Polytene and lampbrush chromosome

Polytene and Lamp brush are the two giant chromosome types which are found in living organisms.

1. POLYTENE CHROMOSOMES
❏ Polytene chromosomes were first observed by Balbiani in Chironomus. Because of
their large size showing numerous strands these are named as polytene
chromosomes by Kollar. These banded chromosomes occur in the larval salivary
glands, midgut epithelium, and rectum and Malpighian tubules of various genera of
dipterans. These are also known as salivary gland chromosomes because they
have been best studied in the salivary gland cells of fly larvae
❏These chromosomes are about 100-200 times larger than those of somatic
chromosomes. They are roughly cylindrical and exhibit a distinct pattern of
transverse striated structures consisting of alternate darkly staining band and light
staining interbands.
❏ Dark bands are rich in DNA along with a small amount of RNA and basic proteins.
They are genetically active. The inter-bands contain less of DNA but more acidic
proteins and hence they are less active.
❏FORMATION :​ The polytene chromosomes are formed by repeated replication of
DNA without division of chromosome into daughter chromosomes. This amplification
without separation is called ​polytenization​. As a result, a thick bundle of parallel
DNA molecules all having the same banding pattern across them is produced. Thus,
there can be as many as several thousands of chromonemata in a giant
chromosome.
❏ During the initial stages of development the bands or inter-bands of chromosomes
exhibit swellings or puffs. During development the puffs appear and disappear in
definite patterns in response to the needs of developing larvae for the RNAs. The
puffs are genetic sites active in RNA synthesis. In some regions of polytene
chromosomes the chromonemata may give out a number of loops at certain places.
Such loops are known as the Balbiani rings. These rings are formed by the lateral
stretching of loops. They are rich in mRNA like the chromosomal puffs

2. Functions of the Giant Polytene Chromosomes
(i) Polytene chromosomes carry genes which ultimately control physiology of an
organism. These genes are formed of DNA molecules. (ii) These chromosomes also help
in protein synthesis indirectly. The RNA present in the nucleolus serves as a means of
transmission of genetic information to the cytoplasm, leading to the formation of specific
protein.


















Lampbrush Chromosomes

❏These are the largest chromosomes which can be seen with naked eyes
and are found in yolk rich oocytic nuclei of certain vertebrates such as
fishes, amphibians, reptiles and birds. They are characterized by the fine
lateral loops, arising from the chromomeres, during first prophase of
meiosis. Because of these loops they appear like brush; that is why they
are called lampbrush chromosomes first discovered by Flemming .
❏Lampbrush chromosome consists of longitudinal axis formed by a single
DNA molecule along which hundreds of bead like chromomeres are
distributed. Two symmetrical lateral loops (one for each chromatid)
emerge from each chromomere, which are able to expand or contract in
response to various environmental conditions.
❏About 5 to 10% of the DNA is in the lateral loops. The axis having
compacted DNA and tightly associated proteins is transcriptionally
inactive. The loops consist of uncompacted DNA and proteins but have a

3. good amount of RNA and they are transcriptionally active. A
chromomere and its associated loop correspond with one gene.
❏ In lampbrush chromosomes the DNA loops are the sites of intensive
RNA synthesis. rRNA and mRNA are synthesized in large amount and
the transcription of rRNA causes the enlargement of nucleolus, or
formation of numerous additional nucleoli. Due to the synthesis of large
amounts of proteins, fats, carbohydrates and other molecules in the
cytoplasm needed for further development of the embryo, the oocyte
grows in size. Synthesis of proteins occurs near the loops