Lab Activity: Sockosomes
ADAPTED by J.Trifunovic from “Mitosis, Meiosis, and Fertilization” by Dr. R. Scott Poethig, Dr. Ingrid
Waldron, and Jennifer Doherty, Department of Biology, University of Pennsylvania
Part 1: Mitosis Review
1. How many cells do you think your body has?
2. Why does your body need to have lots of cells?
3. Each of us began as a single cell, so one important question is: How did that single cell develop
into a body with a trillion cells?
The production of such a large number of body cells is accomplished by many, many repeats of a cycle
of cell division in which one cell gives rise to two cells, each of which in turn gives rise to two cells, etc.
Thus, cell division is needed for growth.
Part 2: Modeling Mitosis
You will model mitosis using sockosomes. Each sockosome represents a chromosome in which the
DNA has been copied, and the two copies of the DNA are in two chromatids which are attached to
each other at the centromere. These chromatids are often called sister chromatids because they are
identical. Model Prophase, Metaphase, Anaphase and Telophase.
Get your teacher’s signature here:
Chromatids
Cell
Chromosome
Part 3: Meiosis
Mitosis gives rise to almost all the cells in the body. A different type of cell division called meiosis gives
rise to sperm and eggs. During fertilization the sperm and egg unite to form a single cell called the
zygote which contains chromosomes from both the sperm and egg. The zygote undergoes mitosis to
begin development of the human embryo which eventually becomes a baby.
1. In humans, how many chromosomes should a zygote have, so the baby's body cells will each
have a normal set of chromosomes? _____
2. If the sperm and egg each contribute all of their chromosomes to the zygote, how many
chromosomes should each sperm and egg have to produce a normal zygote? _____
3. Suppose sperm and eggs were produced by mitosis. If a sperm of this type fertilized an egg of
this type, how many chromosomes would the resulting zygote have? _____
4. Why would this be a problem?
5. How could this problem be avoided?
Meiosis reduces the number of chromosomes by half, so in humans each sperm and each egg has only
23 chromosomes, including one chromosome from each pair of homologous chromosomes. Therefore,
fertilization results in a zygote which has 23 pairs of homologous chromosomes, one in each pair from
the sperm and one from the egg. When the zygote undergoes mitosis to begin to form an embryo,
each cell will have the normal number of 46 chromosomes.
Cells that have two copies of each chromosome (i.e. cells that have pairs of homologous
chromosomes) are called diploid cells. Most of the cells in our bodies are diploid cells. Cells that only
have one copy of every chromosome are called haploid cells. Which types of cells in our bodies are
haploid?
Meiosis consists of two cell divisions, meiosis I and meiosis II. This reduces the chromosome number
by half and produces four haploid daughter cells (instead of two diploid daughter cells as in mitosis).
Sockosomes: Meiosis I
Meiosis I is different from mitosis because homologous chromosomes line up next to each other and
then separate, as shown below. This produces daughter cells with half as many chromosomes as the
parent cell, i.e. haploid cells. Notice that each of the daughter cells has a different chromosome from
the homologous pair of chromosomes. This means that the alleles in each daughter cell are different.
Cell
Now practice with your sockosomes.
Cell
Cell
Meiosis II
Meiosis II is like mitosis. Each chromosome splits in half, so that each daughter cell inherits one
chromatid from each chromosome.
Cell
Cell
Cell
Cell
Cell
Cell
Using your sockosomes, go through each step of meiosis until you are confident that you
understand the difference between Meiosis I and Mitosis and the difference between Meiosis I
and Meiosis II.
1. Describe the differences between the cell that undergoes meiosis and the daughter cells
produced by meiosis.
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Part 4: Fertilization
The following diagram provides an overview of the information covered thus far. Review the diagram,
and fill in the correct number of chromosomes per human cell in each blank.
Mother _____
Meiosis ↓
egg _____
Father _____
Meiosis ↓
sperm _____
Fertilization
zygote _____
Mitosis ↓
Embryo _____
Mitosis ↓
baby _____
Now we will consider fertilization in more detail, in order to provide a basis for understanding genetics.
Events during meiosis and fertilization determine the genetic makeup of the zygote, which in turn
determines the genetic makeup of the baby that develops from the zygote.
You already know that sisters or brothers can have different characteristics, even when they have the
same parents. One major reason for these different characteristics is that the processes of meiosis and
fertilization result in a different combination of genes in each child.
To begin to understand this genetic variability, you will model meiosis and fertilization for a very
simplified case where there is only one pair of homologous chromosomes per cell, and the two
homologous chromosomes carry different alleles of the same genes. One person in your group will be
the mother with two sockosomes that are the same color (one with a stripe and one without) that
represent the two maternal homologous chromosomes with different alleles of the same labeled gene.
Another person will be the father with two sockosomes that are a different color than the mother’s (one
with a stripe and one without) carrying the same two labeled alleles as the mother’s sockosomes. (The
different colors for the mother's and father's sockosomes represent the fact that, although the labeled
alleles are the same for the mother’s and father’s chromosomes, there are many genes on each
chromosome and the mother’s and father’s chromosomes will have different alleles for many of these
genes.)
1. In this simple example, how many different types of eggs will be produced by meiosis? _____
2. How many different types of sperm will be produced by meiosis? _____
The different types of sperm can fertilize the different types of egg to result in zygotes with different
combinations of chromosomes from the mother and the father. Fertilization can be demonstrated by
having the mother and father each contribute one chromatid from one of their sockosomes,
representing one chromosome from the egg and one chromosome from the sperm. Try to produce as
many different types of zygotes as you can by pairing each type of sperm with each type of egg. To
demonstrate fertilization, it works best to lay the chromosomes out on the table, so you can more easily
see the multiple different possible combinations.
1. How many different types of zygotes can be produced by fertilization in this simple case?
2. What different combinations of the labeled alleles can be observed in the zygotes?
A pair of human parents could produce a great many more different genetic combinations than
observed in this simplified example. For example, humans have 23 pairs of homologous
chromosomes, so many, many different combinations of chromosomes can be found in the eggs or
sperm produced by one person, and the different combinations of eggs from one mother and sperm
from one father could produce zygotes with approximately 70 trillion different combinations of
chromosomes! You can see why no two people are genetically alike, except for identical twins who are
derived from the same zygote.
Part 5: Karyotypes
Adapted from Concepts of Genetics 8e by
Klug, Cummings, and Spencer
In a karyotype, the complete set of chromosomes is organized in homologous pairs and numbered.
Each numbered pair of homologous chromosomes carries a specific set of genes. For example, both
copies of human chromosome 11 have a gene for the production of the pigment melanin (a molecule
that contributes to our skin and hair color), but one may have the A allele for melanin production and
skin color, while the other may have the little a allele. If both chromosomes have the little a allele, the
cells do not produce melanin, which results in albinism (as shown in the figure on the previous page).
Introduction
Since the mid 1950s there has been a dramatic increase in our knowledge of human cytogenetics. In
1956 it was discovered that the diploid chromosome number in human beings is 46 rather than 48, as
previously believed. The cause of Down syndrome (an extra chromosome) was discovered in 1959.
This was quickly followed by the discovery of a number of other chromosome abnormalities.
Chromosomal preparations are
most easily obtained from white
blood cells. The preparation is
set up by placing drops of blood
in a tube containing growth
medium supplemented with
phytohemagglutinin to stimulate
the division of lymphocytes. The
tube is capped and incubated at
37C. After the lymphocytes
increase, colchicine is added
and incubation continued for a
short time. Colchicine arrests
dividing cells at metaphase.
Down Syndrome
Sometimes, meiosis does not happen perfectly, so the chromosomes are not divided completely
equally between the daughter cells produced by meiosis. For example, an egg or a sperm may receive
two copies of the same chromosome.
1. In a human egg which has received an extra copy of one chromosome, how many
chromosomes would there be?
2. If this egg is fertilized by a normal sperm, how many chromosomes will there be in the resulting
zygote?
Cells need to have just the right balance of chromosomes and genes in order to function properly and
develop into a normal baby. Therefore, in most cases, a zygote which has an extra chromosome will
die early in embryonic development, resulting in a miscarriage.
However, some babies are born with an extra copy of a small chromosome (chromosome 21), and this
results in the condition known as Down Syndrome. A karyotype of a boy with Down Syndrome is
shown below.1 Multiple abnormalities result from the extra copy of chromosome 21 in each cell,
including mental retardation, a broad flat face, a big tongue, short height, and congenital heart disease.
Circle the TRISOMY below:
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CONCLUDING ANALYSIS
Directions: Answer all questions directly on this sheet.
1. How many chromosomes are there in a human skin cell produced by mitosis? ________
2. How many chromosomes are there in a human sperm cell produced by meiosis? _______
3. If the somatic cell of an apple has 34 chromosomes, how many chromosomes are found in a
gamete of an apple? _________
4. If the gamete of a human has 23 chromosomes, how many chromosomes are found in the
somatic cell of a human? _________
5. If a dog has 39 chromosomes in its sex cells, how many chromosomes are there in its body
cells? _________
6. If a garden pea plant has a diploid number of 8, how many chromosomes are in the haploid
cells of the garden pea plant? _________
7. If the tomato has a haploid number of 12, how many chromosomes are in the diploid cells of the
tomato? _________
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8. If 2n = 1260 for the Adder’s tongue fern, how many chromsomes are in the haploid cells of this
plant? _________
9. What does “2n” mean? _____________________________________________________
10. What does “n” mean? ______________________________________________________
11. How many cells are produced during meiosis? _________
12. In the sexual reproduction of all organisms, what percentage of genetic information comes from
the male? _________
13. In the sexual reproduction of all organisms, what percentage of genetic information comes from
the female? _________
14. What are homologous chromosomes?
15. What is crossing over and why is it important?
16. How are the cells at the end of meiosis different from the cells at the beginning of meiosis? Use
the terms chromosome number, haploid, and diploid in your answer.
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