Unit 2: Genetic Processes
Lesson 4: Meiosis
Offspring produced by asexual reproduction are genetic clones of their parents, whereas offspring
produced by sexual reproduction inherit genetic information from two parents. Each parent contributes
a copy of half of its genetic information. This process of combining genetic information from two
individuals results in offspring that differ genetically from their parents and from each other. In order
for sexual reproduction to take place, gametes (or sex cells) must be produced such that they contain
half the number of chromosomes (haploid - n) as a somatic cell (diploid cell – 2n). If all organisms started
from a fertilized, diploid cell, how is it that we have gametes, such as sperm and ovum that are haploid?
Meiosis: Meiosis is a two-stage cell division in which the resulting daughter cells have
half the number of chromosomes as the parent cell. In every somatic cell, there are
two copies of each chromosome, each from a parent. Homologous chromosomes are
matching pairs of chromosomes, similar in size and carrying information for the same
genes.
Stages of Meiosis:
Meiosis I – division of homologous chromosomes
1. Interphase: As in mitosis, DNA replication occurs during interphase.
2. Prophase I: chromosomes start to shorten and thicken; nuclear membrane starts to dissolve;
centrioles start to move to opposite poles and spindle fibres start to form
- Chromosomes come together in homologous pairs. Each
chromosome of the pair is composed of a pair of sister
chromatids. The whole structure is then referred to as a
tetrad because each pair of chromosomes is composed of
four chromatids. Each sister chromatid intertwines with a
sister chromatid from its matching homologous chromosome,
a process called synapsis.
- Crossing Over occurs during prophase I, whereby
intertwined chromatids from different chromosomes bread
and reattach to eachother – exchanging sections of genetic
material. This results in increased variation as a result of
the recombination of genetic information.
3. Metaphase I: the tetrads (made up of pairs of homologous chromosomes) align their centromeres
across the middle of the cell.
4. Anaphase I: homologous chromosomes move to opposite poles of the cell. This is called reduction
division whereby only one chromosome from each homologous pair will be found in each new
daughter cell.
5. Telophase I: nuclear membrane starts to form around the chromosomes and the cell begins to
divide. These cells are now haploid and are no longer identical.
Meosis II – division of sister chromatids
1. Prophase II: nuclear membrane dissolves and the spindle fibres begin to form
2. Metaphase II: chromosomes align across the middle of the cell such that sister chromatids are
on opposite sides of the metaphase plate
3. Anaphase II: sister chromatids separate and move to opposite poles of the cell. The nuclear
membrane begins to form around the chromatid, now called chromosome.
4. Telophase II: second nuclear division is complete and the second division of cytoplasm occurs
Four haploid daughter cells (gametes) have been produced. The recombination of genetic
information that occurs during crossing over means that all four gametes are genetically
different.
Random Assortment of Homologous
Chromosomes:
In meiosis I, homologous chromosomes line up
and then separate by the end of meiosis I.
When they line up, they line up independently
and randomly which also increases the variety
in the resulting gametes.
For any diploid organism, the number of
possible combinations is 2n. With three pairs
of chromosomes, the number of possible
combinations is 23=8. What would be the
number of possible combinations that could
occur with humans? 223= 8 388 608. This
does not even take into account that
variation is increased with crossing over.
Gametogenesis:
Meiosis takes place in testes and ovaries. The
production of sperm cells is called spermatogenesis and the production of egg cells (ova) is called
oogenesis. While both of these processes follow the general process of meiosis, some differences exist.
In males, the cytoplasm is evenly divided among all four daughter cells, whereas in females, the majority
of the cytoplasm goes to one of the daughter cells resulting in three polar bodies.
Once a male reaches sexual maturity, he can produce hundreds of millions of sperm cells every
day. Only a few weeks before a female is even born, all of her potential eggs stop developing at the end
of prophase I. When she reaches sexual maturity, some of these cells go on to complete meiosis. In an
entire lifetime, a female human produces between 400 and 500 eggs. When the sperm and egg meet
through fertilization, then the two haploid cells join to make a diploid cell known as a zygote.
Homework:
1. Compare and contrast mitosis and meiosis. Choose any format you wish to highlight their
similarities and differences.
2. How do each of the following contributed to genetic variation in offspring?
a. Crossing over
b. Random assortment of homologous chromosomes
c. Fertilization
3. Prior to crossing over, in what ways are homologous chromosomes similar? In what ways do they
differ?
4. Individuals inherit a complete set of genetic instructions from each parent. Explain how this
happens.
5. Complete the following table and
a) Compare the chromosome number in the organisms before, during , and at the end of meiosis
b) Indicate whether the chromosome number is haploid or diploid
Human
Earthworm
Hedgehog
Broccoli
Before Meiosis
Chromosome number (haploid or diploid)
46
?
?
?
Number of pairs of homologous chromosomes
?
?
45
?
After Meiosis I
Chromosome number (haploid or diploid)
?
18
?
?
After Meiosis II
Chromosome number (haploid or diploid)
?
?
?
9
Number of pairs of homologous chromosomes
0
?
?
?
6. At what point in meiosis do cells change from being diploid to being haploid?
7. How many different arrangements of chromosomes are possible in gametes if the cell at the
beginning of meiosis has 10 chromosomes?
8. How can it be beneficial for an organism to produce only one large egg during oogenesis and have
three polar bodies that die? In contrast, how can it be beneficial for males to produce very large
numbers of very small sperm?
Solutions:
1, 2, 3, 4:
5, 6, 7:
8: