what is the main difference between base pairing during DNA replication and base pairing during transcription

Replication involves DNA pairing with DNA, but transcription involves DNA pairing with RNA.

Replication means copying, and it applies to DNA. The two strands in a molecule of DNA separate, and a new strand of DNA is built (synthesized) along each, using the base-pairing rules: A (adenine) with T (thymine); C (cytosine) with G (guanine).

Transcription means the synthesis of a molecule of RNA along with one of the strands of DNA. The base-pairing rules are essentially the same, but RNA has no thymine (T): it has uracil (U) instead. So the base-pairing rules for transcription are (putting DNA first): A with U; C with G; G with C; T with A.

Explanation:

the sun's light combined with the visible wavelenghts form what is called ultraviolet radiation, which are the long waves at the end of a visible light sprectrum.

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