What do the two steps of transcription followed by translation do?
The two steps of transcription followed by translation are essential processes in gene expression, which is the mechanism by which genetic information encoded in DNA is converted into functional proteins.
1. Transcription: In this initial step, the DNA sequence of a gene is copied into a molecule called messenger RNA (mRNA). The process of transcription involves several key players:
- RNA polymerase: This enzyme binds to a specific region on the DNA called the promoter and "reads" the DNA strand. As it moves along the DNA template strand, it synthesizes a complementary mRNA strand by adding the appropriate RNA nucleotides (A, U, G, and C) based on the DNA template.
- DNA template strand: One of the two strands of DNA, known as the antisense or template strand, is used as a template for mRNA synthesis. The mRNA strand is complementary to this template strand.
- mRNA transcript: The resulting mRNA molecule is a complementary copy of the gene's DNA sequence, with thymine (T) in the DNA replaced by uracil (U) in the mRNA. The mRNA transcript is now ready to be processed and exported from the nucleus.
2. Translation: The mRNA transcript formed in transcription carries the genetic code from the nucleus to the ribosomes in the cytoplasm. Translation takes place at the ribosomes and involves the conversion of mRNA into a functional protein. The process of translation consists of three steps:
- Initiation: The mRNA transcript, together with the ribosomal subunits and initiation factors, assembles at the start codon (usually AUG) on the mRNA. This serves as a signal to begin translation.
- Elongation: The ribosome moves along the mRNA molecule, sequentially matching each codon (a triplet of mRNA nucleotides) with the corresponding transfer RNA (tRNA) molecule. Each tRNA carries an amino acid that matches the codon, forming a polypeptide chain. The ribosome catalyzes the formation of peptide bonds between adjacent amino acids, resulting in the growing polypeptide chain.
- Termination: When the ribosome reaches a stop codon (UAA, UAG, or UGA) on the mRNA, it signals the end of translation. The polypeptide chain is then released, and the ribosome dissociates from the mRNA.
In summary, transcription converts the DNA sequence of a gene into an mRNA molecule, while translation uses the mRNA template to synthesize a polypeptide chain, ultimately forming a functional protein.