Video narration
After DNA replication, does a human cell have 46 chromosomes-or 92? Imagine you have 46 instruction books. You photocopy every page, so now you have twice the information. But copying the pages doesn't make the original book count 92. That's the distinction in a cell. During G1, the cell has 46 chromosomes and DNA content of 2C. In S phase, each DNA molecule is copied, so the DNA content rises from 2C to 4C. Each copied chromosome now consists of two identical sister chromatids-matching DNA copies-joined at a centromere, the point holding them together. Here's the counting rule: chromosome number follows the centromeres. As long as those sister chromatids remain joined, they count as one chromosome, not two. So after S phase, the cell has 46 chromosomes-not 92. The DNA has doubled; the chromosome count has not.