Chromosomes are present in cell nucleus and consist of chromatin. Genes are present in linear order on chromosomes. The chromosomes become visible under the microscope as distinct structures during cell division. When cells are not dividing, the chromosomes decondense to loose their individuality and make the mass of chromatin.
Chromatin is complex of DNA and packing proteins. As the cells enter the prophase stage of cell division, condensation of chromatin occurs and individual chromosomes become visible under microscope. Before that (during interphase), chromosomes are not visible as they are present in decondensed form.
Buildup of organisms means that the species number is pretty high. Gradual change means that it was a slow, calm time to make the change. I hope this helps!!
