Genetic information directs the development, functioning, and reproduction of all living organisms. This information is organized and stored, ensuring the continuity of life across generations. It dictates the characteristics and processes within every cell.
DNA The Storage Molecule
Deoxyribonucleic acid, or DNA, is the primary molecule for storing genetic information in most organisms. It serves as the blueprint for life, directing heredity.
DNA is a nucleic acid. In eukaryotic cells (animals, plants, fungi), DNA is primarily located within the nucleus, an organelle. Smaller amounts are also present in mitochondria and, in plants, in chloroplasts.
Conversely, prokaryotic cells (bacteria and archaea) lack a distinct nucleus. Their DNA is found as a single, circular chromosome in a region of the cytoplasm called the nucleoid. Some prokaryotes also contain smaller, circular DNA molecules called plasmids, which can replicate independently.
The Double Helix and Genetic Code
Genetic information is stored within the DNA molecule’s double helix structure. This structure resembles a twisted ladder, composed of two long strands. Each strand has a backbone made of alternating sugar (deoxyribose) and phosphate groups.
Attached to each sugar molecule on the backbone is one of four nitrogenous bases: Adenine (A), Thymine (T), Guanine (G), and Cytosine (C). These bases act as the “letters” of the genetic code. The two complementary DNA strands are held together by hydrogen bonds formed between these bases.
Adenine (A) pairs with Thymine (T), while Guanine (G) pairs with Cytosine (C). This complementary base pairing ensures the sequence of bases on one strand dictates the sequence on the other. This arrangement of bases along the DNA strand constitutes the genetic code, providing instructions for all cellular processes.
From Genes to Proteins
The stored genetic information within DNA is accessed and utilized by the cell. A gene is a segment of DNA that contains instructions to make a particular protein or a functional RNA molecule.
The process of gene expression, which converts genetic information into functional products, involves two main steps: transcription and translation. During transcription, the DNA sequence of a gene is copied into a messenger RNA (mRNA) molecule. This mRNA molecule carries the genetic message from the nucleus (in eukaryotes) to the ribosomes in the cytoplasm.
Translation then occurs at the ribosomes, where the mRNA sequence is used to synthesize proteins. This process involves transfer RNA (tRNA) molecules, which bring specific amino acids to the ribosome, matching them to the mRNA. Ribosomal RNA (rRNA) forms the ribosome’s core, assembling amino acids into a polypeptide chain that then folds into a functional protein.
Copying and Inheriting Genetic Information
The duplication and transmission of stored genetic information to new cells and future generations occurs through DNA replication. This process is semi-conservative, meaning each new DNA molecule consists of one original strand from the parent molecule and one newly synthesized strand.
During replication, the double helix unwinds, and each original strand serves as a template for a new complementary strand. Enzymes like DNA polymerase add new nucleotides following the base pairing rules (A with T, G with C), ensuring accurate copying. This ensures every new cell receives a complete and identical set of genetic instructions.
DNA replication is important for heredity, allowing traits to be passed from parents to offspring. It also supports growth, development, and the repair of damaged tissues, by ensuring new cells have the same genetic information as the original cells.