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An Overview of Meiosis

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An Overview of Meiosis

In human beings, nearly all cells have paired sets of chromosomes, meaning these cells are diploid.
Meiosis is the process by which a single diploid cell divides to produce four haploid cells—cells that contain a single set of chromosomes.

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Overview of Meiosis

The haploid cells produced through meiosis are called gametes

Female gametes are eggs

Male gametes are sperm

Gametes are the reproductive cells of human beings and many other organisms

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Meiosis Compared to Mitosis

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Overview of Meiosis

The haploid sperm and haploid egg fuse
A diploid fertilized egg (or zygote) is produced
This is a new generation of organism

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Understanding Ploidy

Recall that we have 46 chromosomes
This means we have 23 pairs of homologous chromosomes

1 chromosome comes from your father
1 from your mother

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Understanding Ploidy

Once homologous pairs duplicate their DNA they would look like this

Each chromosome now has two sister chromatids

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The Steps in Meiosis

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The Steps in Meiosis

Meiosis has 3 steps:

one round of chromosome duplication
two rounds of cell division

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The Steps in Meiosis

2 primary stages to meiosis:

meiosis I
meiosis II

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Meiosis I

Chromosome duplication occurs
Prophase I:

homologous chromosomes pair with one another
they exchange reciprocal sections of themselves in a process known as crossing over

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Meiosis I

Metaphase I:

Homologous chromosome pairs line up at the metaphase plate
Independent assortment occurs

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Meiosis I

Anaphase I:

Homologous pairs are then separated in the first round of cell division

Telophase I:

Two haploid daughter cells are formed

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Meiosis II

The chromatids of the duplicated chromosomes are separated into separate daughter cells
4 Haploid cells are formed

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10.3 What is the Significance of Meiosis?

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What is the Significance of Meiosis?

Meiosis generates diversity by ensuring that the gametes it gives rise to will differ genetically from one another.
Meiosis is unlike regular cell division, or mitosis, which produces daughter cells that are exact genetic copies of parent cells.

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Meiosis Generates Diversity

Meiosis generates genetic diversity in two ways

Crossover/Recombination during Prophase I
Independent assortment during Metaphase I

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Meiosis Generates Diversity

Crossing over/Recombination
Chromosomes exchange reciprocal chromosomal segments with one another

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Meiosis Generates Diversity

Independent assortment

maternal and paternal chromosomes randomly align on either side of the metaphase plate
This chance alignment determines which daughter cell each chromosome will end up in

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Meiosis Generates Diversity

Evolution is spurred on by differences among offspring, and meiosis and sexual reproduction ensure such differences.
By contrast, asexual reproduction, as is seen in bacteria and other organisms, produces organisms that are exact genetic copies, or clones, of the parental organism.

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10.4 Meiosis and Sex Outcome

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Meiosis and Sex Outcome

How do we become male or female?

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The X and the Y

Figure 10.4

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Meiosis and Sex Outcome

So for meosis:

Female’s produce 4 eggs with a single X chromosome in it

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Meiosis and Sex Outcome

So for meosis:

Male’s produce sperm with either an X or a Y chromosome in it

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Meiosis and Sex Outcome

If a sperm with a Y chromosome fertilizes an egg, the offspring will be male
If a sperm with an X chromosome fertilizes the egg, the offspring will be female

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Gamete Formation in Humans

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Gamete Formation

Spermatogenesis is the formation of gametes in males
Oogenesis is the formation of gametes in females

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Sperm Formation

Spermatogenesis has 3 stages

Production of a primary spermatocyte
Production of 2 secondary spermatocytes
Production of 4 haploid spermatids

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Egg Formation

Oogenesis has 3 stages

Production of a primary oocytes which gets arrested in meosis I until puberty
Production of a secondary oocyte monthly but arrested in meiosis II until fertilization
Production of an egg after fertilization

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Egg Formation

In females there may be no counterparts to the male spermatogonia—no cells that function as reproductive stem cells
Female gamete formation begins with cells called oogonia, most or all of which are produced prior to the birth of the female
Whether or not there are female stem cells is controversial

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Oogenesis and the sexual cycle

After birth females may produce additional oocytes
Study found germline stem cells in the ovary cortex that were able produce oocytes