6 Gene Expression and Regulation

The shape and packaging differ, but each DNA structure preserves information in its base sequence. Before cell division or reproduction, the information must be copied and then distributed to descendant cells or organisms.

Pairing one purine with one pyrimidine keeps the paired structure consistent. Hydrogen bonds hold complementary bases together, yet the strands can separate so each can serve as a template. The conserved A-T and G-C rules therefore connect molecular structure to accurate hereditary copying.

Because the parental strands are antiparallel but DNA polymerase synthesizes only 5′→3′, the two new strands cannot be made in the same physical pattern. The leading strand is synthesized continuously toward the advancing fork. The lagging strand is synthesized discontinuously as short fragments, which ligase joins into one continuous strand.

RNA polymerase uses a single DNA strand as the template. It reads that template in the 3′→5′ direction while adding complementary RNA bases so the RNA grows in the 5′→3′ direction. Thus, template reading and RNA synthesis proceed in opposite directions.

Ribosomes translate mRNA in the cytoplasm of prokaryotic and eukaryotic cells and on the cytoplasmic surface of rough ER in eukaryotes. In prokaryotes, translation can begin while the same mRNA is still being transcribed. Many amino acids have more than one codon, and the genetic code is nearly universal—evidence of common ancestry.

Retroviruses are a special case: reverse transcriptase copies the viral RNA genome into DNA. That DNA integrates into the host genome and can then be transcribed and translated to support production of new viral progeny.

Cells with the same genome can develop different phenotypes because they express different combinations of genes at different levels. Tissue-specific proteins produce observable cell differentiation, and transcription factors induced during development can trigger a sequence of later gene-expression changes.

In an operon, shared control is positioned with the grouped genes, so one regulatory response affects the group. In eukaryotes, a transcription factor can act wherever its matching regulatory sequence occurs, allowing genes at different locations to respond to the same signal.

A promoter or enhancer sequence can be upstream or downstream of the transcription start site. Its regulatory function depends on recognition by the appropriate regulatory molecules, not simply on being located on one particular side of the gene.

Cells can contain the same genetic information yet develop different phenotypes because regulation determines which genes are active. A cell that produces one set and amount of proteins develops different structures and functions from a cell producing another set, supporting cell specialization.

Certain small RNA molecules also regulate gene expression. Their regulatory role adds another way for cells to influence which gene products accumulate and thus which functions are expressed.

A changed codon can alter the type or amount of protein produced, which may alter phenotype. The size of the DNA change alone does not determine severity: a single substitution can create a premature stop, while another substitution can be silent.

Mutations supply genetic variation. The same genotype change can be beneficial, detrimental, or neutral depending on the environment because environmental conditions determine whether the resulting phenotype affects performance, survival, or reproduction.

These processes increase genetic variation; they do not guarantee an advantage. If a resulting phenotype increases survival or reproductive success in a particular environment, individuals carrying that genetic change can contribute more descendants, so the variant may become more common.

Each PCR cycle first separates the DNA strands, then allows primers to anneal to complementary target sequences, and finally extends new DNA. Repeating the cycle amplifies the target fragment rather than merely moving or separating it.