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A short class with Sarah and her students. The other ways are under More.
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Sperm and ova both start from diploid germ cells. What has to happen to their chromosome number?
It must be halved, from 46 to 23, so that the zygote has 46 again.
That happens in the maturation phase, by meiosis. What comes before it?
A multiplication phase by mitosis, then a growth phase that forms primary spermatocytes or primary oocytes.
Good. The same three phases, but very different results at the end.
Four sperm from one primary spermatocyte, but only one ovum from one primary oocyte.
When are a woman's primary oocytes formed?
Before she is born, and they stay paused in prophase I until puberty or later.
Right. Meiosis I finishes only shortly before ovulation. So what is released at ovulation?
A secondary oocyte, not a fully mature ovum.
Correct. Meiosis II finishes only when a sperm enters. Why do you think the divisions are so unequal?
So that almost all the cytoplasm and stored food stays in one cell, which can nourish the early embryo.
What signal starts gametogenesis at puberty?
More GnRH from the hypothalamus, which makes the pituitary release FSH and LH.
In a male, where does LH act?
On the Leydig cells, which secrete testosterone. FSH acts on the Sertoli cells.
And in a female, which hormone surge triggers ovulation?
The surge of LH in the middle of the cycle.
multiplication by mitosis, growth into primary cells, and maturation by meiosis.
begins at puberty, is continuous, and gives 4 sperm from each primary spermatocyte.
begins before birth, pauses at prophase I, and gives 1 ovum and polar bodies from each primary oocyte.
a secondary oocyte is released; meiosis II is completed only after a sperm enters.
GnRH from the hypothalamus causes the pituitary to release FSH and LH, which act on the testes and ovaries.
every ovum carries X, while a sperm carries either X or Y.
10 primary spermatocytes → 20 secondary spermatocytes → 40 spermatids → 40 sperm.
10 primary oocytes → 10 secondary oocytes and 10 first polar bodies → at most 10 ova, one for each primary oocyte.
A sperm carrying 22 + Y that fertilises an ovum carrying 22 + X gives a zygote with 44 + XY, which develops into a boy.
Sperm come four from every one; an egg comes one, the rest are done.
Two cousins began life with 46 chromosomes. The boy started working only at puberty but then produced sperm every day, four from each cell. The girl began before she was even born, paused for years, and finished each egg only when a sperm knocked on the door.
M-G-M - Multiplication, Growth, Maturation, the three phases of gametogenesis in both sexes.
L.T.S. - LH acts on Leydig cells to make Testosterone, while FSH acts on Sertoli cells.
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Where does spermatogenesis take place?
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At what stage do primary oocytes remain paused until puberty?
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What is released from the ovary at ovulation?
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How many sperm does one primary spermatocyte give?
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Which cells does LH act on in the testis?
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The formation of haploid gametes from diploid germ cells.
A diploid germ cell in the seminiferous tubule that multiplies by mitosis.
A diploid cell that undergoes meiosis to form two secondary spermatocytes.
The transformation of a spermatid into a sperm.
A diploid germ cell in the foetal ovary that multiplies by mitosis.
A diploid cell that begins meiosis before birth and remains paused in prophase I.
The large haploid cell formed by meiosis I in the ovary and released at ovulation.
A tiny cell formed by the unequal divisions of oogenesis, which normally degenerates.
Gametogenesis is the formation of haploid gametes from diploid germ cells: sperm by spermatogenesis in the testes and ova by oogenesis in the ovaries. Both follow the same three phases, multiplication, growth and maturation, but they differ sharply in timing, in the number of gametes formed and in how the cytoplasm is shared.
In humans, germ cells with 46 chromosomes give rise to gametes with 23. Spermatogonia and oogonia first multiply by mitosis, then grow into primary spermatocytes and primary oocytes, and finally go through meiosis. Spermatogenesis begins at puberty, runs continuously and gives four sperm from each primary spermatocyte. Oogenesis begins before birth, pauses for years, and gives one ovum and polar bodies from each primary oocyte, with the second meiotic division completed only if a sperm enters. Hormones from the hypothalamus and the pituitary (GnRH, FSH and LH) control both. This lesson sets the two pathways side by side; the next two lessons take each in detail.
Gametogenesis in both sexes passes through three phases:
At puberty the hypothalamus increases its secretion of gonadotropin-releasing hormone (GnRH), which acts on the anterior pituitary to release FSH and LH.
Human body cells have 46 chromosomes: 44 autosomes plus XX in females and 44 autosomes plus XY in males. Every ovum carries 22 + X. Half the sperm carry 22 + X and half carry 22 + Y, which is why the sperm decides whether the child will be a girl or a boy.