Gametogenesis 1 again better

Question 1

What is gametogensis?

Answer 1

It is the process by which gametes (sperm and egg) are formed.

Question 2

What does it mean that gametogenesis is compartmentalized in mammals?

Answer 2

There is separate of the somatic cells and gametes.

Question 3

What are soma?

Answer 3

Somatic cells.

Question 4

What does gametogenesis include in animals?

Answer 4

Meiosis which is a unique cell cycle involving recombination between homologous chromosomes to produce haploid cells.

Question 5

What is the germ plasm?

Answer 5

It is found in eggs in many species, but not mammals. It is part of the egg predetermines germ cells and is localised to one part of the egg. It is differentially inherited by different embryonic cells.

Question 6

What organisms is the germ plasm found in?

Answer 6

Nematodes, insects and amphibians.

Question 7

What are primordial germ cells (PGC)?

Answer 7

Gamete (sperm and egg) precursors.

Question 8

What the features of PGCs in mammals?

Answer 8

There is no germ plasm and PGC genesis is due to exogenous cues.

Question 9

How do PGCs arise in mammals?

Answer 9

Exogenous cues - epigenesis.

Question 10

Where do PGCs form in mammals?

Answer 10

The posterior proximal epiblast.

Question 11

What is the epiblast?

Answer 11

The outermost layer of cells of the embryo before it differentiates into the ectoderm and mesoderm.

Question 12

What is BMP produced by?

Answer 12

Extraembryonic ectoderm.

Question 13

What is BMP?

Answer 13

Growth factor - cytokines.

Question 14

How are proximal epiblast cells sensitive to BMP?

Answer 14

Wnts from visceral endoderm cells render them sensitive.

Question 15

What do Blimp1 and Prdm14 do?

Answer 15

Repress somatic gene expression, activate pluropotency genes and anti-apoptotic gene.

Question 16

What are the pluripotency genes involved in PGC differentiation?

Answer 16

Sox2, Nanog.

Question 17

What is the anti-apoptotic gene involved in PGC differentiation?

Answer 17

Nanos3.

Question 18

What happens if you culture PGCs in vitro?

Answer 18

Embryonic germ cells form which are pluripotent.

Question 19

How can chimeric offspring be formed?

Answer 19

If PGCs are cultured in vitro, EG cells from two different organisms can be injected into a blastocyst to form a mix of the two organisms.

Question 20

What migration occurs with PGCs in mammals?

Answer 20

They migrate to the hindgut endoderm and divide during this migration. They migrate to the anterior from E9 in the mouse, and leave the dorsal gut mesentery and enter genital ridges.

Question 21

What is the purpose of the support cells with PGCs?

Answer 21

They maintain the undifferentiated stem cell phenotype.

Question 22

How do support cells fulfill their function?

Answer 22

The secrete stem cell factor (SCF), steel gene product. The cells follow a fibronectin trail.

Question 23

What are PGCs called when they arrive at the genital ridge?

Answer 23

Gonocytes. They incorporate into sex cords and remain there until maturity.

Question 24

How is meiosis distinct from mitosis?

Answer 24

Cells undergo division without DNA replication and homologous chromosomes recombine.

Question 25

How are parental genomes marked differently?

Answer 25

Imprinting.

Question 26

What are the main phases in meiosis 1?

Answer 26

Prophase 1, metaphase 1, anaphase 1, telophase 1.

Question 27

What are the sub divisions of prophase 1?

Answer 27

Leptotene, zygotene, pachytene, diplotene, diakinesis.

Question 28

What are the phases of meiosis 11?

Answer 28

Prophase 11, metaphase 11, anaphase 11, telophase 11.

Question 29

What happens in leptotene?

Answer 29

DNA replication is finished, sister chromatids are thin and closely associated and the chromosomes begin condensing.

Question 30

What happens in zygotene?

Answer 30

Synapsis (fusing of chromosomes) - tetrad (bivalent) formation which requires the synaptonemal complex. Cohesin rings encircle sister chromatids.

Question 31

What happens in pachytene?

Answer 31

Condensation continues and the chromatids thicken. The synaptonemal complex breaks down and crossing over occurs.

Question 32

What can cause pachytene arrest?

Answer 32

Failure of the synaptonemal complex formation. This also causes chromosome fragmentation.

Question 33

What happens in diplotene?

Answer 33

Chiasmata are visible at the cross-over points and the synaptonemal complex breakdown is complete. Homologous chromosomes separate and there is transcription - lampbrush chromosomes (not in mammals).

Question 34

What are lampbrush chromosomes?

Answer 34

Transcriptionally active chromosomes.

Question 35

What happens in diakinesis?

Answer 35

Kinetochores move away from each other and chromosomes remain joined at the tips.

Question 36

What occurs in metaphase 1?

Answer 36

There is nuclear membrane breakdown, chromosomes assemble on the metaphase plate. (spindle assembly checkpoint - spindles must be properly aligned). Cyclin B prevents degradation until complete and failure causes aneuploidy.

Question 37

What is aneuploidy?

Answer 37

An abnormal number of chromosomes in a cell.

Question 38

What occurs in anaphase 1?

Answer 38

Homologous chromosomes separate.

Question 39

What happens in telophase 1?

Answer 39

Two cytokinetic products form, each having a full set of homologous partners.

Question 40

What is the difference between telophase 1 in males and females?

Answer 40

In males 2 cells are formed, whereas in females a cell and a polar body is formed.

Question 41

What is metaphase 11 arrest in females?

Answer 41

There is arrest after metaphase 11 which is unique to meiosis. THis arrest occurs until fertilisation. This happens in frogs and mammals.

Question 42

What is non-dysjunction?

Answer 42

The failure of chromosomes to separate. This can occur in anaphase 1 or 11.

Question 43

What does non-dysjunction cause?

Answer 43

Chromosome imbalances in the embryo that are usually catastrophic.

Question 44

What happens if non-dysjunction is not catastrophic?

Answer 44

Conditions such as Down syndrome (3 x 21 chromosome) and Turner’s syndrome (part of x chromosome altered or missing in females), Kleinefelter’s syndrome - two X and Y chromosome.