Download Meiosis: A Special Type of Cell Division

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Meiosis: A Special Type of Cell Division
Overview of Meiosis
As viewed from a human perspective, nature has done some ingenious engineering to overcome some of the
obstacles it has faced. Take the evolution of sex, for instance. To make the move from asexual to sexual
reproduction, nature took a system by which parent cells reproduced simply by dividing (asexual
reproduction) and altered it to allow two parent cells to combine to create offspring (sexual reproduction).
It met this challenge by devising (again, speaking from a human perspective) a system by which parent cells
incorporate genetic information from both of its parents but contain half the amount of DNA. With only half
the DNA, when the parent cell combines with another parent cell, the proper amount of DNA is maintained.
This solution is called meiosis.
Mitosis describes the process by which the nucleus of a cell divides to create two new nuclei, each
containing an identical copy of DNA. (Cytokinesis describes the division of the rest of the cell.) Almost all of
the DNA duplication in your body is carried out through mitosis. Meiosis, as described above, is the process
by which certain sex cells (called “gametes”) are created. If you're male, your body uses meiosis to create
sperm cells; if you're female, it uses meiosis to create egg cells. Others cells in your body contain 46
chromosomes: 23 from your father and 23 from your mother. Your gametes contain only half that number—a
total of 23 chromosomes. When an egg and sperm unite to make a fertilized egg, the chromosomes add up
to equal 46.
Consequences of Meiosis
Why do same-sex siblings, sisters or brothers, look similar rather than identical? When we say that one sister
has her mother’s nose or mouth and the other her father’s, we are noting variations caused primarily by the
different genes that each sibling inherited.
Recall that in sexually reproducing organisms, such as humans, each cell has two sets of chromosomes—one
set from each parent. When gametes are formed by meiosis, the homologous chromosomes are split up to
the gametes at random.
Chromosome pairs are oriented along the middle of the cell during metaphase I of meiosis. At this time,
spindle fibers have harnessed and oriented one homologue toward one end of the cell and the other
homologue toward the other end of the cell. The first contacts between spindle fibers and chromosomes are
random. This means that the maternal (mother’s) and paternal (father’s) chromosomes can go into either
daughter cell—there is no particular pattern.
Consider what this means in terms of the number of different gametes possible. A human cell has twentythree pairs of homologous chromosomes. This means there are 223 or 8,388,608 possible combinations of
maternal and paternal chromosomes per gamete. So, the likelihood of any two siblings being dealt the same
chromosome combination from either parent, let alone both, is infinitesimal.
“Crossing Over”
Another important event occurs during meiosis that leads to genetic
variation in offspring. When homologous chromosomes are positioned
side by side during the first meiotic division, sections of the two
chromosomes cross, break, and reattach. This process (called “crossingover”) rearranges the genes on the chromosomes and produces new
combinations of alleles (versions of a gene). This leads to increased
genetic variation in the offspring that receive these altered
chromosomes.
Questions
1. Unlike humans, fruit flies have only 4 pairs of homologous chromosomes. How many possible
combinations of maternal and paternal chromosomes does this represent? Hint: see example in reading.
(2 points)
2. Based on the above information, would you expect fruit fly offspring to be more or less genetically
diverse than human offspring? Explain why. (2 points)
3. What two events occur during meiosis that can lead to genetic variation in the offspring of sexually
reproducing organisms? (2 points)
4. The fact that such a vast majority of animal species reproduce sexually suggests that it must provide
some overall advantage for the survival of a species. What do you think this advantage might be?
(4 points)