Question

Explain the triplet code of DNA. Name the molecule that copies the triplet code of DNA. Name the organelle that is the site of protein synthesis. What other function does this organelle have in protein formation? (p. 59)

Short answer

Answer: mRNA, or messenger RNA, is responsible for copying the DNA triplet code during transcription, creating a complementary sequence of nucleotides that carries the genetic code needed for protein synthesis. This process occurs within the cell's organelle called the ribosome, which is found in the cytoplasm, and is responsible for reading the information in the mRNA and assembling the correct sequence of amino acids to create a protein.

Step-by-step solution

1. Explain the Triplet code of DNA

The triplet code of DNA refers to a set of three nucleotide bases, also known as a codon. Each codon in the DNA sequence corresponds to a specific amino acid, which will be used to create proteins. In other words, the DNA sequence is "read" as a series of three bases at a time, and each unique combination of bases codes for a specific amino acid in a protein chain. There are 64 possible codons, corresponding to 20 amino acids and three stop signals.

2. Name the molecule that copies the DNA triplet code

The molecule responsible for copying the DNA triplet code is the messenger RNA (mRNA). During transcription, the mRNA is synthesized using the DNA template, creating a complementary sequence of nucleotides. This process ensures that the information stored within the DNA molecule is accurately transferred to the mRNA, which then carries the genetic code to the organelle responsible for protein synthesis.

3. Name the organelle where protein synthesis occurs

Protein synthesis occurs in the organelle called the ribosome. Ribosomes are small, dense structures found in the cell's cytoplasm, either free-floating or attached to the endoplasmic reticulum. They are composed of two subunits, the large and the small subunit. Ribosomes are responsible for reading the information in the mRNA and assembling the correct sequence of amino acids to create a protein.

4. Explain the other function of this organelle in protein formation

In addition to protein synthesis, the ribosome has another essential function in protein formation: facilitating the process of folding the polypeptide chain into its correct three-dimensional structure. During translation, the ribosome moves along the mRNA, ensuring the correct anticodon on the transfer RNA (tRNA) pairs with the corresponding mRNA codon. As each amino acid is added to the growing polypeptide chain, the ribosome helps to fold the chain into its proper shape, which is essential for the protein's functionality. Once the protein is properly folded, it is often further processed and transported to its final destination within or outside of the cell.

Key concepts

Protein Synthesis

Protein synthesis is the process through which cells create proteins, essential molecules for various cellular functions. Proteins play critical roles in cell structure, function, enzyme activity, and signaling.
A typical protein synthesis cycle begins with transcription in the nucleus, where DNA is transcribed to produce messenger RNA (mRNA). The mRNA exits the nucleus and travels to the ribosome, the cellular "factory" where proteins are assembled. During this journey, the mRNA holds the instructions for protein creation.

• Transcription: DNA to mRNA
• mRNA travels from the nucleus to the ribosome
• Translation: mRNA to Protein

Each of these steps is crucial for ensuring that proteins are synthesized as per the genetic instructions, maintaining the cell's health and proper functioning.

Messenger RNA (mRNA)

Messenger RNA (mRNA) serves as the vital intermediary between DNA and protein synthesis. Acting as the cellular messenger, mRNA carries the coded instructions copied from DNA out of the cell nucleus and into the cytoplasm. This code determines the sequence of amino acids during protein synthesis.
In the transcription process, enzymes transcribe DNA sequences into mRNA, creating a complementary strand. This strand mirrors the DNA's template strand, except that thymine (T) is replaced with uracil (U) in the RNA.
Once formed, mRNA exits the nucleus and binds to a ribosome, which reads the genetic code and translates it into a specific protein structure.

• Role: Carries genetic information from DNA to ribosomes
• Formation: Transcribed from DNA in the nucleus
• Key differences: RNA uses uracil (U) instead of thymine (T)

Ribosome Function

Ribosomes are essential organelles responsible for the synthesis of proteins. Found in both prokaryotic and eukaryotic cells, they are either floating freely in the cytoplasm or attached to the endoplasmic reticulum, forming the rough ER.
Ribosomes function by reading mRNA's genetic code, which dictates the sequence of amino acids to form a polypeptide chain. This sequence is critical, as it determines the protein's structure and function.
Moreover, ribosomes facilitate the proper folding of the newly synthesized polypeptide chain into its functional three-dimensional structure. This folding process is crucial, as only correctly folded proteins are considered functional and ready to perform their roles in the cell.

• Location: Cytoplasm or on rough ER
• Main roles: Synthesize polypeptides, ensure correct folding of proteins
• Composed of: Large and small subunits working together during translation

Translation in Cells

Translation is the final step in the protein synthesis process, occurring after transcription. During translation, the ribosome interprets the sequence of the mRNA to synthesize a protein. This step takes place in the cytoplasm of the cell, where ribosomes are situated. The ribosome reads the mRNA in sets of three nucleotides, known as codons, which correspond to specific amino acids.
The process of translation begins with the ribosome binding to the mRNA and recruiting transfer RNA (tRNA) molecules. These tRNA molecules carry amino acids corresponding to the codons on the mRNA.
As the ribosome progresses along the mRNA strand, it facilitates the pairing of mRNA codons with tRNA anticodons, adding the appropriate amino acids to the growing polypeptide chain. This chain eventually folds into a functional protein.

• Key elements: mRNA, ribosomes, tRNA, amino acids
• Outcome: Formation of polypeptides; proteins folded into functional shapes
• Significance: Determines the cell's protein output, essential for cellular function