Lampbrush Chromosome

1. Guided by- Submitted by-
Dr. K. K. Panigrahi Ashok Kumar panda
Asst. proffser (PBG) Adm no -25c/15

2.  It was given this name because it is similar in appearance
to the brushes used to clean lamp chimneys in centuries
past.
 First observed by Flemming in 1882.(salamander egg
cells –Amblystoma maxicanum)
 The name lampbrush was given by Ruckert in 1892.
 Lampbrush Chromosomes (LBCs) are present in the
oocytes of birds, lower vertebrata and invertebrates
during the prolonged prophase of the first meiotic
division.
 Also found in plants –but most experiments in oocytes.

6.  These chromosomes are most distinctly observed during
the prolonged diplotene stage of oocytes.
 During diplotene, the homologous chromosomes begin
to separate from each other, remaining in contact only at
several points along their length.
 Each chromosome of a pair has several chromomeres
distributed over its length; from each of a majority of the
chromomeres generally a pair of lateral loops extends in
the opposite directions perpendicular to the main axis of
the chromosome.
 In some cases, more than one pair, even upto 9 pairs of
loops may emerge from a single chromomere
 These lateral loops give the chromosomes the appearance
of a lampbrush which is the reason for their name lamp-
brush chromosomes.

9. Lampbrush chromosomes are up to 800
micrometre long ; thus they provide very
favourable material for cytological studies.
The homologus chromosomes are paired
and each has duplicated to produce two
chromatids at the lampbrush stage.
Each lampbrush chromosome contains a
central axial region , where the two
chromatids are highly condense.

10.  The pairs of loops are produced due to uncoiling of the
two chromatin fibres present in a highly coiled state in
the chromosomes;
 this makes their DNA available for transcription (RNA
synthesis).
 Thus each loop represents one chromatid of a
chromosome and is composed of one DNA double helix.
 One end of each loop is thinner (thin end) than the other
end (thick end).
 There is extensive RNA synthesis at the thin ends of
loops, while there is little or no RNA synthesis at the
thick end

11.  The chromatin fibre of the chromomere is progressively
uncoiled towards the thin end of a loop;
 the DNA in this region supports active RNA synthesis but
 later becomes associated with RNA and protein to become
markedly thicker.
 The DNA at the thick end of a loop is progressively
withdrawn and reassembled into the chromomere.
 The number of pairs of loops gradually increases in meiosis
till it reaches maximum in diplotene.
 As meiosis proceeds further, number of loops gradually
decreases and the loops ultimately disappear
 due to disintegration rather than reabsorption back into the
chromomere.

12.  The loops can be classified according to the type of the
transcriptional polymerase.
 The largest loops include those transcribed by polymerase II. The
smallest loops are transcribed by polymerase III.
 They contain 5S RNA coding units , tRNA or short replication
sequences
 Since 5S RNA sequences are short and divided by noncoding
elements, transcription being basically limited to coding sequences,
 the transcripts of these sequences are also short and, consequently,
do not have the distinctive matrix made up of RNP filaments.
 That is why they are so well visible in the microscopic phase
contrast.
 LBCs can be divided into those with one transcriptional unit and
those with two or more.
 Over the length of 1 μm, one transcriptional unit is transcribed by a
densely compacted package of around 13-20 polymerase molecules .

14.  Transcription activity of LBCs is observed as a mantle of symmetrically
distributed side loops along the chromosome axis.
 Changes in transcriptional activity are reflected in changes in their
morphology.
 Transcriptional activity of LBCs is directly connected with physiological
processes of the body
 Shows in the morphological structure of the chromosomes.
 LBCs are considered as model structures in the studies of transcription
control.
 Changes in their transcriptional activity are reflected as modifications of the
LBC morphological structure and are associated with the physiological
processes of the organism.
 The use of cytogenetic techniques in situ hybridization have made it
possible to identify unique and repeating sequences as well as DNA
replication proteins in LBCs.
 Particularly, interesting prospects are offered by the possibility of using
LBCs in studies of transcriptional activity, cytogenetic investigations of
karyotype evolution and genome mapping.