FERTILIZATION:
1. Sperm
2. Ovum
3. Ovum nucleus
4. Chromatid
The sperm penetrates the ovum (A) and comes to lie alongside the ovum nucleus (B). The chromosomes of both sperm and ovum divide into separate strands known as chromatids. Corresponding chromatids move to opposite ends of the ovum (C), where they are surrounded by nuclear membranes (D). The structure then splits into separate cells (E).
During cell division, when new cells are being formed, the DNA (A) contained within the dividing cell unzips and forms new molecules of DNA along its free ends (B) from the nucleic-acid bases and sugar phosphates contained in the cell nucleus. To produce messenger RNA, the DNA comes apart partially (C) and links with broadly similar material; the sugar phosphate backbone is slightly different chemically and one of the nucleic acids is substituted. The messenger RNA moves to the ribosomes, where it links up with transfer RNA, which carries amino acids (D). The messenger RNA contains the code that ensures that the transfer RNA is linked together in the correct sequence to produce the chain of amino acids that form the desired protein.