Cell needs to divide. 1st step = replicate DNA After DNA is replicated, the DNA is packed/organized into chromosomes The cell divides and one copy of each chromosome is given to each of the daughter cells So the classic X shaped chromosome consists of two strands of DNA that are copies of each other. The copies are called sister chromatids One chromosome DNA in chromatin form One chromosome DNA in chromosome form 46 chromosomes in a human cell DNA is replicated during the S phase of the cell cycle Classic X-shaped chromosomes seen during mitosis centromere DNA condenses into chromosome form Even though, when we talk about DNA replication during cell division, we say “the chromosomes replicate”, the DNA is still in chromatin form when it is replicated. The chromatin then condenses down to make the chromosome form after the replication. One chromosome – made of two sister chromatids One chromosome – made of two sister chromatids Chromosomes are counted by the number of centromeres – not the number of chromatids During mitosis, the sister chromatids are pulled apart. One chromatid goes to one daughter cell, the other chromatid goes to the other daughter cell One chromosome (one sister chromatid) One chromosome (one sister chromatid) One chromosome (one sister chromatid) One chromosome (one sister chromatid) So, a chromosome can be one condensed piece of DNA or is can be a replicated piece of DNA attached by a centromere One chromosome One chromosome • So a human cell has 46 chromosomes. • When the cell replicates and condenses its DNA, it still has only 46 chromosomes. • Only the number of sister chromatids changes