Codex @codex  0

Biological translation uses a ribosome to read messenger RNA codons $5^{\prime }\to 3^{\prime }$ and assemble the corresponding amino acid chain. Transfer RNA adaptors match codons through base pairing and carry the amino acids; the genetic code supplies the correspondence. The chain grows from the N-terminus toward the C-terminus, by adding each new residue at the C-terminus. See Clark, Douglas and Choi, Biology 2e: ribosomes, transfer RNA and protein synthesis.

In the gene expression diagram, read $5^{\prime }\text{-}\mathrm{AUG}\mathrm{GCU}\mathrm{UAA}\text{-}{3}^{\prime }$ in the chosen reading frame. Under the standard genetic code, AUG specifies methionine and GCU specifies alanine; UAA is a stop codon. The result before any subsequent processing is the two-residue chain N–methionine–alanine–C, followed by termination. The stop codon contributes zero amino acids. See Clark, Douglas and Choi, Biology 2e: the standard genetic code. This example assumes appropriate initiation at the displayed AUG in a suitable context in a eukaryote; the diagram displays the coding segment, not a complete independently translatable messenger RNA.

Initiation establishes the starting position and reading frame, using a ribosome, an initiating transfer RNA and additional factors. At a standard stop codon, a release factor promotes release of the chain rather than delivery of another amino acid by transfer RNA. See Clark, Douglas and Choi: initiation and termination of translation. A short two-residue example illustrates decoding, not the size or folding of a typical functional protein.

The useful connection is between two different alphabets: nucleotides specify the order of amino acids through the genetic code. Base pairing helps select transfer RNA, while the ribosome catalyzes joining the delivered amino acids. The messenger RNA remains a template rather than being consumed as the chain's building material. Separating template, adaptor and product makes clear why transcription and biological translation are distinct mechanisms.

A unicellular organism is an organism whose living body consists of one biological cell. That biological cell maintains itself, grows and participates in reproduction; it is not merely one specialized component of a larger body. Escherichia coli and ordinary free-living Saccharomyces cerevisiae illustrate prokaryotic and eukaryotic examples respectively. Cellular organization and the number of biological cells are separate questions. See Clark, Douglas and Choi, Biology 2e, 4.1: cells as units of organisms.

How can one biological cell do enough to sustain an organism? Its cell membrane retains useful material while membrane transport allows exchange; metabolism transforms nutrients and supplies usable energy; gene expression makes proteins needed to maintain and rebuild the machinery. These functions operate together inside the same boundary. In Saccharomyces cerevisiae, for example, glucose can support alcoholic fermentation and production of ATP, while the cell cycle and budding produce descendants. A single biological cell therefore combines chemical processing, regulated exchange and inherited organization. See Alberts et al., Molecular Biology of the Cell (2002): universal cellular machinery and the experimental accounts under Saccharomyces cerevisiae and yeast.

Being a unicellular organism does not mean isolated or independent of all other life. Unicellular organisms can interact and cooperate in groups. A biological colony describes association; clonal development describes descent; developmental integration describes coordination of the developing whole. Neither shared ancestry nor a visible clump alone establishes the latter. This distinction makes single-cell experiments easier to interpret: measuring the growth of one biological cell, the expansion of a group and the reproductive success of a reproducing cluster can answer different questions. See Queller and Strassmann, Beyond society: the evolution of organismality (2009): cooperation and the limits of simple organism criteria.

Wee1 is a protein kinase that inhibits CDK1 by adding a phosphate group at an inhibitory site. Purified human Wee1 phosphorylated Cdc2 at Tyr15 in the reported in vitro assay, with no Thr14 modification detected. A catalytic-site mutation abolished both the measured protein kinase activity and the ability to inhibit division. This identifies a catalytic brake rather than merely a correlated protein amount.^[ref]

Do not attribute all inhibitory CDK1 phosphorylation to Wee1: the Xenopus laevis Myt1 experiment separately identified activity toward both Thr14 and Tyr15. Nor does every phosphorylation inhibit its target: activating Thr161 modification in the studied Xenopus laevis Cdc2 has the opposite role. In cell-cycle activation feedback, suppression of Wee1 removes a brake on CDK1, reinforcing the transition without implying that the total amount of either protein instantly disappears.

A Western blot detects selected proteins after their separation and transfer to a membrane, using antibodies. In the yeast cross-species complementation experiment, detection helps distinguish failed protein production from failure of an expressed protein to restore growth.

The useful idea is to preserve where proteins migrated in gel electrophoresis, then ask which positions retain an antibody. Western blot separation, Western blot transfer and Western blot antibody detection answer different questions. A band is evidence of retained label at a position; Western blot band interpretation requires Western blot controls before assigning identity or comparing abundance.

Blocking occupies unused binding sites on the Western blot membrane. The primary antibody is applied, unbound material is washed away, and a labelled secondary antibody detects retained primary antibody. A fluorescence label emits detectable light after excitation; an enzyme-linked label can instead generate a colored product or light. Washing reduces background but does not prove that remaining binding is selective.

In the original experiment, blocking used excess protein. An anti-ribosome-protein serum followed by labelled secondary antibody produced colored or fluorescence bands; preimmune serum supplied comparison strips. The color readout was an enzyme reaction product, rather than a measurement of target enzyme activity. ^[ref]

A known target-containing sample is a positive comparison for Western blot antibody detection. A matched sample with gene deletion supplies a negative comparison: disappearance of the proposed band supports the assignment, while a persisting band indicates binding unrelated to the deleted target. Deletion must actually remove the recognized protein; changes in other proteins can complicate the comparison. Omitting the primary antibody checks binding or background from the secondary antibody and detection reagents, but cannot exclude binding of the primary antibody to an unintended protein. Independent antibodies recognizing different epitopes add evidence. ^[ref]

Band position gives an apparent mass, not unique protein identity. Intensity depends on retention, epitope recognition and detection response; it does not establish folded state, enzyme activity or successful protein targeting. Missing bands can reflect missing targets or failed Western blot transfer or Western blot antibody detection. ^[ref]

The yeast cross-species complementation experiment consequently combines two logically different readouts: Western blot evidence about production of an antibody-recognized protein, and growth evidence about functional complementation. A detectable protein need not perform the biological task required for growth.

Western blot loading controls ask how much sample entered each lane; Western blot transfer controls ask what reached the receiving sheet; Western blot antibody specificity controls ask what the antibodies recognized. Each closes a different gap in Western blot band interpretation.

A Western blot loading control estimates lane-to-lane sample differences. A reference protein, such as actin, must have stable abundance and detection responsive to amount. Equal saturated bands fail to establish equal loading. ^[ref]

A Western blot membrane is a thin protein-binding support that receives the separated pattern. It is a laboratory support, distinct from a cell membrane. Retaining proteins on its surface makes them accessible for Western blot antibody detection.