Chromosomes
Types of Chromosomes
Lampbrush chromosomes
Lampbrush chromosomes are formed at the end of oogenesis (when meiosis produces egg cells) in all animals except for mammals.
They are meiotic half-bivalent – basically, a pair of homologous chromosomes that is split in two. They contain chromomeres (condensed parts of DNA), from which asymmetric loops extend.
The loops are the location of active RNA transcription. They help in rapid development and growth of the egg.
Polytene chromosomes
Polytene chromosomes are extra-large chromosomes. They are found in the salivary glands of Drosophila, and contain characteristic light and dark bands.
Polytene chromosomes are formed through endomitosis, in which DNA replicates, skipping the mechanism of mitosis isself. They form in organs that need to undergo rapid growth.
Many times, many polytene chromosomes join their centromeres to form a common chromocenter:
Types of polyteny
In classical polyteny, all chromatids conjugate (join together) perfectly (a).
In cryptic polyteny, chromatid conjugation is only partial (b).
If only one pair of chromatids is connected, “pompon-like” chromosomes form (c).