Question

complete the sentence.
at the end of the s phase, cells have twice as much dna and twice as many
as cells at the end of the
phase.
options: nuclear envelopes, centrioles

Explanation:

The S phase (synthesis phase) of the cell cycle is when DNA replication occurs, so cells have twice as much DNA. Centrioles are also replicated during the S phase (in animal cells), so the number of centrioles doubles. The G₁ phase is before S phase, so at the end of S phase, cells have twice as much DNA and twice as many centrioles as at the end of G₁ phase. Nuclear envelopes don't double in number during S phase in terms of the relevant comparison here (they break down in prophase of mitosis, and their replication is not the main change in S phase like DNA and centrioles). So the first blank (after "twice as many") should be "centrioles" and the second blank (after "end of the") should be "G₁".

Answer:

First blank: centrioles
Second blank: G₁