How many DNA molecules in a chromosome?Watch
I am confused as different websites say different things. I thought that a chromosome is made up of 2 chromatids, and each chromatid has 1 DNA molecule, therefore one chromosome has 2 DNA molecules. But some websites say chromosome has 1 DNA molecule. Which is correct?
During interphase (where the cell spends most of it's time) the DNA is found in a loose form known as chromatin. All the chromosomes are in a loose form and so it would be incorrect to call the DNA chromosomes at this point - the structure is known as chromatin. During the S phase (synthesis phase) of interphase, the DNA is replicated. This means that the soon-to-be chromosomes (remember, there are no chromosomes yet) have replicated. Before they replicated, there was only one copy of the DNA and so there was only one molecule of DNA for every soon-to-be chromosome. After S phase, the DNA has replicated and so there are now two molecules of DNA for every soon-to-be chromosome. Then, in prophase, the DNA coils and condenses into tightly packed structures - these are chromosomes. Each chromosome has two chromatids that are joined by a centromere. These chromatids are identical to one another. You could view each chromatid as a separate molecule of DNA. Although there are two molecules of DNA attached to one another, the two sister chromatids are still referred to as one chromosome. Homologous chromosomes then pair up. During metaphase, the spindle attaches to the centromeres of the chromosomes and aligns chromosomes in homologous pairs at the equator of the cell. The spindle then pulls the sister chromatids apart to opposite poles of the cell. The sister chromatids are now separate from one another and now become separate chromosomes.