List the steps in protein synthesis and briefly describe what occurs at each step.
1. Transcription: Protein synthesis begins with transcription. In this step, the DNA sequence of a specific gene is transcribed into a complementary RNA molecule called messenger RNA (mRNA). RNA polymerase enzyme binds to a specific region of the gene, unwinds the DNA helix, and synthesizes the mRNA molecule by adding complementary RNA nucleotides according to the base pairing rules. This results in the formation of pre-mRNA.
2. RNA processing: The pre-mRNA molecule undergoes a series of modifications known as RNA processing. It includes the removal of non-coding regions (introns) from the pre-mRNA and the joining of the remaining coding regions (exons) to form a mature mRNA. Additionally, a modified 5' cap and a poly-A tail are added to stabilize the mRNA molecule and assist in its export from the nucleus.
3. mRNA export: The mature mRNA molecule is transported from the nucleus to the cytoplasm through nuclear pores for further processing and translation. This step ensures that the mRNA reaches the ribosomes, where protein synthesis occurs.
4. Translation initiation: In translation initiation, the mRNA molecule binds to a ribosome, which consists of two subunits. The small ribosomal subunit recognizes and binds to a specific sequence on the mRNA called the start codon (typically AUG), while the large subunit attaches to the small subunit, forming a functional ribosome.
5. Elongation: During the elongation phase, the ribosome moves along the mRNA molecule, one codon at a time, and matches each codon with its complementary anticodon on a transfer RNA (tRNA) molecule. Each tRNA molecule carries a specific amino acid that corresponds to the codon it recognizes. The ribosome catalyzes the formation of peptide bonds between adjacent amino acids, assembling them into a growing polypeptide chain.
6. Termination: Termination signals the end of protein synthesis. When a stop codon (UAA, UAG, or UGA) is encountered on the mRNA, a release factor binds to the ribosome instead of a tRNA. This causes the ribosome to dissociate, and the newly synthesized polypeptide is released.
7. Post-translational modifications: After protein synthesis, the newly formed polypeptide may undergo post-translational modifications to become a functional protein. These modifications can include folding, cleavage of excess amino acids, attachment of chemical groups (such as phosphate or carbohydrate), or association with other protein subunits to form a complex protein structure.
Overall, the steps of protein synthesis involve transcribing the DNA sequence into mRNA, processing and exporting the mRNA, initiating translation, elongating the polypeptide chain, terminating translation, and potentially modifying the resulting protein to achieve its final form and function.
The process of protein synthesis can be divided into two main steps: transcription and translation. Here are the steps involved in protein synthesis, along with a brief description of what occurs at each step:
1. Transcription:
- Initiation: The enzyme called RNA polymerase binds to the DNA at a specific region called the promoter. This signals the beginning of transcription.
- Elongation: RNA polymerase moves along the DNA strand, unwinding it and synthesizing a complementary RNA molecule. This process continues until the entire DNA sequence (gene) has been transcribed into RNA.
- Termination: Transcription concludes when RNA polymerase reaches a terminator sequence on the DNA. At this point, RNA polymerase and the newly synthesized RNA molecule are released from the DNA.
2. mRNA Processing (in eukaryotes):
- Capping: A modified guanine nucleotide is added to the 5' end of the mRNA, providing stability and aiding in the export of mRNA from the nucleus.
- Polyadenylation: A stretch of adenine nucleotides is added to the 3' end of the mRNA, which helps protect the mRNA from degradation and facilitates its translation.
- Splicing: Introns, non-coding regions within the pre-mRNA, are removed, and the remaining exons (coding regions) are joined together to form the mature mRNA. This process is carried out by a complex called the spliceosome.
3. Translation:
- Initiation: The mRNA binds to a ribosome, and the ribosome scans the mRNA until it reaches the start codon (AUG). The start codon signals the recruitment of the initiator tRNA, carrying the amino acid methionine, to the ribosome.
- Elongation: The ribosome moves along the mRNA in a 5' to 3' direction, reading the sequence of codons. Each codon is recognized by a specific tRNA molecule that carries the corresponding amino acid. The ribosome catalyzes the formation of peptide bonds between adjacent amino acids, creating a growing polypeptide chain.
- Termination: When a stop codon (UAA, UAG, or UGA) is encountered on the mRNA, it does not code for an amino acid but instead signals the end of translation. A release factor binds to the ribosome, causing the polypeptide chain to detach, and the ribosome and mRNA dissociate.
These steps collectively result in the synthesis of a functional protein from the genetic information encoded in DNA.