A.Natural selection, crossing over, and independent assortment.
B.Crossing over, independent assortment, and fertilization.
C.Independent assortment, fertilization, and natural selection.
D.Fertilization, natural selection, and crossing over.
The correct answer is B. Crossing over, independent assortment, and fertilization.
Genetic recombination is the process in which genes are recombined in such a way as to create a genetically different structure. This recombination is important since it produces a unique individual cell.
It is important for there to be genetically variable individuals in a population because of natural selection. Selection works by ensuring only the fittest individuals survive and therefore reproduce.
There are three ways in which genetic recombinants are generated. These three processes are crossing over, independent assortment and fertilization. Crossing over and independent assortment both happen during the process of meiosis, which occurs in the sex cells or gametes. First crossing over occurs, then independent assortment.
It is only after these two processes have occurred that the third method becomes important. Fertilization is the final way that genetic recombination occurs. This is because it is a random process in which any one of many sperm cells will fuse with any one particular egg cell.
The crossing over stage occurs during the first step of meiosis and really involves homologous chromosomes coming together and switching genes.
The next step of meiosis involves these genetically different homologs now lining up at the center of the cell. The way in which they line up is random and hence is known as independent assortment.
This is the type of cell division that only occurs in organisms that have sexual reproduction. The process takes place in the gametes or sex cells which are the sperm of males and eggs of females.
There are two main divisions that occur when a cell divides in meiosis. The first division is known as the reduction division because the chromosome number is halved.
The genetic material is copied before this division begins, and the result of this is that each chromosome has two arms known as chromatids. The stages include prophase I, metaphase I, anaphase I, telophase I and cytokinesis.
Prophase I to Metaphase I
During prophase I, the nucleolus and nuclear envelope disintegrate and chromosomes become visible. The process of crossing over also occurs during this stage.
Chromosomes are found in pairs known as homologs. This is because you inherit chromosomes from two parents, therefore one would be from your mother and one from your father.
These homologous chromosomes have the same genes found in the same places in each case. When the pair comes together it forms a tetrad because four chromosome arms are evident.
The two separate homologs now switch pieces of DNA, which means they become genetically variable. These new chromosomes are now genetic recombinants.
During metaphase I, the next stage, the homologs line up at the center of the cell. They line up opposite each other at random, which is why this is called independent assortment.
Since the process is random it means there is no way to know which chromosome will go to which daughter cell during the next stage when they move apart. This is an effective way to further introduce genetic variation during the cell division.
Anaphase I to Cytokinesis
Each of these chromosomes also attaches to a spindle fiber which contracts during anaphase I to pull them apart. The spindle is made of microtubules which contract and shorten with the effect of separating the homologs. It is important to note that the entire chromosome moves to the end of the cell.
In telophase I then the chromosomes are at the ends of the cell, and a nuclear envelope and nucleolus reforms. Cytokinesis then occurs in which the cytoplasm divides at the center of the cell to form two cells.
These daughter cells have half the number of chromosomes as the parent cell that divided. In other words, in humans, we would have 23 chromosomes in each of these cells since the parent cell that divided had 46 chromosomes. Each one of these daughter cells now undergoes meiosis II.
Each cell formed during meiosis I divides into two so that we end up with four cells at the end of all the stages. Meiosis II is similar to mitosis in that the chromatids separate and no genetic variation is introduced.
The first step is prophase II during which, once again, the chromosomes become visible and nucleolus and nuclear envelope disintegrate.
During metaphase II the chromosomes form one line at the center of the cell. This is different from metaphase I where there were two lines of chromosomes, one on each side of the center of the cell. These then attach to spindle fibers and chromatids are moved apart in anaphase II.
This is different from anaphase I where the chromosome with both chromatids still attached, moved on the spindle to the end of the cell. The final stage is cytokinesis or division of the cytoplasm.
After the cytoplasm divides we have two new daughter cells formed from each previous cell. This means that the total number formed from the one cell we started with is four cells.
These four cells are genetically different and have half the number of chromosomes as the parent cell. In fact, these four will become the gametes or sex cells of the organism. In males, they will form sperm, and in females, eggs.
The final way that genetic recombinants are formed is through random fertilization. It is totally random as to which sperm fuses with which egg cell. Since animals tend to produce many sperm cells; the odds of a particular cell fertilizing an egg is very small.
Once the gametes fuse a series of mitotic divisions occur to form an embryo. The new individual that is formed will be genetically unique and will contain genetic material from both parents.
The chromosome number is restored at fertilization, so in humans, we are back to 46 chromosomes in each cell.
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