Explain the difference between chromatin, chromatids, chromosomes and homologous pairs.

Genetic information is stored in the nucleus as DNA. Humans have 46 molecules of DNA in each cell; each molecule known as a chromosome. When the cell is in interphase of the cell cycle, the DNA is in the chromatin form: very loosely packaged on histone proteins and almost impossible to distinguish between separate strands of DNA by using a microscope. During interphase, the DNA is replicated, resulting in 2 copies of each DNA strand. In prophase of mitosis, each DNA strand condenses down to become much shorter and thicker by winding up much more tightly, in a process called supercoiling. This allows each strand of DNA to become visible, which is when it resembles the sausage shape that most would associate as being a chromosome. In prophase, each chromosome is visible as an X shaped sausage as the DNA has just been replicated. The two halves of the X are identical to each other as one is a copy of the other. These are known as sister chromatids, and are held together by a centromere. Each chromosome has a homologous pair which contains genes which code for the same proteins but with different alleles of the genes as one chromosome of the pair is inherited from each parent.

Answered by Maggie B. Biology tutor

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