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Why does crossing over take place at all?

Why does crossing over take place at all?


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I already know that crossing over causes genetic variation. My question is that since DNA is a stable molecule, why would it undergo process like crossing over during which it becomes so unstable and develop a chance of mutation?


Crossing over is believed to be beneficial for evolution. It does not cause only mutations, it allows for the switching and mixing of genetic information. Thanks to crossing over entire protein families evolved, like the hemoglobins. Indeed it is one of the mechanisms that shaped the chromosomes. Some in-silico simulation proved that, in general, adaptation take place faster if crossing over is applied.

Here some extra references for further readings.

https://www.ncbi.nlm.nih.gov/pubmed/28565671

https://www.ncbi.nlm.nih.gov/pubmed/28564047

https://www.ncbi.nlm.nih.gov/pubmed/28563027

https://www.ncbi.nlm.nih.gov/pubmed/28557225

https://www.ncbi.nlm.nih.gov/pubmed/28555659


7.6: Genetic Variation

  • Contributed by Suzanne Wakim & Mandeep Grewal
  • Professors (Cell Molecular Biology & Plant Science) at Butte College

What helps ensure the survival of a species?

Genetic variation. It is this variation that is the essence of evolution. Without genetic differences among individuals, "survival of the fittest" would not be likely. Either all survive, or all perish.

Figure (PageIndex<1>): Profile photos


Chromosomes Crossing Over - Linked Genes

Patrick has been teaching AP Biology for 14 years and is the winner of multiple teaching awards.

When two genes are located on the same chromosome they are called linked genes because they tend to be inherited together. They are an exception to Mendel's law of Segregation because these genes are not inherited independently. When chromosomes cross over, two different chromosomes trade pieces of genetic information during prophase I of meiosis. If the linked genes are far apart on the chromosome, it is more likely that crossing over will separate them.

When my students start trying to study linked genes initially they get kind of confused but when you first after you get past that first confusion and you start to learn what it really is it's actually pretty simple concept.

The Human Genome Project has determined that humans have somewhere in the range of about 25,000 genes now if we only have 23 different kinds of chromosomes that tells you could do the same simple Math that we have roughly a 1000 genes plus or minus a bit on average per chromosomes so linked genes is simply the idea that if you have two genes on the same chromosomes, when that chromosome goes into a sperm or an egg those two genes travel together and windup being inherited together so they are "linked." This would violate Mendel's second law, the law of independent Assortment, but luckily there's this process during meiosis during the first step called prophase I that does this process called crossing over where physically a chromosome will break and exchange parts with its homologous pair so that even though two genes were right next to each other, maybe they get swapped over so your mum's version of a particular allele will get swapped over to the DNA that originally came from your dad and vice versa so that all of your children don't look the same.

Now, one thing about this is that genes that are far apart on the chromosome they tend to get crossed over very easily because they have lots of room for those breaks and exchanges to occur but if they're right smacked up next to each other it's very unlikely that you'll get the crossing over even between them so they tend to be inherited together. A lot of genetic test where they test your DNA to see if you have a particular disease or a chance for a particular disease, they're not actually testing for that gene they're actually testing for a linked gene that they found tends to be passed on along with the disease because they are still trying to figure out where exactly the disease gene is, so let's take a look at this and see how it works.

Now, here is a standard Mendelian example here we have somebody's homozygote recessive for two genes the r gene and the e gene I don't really care in this context what they stand for and this person is having a crossing event with somebody who is big R little r big E little e heterozygotes for both traits. Now this person can only create one possible kind of gamete, a gamete sperm or egg, that has little r and little e in it, this person on the other hand through the process of meiosis can create four possible gametes for example here is their chromosomes with the big R's and the larger I'm sorry the r gene is on the larger chromosomes the e gene is on the smaller chromosomes and if they are aligned this way when these separate we windup getting our r's and our e's like this. Now that was with the big r's and big e's together on one side. What if they work the other way? What if the big e happens to be on that side? On the opposite side from the big r what that means happens is that when they separate I windup creating a different possible combination than with the earlier combo, so what I'm going to do is I'm going to create for you here the four possible gametes that this one individual could create so they could make the big r little e that we see right here, they could see the big r big e version that's right here we could create a little r little e right here and our last one our little r big e right there and so when we do this cross when we have these two individuals have their offspring we would expect 25% would be this way 25, 25 and 25 so we'd have equal distributions of our genotypes and phenotypes make sense?

Alright, let's take a look at what happens when they're not on different chromosomes when instead the r and the e are on the same chromosome so here I have again same individuals same genotype but now the little r and little e are on physically the same piece of paper or DNA molecule similarly over here we have them together on the same DNA molecule so now when they pair up yes the red one could be on my left or on the right or can swap around it doesn't matter however they windup only creating two kinds of gametes what we see here alright? So we have only our big r big e gametes being formed these guys here absolutely none of the big r little e, fail, we have some little r's little e's right here but none of our little r big e's so 50% of our offspring will be this way 50% that way.

Now what's their crossing over stuff well let's put them back you recall meiosis during prophase I, the homologous chromosomes come together and then randomly they get broken by enzymes and then other enzymes specifically the enzyme used in your cells is an enzyme called ligase in this case I call it the enzyme tape and so they cross over like that and we windup getting this being formed and this being formed and this being formed, and our tape is running out too bad so sad, this being formed but wait even though these two chromosomes are now different from those the crossing over didn't occur between my r's and my e's so my eventual outcome winds-up being the same and that's because these guys are so close to each other. What if they were further apart? Let's take a look at that, here I have again two chromosomes again the r's and the e's are together on the same chromosome but now there's much further distance between them which gives, if I make the break at roughly the same spot there's a greater chance that the crossing over can occur so now I make the break at roughly the spot and now when I separate them I wind up going back to having something approaching a quarter or a quarter or a quarter so now I do start to see these sets of offspring when I do the breeding now it may not be 25, 25, 25, 25 percent it maybe 15% and 15% of this and then hmm let's see of 40 oh sorry, let me do my Math, 35 and 35% there but again we've de-linked them because of this crossing over event so that's linked genes.


How Does Crossing Over Create Genetic Variation?

Crossing over creates genetic variation by exchanging DNA between two nonsister chromatids to produce genetically unique chromosomes. The process of crossing over occurs during meiosis.

Meiosis is a type of cell division that produces four haploid gametes from a parent cell. These gametes contain 23 chromosomes, which is half of the genetic information of the parent. During fertilization, a zygote is formed when these gametes combine with the gametes of the opposite sex. The zygote will have the complete or diploid number of chromosomes, which is 46. The two ways wherein meiosis increases genetic diversity in a species are crossing over and independent assortment of homologous chromosomes.

Crossing over takes place during meiosis I when two nonsister chromatids exchange DNA material. Approximately two or three crossovers occur on a single chromosome. A homologous pair refers to two chromosomes that paired up in a process called synapsis. During synapsis, a nucleoprotein lattice is created to ensure that the DNA of non-sister chromatids are aligned. As the lattice breaks down, the chromatids remain attached at regions called chiasmata. These points of cross allow the exchange of genes between the chromatids. When the chromatids separate as chromosomes, the resulting haploid gametes will be genetically unique and varied.


What Is Crossing Over?

Another way to increase genetic diversity within an individual's gametes is a process called crossing over. During Prophase I in Meiosis I, homologous pairs of chromosomes come together and may exchange genetic information. While this process is sometimes difficult for students to grasp and visualize, it is easy to model using common supplies found in pretty much every classroom or home. The following lab procedure and analysis questions can be used to help those struggling to grasp this idea.


Review

  1. Explain how sexual reproduction occurs at the cellular level.
  2. Summarize what happens during meiosis.
  3. Compare and contrast gametogenesis in males and females.
  4. Explain mechanisms that increase genetic variation in offspring produced by sexual reproduction.
  5. Why do gametes need to be haploid? What would happen to the chromosome number after fertilization if they were diploid?
  6. Describe one difference between prophase I of meiosis and prophase of mitosis.
  7. Do all of the chromosomes that you got from your mother go into one of your gametes? Why or why not?
  8. True or False. Crossing-over is the exchange of genetic material between sister chromatids.
  9. True or False. Human sperms are haploid.
  10. True or False. Sister chromatids separate from each other during meiosis I.
  11. How many cells are produced after a single cell goes through meiosis?
  12. Which stage of meiosis (prophase I or II metaphase I or II anaphase I or II telophase I or II) best fits the descriptions below? Choose only one for each description.
    1. Pairs of homologous chromosomes line up along the equator of the cell
    2. Sister chromatids separate
    3. Homologous chromosomes separate from each other

    Why is Meiosis Important in Survival of Life?

    Meiosis is a phase in sexually reproductive organisms, wherein cell-division takes place. It is of great importance, because it creates genetic diversity in the population.

    Meiosis is a phase in sexually reproductive organisms, wherein cell-division takes place. It is of great importance, because it creates genetic diversity in the population.

    Meiosis is a process of gamete formation in which diploid germ-line cells, i.e., the cells that are set aside early in animal development for sexual reproduction, yield four genetically different haploid cells. It occurs only in sex cells, which are eggs and sperms.

    Phases

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    Meiosis takes place in two stages – Meiosis I, where DNA replication takes place and crossing-over occurs and Meiosis II, which lacks DNA replication, but is similar to Mitotic cell division.

    The Process

    • In meiosis, during the formation of gametes in animals and spores in plants, the chromosome number is reduced to half. These chromosomes contain the basic DNA chain.
    • During the first meiotic reduction division, the chromosomal pairs are divided so that each gamete or spore contains one of each chromosomal pair, it becomes a haploid.
    • When haploid gametes unite during fertilization, they form a zygote. Zygotes, having received one chromosome of each pair from each parent become diploid.
    • Meiosis involves two successive nuclear divisions, which produce four haploid cells. The meiosis I is the reduction division, meiosis II separates the chromatids, which are the daughter strands of a duplicated chromosome joined together by a centromere.
    • In mitotic cell division, new cells genetically identical to the parent cell are produced. Meiosis is responsible for increasing genetic variation in the population.
    • Each diploid cell, which undergoes meiosis can produce 2n different chromosomal combinations, where ‘n’ is the haploid number.
    • In humans, the number is 223, because there are 23 pairs of chromosomes. This number is greater than eight million different combinations.
    • The variation increases, because, during meiosis I, each pair of homologous chromosomes comes together.
    • In a process known as synapsis, each pair of homologous chromosomes may exchange parts.
    • The relative distance between two genes on a given chromosome can be estimated by calculating the percentage of crossing-over that takes place between them.

    Tasks of Meiosis

    • Production of haploid gametes to maintain the diploid number of species, generation after generation.
    • Crossing-over, which brings together new gene combination of chromosomes.
    • A mechanism for comparing the two copies of each chromosome, provided with the purpose of error correction or repairing.

    Importance

    • In meiosis, variation occurs, because each gamete (either sperm or egg) contains a mixture of genes from two different parent chromosomes in sexual reproduction. In other words, the genetic coupling of non-identical DNA takes place in meiosis.
    • It results in an offspring, which has the genetic material of two different individuals.
    • These chromosomes contain the basic DNA chain, which determines the physical and genetic characteristics of the child.
    • A new combination of genetic information is produced in the gametes. Therefore, in meiosis, the characteristics of parent chromosomes are combined with the characteristics of offspring chromosomes, which ultimately results in a new and unique set of chromosomes.
    • It enables individuals to produce physically and genetically unique offspring. Because of this, a high genetic diversity of a population is maintained.
    • With mitosis only division would have been possible and there would have been no sharing of genetic information.
    • In such a situation, there would have been only clonal populations, which would eventually suffer from diseases or natural disasters.
    • What is the explanation for the diversity in populations? How can they survive variations in the environment? The reason is meiosis. Genetic variation plays the role of a raw material for natural selection.
    • Some individuals who are favored by natural selection have greater fitness than others because of their alleles (pair of alternative forms of gene).
    • In case of animals, males that are unable to compete for mates, for example, succumb to predation or disease or fail to reproduce small and weak organisms don’t survive for long time. These are the best examples of natural selection.
    • You can also take an example of a disease to which some individuals will be at least partially resistant while others are susceptible to it.
    • A population can adapt to changes in the environment as a result of the genetic variation resulting from meiosis. However, in clonal asexual populations, organisms are not able to adapt to changes without mutations.
    • Organisms which adapt to changes in the environment, survive, while others get eliminated by natural selection. In this way, a population contains fit individuals and the process continues for generations together.
    • The diversity afforded by meiosis is beneficial for the population as a whole.

    Thus, meiosis helps to create a population that is not only physically and genetically different but also one, which is perfectly fit to survive.

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    What is a Test Cross: Why is it used (Biology)

    If I handed you a black guinea pig & asked, “What’s its phenotype for fur color?” You would gently hold the guinea pig, look at it and reply, “Black, you dummy … all you gotta do is look at it”. And I would say, “Correct, & please don’t call me dummy”.

    If I handed you the same guinea pig & asked, “What’s the genotype of this guinea pig with respect to its fur color?” You wouldn’t be able to tell me, and I wouldn’t be able to tell you either.

    The reason we don’t know is that there are two genotypes that BOTH produce a dominant trait phenotype, homozygous dominant (BB) & heterozygous (Bb), & we can’t see the actual alleles (letters) without serious scientific chromosomal-type analysis — and that’s assuming that a Guinea Pig Genome Project has been completed for us to refer to, & I don’t think it has.

    So how do we figure it out? We perform a TEST CROSS !

    Test cross = the cross of an organism with an unknown dominant genotype with an organism that is homozygous recessive for that trait

    What does it do?

    A test cross can determine whether the individual being tested is homozygous dominant (pure bred) or heterozygous dominant (hybrid).

    To perform an actual test cross with our black guinea pig, we would need a guinea pig (of the opposite sex) that is homozygous recessive (“bb”). In other words, we would need a white guinea pig to mate with our black guinea pig.

    We would give them a little privacy, hope that the female becomes pregnant, wait for however long the gestation period of a guinea pig is, & THEN we would look at the offspring .

    If any of the offspring from a test cross have the recessive trait, the genotype of the parent with the dominant trait must be heterozygous The reliability of a test cross increases with the number of offspring produced.

    “Key Points” to remember about a TEST CROSS :

    1. the organism with the dominant trait is always crossed with an organism with the recessive trait

    2. if ANY offspring show the recessive trait, the unknown genotype is heterozygous

    3. if ALL the offspring have the dominant trait, the unknown genotype is homozygous dominant


    Biology calculation

    the haploid chromosome number in the koala is 8. independent assortment of chromosomes in meiosis contributes to genetic variation in the gametes of the koala. how many genetically different versions of koala gamete would it be possible for one individual to produce if independent assortment were the only source of variation?

    Not sure what to do, please help.

    Not what you're looking for? Try&hellip

    (Original post by A*my)
    the haploid chromosome number in the koala is 8. independent assortment of chromosomes in meiosis contributes to genetic variation in the gametes of the koala. how many genetically different versions of koala gamete would it be possible for one individual to produce if independent assortment were the only source of variation?

    Not sure what to do, please help.

    I believe the equation you're looking for is 2 n where n = the number of pairs of homologous chromosomes

    I'm not sure whether or not the number of homologous pairs would be the number of homologous pairs but presuming it is?


    ** edit ** my textbook suggests it would be 16 as meiosis starts with a cell that contains the diploid number.
    2 16 = 65536??


    do you have a mark scheme for your question?

    (Original post by LiiHyde)
    I believe the equation you're looking for is 2 n where n = the number of pairs of homologous chromosomes

    I'm not sure whether or not the number of homologous pairs would be the number of homologous pairs but presuming it is?


    ** edit ** my textbook suggests it would be 16 as meiosis starts with a cell that contains the diploid number.
    2 16 = 65536??


    do you have a mark scheme for your question?

    Oooh so 256 so that means you use the haploid number as the number of homologous pairs - I shall be making note of that thank you

    This equation is in my AQA oxford published A level biology textbook

    If you have the textbook I'm talking about it's on page 228 but I'll presume not and tell you what it says

    Basically for 2 n you're working out the different combinations for each one in the homologous pair going into one of the two cells that forms which is why you have 2 at the start always.

    So 2 n is what you use to work out only independent segregation

    for working out the number of combinations when you factor in the random pairing of male and female gametes where you'd have done 2 n individually in both the sperm and the egg you can do (2 n ) 2 to work out the number of different combinations that you result in an offspring

    A question might ask you about problems with this method of calculating as obviously it is flawed in that it can't take into account crossing over into it's calculations

    Therefore these calculations are assuming that the chromosomes stay completely intact and no crossing over occurs

    (Original post by LiiHyde)
    Oooh so 256 so that means you use the haploid number as the number of homologous pairs - I shall be making note of that thank you

    This equation is in my AQA oxford published A level biology textbook

    If you have the textbook I'm talking about it's on page 228 but I'll presume not and tell you what it says

    Basically for 2 n you're working out the different combinations for each one in the homologous pair going into one of the two cells that forms which is why you have 2 at the start always.

    So 2 n is what you use to work out only independent segregation

    for working out the number of combinations when you factor in the random pairing of male and female gametes where you'd have done 2 n individually in both the sperm and the egg you can do (2 n ) 2 to work out the number of different combinations that you result in an offspring

    A question might ask you about problems with this method of calculating as obviously it is flawed in that it can't take into account crossing over into it's calculations

    Therefore these calculations are assuming that the chromosomes stay completely intact and no crossing over occurs