mitosis produces two identical daughter cells, but meiosis produces 4 very different cells....

• Mitosis produces two identical daughter cells, but meiosis produces 4 very different cells.

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Fig. 13.8

• Meiosis reduces chromosome number by copying the chromosomes once, but dividing twice.

• The first division, meiosis I, separates homologous chromosomes.

• The second, meiosis II, separates sister chromatids.


Fig. 13.6

• Division in meiosis I occurs in four phases: prophase, metaphase, anaphase, and telophase.

• During the preceding interphase the chromosomes are replicated to form sister chromatids.– These are genetically identical

and joined at the centromere.

• Also, the single centrosome is replicated.


Fig. 13.7

• In prophase I, the chromosomes condense and homologous chromosomes pair up to form tetrads.– In a process called synapsis, special proteins attach

homologous chromosomes tightly together.– At several sites the chromatids of

homologous chromosomes are crossed (chiasmata) and segments of the chromosomes are traded.

– A spindle forms from each centrosome and spindle fibers attached to kinetochores on the chromosomes begin to move the tetrads around.


Fig. 13.7

• At metaphase I, the tetrads are all arranged at the metaphase plate.– Microtubules from one pole are attached to the

kinetochore of one chromosome of each tetrad, while those from the other pole are attached to the other.

• In anaphase I, the homologous chromosomes separate and are pulled toward opposite poles.


Fig. 13.7

• In telophase I, movement of homologous chromosomes continues until there is a haploid set at each pole.– Each chromosome consists of linked sister

chromatids.

• Cytokinesis by the same mechanisms as mitosis usually occurs simultaneously.

• In some species, nuclei may reform, but there is no further replication of chromosomes.


Fig. 13.7

• Meiosis II is very similar to mitosis.– During prophase II a spindle apparatus forms,

attaches to kinetochores of each sister chromatids, and moves them around.• Spindle fibers from one pole

attach to the kinetochore of one sister chromatid and those of the other pole to the other sister chromatid.


Fig. 13.7

• At metaphase II, the sister chromatids are arranged at the metaphase plate.– The kinetochores of sister chromatids face opposite

poles.

• At anaphase II, the centomeres of sister chromatids separate and the now separate sisters travel toward opposite poles.


Fig. 13.7

• In telophase II, separated sister chromatids arrive at opposite poles.– Nuclei form around

the chromatids.

• Cytokinesis separates the cytoplasm.

• At the end of meiosis, there are four haploid daughter cells.


Fig. 13.7

Primordial Germ Cells (PGCs) retain the ability to undergo meiosisFrogs & Drosophila - Germ plasm/polar granules determination of germ line and inhibition of somatic gene expressionm-RNA/protein in vegetal/posterior cytoplasm from egg

Mammals-future posterior region of embryo/extraembryonic regiongene expression and interaction produces PGCs which then migrate to the gonads

Fig. 46.11Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Primordialgerm cellPGC

Sperm structure: Haploid nucleus. Tipped with an acrosome.

Contains enzymes that help the sperm penetrate to the egg.

A large numberof mitochondriaprovide ATP topower theflagellum.


Fig. 46.12

Fig. 46.13


March 11, 2004 Nature

For nearly a century, scientists have firmly believed that whereas men can produce sperm throughout their lives, women are born with all the eggs they will ever have. But new research suggests that this basic tenet of reproductive biology is wrong, a discovery that could have enormous repercussions for fertility treatment. Egg cells, or oocytes, that are not fertilized are known to undergo a natural process of cell death. While investigating the effects of chemotherapy drugs on fertility in mice, Jonathan L. Tilly of Massachusetts General Hospital and his colleagues began to count the normal rate at which oocytes die. "We certainly didn’t set out two years ago to overturn dogma," says Tilly, lead author of the report detailing the findings, published today in Nature. But to their surprise, the scientists found that the number of oocytes dying over a period of several days was far greater than would be sustainable over the long term if the egg supply was not being replenished. In fact, at that rate, mice would be fertile for just two weeks following birth, as opposed to more than a year. The team subsequently conducted a series of experiments to verify the observation. Careful examination of the lining of adult mouse ovaries revealed cells that closely resemble germline stem cells, those continuously dividing cells that give birth to oocytes. The researchers determined that these ovarian cells express a protein that is associated with meiosis, the process by which sex cells divide. When they transplanted the putative germline stem cells into a strain of transgenic mice whose cells all express a green fluorescent marker, they found that the transplanted cells had divided and produced oocyte follicles in the host tissue. Tilly and his collaborators also analyzed the effects of a chemotherapy agent called Busulfan on the mouse ovaries. The drug, which has been used to study sperm proliferation, destroys the ability of male germline stem cells to divide into new sperm, but does not harm existing sperm. Several weeks after injecting the ovaries with Busulfan, the researchers found that the number of oocytes had decreased dramatically. There was no sign of the cell death that marks oocyte degeneration, however. The drug apparently targeted the female stem cells, preventing their ability to produce oocytes.

Early in the 20th century, some scholars had suggested that eggs could in fact be replenished in adult mammals. But a 1951 study definitively argued that egg numbers are determined at birth, shutting the door to further work for the next half century. "People were viewing ovaries very differently than we do now," Tilly reflects. "The technology was just based on histological analysis." Although biological markers for germline stem cells have since been developed, conclusions about mammalian ovaries were never reexamined because they were believed to be "as sound as telling people that the sun sets in the west." The team is now working to demonstrate the existence of germline stem cells in human ovaries, and it is confident that the finding will carry over. The next step is to figure out which genes instruct a germ stem cell to differentiate into an egg. Ultimately, Tilly says, transplanting these cells into the ovaries of menopausal women or women whose ovaries have been prematurely damaged by cancer treatment could restore their fertility. --Alla Katsnelson

Usually one follicle matures and releases its egg during each menstrual cycle. After ovulation the remaining follicular tissue develops

into the corpus luteum. Secretes estrogens and progesterone. Maintain the uterine

lining during pregnancy. If pregnancy does not

occur the corpus luteumdisintegrates.

Growth of Primary Oocyte

Lampbrush Chromosomes 5% of genome

Increased number of Nucleoli gene amplification

Increased number of Organelles

Yolk Production Egg Types

IsolecithalMesolecithalTelolecithal CentrolecithalIsolecithal

Gene for Twinning?

Fraternal (non-identical) twinning occurs when two eggs are ovulated simultaneously. Thus, the twins derive from two separate fertilization events. Multiple ovulations are uncommon in humans, cattle, and many breeds of sheep. (Obviously multiple births are normal in mice, which have eight to sixteen live births at a time, depending on the strain of mouse). It has long been thought that pituitary gonadotropin levels can regulate the number of eggs that mature, but it has also been thought that the ovary must also be able to regulate egg maturation.

The clue to the ovarian regulator came in 1968, when Derek Weir, who farmed in the South Island of New Zealand, noticed he had a prolific breeder on his hands. This particular ewe produced 33 lambs in 11 years. Hearing about a research project on good breeders, Weir offered up his ewe. That sheep was used to start a colony, and the gene for multiple births was found to reside on the X chromosome of this "Inverdale" strain. In 1998, the team heard of the work of Finnish researchers who had mapped the mouse and human versions of a GDF9B to the X chromosome. The function of the GDF9B gene was not known, but it was known to be expressed in primary oocytes.

The New Zealanders thought the sheep equivalent of GDF9B (also known as BMP 15) might be causing the Inverdale effect. Using human primers from the Finnish group, the New Zealand group used PCR to clone the homologous gene in the sheep. They demonstrated that the BMP15 gene is expressed in the sheep oocytes and that there was a one base pair difference between the BMP15 genes of wild-type and Inverdale sheep (Galloway et al. 2000; Griggs 2000). It appears that one dose of the mutant BMP15 gene causes twin and triplet births, while homozygosity at that locus produces ovarian failure and sterility.

The BMP15 gene may possibly explain X-linked ovarian dysgenesis syndromes. In XO mice and humans (Turner’s syndrome), premature ovarian failure is caused by the absence of an X chromosome. Thus, in addition to the pituitary factors regulating ovulation, the oocytes,

themselves, help regulate the number of eggs that will be released at ovulation. The basis for this cooperation has yet to be found.

Egg Envelopes

I. Produced within the ovaryVitelline Membrane sea urchin, frog, bird

Zona pellucida mammal

II. Produced outside of the ovaryjellyalbumin & shell membranes

mitosis produces two identical daughter cells, but meiosis produces 4 very different cells....

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