05-05-2021 drafting: ' Primordial germ cells migrate to the forming gonad to become oogonia, which proliferate by mitosis for a period before differentiating into primary oocytes. At this stage (usually before birth in mammals), the first meiotic division begins: the DNA replicates so that each chromosome consists of two sister chromatids, the duplicated homologous chromosomes pair along their long axes, and crossing-over occurs between nonsister chromatids of these paired chromosomes.' (nbk26842)

' Primordial germ cells migrate to the forming gonad to become oogonia, which proliferate by mitosis for a period before differentiating into primary oocytes. At this stage (usually before birth in mammals), the first meiotic division begins: the DNA replicates so that each chromosome consists of two sister chromatids, the duplicated homologous chromosomes pair along their long axes, and crossing-over occurs between nonsister chromatids of these paired chromosomes.'

'oocyte

The developing egg. It is usually a large and immobile cell.

differentiation

Process by which a cell undergoes a change to an overtly specialized cell type.

oogenesis

Formation and maturation of oocytes in the ovary.

operator

Short region of DNA in a bacterial chromosome that controls the transcription of an adjacent gene.

operon

In a bacterial chromosome, a group of contiguous genes that are transcribed into a single mRNA molecule.

ORC

see origin recognition complex

organelle

Membrane-enclosed compartment in a eucaryotic cell that has a distinct structure, macromolecular composition, and function. Examples are nucleus, mitochondrion, chloroplast, Golgi apparatus.

Organizer

see Spemann’s Organizer

origin recognition complex (ORC)

Large protein complex that is bound to the DNA at origins of replication in eucaryotic chromosomes throughout the cell cycle'

fertilization

Fusion of a male and a female gamete (both haploid) to form a diploid zygote, which develops into a new individual.

meiosis

Special type of cell division by which eggs and sperm cells are produced. It comprises two successive nuclear divisions with only one round of DNA replication, which produces four haploiddaughter cells from an initial diploid cell.

https://www.ncbi.nlm.nih.gov/books/NBK21052/#A5578

' Primordial germ cells migrate to the forming gonad to become oogonia, which proliferate by mitosis for a period before differentiating into primary oocytes. At this stage (usually before birth in mammals), the first meiotic division begins: the DNA replicates so that each chromosome consists of two sister chromatids, the duplicated homologous chromosomes pair along their long axes, and crossing-over occurs between nonsister chromatids of these paired chromosomes. After these events, the cell remains arrested in prophase of division I of meiosis (in a state equivalent, as we previously pointed out, to a G2 phase of a mitotic division cycle) for a period lasting from a few days to many years, depending on the species. During this long period (or, in some cases, at the onset of sexual maturity), the primary oocytes synthesize a coat and cortical granules. In the case of large nonmammalian oocytes, they also accumulate ribosomes, yolk, glycogen, lipid, and the mRNA that will later direct the synthesis of proteins required for early embryonic growth and the unfolding of the developmental program. In many oocytes, the intensive biosynthetic activities are reflected in the structure of the chromosomes, which decondense and form lateral loops, taking on a characteristic “lampbrush” appearance, signifying that they are very busily engaged in RNA synthesis (see Figures 4-36 and 4-37).'

https://www.ncbi.nlm.nih.gov/books/NBK26842/ 

'The next phase of oocyte development is called oocyte maturation. It usually does not occur until sexual maturity, when the oocyte is stimulated by hormones. Under these hormonal influences, the cell resumes its progress through division I of meiosis. The chromosomes recondense, the nuclear envelope breaks down (this is generally taken to mark the beginning of maturation), and the replicated homologous chromosomes segregate at anaphase I into two daughter nuclei, each containing half the original number of chromosomes. To end division I, the cytoplasm divides asymmetrically to produce two cells that differ greatly in size: one is a small polar body, and the other is a large secondary oocyte, the precursor of the egg. At this stage, each of the chromosomes is still composed of two sister chromatids. These chromatids do not separate until division II of meiosis, when they are partitioned into separate cells, as previously described. After this final chromosome separation at anaphase II, the cytoplasm of the large secondary oocyte again divides asymmetrically to produce the mature egg (or ovum) and a second small polar body, each with a haploid set of single chromosomes (see Figure 20-22). Because of these two asymmetrical divisions of their cytoplasm, oocytes maintain their large size despite undergoing the two meiotic divisions. Both of the polar bodies are small, and they eventually degenerate.

In most vertebrates, oocyte maturation proceeds to metaphase of meiosis II and then arrests until fertilization. At ovulation, the arrested secondary oocyte is released from the ovary and undergoes a rapid maturation step that transforms it into an egg that is prepared for fertilization. If fertilization occurs, the egg is stimulated to complete meiosis.'

https://www.ncbi.nlm.nih.gov/books/NBK26842/