Genetic Code, DNA Replication and Gene Expression: Key Concepts

Gene Expression and the Control of Protein Synthesis

Take care genetic: the DNA must contain all the information on the role of the infrastructure and organization of the cell, the nature of the stable phenotype, form of the cell and organism, and the determination of protein synthesis. Therefore, genetic information must determine the presence and quantity of effective proteins in the cell.

• Replication: the DNA must be very precise so that the progeny have the same genetic information. The parent cell copies itself with fidelity, so each daughter cell receives an exact copy of the first.
• Variation: without the ability to vary, bodies would not change and thus could not evolve.

DNA Transcription

Through the process of transcription, that information is transferred from the DNA molecule to a single-stranded molecule called messenger RNA (mRNA). Translation converts the information in the mRNA sequence into an amino acid sequence.

Example: Gene Control in Escherichia coli

An example of the control of gene expression is found in Escherichia coli, the enzymes that metabolize lactose. This bacterium uses glucose as its carbon source. If we use lactose instead of glucose in the medium, the bacterium synthesizes enzymes capable of degrading lactose. These enzymes are not present or are synthesized only in response to the presence of lactose and the absence of glucose.

In short: these genes encode enzymes that are active only if lactose is present and glucose is absent; otherwise they remain inactive.

Recombinant DNA

Recombinant DNA is a new molecule formed by combining DNA from two different biological sources. Recombination already occurs naturally in meiosis, when chromosomes exchange DNA fragments. However, the term recombinant DNA refers to the artificial combination of DNA from different sources.

To produce recombinant DNA, scientists usually use certain enzymes called restriction enzymes (restriction endonucleases). Bacteria produce these enzymes to degrade foreign DNA. They recognize specific DNA sequences, generally 4 to 10 base pairs long, and cut the DNA at those sites. While the bacterial DNA is protected by methylation, the DNA of the virus is degraded, so the bacterium is not harmed.

Various types of restriction enzymes are known. Types I and III cut the DNA at a site different from their recognition sequence, while type II enzymes cut the DNA at the same place they recognize. The recognition sites of type II enzymes are inverted repeats.

DNA Replication

DNA must replicate accurately so that the daughter cells have the same information as the parent cells. Watson and Crick's double helix model allows the strands to separate, and each strand serves as a template for a new complementary strand. Several replication models have been proposed: conservative, semiconservative, and dispersive.

• Conservative model: the original strands remain together after replication.
• Semiconservative model: each new DNA molecule consists of one original strand and one newly synthesized strand.
• Dispersive model: the original double helix breaks into segments, and new segments are synthesized between the original fragments.

The experiment by Meselson (1930–) and Stahl (1929–) in 1958 allowed scientists to determine how DNA replicates. They used Escherichia coli cultured in a medium whose only nitrogen source was ¹⁵NH₄Cl (ammonium chloride). The normal nitrogen isotope (¹⁴N) was replaced by the heavy isotope (¹⁵N). The DNA of these bacteria was denser and was marked with ¹⁵N. The bacteria were then transferred to a medium with ¹⁴N, allowing replication to proceed normally.

Over time, samples of bacteria were taken from the first and second generations, lysed, and their DNA was analyzed by density-gradient centrifugation in cesium chloride. In the first generation, the DNA had an intermediate position, showing one heavy and one light strand. In the second generation, two bands were observed: one with intermediate density and another with the density of ¹⁴N DNA. These results are consistent with the semiconservative model.

The Genetic Code

The information needed to produce proteins is encoded in the genes found in the cell's genome. Structural information is transcribed from DNA to mRNA, and then translated into an amino acid sequence. The genetic code is the set of rules by which the nucleotide sequence of DNA is converted into the amino acid sequence of a protein. Each three-nucleotide sequence, called a codon, specifies a particular amino acid.

The genetic code is virtually universal among all studied organisms. The only exceptions found are in some mitochondrial DNA. This similarity supports the hypothesis of a common ancestor, since the code is biochemically operational in nearly all living beings.