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.
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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.
Now practice with your sockosomes.
Meiosis II
Meiosis II is like mitosis. Each chromosome splits in half, so that each daughter cell inherits one chromatid from each chromosome.
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 _____ Father _____
Meiosis ↓ Meiosis ↓
egg _____ 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.
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.
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:
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? ______
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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