes do not have membranes, even when they are attached to rough endoplasmic reticulum.
- Thinking one codon equals one nucleotide is wrong because each codon contains 3 nucleotides and usually codes for one amino acid.
- Saying tRNA reads DNA directly is wrong because tRNA matches anticodons to codons on mRNA during translation.
- Assuming free and bound ribosomes are different types of ribosomes is wrong because the same ribosome can be free in the cytoplasm or attached to rough ER depending on the protein being made.
Practice Questions
- 1 An mRNA strand has ninety nucleotides in its coding region, not counting the stop codon. How many amino acids will be added to the polypeptide?
- 2 A polypeptide contains 120 amino acids. How many mRNA nucleotides were needed to code for those amino acids, not counting the stop codon?
- 3 A cell is making a protein that will be secreted outside the cell. Explain whether the ribosome making it is more likely to be free in the cytoplasm or bound to rough endoplasmic reticulum, and why.