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Meiosis

No DNA Replication

Big Questions:

Make Sure You Can:

How is sex possible?

Why does sex exist?

Where does variation in a population come from?

Identify similarities and differences in sexual life cycles among various groups of organisms.

Explain how meiosis leads to the transmission of genetic information from parent to offspring.

Compare the events and outcome of meiosis with mitosis.

Explain the process and function of crossing over.

Explain how various aspects of meiosis and sexual reproduction increases variation in a species.

SEX! SEX! SEX!

What Sex Is

Why Sex Is

Sexual Reproduction is highly varied

Nobody knows why sex evolved.

Here's what we do know:

Most organisms do not sexually reproduce

At least in terms of mechanics, & life-cycles.

Sex increases variation exponentially

The "Reproductive handicap" of sex

3 Ways to make clones:

Sexual reproduction leads to a tremendous amount of variation in a population.

Asexually reproducing organisms only generate variation through mutations and horizontal genetic transfer.

Sexually reproducing organisms generate variation through the events of meiosis. Let's consider humans (n = 23, 2n = 46):

The variety of sexual life cycles seen among organisms

Parthenogentic Rotifer

Budding Hydra

Sprouting Redwood

  • "independent assortment" of homologues during metaphase I

23

Fundamentally all sexual reproduction involves the same cellular process ("fertilization"):

2

=

The easiest to understand mathematically.

  • Each homologous pair has a 50/50 chance of lining up so that "mom's" pair or "dad's" pair winds up in either of the cells produced.
  • Mathematically, this means that there are 2 possibilities for each of the 23 tetrads.

8,388,608 possible combinations per gamete

In sexually reproducing organisms, one gender ("males") is not capable of producing an offspring.

And yet, sex has evolved many, many times in many different lineages...why?

Sexually reproducing organisms need to make haploid cells ("gametes") from diploid cells...or there would be problems during fertilization

+

Haploid cell

Diploid cell

  • The random nature of fertilization

23

A bit more on chromosomes:

(n)

(2n)

(2 )

=

Analysis of chromosomes can tell us a lot about an individual.

"Karyotype": A picture of an individuals chromosomes.

One male gamete will combine with one female gamete

  • If there are 8,388,608 possible combinations in a gamete, and each gamete has an equal chance of combining with a gamete from the opposite gender, then our possible genetic combinations for an offspring is equal to the possible number of combinations for each gamete, multiplied by each other

n - the "haploid number"

2n - the "diploid number"

70,368,744,177,664 possible combinations per offspring

A technician making a karyotype

Pre-sorted:

Post-sorted:

  • Crossing Over during prophase I

Autosomes: Chromosomes that both genders have in equal numbers (humans: 1-22)

Sex Chromosomes: Chromosomes that determine gender (humans: X & Y)

Functionally

Infinite

Crossing over produces genetically unique chromatids. It is a random process, occurring an unpredictable number of times per meiotic cycle.

  • Due to this, it is not easy to mathematically model, but it is easy to draw a fundamental conclusion about the number of possible variants produced.

Different species have different numbers of chromosomes

Humans: n = 23 2n = 46

Fruit Flies: n = 4 2n = 8

Dogs: n = 39 2n = 78

Normal human female karyotype:

Normal human male karyotype:

genetically unique offspring produced in a sexually reproductive species

Meiosis

"Reductive"

Eukaryotic cell division

Stages of Meiosis:

Diploid Chromosomes

With a focus on differences from mitosis

Remember that every diploid cell has two copies of each chromosome.

During S phase each of these chromosomes is replicated.

"Sister Chromatids": The replicated copies of a particular chromosome

"Homologous Pairs": The set of 2 replicated copies of a particular chromosome (sometimes shortened to "homologues")

Is it making sense yet?

Interphase

How about now?

The fate of chromosomes during meiosis

Prophase I:

Metaphase I:

Meiosis at a Glance:

"Crossing Over"

Fundamentally, meiosis serves two major purposes:

  • Produce haploid cells
  • Create cells with unique combinations of genetic information.

How does meiosis lead to these outcomes?

Why does meiosis look so similar to mitosis?

During metaphase 1, homologous pairs of chromosomes line up at the metaphase plate still attached to each other.

When they separate during anaphase 1, the homologous pairs will separate.

Sister chromatids will remain attached.

This is a major difference from mitosis, where chromosomes line up "single file" during metaphase.

In meiosis, that doesn't happen until metaphase II.

When chromosomes condense during prophase 1, homologous pairs physically connect to eachother ("synapsis"), forming structures called "tetrads".

At each connection ("chiasma"), DNA is exchanged between the homologous pairs.

Every chromatid that is produced has a unique combination of DNA from both chromosomes in the pair.

This results in every gamete produced having a unique sequence of DNA in each chromosome.

"Recombination": Combining DNA from 2 different sources.

If only there were an awesomely cheesy/bizarre cartoon...

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