The homologous pair exchanges genes via genetic recombination so that genetic diversity may be promoted. This is regarded as one of the advantages of having been able to reproduce sexually. Those that reproduce asexually create a clone of themselves. Thus, this could reduce the gene pool. A small gene pool means low genetic diversity. It could be unfavorable because it means there is less opportunity in acquiring genes essential for adapting to an environment prone to inexorable physicochemical changes.
In contrast, greater genetic variability means a higher propensity to acquire better genes. High genetic diversity also means a large gene pool. This, in turn, implicates increased chances of acquiring genes that could enhance biological fitness and survival. Got questions on homologous chromosomes? Our community may be able to help! Mutations can also influence the phenotype of an organism. This tutorial looks at the effects of chromosomal mutations, such as nondisjunction, deletion, and duplication.
Read More. Plants are characterized by having alternation of generations in their life cycles. This tutorial is a review of plant mitosis, meiosis, and alternation of generations. This tutorial looks at sex determination via the sex chromosomes, X and Y.
Read it to get more info on X and Y chromosomes and the genetic traits inherited via these two This tutorial describes the independent assortment of chromosomes and crossing over as important events in meiosis.
Read this tutorial to know more details in each of these meiotic events and how they promote genetic diversity in sexually-reproducing organisms Humans are diploid creatures. This means that for every chromosome in the body, there is another one to match it. However, there are organisms that have more than two sets of chromosomes. The condition is called polyploidy.
Know more about this topic through this tutorial Genes are expressed through the process of protein synthesis. This elaborate tutorial provides an in-depth review of the different steps of the biological production of protein starting from the gene up to the process of secretion. Also included are topics on DNA replication during interphase of the cell cycle, DNA mutation and repair mechanisms, gene pool, modification, and diseases Skip to content Main Navigation Search.
Dictionary Articles Tutorials Biology Forum. Homologous chromosome — definition. Table of Contents. Homologous chromosomes showing sister and non-sister chromatids. The main differences between mitosis and meiosis occur in meiosis I, which is a very different nuclear division than mitosis. In meiosis I, the homologous chromosome pairs associate with each other, are bound together, and undergo crossing over between nonsister chromatids.
They line up along the metaphase plate as tetrads. With pulling apart of the tetrad during anaphase I, the number of chromosomal sets has been reduced.
Mitosis has not chromosomal reduction. Meiosis II is much more analogous to a mitotic division. In this case, the duplicated chromosomes line up on the metaphase plate. During anaphase II, as in mitotic anaphase, the centromeres divide and one sister chromatid is pulled to one pole while the other sister chromatid is pulled to the other pole.
If not for crossing over, the two products of each individual meiosis II division would be identical like in mitosis. But there will always be some crossing over. Meiosis II is not a reduction division because although there are fewer copies of the genome. There is still one set of chromosomes, as at the end of meiosis I.
Figure 6. Meiosis and mitosis are both preceded by one round of DNA replication; however, meiosis includes two nuclear divisions.
The four daughter cells resulting from meiosis are haploid and genetically distinct. The daughter cells resulting from mitosis are diploid and identical to the parent cell. Click through the steps of this interactive animation to compare the meiotic process of cell division to that of mitosis: How Cells Divide.
Sexual reproduction requires that diploid organisms produce haploid cells. These then fuse during fertilization to form diploid offspring. Meiosis is a series of events that arrange and separate chromosomes and chromatids into daughter cells. During the interphases of meiosis, each chromosome is duplicated. In meiosis, there are two rounds of nuclear division resulting in four nuclei and usually four daughter cells, each with half the number of chromosomes as the parent cell.
The first separates homologous chromosomes, and the second separates chromatids into individual chromosomes. During meiosis, variation in the daughter nuclei can occur due to crossing over prophase I and random alignment of tetrads metaphase I. The cells produced by meiosis are genetically unique.
Meiosis and mitosis share similarities, but have distinct outcomes. Mitotic divisions are single. Meiotic divisions include two nuclear divisions producing four daughter nuclei that are genetically different, having one chromosome set instead of the two sets like the parent cell. The main differences between the processes occur in the first division of meiosis. The second division of meiosis is more similar to a mitotic division. In a comparison of the stages of meiosis to the stages of mitosis, which stages are unique to meiosis and which stages have the same events in both meiosis and mitosisAnswers.
All of the stages of meiosis I are unique because homologous chromosomes are separated, not sister chromatids. Skip to main content. Chapter 8: Genes, Chromosomes, and the Cell Cycle. Search for:. The Process of Meiosis Learning Objectives By the end of this section, you will be able to: Describe the behavior of chromosomes during meiosis Describe cellular events during meiosis Compare the differences between meiosis and mitosis Distinguish between the two instances of genetic variation.
Meiosis I Meiosis I is preceded by an interphase consisting of the G 1 , S, and G 2 phases, which are very similar to the phases preceding mitosis.
Prophase I Figure 1. Link to Learning Click through the steps of this interactive animation to compare the meiotic process of cell division to that of mitosis: How Cells Divide. Additional Self Check Questions 1.
Define tetrad. Name two methods of variation in cell division. A tetrad forms when homologous chromosomes pair up during synapsis. For example, there are genetic variations that arise in clonal species , such as bacteria , due to spontaneous mutations during mitotic division.
Furthermore, chromosomes are sometimes replicated multiple times without any accompanying cell division. This occurs in the cells of Drosophila larvae salivary glands, for example, where there is a high metabolic demand. The chromosomes there are called polytene chromosomes, and they are extremely large compared to chromosomes in other Drosophila cells.
These chromosomes replicate by undergoing the initial phases of mitosis without any cytokinesis Figure 2. Therefore, the same cell contains thick arrangements of duplicate chromosomes side by side, which look like strands of very thick rope. Scientists believe that these chromosomes are hyper-replicated to allow for the rapid and copious production of certain proteins that help larval growth and metamorphosis Gilbert, The greatest impact of Sutton's work has far more to do with providing evidence for Mendel's principle of independent assortment than anything else.
Specifically, Sutton saw that the position of each chromosome at the midline during metaphase was random, and that there was never a consistent maternal or paternal side of the cell division. Therefore, each chromosome was independent of the other. Thus, when the parent cell separated into gametes, the set of chromosomes in each daughter cell could contain a mixture of the parental traits, but not necessarily the same mixture as in other daughter cells.
To illustrate this concept, consider the variety derived from just three hypothetical chromosome pairs, as shown in the following example Hirsch, Each pair consists of two homologues: one maternal and one paternal.
Here, capital letters represent the maternal chromosome, and lowercase letters represent the paternal chromosome:. When these chromosome pairs are reshuffled through independent assortment , they can produce eight possible combinations in the resulting gametes:. A mathematical calculation based on the number of chromosomes in an organism will also provide the number of possible combinations of chromosomes for each gamete. In particular, Sutton pointed out that the independence of each chromosome during meiosis means that there are 2 n possible combinations of chromosomes in gametes, with "n" being the number of chromosomes per gamete.
Thus, in the previous example of three chromosome pairs, the calculation is 2 3 , which equals 8. Furthermore, when you consider all the possible pairings of male and female gametes, the variation in zygotes is 2 n 2 , which results in some fairly large numbers. But what about chromosome reassortment in humans?
Humans have 23 pairs of chromosomes. That means that one person could produce 2 23 different gametes. In addition, when you calculate the possible combinations that emerge from the pairing of an egg and a sperm, the result is 2 23 2 possible combinations. However, some of these combinations produce the same genotype for example, several gametes can produce a heterozygous individual.
Of course, there are more than 23 segregating units Hirsch, While calculations of the random assortment of chromosomes and the mixture of different gametes are impressive, random assortment is not the only source of variation that comes from meiosis. In fact, these calculations are ideal numbers based on chromosomes that actually stay intact throughout the meiotic process. In reality, crossing-over between chromatids during prophase I of meiosis mixes up pieces of chromosomes between homologue pairs, a phenomenon called recombination.
Because recombination occurs every time gametes are formed, we can expect that it will always add to the possible genotypes predicted from the 2 n calculation.
In addition, the variety of gametes becomes even more unpredictable and complex when we consider the contribution of gene linkage.
Some genes will always cosegregate into gametes if they are tightly linked, and they will therefore show a very low recombination rate. While linkage is a force that tends to reduce independent assortment of certain traits, recombination increases this assortment. In fact, recombination leads to an overall increase in the number of units that assort independently, and this increases variation. While in mitosis, genes are generally transferred faithfully from one cellular generation to the next; in meiosis and subsequent sexual reproduction , genes get mixed up.
Sexual reproduction actually expands the variety created by meiosis, because it combines the different varieties of parental genotypes. 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.
The crossover events are the first source of genetic variation produced by meiosis. A single crossover event between homologous non-sister chromatids leads to an exchange of DNA between chromosomes.
Following crossover, the synaptonemal complex breaks down and the cohesin connection between homologous pairs is also removed. At the end of prophase I, the pairs are held together only at the chiasmata; they are called tetrads because the four sister chromatids of each pair of homologous chromosomes are now visible. Crossover between homologous chromosomes : Crossover occurs between non-sister chromatids of homologous chromosomes.
The result is an exchange of genetic material between homologous chromosomes. Synapsis holds pairs of homologous chromosomes together : Early in prophase I, homologous chromosomes come together to form a synapse. The chromosomes are bound tightly together and in perfect alignment by a protein lattice called a synaptonemal complex and by cohesin proteins at the centromere. The key event in prometaphase I is the formation of the spindle fiber apparatus where spindle fiber microtubules attach to the kinetochore proteins at the centromeres.
Microtubules grow from centrosomes placed at opposite poles of the cell. The microtubules move toward the middle of the cell and attach to one of the two fused homologous chromosomes at the kinetochores.
At the end of prometaphase I, each tetrad is attached to microtubules from both poles, with one homologous chromosome facing each pole. In addition, the nuclear membrane has broken down entirely. During metaphase I, the tetrads move to the metaphase plate with kinetochores facing opposite poles. The homologous pairs orient themselves randomly at the equator. This event is the second mechanism that introduces variation into the gametes or spores. In each cell that undergoes meiosis, the arrangement of the tetrads is different.
The number of variations is dependent on the number of chromosomes making up a set. There are two possibilities for orientation at the metaphase plate. The possible number of alignments, therefore, equals 2n, where n is the number of chromosomes per set. Given these two mechanisms, it is highly unlikely that any two haploid cells resulting from meiosis will have the same genetic composition.
In this case, there are two possible arrangements at the equatorial plane in metaphase I. The total possible number of different gametes is 2n, where n equals the number of chromosomes in a set.
In this example, there are four possible genetic combinations for the gametes. In anaphase I, the microtubules pull the attached chromosomes apart. The sister chromatids remain tightly bound together at the centromere. The chiasmata are broken in anaphase I as the microtubules attached to the fused kinetochores pull the homologous chromosomes apart.
In telophase I, the separated chromosomes arrive at opposite poles. In some organisms, the chromosomes decondense and nuclear envelopes form around the chromatids in telophase I. Then cytokinesis, the physical separation of the cytoplasmic components into two daughter cells, occurs without reformation of the nuclei.
In nearly all species of animals and some fungi, cytokinesis separates the cell contents via a cleavage furrow constriction of the actin ring that leads to cytoplasmic division. In plants, a cell plate is formed during cell cytokinesis by Golgi vesicles fusing at the metaphase plate.
This cell plate will ultimately lead to the formation of cell walls that separate the two daughter cells. Two haploid cells are the end result of the first meiotic division. The cells are haploid because at each pole there is just one of each pair of the homologous chromosomes. Therefore, only one full set of the chromosomes is present.
Although there is only one chromosome set, each homolog still consists of two sister chromatids. During meiosis II, the sister chromatids within the two daughter cells separate, forming four new haploid gametes.
Meiosis II initiates immediately after cytokinesis, usually before the chromosomes have fully decondensed. In contrast to meiosis I, meiosis II resembles a normal mitosis.
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