7.2: DNA Replication and Gene Expression

Genetic information flows through two fundamental processes: replication and expression.

DNA replication occurs when cells create exact copies of DNA molecules before cell division. The double helix unwinds, separating the two strands at their base pairs and exposing each base. The enzyme DNA polymerase then moves along each exposed strand, using it as a template to synthesize a new complementary strand. This ensures that each daughter cell receives a complete, identical copy of the genetic information.

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Protein synthesis, or gene expression, is the process by which genetic information encoded in DNA is ultimately translated into functional proteins. This occurs in two main stages:

During transcription, a specific DNA segment serves as a template for the synthesis of a complementary strand of ribonucleic acid (RNA). The initial RNA transcript is then processed to remove non-coding regions, yielding messenger RNA (mRNA) that carries the instructions for protein synthesis. Simultaneously, DNA codes for transfer RNA (tRNA) molecules, each designed to transport specific amino acids.

During translation, the mRNA exits the nucleus and binds to a ribosome in the cytoplasm. As the ribosome moves along the mRNA sequence, it reads the genetic code in groups of three bases called codons. Each codon signals a specific tRNA molecule to deliver its corresponding amino acid. The ribosome links these amino acids together in the precise order dictated by the mRNA, assembling them into a functional protein chain.

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