Introduction: The Blueprint of Life and Its Expression
Welcome to Lesson 12. In this lesson, we delve into the core processes that govern the continuity and expression of life's genetic blueprint. First, we will examine DNA replication, the elegant mechanism by which a cell faithfully copies its entire genome before dividing. Then, we will explore gene expression—the "Central Dogma" of molecular biology—which describes the two-step process of transcription and translation, by which the genetic information encoded in DNA is used to synthesize functional proteins.
Part 1: DNA Replication
Before a cell can divide, it must make a complete and accurate copy of its DNA. This process ensures that each daughter cell receives a full set of genetic instructions.
1.1 The Semiconservative Mechanism
DNA replication follows a semiconservative model. This means that when the double helix unwinds, each of the two parental strands serves as a template for the synthesis of a new, complementary strand. The result is two new DNA molecules, each consisting of one "old" (parental) strand and one "new" strand.
Diagram: The Semiconservative Model of DNA Replication
1.2 Key Enzymes and the Replication Fork
Replication is a complex process orchestrated by a team of enzymes.
| Enzyme/Protein | Function |
|---|---|
| Helicase | Unwinds the DNA double helix at the replication fork. |
| Single-Strand Binding Proteins | Bind to the separated DNA strands to keep them from re-pairing. |
| Topoisomerase | Relieves the strain caused by unwinding by breaking, swiveling, and rejoining DNA strands ahead of the replication fork. |
| Primase | Synthesizes a short RNA primer to provide a 3' end for DNA polymerase to start from. |
| DNA Polymerase III | Synthesizes the new DNA strand by adding nucleotides to the 3' end of a pre-existing chain. |
| DNA Polymerase I | Removes the RNA primers and replaces them with DNA nucleotides. |
| DNA Ligase | Joins the Okazaki fragments on the lagging strand into a continuous strand. |
DNA polymerases can only add nucleotides to the free 3' end of a growing strand. This means a new DNA strand can only elongate in the 5' to 3' direction. This creates a challenge at the replication fork:
- The leading strand is synthesized continuously in the 5' to 3' direction, moving toward the replication fork.
- The lagging strand is synthesized discontinuously as a series of short segments called Okazaki fragments, each synthesized in the 5' to 3' direction away from the replication fork. DNA ligase then joins these fragments together.
Diagram: The Replication Fork
Part 2: Gene Expression - The Central Dogma
Gene expression is the process by which information from a gene is used in the synthesis of a functional gene product, often a protein. This is summarized by the Central Dogma of molecular biology.
Diagram: The Central Dogma
2.1 Transcription: From DNA to RNA
In transcription, the enzyme RNA polymerase synthesizes a messenger RNA (mRNA) molecule that is complementary to a gene on one of the DNA strands (the template strand). In eukaryotes, transcription is followed by RNA processing, where introns are removed (splicing), and a 5' cap and a poly-A tail are added to the mRNA before it leaves the nucleus.
2.2 Translation: From RNA to Protein
In translation, the sequence of nucleotides in mRNA is "read" by ribosomes in the cytoplasm. The genetic information is read as three-nucleotide "words" called codons. Transfer RNA (tRNA) molecules, each carrying a specific amino acid and having an anticodon that complements an mRNA codon, deliver the amino acids to the ribosome. The ribosome catalyzes the formation of peptide bonds, linking the amino acids to form a polypeptide chain.
Diagram: Translation at the Ribosome
Interactive Practice Quiz
Test your understanding of DNA replication and gene expression. Choose the best answer for each question (A-E) and then submit to see your results.