Kinetochore proteins are multiprotein complexes that bind the centromeres of a chromosome to the microtubules of the mitotic spindle. 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.
With each member of the homologous pair attached to opposite poles of the cell, in the next phase, the microtubules can pull the homologous pair apart.
A spindle fiber that has attached to a kinetochore is called a kinetochore microtubule. At the end of prometaphase I, each tetrad is attached to microtubules from both poles, with one homologous chromosome facing each pole. The homologous chromosomes are still held together at chiasmata.
In addition, the nuclear membrane has broken down entirely. During metaphase I, the homologous chromosomes are arranged in the center of the cell with the kinetochores facing opposite poles. The homologous pairs orient themselves randomly at the equator. For example, if the two homologous members of chromosome 1 are labeled a and b, then the chromosomes could line up a-b, or b-a. This is important in determining the genes carried by a gamete, as each will only receive one of the two homologous chromosomes.
Recall that homologous chromosomes are not identical. They contain slight differences in their genetic information, causing each gamete to have a unique genetic makeup. This randomness is the physical basis for the creation of the second form of genetic variation in offspring. Consider that the homologous chromosomes of a sexually reproducing organism are originally inherited as two separate sets, one from each parent. Using humans as an example, one set of 23 chromosomes is present in the egg donated by the mother.
The father provides the other set of 23 chromosomes in the sperm that fertilizes the egg. Every cell of the multicellular offspring has copies of the original two sets of homologous chromosomes. In prophase I of meiosis, the homologous chromosomes form the tetrads. In metaphase I, these pairs line up at the midway point between the two poles of the cell to form the metaphase plate.
Because there is an equal chance that a microtubule fiber will encounter a maternally or paternally inherited chromosome, the arrangement of the tetrads at the metaphase plate is random.
Any maternally inherited chromosome may face either pole. Any paternally inherited chromosome may also face either pole. The orientation of each tetrad is independent of the orientation of the other 22 tetrads. This event—the random or independent assortment of homologous chromosomes at the metaphase plate—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 2 n , where n is the number of chromosomes per set. Humans have 23 chromosome pairs, which results in over eight million 2 23 possible genetically-distinct gametes.
This number does not include the variability that was previously created in the sister chromatids by crossover. Given these two mechanisms, it is highly unlikely that any two haploid cells resulting from meiosis will have the same genetic composition Figure 3. Figure 3. In this case, there are two possible arrangements at the equatorial plane in metaphase I.
The total possible number of different gametes is 2 n , where n equals the number of chromosomes in a set. In this example, there are four possible genetic combinations for the gametes. Then, if required, a synaptonemal complex is formed, and exchange can take place. Abstract The conditions re reviewed that must be met by any model of long distance attraction and transport of homologous chromosomes to the points of intimate DNA synapsis. Publication types Review. In meiosis, a diploid 2n cell will give rise to four haploid n cells.
The cells that undergo meiosis are the gametes producing haploid sperm cell and egg cell. Haploidy is essential so that at fertilization the chromosomal number remains the same throughout generations.
In order to achieve haploidy, the cell undergoes two consecutive nuclear divisions. They are referred to as meiosis I and meiosis II. To prepare the cell to meiosis, one of the major preparatory steps is DNA replication. The chromosomes duplicate their DNA, particularly in the S phase of interphase. At this point, each of the chromosomes will consist of two strands sister chromatids joined at the centromere.
The pairing synapse of homologous chromosomes will occur at prophase I. DNA exchanges occur between homologous chromosomes via homologous recombination and crossover at chiasmata between non-sister chromatids. Then, the homologous pairs line up at the metaphase plate. Next, the homologous chromosomes separate during anaphase I and move to the opposite poles of the cell.
Then, the cell divides for the first time during telophase I resulting in two genetically non-identical daughter cells but with sister chromatids still intact.
Each cell will undergo meiosis II so that the resulting daughter cells will each have a chromosomal number reduced by half. In humans, the nucleus typically contains 46 chromosomes. Thus, there are 22 pairs of autosomes with approximately the same length, staining pattern, and genes with the same loci. As for the sex chromosomes, the two X chromosomes are considered as homologous whereas the X and Y chromosomes are not.
Thus, females have 23 homologous chromosomes i. They also bear the genetic information that determines the trait of an organism. Homologous chromosomes, therefore, are vital in the same way. They carry genetic information that has been passed down from one generation to the next.
And since alleles may possibly be different in the same gene, the result is varying phenotypes. Thus, the distinctiveness of an individual of the same species is established.
Apart from this, the organism is capable of reproducing offspring that is genetically different from itself as well as from the rest of its descendants. This event is crucial to promote genetic variation. 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.
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