DNA Replication, Transcription, and Genetic Information Flow
DNA: The Blueprint of Life
Griffith’s Experiment
Griffith worked with two strains of bacteria: a rough strain (R) and a smooth strain (S). The R strain lacked a polysaccharide capsule and was harmless to mice. The S strain, possessing a polysaccharide capsule, caused fatal infections. When dead S strain bacteria were mixed with live R strain bacteria and injected into mice, the mice died. This suggested a transforming principle from the dead S strain bacteria had altered the live R strain.
Hershey-Chase Experiment
Hershey and Chase used radioactive markers to determine whether DNA or protein was the genetic material. They labeled phage DNA with 32P and phage protein with 35S. After infecting bacteria with the labeled phages, they found that the radioactive DNA had entered the bacterial cells, while the protein remained outside. This confirmed that DNA carries genetic information.
The Gene: Unit of Heredity
A gene is the basic unit of heredity, a region of a chromosome that carries genetic information. Genes contain coding regions (exons) and non-coding regions (introns). Prokaryotic genes are typically continuous, while eukaryotic genes often have exons interrupted by introns. Introns may play a role in increasing genetic variability through meiotic recombination.
Genetic Information Flow
The central dogma of molecular biology describes the flow of genetic information from DNA to RNA to protein. Reverse transcriptase, an enzyme found in some RNA viruses, can synthesize DNA from an RNA template. This enzyme is important in viral infections and the development of some cancers.
DNA Replication
DNA replication is the process of creating two identical DNA molecules from a parent molecule. It follows a semi-conservative model, where each new molecule contains one original strand and one newly synthesized strand.
Replication in Prokaryotes
Several enzymes are involved in prokaryotic DNA replication:
- DNA polymerases: Catalyze the formation of phosphodiester bonds.
- Helicases: Unwind the DNA double helix.
- Primase: Synthesizes RNA primers to initiate replication.
- SSB proteins: Keep the DNA strands separated.
- DNA ligase: Joins DNA fragments.
Replication is bidirectional and occurs continuously on the leading strand and discontinuously on the lagging strand, forming Okazaki fragments.
Replication in Eukaryotes
Eukaryotic DNA replication is similar to prokaryotic replication but involves multiple replication origins and five types of DNA polymerases. Telomeres, the ends of chromosomes, shorten with each replication cycle, a phenomenon associated with aging.
Transcription: DNA to RNA
Transcription is the process of synthesizing mRNA from a DNA template. This process requires:
- A DNA template strand.
- RNA polymerase enzymes.
- Ribonucleotide triphosphates (A, G, C, U).
The mRNA carries the genetic information from DNA to the ribosomes for protein synthesis.