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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!
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.
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):
Fundamentally all sexual reproduction involves the same cellular process ("fertilization"):
=
The easiest to understand mathematically.
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
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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
n - the "haploid number"
2n - the "diploid number"
Autosomes: Chromosomes that both genders have in equal numbers (humans: 1-22)
Sex Chromosomes: Chromosomes that determine gender (humans: X & Y)
Crossing over produces genetically unique chromatids. It is a random process, occurring an unpredictable number of times per meiotic cycle.
Different species have different numbers of chromosomes
Humans: n = 23 2n = 46
Fruit Flies: n = 4 2n = 8
Dogs: n = 39 2n = 78
"Reductive"
Eukaryotic cell division
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")
"Crossing Over"
Fundamentally, meiosis serves two major purposes:
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.