Polytene chromosomes are large chromosomes found in secretory cells like salivary glands that contain thousands of identical DNA strands aligned in parallel. This gives them a banded appearance with dark bands and clear interbands when viewed under a microscope. The bands represent regions of condensed and transcriptionally active DNA. B chromosomes are nonessential supernumerary chromosomes that are found in some populations but not others and can provide adaptive advantages in some species and environments.

1. Polytene Chromosomes
• E.G. Balbiani(1881): str. In nuclei of
secretory cells
• Kollar: polytene chr.
• Found in salivary glands and other
tissues of flies and species of
protozoans
• Seen in the nucleus during
interphase
• linear series of alternating bands
and interbands, distinctive banding
for each chromosome in a given
species
• High DNA content (repl.Without
nuclear division)

2. Polytene continued
• Chromomeres – individual bands seen in chromosome are lateral
condensations of material along the chromosome
• Polytene are believed to be:
• paired homologs which are usually dispersed as chromatin in
somatic cell and not as chromosomes
• large size and distinctiveness result from many strands of DNA
that replicate but do not separate or do not undergo cytoplasmic
division
• 1000-5000 DNA strands in parallel alignment that gives to
banding pattern(dark transverse bands and clear zones:
interbands)
• Eg: Drosophila-5000 bands and 5000 interbands
representing1024 identical DNA sequences.

3. A polytene chromosome from Drosophila
salivary gland
Dark bands and interbands

4. Puffs/
Balbiani rings
•Bands are believed to be unfolded DNA into loops; uncoil to
undergo transcription and creates a puff
•Study of the bands reveal differential gene activity –
characteristic of pattern of activation during development
•Appear/ disaapear in relation to protein production.

5. Lampbrush Chromosomes
• Looks like a brush
• 1st observed in salamander oocytes; in most vertebrate oocytes and
spermatocytes of some insects
• Occur in diplotene stage of 1st prophase – direct metabolic
activities in developing cells
• Present in form of bivalents: maternal and paternal chr. r held by
chiasmata after crossing over.
• they do not condense like the rest of chromosomes initially, later
they coil into a normal chromosome

6. Lampbrush Structure
• Condensed areas in loops along a
central axis – chromomeres pair in
lateral loops – adjacent loops
present along 1 of 2 axes of
chromosome
• Axis of each homologue consist of
chromomeres: form lateral loops
• Loop: single DNA molecule
unfolded; transcriptionally active
• Covered by matrix: RNA and
proteins

7. A model for the structure of a lampbrush
chromosome
Chromomeres: highly condensed and in
general not expressed until unfolding

8. B chromosomes:
• In addition to the normal karyotype, wild populations of many animal,
plant, and fungi species contain B chromosomes (also known
assupernumerary or accessory chromosomes).
• These chromosomes are not essential for the life of a species, and are
lacking in some (usually most) of the individuals.
• Thus a population would consist of individuals with 0, 1, 2, 3 (etc.)
supernumeraries.
• Most B chromosomes are mainly or entirely hetrochromatin (and so
would be largely non-coding.
• Positive adaptive advantage : British grasshopper ( Myrmeleotettix
maculatus) has two structural types of B chromosomes: metacentrics and
submetacentrics.
• The supernumeraries, occur in warm, dry environments, and are scarce
or absent in humid, cooler localities.

9. • In plants there is a tendency for B chromosomes to be present
in the germ-line, but to be lost from other tissues such as root tips
and leaves.
• Deleterious effects of supernumeraries on pollen fertility, and
favourable effects or associations with particular habitats are also
known in a number of species.