02 Molecular Biology for Biotechnology

DNA stores hereditary information

DNA stores hereditary information in the order of its four bases: adenine (A), cytosine (C), guanine (G), and thymine (T). It usually forms a double helix with complementary strands: A pairs with T, and C pairs with G. Because each strand can guide construction of its complement, cells copy DNA by using the original strands as templates before cell division. Complementary pairing also underlies laboratory methods for detecting or copying chosen DNA sequences.

Genomes and gene structure

A genome is an organism’s complete set of DNA. It includes genes, regulatory sequences, and other noncoding sequences; not every stretch of DNA encodes a protein. In eukaryotic cells, most DNA is organized into chromosomes in the nucleus, with additional DNA in some organelles. Bacteria typically have a main chromosome and may also carry smaller DNA molecules called plasmids. Genome size does not by itself indicate an organism’s complexity.

Genes, RNA products, and sequence variation

A gene is a DNA region that produces a functional product: an RNA molecule or, through RNA, a protein. Protein-coding genes commonly contain information for an RNA that is translated into a protein. Other genes produce functional RNAs, including ribosomal RNA and transfer RNA. A gene’s surrounding regulatory sequences help determine when, where, and how much product is made.

In eukaryotes, many genes contain exons and introns. Introns are removed from the initial RNA transcript, while exons are joined together. Alternative splicing can join exons in different combinations, allowing one gene to yield different RNA products.

Different versions of a DNA sequence are called alleles. A change in sequence—a mutation or variant—may have no noticeable effect, alter gene activity, change a protein, or disrupt a gene product. Its effect depends on where the change occurs and the biological context.

From gene expression to protein

Gene expression is the use of genetic information to make a functional RNA or protein. For a protein-coding gene, it commonly proceeds through transcription, RNA processing, and translation.

1. Transcription: RNA polymerase uses one DNA strand as a template to synthesize a complementary RNA molecule. The promoter and other regulatory elements help control where transcription begins and how actively it proceeds.

2. RNA processing: In eukaryotes, an initial RNA transcript is generally processed to produce mature messenger RNA (mRNA). Processing commonly includes adding a cap and poly-A tail and removing introns. Bacterial gene expression typically lacks this same nuclear RNA-processing stage.

3. Translation: Ribosomes read mRNA in three-base units called codons. Transfer RNAs help match codons to amino acids, which are joined into a protein.

The relationship is often summarized as $\text{DNA} \to \text{RNA} \to \text{protein}$, but this is a useful outline rather than a complete description of all biology: some genes function through RNA, and gene activity is regulated at multiple stages. Cells control which genes are active and how much product they make, so cells with essentially the same genome can have different characteristics and functions.

Applying molecular biology in biotechnology

Biotechnology uses the cell’s information systems to study, alter, or produce biological molecules. Researchers can identify and amplify DNA sequences, determine their order by sequencing, or combine a chosen gene with a DNA vector to make recombinant DNA. A vector can carry the gene into a host cell; if the gene is expressed, the cell may produce the encoded RNA or protein.

A production strain can be engineered to make a useful protein by introducing an appropriate gene and providing the conditions needed for its expression. Success depends on more than inserting the coding sequence: regulatory signals, the host organism, RNA processing, and protein folding can all affect the final product.

Molecular methods answer different questions

Molecular tests use different methods to answer different questions. PCR uses sequence-specific primers and DNA polymerase to amplify a selected region. DNA sequencing reads nucleotide order. Analysis of RNA or protein can help determine whether a gene is expressed and what product is present.

DNA analysis examines genetic sequence, while RNA and protein measurements provide evidence about gene activity and its products.