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Copy of AP Bio- Cell Cycle 3: Meiosis
Transcript of Copy of AP Bio- Cell Cycle 3: Meiosis
Make Sure You Can:
What Sex Is
Why Sex Is
Sexual Reproduction is highly varied
At least in terms of mechanics, & lifecycles.
Fundmentally all sexual reproduction involves the same cellular process ("fertilization"):
Sexually reproducing organisms need to make haploid cells ("gametes") from diploid cells...or there would be problems during fertilization
Most organisms do not sexually reproduce
Eukaryotic cell division
Stages of Meiosis:
No DNA Replication
Meiosis at a Glance:
The variety of sexual life cycles seen among organisms
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?
How about now?
The fate of chromosomes during meiosis
n - the "haploid number"
2n - the "diploid number"
Different species have different numbers of chromosomes
Humans: n = 23 2n = 46
Fruit Flies: n = 4 2n = 8
Dogs: n = 39 2n = 78
With a focus on differences from mitosis
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.
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.
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?
3 Ways to make clones:
Nobody knows why sex evolved.
Here's what we do know:
The "Reproductive handicap" of sex
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?
Sex increases variation exponentially
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):
"indendent assortment" of homologues during metaphase I
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
The random nature of fertilization
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
70,368,744,177,664 possible combinations per offspring
Crossing Over during prophase I
Crossing over produces genetically unique chromatids. It is a random process, occuring 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.
genetically unique offspring produced in a sexually reproductive species
A bit more on chromosomes:
Analysis of chromosomes can tell us a lot about an individual.
"Karyotype": A picture of an individuals chromosomes.
A technician making a karyotype
Autosomes: Chromosomes that both genders have in equal numbers (humans: 1-22)
Sex Chromosomes: Chromosomes that determine gender (humans: X & Y)
Normal human male karyotype:
Normal human female karyotype:
How is sex possible?
Why does sex exist?
Where does variation in a population come from?
SEX! SEX! SEX!
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.
If only there were an awesomely cheesy/bizarre cartoon...