Growing oocytes actively transcribe genes to produce maternal transcripts that are (1) necessary for cell metabolism, (2) needed for oocyte-specific processes, and (3) required for early embryonic development. In mice, genes for zona pellucida proteins ZP1, ZP2, and ZP3 are transcribed only in oocytes. Amphibian oocytes have periods of intense RNA synthesis, and their lampbrush chromosomes are sites of this RNA transcription. Ribosomal RNA genes are amplified in amphibian oocytes, resulting in thousands of extra-chromosomal rRNA gene copies distributed among nucleoli not associated with chromosomal regions. This rDNA amplification allows amphibian oocytes to support unusually

1. Dr. Shoeb Ahmad, Assistant Professor, Department of Zoology, AKI’s Poona College of Arts, Science & Commerce, Camp, Pune -01
e-mail: shoeb.zoology@gmail.com Mobile no 9919509673
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OOGENESIS
Synthesis and storage of maternal transcripts
In most animals (insects being a major exception), the growing oocyte is active
in transcribing genes whose products are
(1) necessary for cell metabolism,
(2) necessary for oocyte-specific processes,
(3) needed for early development before the zygote-derived nuclei begins to
function.
In mice, for instance, the growing diplotene oocyte is actively transcribing the
genes for zona pellucida proteins ZP1, ZP2, and ZP3.
Moreover, these genes are transcribed only in the oocyte and not in any other
cell.
The amphibian oocyte has certain periods of very active RNA synthesis. During
the diplotene stage, certain chromosomes stretch out large loops of DNA,
causing the chromosome to resemble a lampbrush (a handy instrument for
cleaning test tubes in the days before microfuges).
These lamp brush chromosomes (Figure 1 d) can be revealed as the sites of
RNA synthesis by in situ hybridization.
rDNA Amplification
Ribosomal RNA genes are amplified in amphibian oocytes.
As a consequence. a huge number of extra-chromosomal rRNA genes occur (in
addition to the few hundred rRNA genes clustered at the chromosomal
nucleolus organizer regions) in numerous nucleoli that are not associated with
the lampbrush chromosomes (Fig. 1 a.b).
Although the amplification process begins in premeiotic oogonia. the main
period for the selective replication of rDNA is pachytene of meiotic prophase.
Followed by completion in early diplotene. In Xenopus laevis the amount of
extrachromosomal rDNA per oocyte nucleus is about 30 pg (Picograms
[trillionths] of a gram). corresponding to approximately 2 million copies of

2. Dr. Shoeb Ahmad, Assistant Professor, Department of Zoology, AKI’s Poona College of Arts, Science & Commerce, Camp, Pune -01
e-mail: shoeb.zoology@gmail.com Mobile no 9919509673
Page 2
rRNA genes. which are distributed in about 1000 amplified nucleoli. It is
obvious that amplification of the rDNA enables amphibian oocytes to support
unusually high rates of rRNA synthesis. Thus, a single Xenopus oocyte
synthesizes in stages of maximal growth about 300.000 ribosomes per second.
Whereas a normal somatic cell produces in the same time span only 10-100
ribosomes. The majority of the oocyte ribosomes are stored in the cytoplasm as
monosomal particles for use in future embryonic growth.

3. Dr. Shoeb Ahmad, Assistant Professor, Department of Zoology, AKI’s Poona College of Arts, Science & Commerce, Camp, Pune -01
e-mail: shoeb.zoology@gmail.com Mobile no 9919509673
Page 3