Review Questions
Meiosis
1.
Asexual reproduction versus sexual reproduction: which is better?
Asexual reproduction is much more efficient than sexual reproduction in a
number of ways. An organism doesn’t have to find a mate. An organism donates
100% of its’ genetic material to its offspring (with sex, only 50% end up in the
offspring). All members of a population can produce offspring, not just females,
enabling asexual organisms to out-reproduce sexual rivals.
2. So why is there sex? Why are there boys? If females can reproduce easier and
more efficiently asexually, then why bother with males?
Sex is good for evolution because it creates genetic variety. All organisms
depend on mutations for genetic variation. Sex takes these preexisting traits
(created by mutations) and shuffles them into new combinations (genetic
recombination). For example, if we wanted a rice plant that was fast-growing but
also had a high yield, we would have to wait a long time for a fast-growing rice to
undergo a mutation that would also make it highly productive. An easy way to
combine these two desirable traits is through sexually reproduction. By breeding
a fast-growing variety with a high-yielding variety, we can create offspring with
both traits.
In an asexual organism, all the offspring are genetically identical to the parent
(unless there was a mutation) and genetically identically to each other. Sexual
reproduction creates offspring that are genetically different from the parents and
genetically different from their siblings. In a stable environment, asexual
reproduction may work just fine. However, most ecosystems are dynamic places.
Conditions are in constant flux: changes in weather, climate, and landscape; new
competitors, new diseases, new predators, new parasites, etc. In a changing
world, it is dangerous for an organism to put all their genetic “eggs in one
basket”. So, most organisms use sex to hedge their bets. By producing
genetically unique offspring, parents have a better chance that a least some of
their offspring will have the right set of traits to survive a change.
So why are there boys? Females need males to produce offspring with greater
genetic diversity. Biologists see sex as a way a living organism can maintain the
evolutionary balance in ecosystems. Researchers studying parasite loads on
sexual- and asexual-reproducing populations discovered that the sexual
reproducers suffer less parasitism than their asexual counterparts.
Life is an evolutionary arms race. Parasites, for example, can quickly adapt to
host organisms with a single genotype. However, if the host genotype constantly
changes and varies every generation, adaptation, for the parasite, becomes
difficult. Sex bestows the benefit of a moving target onto these organisms. For a
female then, a male is worth the trouble if his shared diversity helps her children
evolve, survive and compete in a changing world.
3.
So if sex is great for creating genetic diversity, then how can critters like
bacteria and yeast maintain diversity while reproducing asexually 99% of the
time?
Microbes can rely mostly on mutations for their genetic diversity because they
reproduce often. When you have thousands of generations per year, you can
keep up with environmental change by relying solely on mutations. Slower
reproducing organisms have to reproduce sexually because the just can’t get
enough diversity through mutations alone.
4. Compare and contrast haploid and diploid cells.
Diploid cells have two sets of chromosomes. Diploid is abbreviated “2n”. Somatic
cells (body cells) like bone cells, muscle cells, neurons are all diploid. The
gametes or sex cells (eggs and sperm) are haploid. They have only one set of
chromosomes. Haploid is abbreviated “n”.
Where did these sets of chromosomes come from? In human diploid cells, we
inherited one set from mom and one set from dad. We all have one set and a
backup.
If you examined the karyotype of a human somatic cell, you would see 23 types
of chromosomes. Karyotyping is a way of visually organizing the chromosomes.
They are arranged by size, the longest type being chromosome #1. There are
two of each type—one from mom and one from dad. Each pair is called
homologous chromosomes. You’ll notice homologues have the same length, the
same centromere location, the same banding pattern, and if you were able to
look at the DNA sequence, you would discover that they have genes from the
same trait at the same location on the chromosome.
5. What is meiosis?
Meiosis is a special kind of cell division in eukaryotes. Meiosis creates haploid
gametes. Often referred to as reduction division, meiosis starts with a diploid cell
and ends up after two cell divisions with four haploid daughter cells. Meiosis in
humans occurs in the ovaries of females and the testes of males.
6. List the three functions of meiosis.
Meiosis has three functions. The first function is to keep the chromosome
number constant generation after generation. If meiosis did not occur, the sex
cells would remain diploid and, after fertilization, the offspring would have double
the number of chromosomes (tetraploid—4n). If this were allowed to continue,
there would be a doubling of chromosomes every generation. Not good.
The second function is the creation of genetic variation in the sex cells and in
turn diversity in the offspring. Meiosis creates sex cells that are genetically
different from the parent cell and genetically different from each other.
The third function is genetic integrity. Meiosis insures that every sex cell gets one
complete set of chromosomes. For humans, that would be one of every 23 types
of chromosomes.
7. Diagram the events in meiosis.
The chromosomes are duplicated in interphase.
During prophase I the chromosomes appear, the nuclear envelope fragments,
the spindle fibers form and attach to the centromeres of the chromosomes, the
nucleoli disappear just like prophase in mitosis.
However, there are a couple of events in prophase I that are unique. A duplicated
chromosome is also called a dyad (refers to the two chromatids). Homologous
pairs of dyads are pulled together into close proximity to one another in a
process called synapsis. In fact, in synapsis, the homologous dyads wrap around
one another and form a structure we call a tetrad (refers to the four chromatids).
While the homologous chromosomes are locked inside a tetrad, parts of the nonsister chromatids break off, exchange places, and reanneal. This is called
crossing over. The first source of variation, crossing over creates genetically
unique sex cells. Not only does crossing over recombine genes it also creates
mutations if the breaks occur within a gene. Crossing over doesn’t just occur on
one location on a chromosome. Crossing over events may span the entire
chromosome.
In metaphase I the pairs of homologous chromosomes are lined up side by side
on the equator. This is different from mitosis where the chromosomes line up
singly. Although crossing over is great at creating variation, the major gene
shuffling takes place in metaphase I during a process called independent
assortment. Each pair of chromosomes can line up in two different ways: mom
on the left—dad on the right or visa versa. Whether a chromosome lines up on
the left or right of the metaphase plate is unpredictable. How they line up dictates
what genetic combination ends up in the daughter cells: the greater the number
of chromosomes, the greater the number of possible combinations.
The tetrad is split and the dyads are pulled to opposite poles during anaphase I.
In telophase I, the chromosomes unwind, spindle fibers disappear, nuclear
envelope reforms, and the nucleoli reappear.
The cell divides into two (we call this interkinesis).
Meiosis II begins with prophase II. As predicted the chromosomes appear,
nuclear envelope fragments, spindle fibers appear, the nucleoli disappear. The
spindle fibers attach to the chromosomes and move them toward the equator.
In metaphase II, the dyads are lined up singly on the equator.
The spindle fibers shorten, the sister chromatids split and are pulled to opposite
poles in anaphase II.
Telophase II reforms the nuclear envelope, breaks down the spindle fibers,
uncoils the chromosomes, and the nucleoli reappear.
Cytokinesis divides the daughter cells and meiosis is complete. We are left with
four haploid cells; each one genetically different from each other and the parent
cell.
8. Describe the three ways meiosis produces genetic variability.
We have seen that meiosis creates variation three ways: crossing over,
mutations caused during crossing over, and independent assortment.
9. Describe spermatogenesis and oogenesis.
Spermatogenesis is the production of mature sperm. Sperm cells are made in the
seminiferous tubules within the testes. The average adult male makes 300,000
new sperm cells every minute. The average human ejaculate contains 250-400
million sperm. Males begin spermatogenesis at puberty and continue throughout
life. From start to finish, it takes 65-75 days for a mature sperm to be made.
Spermatogenesis begins with a diploid stem cell called a spermatogonium. Stem
cells can make other stem cells, as well as, differentiate and become a
specialized cell. Spermatogonia are no different. A spermatogonium can divide
(through mitosis) to make new spermatogonia. When a spermatogonium
differentiates to become a sperm it first transforms into a primary spermatocyte
(also diploid). The first meiotic division occurs and the primary spermatocyte
divides into two secondary spermatocytes that are haploid. The second meiotic
division occurs and the secondary spermatocytes divide resulting in a total of four
cells called spermatids (all haploid). The spermatids specialize and transform into
sperm. They build their tails, rearrange their cytoplasm, and stuff mitochondria
into their mid-piece.
Oogenesis is the production of mature eggs. Eggs are made in the ovaries of a
female. Eggs production begins before a woman is born. The stem cell oogonia
have all ready transformed into primary oocytes. At birth, the primary oocytes are
in prophase I (synapsis, tetrads, crossing over).
Women are born with 2 million primordial follicles (each one with a primary
oocyte). By puberty, the number of follicles has dropped to 300,000 to 400,000.
The first meiotic division also occurs at puberty forming two new daughter cells.
One daughter cell, the secondary oocyte is large but the other, called a polar
body, is small and tiny. Unlike spermatogenesis, oogenesis only produces one
egg from one primary oocyte. The other three daughter cells (polar bodies) are
discarded. The egg has to be large (lots of cytoplasm) to support the developing
embryo.
The second meiotic division only occurs if the secondary oocyte is fertilized by a
sperm. Without fertilization, meiosis never goes to completion. The final result of
oogenesis is four haploid cells; one functional egg and three discarded polar
bodies.
10. How many genetically different types of human gametes can be produced?
We can calculate the genetic diversity produced by meiosis by calculating the
number of possible combinations made during independent assortment. Humans
have 23 pairs of chromosomes. Each pair in metaphase I has two possible
combinations (one side or the other). Combine that 23 times and you get the
following:
223 = 8 million
So, independent assortment can produce 8 million different sperm or 8 million
different eggs. Therefore, the chances that one sperm is identical to another is 1
in 8 million. The chances that one egg is identical to another egg is also 1 in 8
million.
11. What are the chances that two offspring are genetically identical?
We can calculate those odds by multiplying the number of possible combinations
of a sperm with an egg.
223 x 223 = 64 trillion
The chances that two siblings will be genetically identical is 1 in 64 trillion
(assume two different sperm and two different eggs). If we asked your parents to
make another child genetically identical to you, they would only have a 1 in 64
trillion chance of doing it. Throw in the variation made by crossing over and the
odds sky rocket. Truly, there is no one like you.