lab 3 meiosis and trait inheritance overview this lab will introduce s
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Lab 3- Meiosis and Trait Inheritance
Overview
This lab will introduce students to the purpose of meiosis. Students will also learn how
chromosomes segregate according to Mendelian laws.
Learning objectives
By the end of this lab, students will be able to:
State the purpose of meiosis
Differentiate mitosis and meiosis
Explain the difference between haploid and diploid cells
Describe key events that occur in the stages of meiosis I and meiosis II
Learn to compute a Punnett square
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Exercise 1: Fertilization
A. Comparing Mitosis and Meiosis
The diagrams in Figure 1 show mitosis and meiosis after DNA has been replicated and
wound tightly into sister chromatids. The dotted lines represent cytokinesis.
Figure 1. Mitosis and meiosis
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A
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Sex
B
sex
D Activity A questions
1. In Figure 1, which column shows meiosis and which column shows mitosis.
Meiosis Click/tap to enter A or B
Mitosis Click/tap to enter A or B
2. What are some similarities between cell division by mitosis and cell division by
meiosis?
Click or tap here to enter text.
3. Complete Table 1 to describe some important differences between mitosis and
meiosis.
Table 1. Differences between mitosis and meiosis
Characteristic
Type of cells produced
Number of daughter cells Click or tap here to enter Click or tap here to enter
text.
text.
Genetic differences or
similarities between
daughter cells
Mitosis
Number of cell divisions
Meiosis
Click or tap here to enter Click or tap here to enter
text.
text.
Click or tap here to enter Click or tap here to enter
text.
text.
Click or tap here to enter Click or tap here to enter
text.
text. B. Meiosis - Cell Divisions to Produce Haploid Gametes
Before meiosis, the cell makes a copy of the DNA in each chromosome. Then, during
meiosis there are two cell divisions, Meiosis I and Meiosis II. These two cell divisions
produce four haploid daughter cells.
Meiosis I is different from mitosis because each pair of homologous chromosome lines
up next to each other and then the two homologous chromosomes separate.
Figure 2 shows Meiosis I for a cell with a single pair of homologous chromosomes. The
stripes on the chromatids of one of the chromosomes indicates that this chromosome
has different alleles than the other homologous chromosome.
Meiosis I will produce daughter cells with half as many chromosomes as the parent cell,
so the daughter cells are haploid. Each daughter cell has a different chromosome from
the original pair of homologous chromosomes.
Activity B questions
Examine Figure 2 which shows a diploid cell (2n) with a pair of homologous
chromosomes. After completing meiosis I, the resulting cells each contain one
chromosome and are now considered haploid (n).
Figure 2. Meiosis I
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xx
X
1. Do the chromosomes in the two daughter cells produced by Meiosis I have the same
alleles for each gene? Explain your reasoning.
Click or tap here to enter text. Meiosis II is like mitosis since the sister chromatids of each chromosome are
separated. As a result, each daughter cell gets one copy of one chromosome from the
pair of homologous chromosomes that was in the original cell. These haploid daughter
cells are the gametes.
2. Use asterisks (*) to indicate the cells in Figure 3 that represent sperm produced by
meiosis.
Figure 3. Meiosis II
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C. Meiosis and fertilization
Almost all the cells in your body were produced by mitosis. The only exceptions are the
gametes - sperm or eggs - which are produced by a different type of cell division
called meiosis.
During fertilization the sperm and egg unite to form a single cell called the zygote
which contains all the chromosomes from both the sperm and the egg. The zygote
divides into two cells by mitosis, then these cells each divide by mitosis, and mitosis is
repeated many times to produce the cells in an embryo which develops into a baby
(Figure 4).
Figure 4. Meiosis and fertilization
mother's cells
meiosis↓↓
egg
father's cells
meiosis↓↓
fertilization
zygote
repeated mitosis↓↓
embryo's cells
repeated mitosis ↓
baby's cells
sperm Activity C questions
1. Each cell in a normal human embryo has 23 pairs of homologous chromosomes, for
a total of 46 chromosomes per cell. How many chromosomes are in a normal human
zygote? Explain your reasoning.
Click or tap here to enter text.
2. What would happen if human sperm and eggs were produced by mitosis? Explain
why this would result in an embryo, which had double the normal number of
chromosomes in each cell.
Click or tap here to enter text.
A human embryo with that many chromosomes in each cell would be abnormal and
would die before it could develop into a baby. So, gametes cannot be made by mitosis.
For questions 3 and 4, click or tap number to enter the correct number.
3. Each human sperm and egg should have number_chromosomes, so fertilization will
produce a zygote with number_chromosomes. This zygote will develop into a
healthy embryo with number chromosomes in each cell.
4. Each sperm and each egg produced by meiosis has only one chromosome from
each pair of homologous chromosomes. When a sperm and egg unite during
fertilization, the resulting zygote has number pairs of homologous chromosomes.
For each pair of homologous chromosomes in a zygote, one chromosome came
from the egg and the other chromosome came from the Click or tap to enter text.