5 Genetics and Molecular Biology

How genetic information works

Genetics follows the flow of biological information: DNA can be copied, used to make RNA, and expressed through proteins. Inheritance passes genetic information to the next generation, while mutations introduce changes. Proteins perform many structural and chemical functions in cells, linking the information in DNA to cellular traits and activities.

DNA replication

Before a cell divides, it copies its DNA so that each daughter cell can receive a genome. DNA has two complementary strands: adenine pairs with thymine, and cytosine pairs with guanine. In semiconservative replication, the strands separate, and each serves as a template for a new complementary strand. The result is two DNA molecules, each with one original strand and one new strand.

Several enzymes coordinate the process. Helicase opens the double helix, and DNA polymerase adds complementary nucleotides. DNA polymerase builds DNA only in the 5′-to-3′ direction, so the two strands are copied differently at the replication fork. Short fragments made on one strand are later joined by ligase. Proofreading and repair systems reduce copying errors, but do not eliminate them.

Takeaway: Complementary base pairing enables DNA copying, while enzymes coordinate and help check the process.

From DNA to RNA

A gene is a region of DNA that produces a functional RNA, often one that carries instructions for making a protein. During transcription, RNA polymerase uses one DNA strand as a template to make a complementary RNA molecule. RNA uses uracil in place of thymine. For example, the DNA template sequence 3′-TAC-5′ can produce the RNA sequence 5′-AUG-3′.

In eukaryotic cells, an initial RNA transcript is usually processed before it is translated. Introns are removed, exons are joined, and protective modifications are added to the RNA ends. Mature messenger RNA then leaves the nucleus. Gene regulation at multiple stages means that a gene is not necessarily active in every cell or at every time.

Takeaway: Transcription transfers information from DNA into RNA, and RNA processing and gene regulation influence what happens next.

From RNA to protein

During translation, a ribosome reads messenger RNA in groups of three nucleotides called codons. Each codon specifies an amino acid or a stop signal. Transfer RNAs pair their anticodons with messenger RNA codons and deliver the corresponding amino acids. The ribosome links the amino acids into a polypeptide, which folds and may undergo further modification to become a functional protein.

For example, AUG is a start codon and specifies methionine. The ribosome reads subsequent codons in order until it encounters a stop codon. DNA sequence therefore influences a protein’s amino-acid sequence, while gene regulation and RNA processing also affect the final product.

Takeaway: Translation converts the codon sequence of messenger RNA into an amino-acid chain.

Mutations and their effects

A mutation is a lasting change in DNA sequence. Substitutions change one or more bases; insertions add bases, and deletions remove them. A substitution may be silent, leaving the encoded amino acid unchanged; missense, changing an amino acid; or nonsense, creating a premature stop codon. An insertion or deletion that is not a multiple of three can shift the reading frame and alter many downstream codons.

Effects depend on a mutation’s location and consequences. A mutation may alter a protein, change when or how much a gene is expressed, or have no noticeable effect. Mutations can arise during replication or from DNA damage. A mutation in a cell that contributes to eggs or sperm may be inherited by offspring. A mutation in an ordinary body cell is generally not passed to offspring, although it can affect the individual’s tissues.

Takeaway: Mutations vary in their effects, and whether they can be inherited depends in part on which cells carry them.

Inheritance and allele combinations

Genes occupy particular locations, called loci, on chromosomes. Alternative versions of a gene are alleles. Many animals, including humans, inherit one set of chromosomes from each parent and commonly carry two alleles for a gene on an autosome. An individual’s genotype is its allele combination; its phenotype is an observable characteristic that can reflect both genes and environment.

During meiosis, homologous chromosomes and their alleles separate into different reproductive cells. This is the basis of the law of segregation. Different chromosome pairs usually assort independently, although genes close together on the same chromosome can be inherited together more often. Recombination can separate them. Fertilization combines reproductive cells and restores paired chromosome sets.

For a simple example, suppose an allele represented by $A$ is dominant and an allele represented by $a$ is recessive. A parent with genotype $Aa$ can pass either allele to a reproductive cell. If two $Aa$ individuals have offspring, each possible combination—$AA$, $Aa$, $aA$, or $aa$—has probability $\frac{1}{4}$ under this simple model. Dominance describes how alleles affect a particular trait in a heterozygote; it does not mean an allele is more common, stronger, or better. Many traits involve multiple genes and environmental influences, so they do not follow this simple pattern.

Takeaway: Inheritance combines the separation and reshuffling of alleles, while traits may also be shaped by environmental factors.