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. This are sex cells. These are gametes.
This can now be used in fertilization. Thus, because of independent assortment, recombination, and sexual reproduction, there are trillions of possible genotypes in the human species. During cell division, chromosomes sometimes disappear.
This occurs when there is some aberration in the centromere , and spindle fibers cannot attach to the chromosome to segregate it to distal poles of the cell.
Consequently, the lost chromosome never properly groups with others into a new nuclear envelope , and it is left in the cytoplasm , where it will not be transcribed. Also, chromosomes don't always separate equally into daughter cells. This sometimes happens in mitosis, when sister chromatids fail to separate during anaphase. One daughter cell thus ends up with more chromosomes in its nucleus than the other. Likewise, abnormal separation can occur in meiosis when homologous pairs fail to separate during anaphase I.
This also results in daughter cells with different numbers of chromosomes. The phenomenon of unequal separation in meiosis is called nondisjunction. If nondisjunction causes a missing chromosome in a haploid gamete, the diploid zygote it forms with another gamete will contain only one copy of that chromosome from the other parent, a condition known as monosomy.
Conversely, if nondisjunction causes a homologous pair to travel together into the same gamete, the resulting zygote will have three copies, a condition known as trisomy Figure 3. The term " aneuploidy " applies to any of these conditions that cause an unexpected chromosome number in a daughter cell. Aneuploidy can also occur in humans. For instance, the underlying causes of Klinefelter's syndrome and Turner's syndrome are errors in sex chromosome number, and Down syndrome is caused by trisomy of chromosome However, the severity of phenotypic abnormalities can vary among different types of aneuploidy.
In addition, aneuploidy is rarely transferred to subsequent generations, because this condition impairs the production of gametes. Overall, the inheritance of odd chromosome number arises from errors in segregation during chromosome replication. Often, it is these very exceptions or modifications of expected patterns in mitosis and meiosis that enrich our understanding of how the transfer of chromosomes is regulated from one generation to the next.
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Epistasis: Gene Interaction and Phenotype Effects. Genetic Dominance: Genotype-Phenotype Relationships. Phenotype Variability: Penetrance and Expressivity. Citation: Miko, I. Diploid cells all the cells in our body except our gametes have 2N chromosomes, because a diploid organism is created by union of 2 gametes each containing 1N chromosomes. A pair of sister chromatids is one chromosome because it has genetic information alleles inherited from only one parent.
A pair of homologous chromosomes, each consisting of a single chromatid in a daughter cell at the end of mitosis, has alleles from the father and from the mother, and counts as 2 chromosomes. This chromosome number stays the same after chromosome replication during S phase: each chromosome entering cell division now consists of a pair of sister chromatids joined together at the centromere.
Then in mitosis, the sister chromatids of each chromosome separate, so each daughter cell receives one chromatid from each chromosome. The result of mitosis is two identical daughter cells, genetically identical to the original cell, all having 2N chromosomes.
So during a mitotic cell cycle, the DNA content per chromosome doubles during S phase each chromosome starts as one chromatid, then becomes a pair of identical sister chromatids during S phase , but the chromosome number stays the same. A chromatid, then, is a single chromosomal DNA molecule.
The number of chromatids changes from 2X in G1 to 4X in G2 and back to 2X, but the number of chromosomes stays the same. The chromosome number is reduced from 2N to 1N in the first meiotic division, and stays at 1N in the second meiotic division. Because homologous chromosomes separate in the first division, the daughter cells no longer have copies of each chromosome from both parents, so they have haploid genetic information, and a 1N chromosome number.
The second meiotic division, where sister chromatids separate, is like mitosis. Chromosome number stays the same when sister chromatids separate. Using the information above, compare these two simplified diagrams of mitosis and meiosis to visualize why cells are haploid after meiosis I.
Specifically, compare the chromosomes in cells at the end of mitosis vs the end of meiosis I, recognizing that the diagram of mitosis tracks just a single pair of homologous chromosomes, whereas the diagram of meiosis tracks two pairs of homologous chromosomes one long chromosome and short chromosome :.
The video below is geared toward a high school audience, but it does present a helpful way for recognizing how many chromosomes are present in a cell and thus the ploidy level of that cell. While watching, see if you can recognize why the products of meiosis 1 are haploid cells:. Pingback: Revision — bbkamodules. You must be logged in to post a comment. Biological Principles. Skip to content. Home Evolution Strong Inference What is life? What is evolution? Evolution by Natural Selection Other Mechanisms of Evolution Population Genetics: the Hardy-Weinberg Principle Speciation Phylogenetic Trees Earth History and History of Life on Earth Origin of Life on Earth Molecules and Metabolism Chemical context for biology: origin of life and chemical evolution Biological molecules Membranes and Transport Cells Energy and enzymes Respiration, chemiosmosis and oxidative phosphorylation Oxidative pathways: electrons from food to electron carriers Fermentation, mitochondria and regulation Why are plants green, and how did chlorophyll take over the world?
Cell division: mitosis and meiosis Learning Objectives Describe the chromosomal makeup of a cell using the terms chromosome, sister chromatid, homologous chromosome, diploid, haploid, and tetrad Recognize the function and products of mitosis and meiosis Compare and contrast the behaviors of chromosomes in mitosis and meiosis Recognize when cells are diploid vs.
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