Gene expression is the fundamental process by which the information in our DNA is converted into functional products like proteins. This process occurs in two main steps: transcription and translation. Transcription is the first step, where DNA is used as a template to create RNA. Translation is the second step, where the RNA is used to synthesize proteins. Together, these processes form what's known as the central dogma of molecular biology.
Transcription is the first step in gene expression, where a segment of DNA is copied into RNA by the enzyme RNA polymerase. This process begins with initiation, where RNA polymerase binds to a specific DNA sequence called the promoter. During elongation, RNA polymerase moves along the DNA template strand, adding complementary RNA nucleotides to create a growing RNA strand. In RNA, the base uracil replaces thymine found in DNA. Finally, during termination, the RNA polymerase reaches a termination sequence and releases the newly synthesized RNA molecule.
Translation is the second major step in gene expression, where the genetic code in mRNA is decoded to produce a specific amino acid chain, or polypeptide. This process occurs on ribosomes in the cytoplasm. Translation begins with initiation, where the ribosome assembles at the start codon, typically AUG, which codes for the amino acid methionine. During elongation, transfer RNAs or tRNAs bring specific amino acids to the ribosome based on the mRNA codon sequence. Each tRNA has an anticodon that pairs with a complementary mRNA codon. As the ribosome moves along the mRNA, amino acids are linked together by peptide bonds, forming a growing polypeptide chain. Translation ends at a stop codon, where the completed protein is released from the ribosome.
The genetic code is the set of rules by which information encoded in mRNA sequences is translated into proteins. It has several key features. First, it's a triplet code, where each group of three nucleotides, called a codon, specifies one amino acid. Second, the code is degenerate, meaning multiple codons can code for the same amino acid. For example, both CUU and CUC code for leucine. Third, the genetic code is nearly universal, being almost identical across all organisms from bacteria to humans. Fourth, it's unambiguous - each codon specifies only one amino acid. The start codon AUG, which codes for methionine, initiates protein synthesis, while three stop codons - UAA, UAG, and UGA - signal the end of translation.
To summarize what we've learned about gene expression: First, gene expression is the process by which the information in DNA is converted into functional proteins through the two key steps of transcription and translation. During transcription, which occurs in the nucleus of eukaryotic cells, RNA polymerase creates an RNA copy of a specific DNA segment. In translation, which takes place in the cytoplasm, ribosomes read the mRNA sequence and build proteins according to the genetic code. This genetic code uses triplet codons to specify amino acids and has universal properties that are largely consistent across all living organisms. These fundamental processes of transcription and translation are essential for all cellular functions, from basic metabolism to complex developmental processes, and are highly conserved across species throughout evolution.