Name: Dina Date: Tuesday, December 1, 2020
Student Exploration: Meiosis
Vocabulary : anaphase, chromosome, crossover, cytokinesis, diploid, DNA, dominant, gamete, genotype, germ cell, haploid, homologous chromosomes, interphase, meiosis, metaphase, mitosis, ovum, phenotype, prophase, recessive, sister chromatid, sperm cell, telophase, zygote
Prior Knowledge Questions (Do these BEFORE using the Gizmo.)
- During mitosis , a single cell divides to produce two daughter cells. What must happen in the original cell so that each of the daughter cells has a complete set of chromosomes ?
It is important that the daughter cells have a copy of every chromosome, so the process involves copying the chromosomes first and then carefully separating the copies to give each new cell a full set.
- During sexual reproduction, two sex cells fuse to create a fertilized cell with a complete set of chromosomes. What must be true about the number of chromosomes in each sex cell?
The DNA must be copied so there is a full set of DNA to pass on to each daughter cell.
Gizmo Warm-up Meiosis is a type of cell division that results in four daughter cells with half as many chromosomes as the parent cell. These daughter cells mature into gametes , or sex cells. In the Meiosis Gizmo, you will learn the steps in meiosis and experiment to produce customized sex cells and offspring.
On the STEPS tab, click Male . You are looking at a germ
cell , or a cell that will undergo meiosis to become gametes.
- Read the description of interphase at the bottom of the Gizmo. What happens to the cell at
the beginning of interphase? cells grow synthesize mRNA and proteins required for
- Click on the DNA in the nucleus of the cell. Describe what happens. DNA is copied and the dell grows some more
- Why is it necessary for the cell to grow and duplicate its DNA before the start of meiosis? 2 DNA sets
Introduction: Unlike mitosis, which produces two identical daughter cells from one parent cell, meiosis creates four unique daughter cells with half the amount of DNA as the parent cell.
Question: How does meiosis create four daughter cells from one parent cell?
- Observe: ( Prophase I) Click on the nucleus to break it down then click on the DNA to condense it into chromosomes. Drag the centrosomes to the top and bottom of the cell.
A. How many chromosomes does this cell have? 4 pairs
Each chromosome consists of a pair of sister chromatids , two identical strands of DNA that formed when DNA replicated during interphase.
B. On the image to the right, draw two lines connecting the pairs of homologous chromosomes (chromosomes of similar size with a matching set of genes).
In the Gizmo, drag the homologous chromosomes together. Click Continue.
- Observe: ( Metaphase I and Anaphase I) – Drag the groups of homologous chromosomes to the metaphase plate, then drag spindle fibers from each of the centrosomes to the chromosomes. Click the centrosome to pull the chromosomes apart.
How do the chromosomes separate in anaphase I? sister chromatids het pulled to either end of the cell
- Compare: An image of the anaphase step in mitosis is shown to the right.
A. How does anaphase I in meiosis differ from anaphase
in mitosis? mitosis breaks the chromatids
into 4. meiosis pulls 2 chromatids apart.
Steps in meiosis
Get the Gizmo ready: ● Make sure the STEPS tab is selected. ● If necessary, choose the Male cell. Click on the DNA to copy it to proceed to prophase I.
Activity A (continued from previous page)
- Observe: Telophase I and cytokinesis are the final steps of the first half of meiosis.
A. Describe what happens when you click on the chromosomes during telophase I.
chromosomes unravel and the nuclear envelope reforms around them
B. Click and drag on the contractile ring. Describe what happened during cytokinesis.
Structure made of actin and myosin filaments that forms a belt around a dividing cell, pinching it in two.
- Observe: Go through the steps of the second half of meiosis until you reach the end of telophase II, following the instructions at the top right corner. As you proceed, answer the questions below. Use the Back button if you need to see a step again.
A. Before prophase II begins, does the DNA in the cell duplicate itself? No
B. During metaphase II, do homologous chromosomes pair up as in metaphase I? No
C. How does anaphase II differ from anaphase I? anaphase I has chromosomes, anaphase II has sister chromatids
D. At the end of anaphase II, how many chromatids are on each side of the cell? 2
E. After cytokinesis, how many cells have been formed from the parent cell? 4
F. Are all of the cells the same size? yes
The original parent cell is called diploid because it contains a complete set of homologous chromosome pairs. Each of the four daughter cells is haploid , meaning that each contains half of the original parent cell’s chromosomes. Each daughter cell contains one chromatid from each homologous pair.
- Observe: Click on the spermatids. Spermatids that formed from meiosis will develop into mature male gametes called sperm cells . Sketch a mature sperm cell in the space to the right.
Mature sperm cells have only a small amount of cytoplasm and use their flagella, or “tails,” to propel themselves forward. Sperm are designed for one purpose, to deliver genetic material to the egg cell during fertilization.
Introduction: Although both male and female gametes contain genetic material from the parent
organism, they perform different functions. A male gamete delivers genetic material to a female gamete. The fertilized female gamete, called a zygote , then grows into the offspring.
Question: What are the differences in meiosis between male and female cells? Male meiosis takes place in the testicles, while female meiosis takes place in the ovaries.
- Compare: Click on the Female button. For the female cell, proceed through meiosis until you reach the end of anaphase I.
Up to this point, did you notice any differences between the development of male and
female gametes? In species with two separate sexes, the sex that produces the
smaller and more motile sex cell or gamete is called the male. Explain: Male mammals
produce gametes called sperm while female mammals produce gametes called eggs.
- Compare: Proceed through telophase I and cytokinesis I.
A. What do you notice about the size of the two resulting cells? 3 small, 1 large
B. How does this compare to the two cells at the end of telophase I and cytokinesis I in
male cells? Many cells that undergo rapid meiosis do not decondense the
chromosomes at the end of telophase I. Other cells do exhibit chromosome
decondensation at this time; the chromosomes recondense in prophase II.
- Compare: Continue through meiosis until you finish telophase II and cytokinesis II.
A. What do you notice about the four cells now? All 3 of the cells are the same size an their is one larger one
B. What is the largest cell called? OVUM
The ovum is the largest cell in the human body. In contrast, the sperm cell is the smallest cell in the human body.
Comparing female and male gametes
Get the Gizmo ready: ● Make sure the STEPS tab is selected.
● Click Reset .
Introduction: The activities above shows that organisms can produce at least four different
gametes. In reality, organisms can produce millions of genetically unique gametes.
Question: How can meiosis create an unlimited number of unique gametes?
- Experiment: Use the following abbreviations for the chromosomes. Dark green – DG; Light green – LG; Dark purple – DP, Light purple – LP. Choose a Male or Female cell.
A. Proceed though meiosis to anaphase I. Which chromosomes went up and which
went down? Up: chromosomes Down: anaphase
B. Click Back and run anaphase I again a few times. Did the results ever change?
Explain. Chromosomes are distributed randomly during anaphase I.
C. Chromosomes are distributed randomly during anaphase I. What are the possible chromosome combinations in the two daughter cells? (Use DG, LG, DP, and LP.)
There are (223) possible combinations of maternal and paternal chromosomes.
- Experiment: Click Reset . Choose a Male or Female cell. Proceed through meiosis until the chromosomes are condensed in Prophase I.
Drag the LG (light green) chromosome to the Allele map on the left. This shows the alleles (or variations of a gene) that are present on the chromosome. A genotype is a list of alleles. The genotype of the LG chromosome, for example, is EEFFGGHHJJ.
A. What are the genotypes of the remaining chromosomes? DG: Light green
LP: Dark green DP: Light purple
B. After moving the centrosomes, drag the pairs of homologous chromosomes together.
Click on a chromosome. What happens? It creates a crossover
Get the Gizmo ready: ● Make sure the STEPS tab is selected. ● Click Reset .
When homologous chromosomes are paired up, they can exchange sections. This exchange of genes is called a crossover .
C. Click on several segments to create crossovers, and then click Continue . Proceed to anaphase I. Drag each chromosome to the Allele map and write its genotype.
LG: DG DG: LP LP: DP DP: LP
(Activity C continued on next page)
Introduction: Earlier, you learned how crossovers can result in genetically diverse gametes. In this activity, you will perform crossovers in parent cells undergoing meiosis and combine the resulting gametes to produce offspring with specific genotypes.
Question: How can offspring be created that have a specific phenotype and genotype?
- Explore: The EXPERIMENTATION tab shows a simplified fruit fly genome, with a single pair of homologous chromosomes. Each chromosome has genes that control wing shape, body color, antenna type, and eye color. The uppercase alleles are dominant and the lower case alleles are recessive . The allele key is given at lower left. (Note that real fruit flies have eight chromosomes and many more genes.)
A. Click Reset . Without creating any crossovers, click Divide into gametes . What are
the possible genotypes of the gametes? CBLR or cbrl
B. Drag a gamete from each parent into the box below to create a zygote. What are the
different combinations of possible offspring genotypes? Bb Aa Ab
C. Click Show phenotype for each combination. What are the resulting phenotypes?
- Experiment: Click Reset . You can create crossovers by clicking on the middle chromatids in each of the parent cells.
A. Create a gamete with the genotype C b l r. First, click on the c gene in one of the parent cells to create the crossover. Then, click Divide into gametes .
Did you create a gamete with the genotype C b l r? yes
B. Click Reset . Create a gamete with the genotype: c b L R. How many crossover were
needed to create this gamete? Just one
When a crossover occurs, the entire portion of genetic material is swapped between the two homologous chromosomes, so gene C is swapped along with gene B and gene R is swapped along with gene L.
C. Click Reset . Create a c B L r gamete. How many crossovers were needed? two
(Activity D continued on next page)
Activity D (continued from previous page)
- Challenge: Select the Challenge radio button. Make sure that Target offspring 1 is selected in the dropdown menu.
Target offspring 1 is a fruit fly with normal wings (cc), a black body (bb), normal antenna (ll) and red eyes (Rr). Because the offspring receives one chromatid from each parent, each chromatid should come from a different parent.
A. Using the Gizmo, create a fruit fly with the correct genotype. Explain how you did it.
I crossed over with a losercase r and an uppercase.
B. Is there another way to get the correct phenotype, but not the correct genotype? Yes
Explain. Since some genes are recessive, dominant ones will show up on top.
- Challenge: Use the dropdown menu to switch to the next target offspring. While creating target offspring 2-5, fill out the table below.
To produce target offspring 5, why were two crossovers needed on one chromatid arm?
Two crossover were needed because the chromosomes switched out the inside parts.
5. Think and discuss: Suppose there are two homologous chromosomes. Each chromosome
contains a single mutant allele in different parts of the chromosome. How can crossovers be beneficial in this situation? (Hint: How can you create a single, mutation-free chromosome?)
If the crossover results in the pair of the chromosome, in which one contains no mutant allele while others contain two mutant alleles.
Target offsprin g
Genotype of chromatid 1
Genotype of chromatid 2
Number of crossovers Parent 1
Number of crossovers Parent 2