Final answer:
Thymine is the base not found covalently linked to a ribose sugar, as it is exclusive to DNA, which contains deoxyribose; whereas RNA, which has ribose, uses uracil instead.
Step-by-step explanation:
In a polynucleotide chain, which base is not found covalently linked to a ribose sugar? The answer is thymine. Thymine is a base found exclusively in DNA, and it is attached to the sugar deoxyribose rather than ribose. In RNA, which contains ribose sugar, thymine is replaced by uracil.
RNA and DNA differ in the sugar present in their nucleotides: RNA has a ribose sugar while DNA has deoxyribose. Deoxyribose lacks the hydroxyl group (-OH) on the 2' carbon that is present in ribose, which is one of the key differences between these two nucleic acids.