Physical Basis of Heredity 1.  The Nucleus and the Chromosome
The Nucleus 
Significance of nucleus : Hammerling's experiment
Number, shape and size of nucleus
Nucleus in prokaryotes and eukaryotes
Nuclear envelope
Nuclear pore complex and nucleocytoplasmic traffic
Number, size and shape of chromosomes
Morphology of chromosomes
Euchromatin and heterochromatin
Constitutive and facultative heterochromatin
Single-stranded and multi-stranded hypotheses for chromosomes
Chemical composition of chromosomes
Infrastructure of chromosomes
Function of chromosomes
Special types of chromosomes 
Lampbrush chromosomes
Salivary gland chromosomes
Prokaryotic Nucleoids

E. Strasburger in 1875 discovered thread-like structures which appeared during cell division. These thread-like structures were called chromosomes (chroma = colour) due to their affinity for basic dyes. In all types of higher organisms (eukaryota), the well organized nucleus contains definite number of chromosomes of definite size, and shape. The chromosomes cannot clearly seen in the nucleus but can be easily seen during cell division, whether mitosis or meiosis.

At leptotene stage of meiotic prophase, chromosomes appear as beaded structures, bead-like nodules being known as chromomeres. Size of chromomeres and interchromomeric regions are not constant, so that every leptotene has its own particular pattern. The DNA is though known to concentrate in the chromomeres, but is believed to be present in the inter-chfomomeric regions also. A diploid nucleus has two chromosomes of each type. Two similar chromosomes, are known as homologous chromosomes, which come in contact at zygotene and pair lengthwise throughout their length.