The nucleus and its structure
The nucleus stores, copies and passes on hereditary information and controls metabolism in the cell. An interphase nucleus consists of the nuclear envelope (two membranes with pores), karyoplasm (nuclear sap), chromatin and the nucleolus. Chromatin consists of DNA and protein; during division it coils up into compact chromosomes. The centromere divides a chromosome into arms: metacentric (equal arms), submetacentric (unequal arms) and acrocentric (rod-like) chromosomes are distinguished. All somatic cells of one species have the same number of chromosomes (46 in humans, 8 in Drosophila), while sex cells have half as many; the karyotype is the number, size and shape of the chromosome set. Somatic cells are diploid (2n) and gametes haploid (n). In the nucleolus ribosomal RNA is made and ribosomes are assembled.
“Chromosome game”. Cut out paper homologous chromosomes (each pair the same colour and length), arrange them in pairs to make a “karyotype”, then take one from each pair to make a haploid set. Use scissors with the teacher’s help.