Questions: Multiple Sequence Alignment

Question 1

Why do progressive alignment methods like ClustalW build a guide tree before constructing the multiple sequence alignment?

• A. The guide tree determines the final phylogeny reported in the output
• B. The guide tree determines the order in which sequences are added to the growing alignment, reducing error propagation
• C. The guide tree is required by the Needleman-Wunsch algorithm
• D. The guide tree eliminates the need for gap penalties

Answer: (B)

Progressive alignment works by first aligning the most similar pair of sequences, then progressively adding more distant sequences. The guide tree (built from pairwise distances) determines this order. Aligning closely related sequences first reduces the chance of early errors that would propagate through the rest of the alignment. The guide tree is not the final phylogenetic tree — it is a rough clustering used only to order the alignment steps.

Question 2

In a multiple sequence alignment of 20 homologous proteins, a column is perfectly conserved (all 20 sequences have the same amino acid). This always indicates that the residue is essential for protein function.

• T. True
• F. False

Answer: False

Perfect conservation is strong evidence for functional or structural importance, but it is not proof. The column could be conserved due to insufficient divergence time (if all 20 species are closely related), phylogenetic bias (sampling only from one clade), or structural constraints that are not directly related to catalytic function. Additionally, some perfectly conserved residues contribute to protein folding stability rather than enzymatic activity. Conservation analysis is most informative when the sequences span broad evolutionary distances.

Question 3

Explain why the quality of a multiple sequence alignment can significantly affect downstream phylogenetic analysis.

Model answer: Phylogenetic methods infer evolutionary relationships from the pattern of substitutions across alignment columns. If the MSA incorrectly aligns non-homologous positions, the phylogenetic method will interpret random differences as evolutionary signal, producing an incorrect tree. Misplaced gaps are particularly damaging because they create false patterns of shared insertions or deletions. Regions of uncertain alignment are often excluded from phylogenetic analysis for this reason.

The MSA is the hypothesis of positional homology — it asserts which residues in different sequences descended from the same ancestral residue. Every downstream analysis (phylogenetics, conservation scoring, positive selection tests) depends on this hypothesis being correct. Garbage in, garbage out applies directly.