Comparing mitosis and meiosis (video) | Khan Academy
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Now what happens in meiosis? What happens in meiosis? I'll do that over here. In meiosis, something slightly different happens and it happens in two phases. You will start with a cell that has a diploid number of chromosomes. So you will start with a cell that has a diploid number of chromosomes.
And in it's interphase, it also replicates its DNA. And then it goes through something called Meiosis One. And in Meiosis One, what you end up with is two cells that now have haploid number of chromosomes. So you end up with two cells, You now have two cells that each have a haploid number of chromosomes.
So you have n and you have n. So if we're talking about human beings, you have 46 chromosomes here, and now you have 23 chromosomes in this nucleus. And now you have 23 in this nucleus. But you're still not done. Then each of these will go through a phase, which I'll talk about in a second, which is very similar to mitosis, which will duplicate this entire cell into two.
So actually, let me do it like this. So now, this one, you're going to have four cells that each have the haploid number that each have the haploid number of chromosomes. And they don't all necessarily have the same genetic informatioin anymore. Because as we go through this first phase, right over here of meiosis, and this first phase here you go from diploid to haploid, right over here, this is called Meiosis One. Meiosis One, you're essentially splitting the homologous pairs and so this one might get some of the ones that you originally got from your father, and some that you originally got from your mother, some that you originally got from your father, some that you originally got from your mother, they split randomly, but each homogolous pair gets split up.
And then in this phase, Meiosis Two, so this phase right over here is called Meiosis Two, it's very similar to mitosis, except your now dealing with cells that start off with the haploid number. It's important to realize that meiosis is not a cycle. These cells that you have over here, these are gametes.
Comparing mitosis and meiosis
This are sex cells. This can now be used in fertilization. If we're talking about, if you're male, this is happening in your testes, and these are going to be sperm cells If you are female, this is happening in your ovaries and these are going to be egg cells.
If you a tree, this could be pollen or it could be an ovul. Chromatin can be wound into higher-order fibers and into fully condensed chromosomes visible by light microscopy during mitosis or meiosis. In G2, after DNA replication in S phase, as cell enter mitotic prophase, each chromosome consists of a pair of identical sister chromatids, where each chromatid contains a linear DNA molecule that is identical to the joined sister.Carbon-14 Radioactive Dating Worked Example
The sister chromatids are joined at their centromeres, as shown in the image below. A pair of sister chromatids is a single replicated chromosome, a single package of hereditary information.
These mitotic chromosomes each consist of a pair of sister chromatids joined at their centromeres. The images of the homologous chromosome pairs e.
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Image from Bolzer et al. PLoS Biol 3 5: We inherited one copy of each chromosome from other mother, and one copy of each from our father.
Gametes sperm cells or egg cells are haploid, meaning that they have just one complete set of chromosomes. Chromosomes that do not differ between males and females are called autosomes, and the chromosomes that differ between males and females are the sex chromosomes, X and Y for most mammals.
Humans most commonly have 22 pairs of autosomes and 1 pair of sex chromosomes XX or XYfor a total of 46 chromosomes. Cells with complete sets of chromosomes are called euploid; cells with missing or extra chromosomes are called aneuploid.
The most common aneuploid condition in people is variation in the number of sex chromosomes: Having no X chromosome results in early embryonic death. The two copies of a particular chromosome, such as chromosome 1, are called homologous. The karyotype image above shows the homologous pairs for all the autosomes. Homologous chromosomes are not identical to each other, unlike sister chromatids. They frequently have different variants of the same hereditary information — such as blue eye color vs brown eye color, or blood type A versus blood type B.
Mitosis Mitosis produces two daughter cells that are genetically identical to each other, and to the parental cell. Each chromosome now consists of a joined pair of identical sister chromatids. During mitosis the sister chromatids separate and go to opposite ends of the dividing cell. This animation below shows the packaging of DNA and condensation of chromosomes as a cell undergoes mitosis. The video narration has a major error at time 1: Meiosis This is a special sequence of 2 cell divisions that produces haploid gametes from diploid germline cells.
It starts with a diploid cell that has undergone chromosomal DNA replication: Two successive divisions, with no additional DNA replication, results in 4 haploid gametes: NOVA has a good interactive side-by-side comparison of mitosis and meiosis on this page: How cells divide Meiosis sets the stage for Mendelian genetics.
Consider the X and Y chromosomes. They pair in prophase I, and then separate in the first division. Each cell now has only one sex chromosome, like a haploid cell. One way of thinking about ploidy is the number of possible alleles for each gene a cell can have.
Right after meiosis I, the homologous chromosomes have separated into different cells. Each homolog carries one copy of the gene, and each gene could be a different allele, but these two homologs are now in two different cells.
Though it looks like there are two of each chromosome in each cell, these are duplicated chromosomes; ie, it is one chromosome which has been copied, so there is only one possible allele in the cell just two copies of it. The second meiotic division is where sister duplicated chromatids separate. It resembles mitosis of a haploid cell. At the start of the second division, each cell contains 1N chromosomes, each consisting of a pair of sister chromatids joined at the centromere.
Here is a simplified diagram illustrating the overall process and products of meiosis: Meiosis Overview from Wikipedia by Rdbickel. In prophase I, homologous chromosomes pair and separate in the first division Meiosis I.
In Meiosis II, sister chromatids separate. And here is a video that walks through the steps of meiosis: It is very important that you recognize how and why cells become haploid after meiosis I.