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Lesson Overview
11.4 Meiosis
Lesson Overview
Meiosis
Question #1
What must occur in order for Mendel’s principles to hold true?
An organism with two parents must inherit a single copy of
every gene from each parent
When that organism produces gametes, those two sets of
genes must be separated so that each gamete contains
just one set of genes
Chromosomes are the carriers of genes, therefore,
chromosomes must be separated
Lesson Overview
Meiosis
Question #2
What is a diploid cell?
Diploid = “Two sets”
Diploid cells of most adult organisms contain two complete
sets of inherited chromosomes (homologous
chromosomes) and two complete sets of genes
Homologous chromosomes are the two sets of chromosomes
(one from the male parent and one from the female parent)
Lesson Overview
Meiosis
Question #2
What is a diploid cell?
Lesson Overview
Meiosis
Question #3
What is a haploid cell?
Haploid = “One set”
Haploid cells contain only a single set of chromosomes, and
therefore a single set of genes
The gametes of sexually reproducing organisms are haploid
Lesson Overview
Meiosis
Question #4
What is meiosis?
Process in which the number of chromosomes per cell is cut
in half through the separation of homologous chromosomes
in a diploid cell
One diploid cell goes through two separate divisions,
producing four haploid cells
Lesson Overview
Meiosis
Question #5
Human Cell 2N = 46
Sperm cells (N) have 23 chromosomes
Egg cells (N) have 23 chromosomes
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Meiosis
Question #6
What happens prior to meiosis I?
The cell undergoes a round of
chromosome replication
during interphase
Each replicated chromosome
consists of two identical
chromatids, joined at the
centromere
Lesson Overview
Meiosis
Question #7
PROPHASE I
Spindle forms in the cell
Each replicated chromosome pairs with its corresponding
homologous chromosome
This pairing forms a structure called a tetrad
Crossing over occurs during prophase I
In crossing-over, sections of non-sister chromatids are
exchanged (this produces new combinations of alleles in
the cell)
Lesson Overview
Meiosis
Question #7
PROPHASE I – CROSSING OVER
Lesson Overview
Meiosis
Question #7
PROPHASE I
Tetrad (homologous
chromosomes) consisting of 2
sets of sister chromatids
Lesson Overview
Meiosis
Question #7
METAPHASE I
Paired homologous
chromosomes line up
across the center of the cell
The spindle fibers attach to
each tetrad
Lesson Overview
Meiosis
Question #7
ANAPHASE I
Spindle fibers pull each
homologous chromosome
pair toward opposite ends
of the cell
Lesson Overview
Meiosis
Question #7
TELOPHASE I and CYTOKINESIS
A nuclear membrane forms
around each cluster of
chromosomes
Cytokinesis follows telophase
I, forming two new cells
Each cell has ½ the genetic
material of the original
parent cell
Lesson Overview
Meiosis
Question #7
PROPHASE II
The spindle reforms in both of
the cells that were created
during meiosis I
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Meiosis
Question #7
METAPHASE II
Chromosomes (paired sister
chromatids) line up in the
center of the cell
Lesson Overview
Meiosis
Question #7
ANAPHASE II
The paired chromatids
separate
Lesson Overview
Meiosis
Question #7
TELOPHASE II and CYTOKINESIS
The nucleus begins to reform
around the chromosomes
in each cell
Cytokinesis of meiosis II
results in four haploid
daughter cells
Lesson Overview
Meiosis
Question #8
Gamete to Zygote
MEIOSIS
MITOSIS
MITOSIS
Sperm (N)
Egg (N)
Zygote (2N)
Lesson Overview
Meiosis
Question #9
Mitosis and Meiosis – Similarities
Both are preceded by a complete copying, or replication, of
the genetic material
Both have spindles
Both have the movement of chromosomes
Lesson Overview
Meiosis
Question #9
Mitosis and Meiosis – Differences
Mitosis has one cell division (meiosis has two cell divisions)
Mitosis produces two cells (meiosis produces four cells)
Mitosis only has sister chromatids line up and separate (meiosis has
homologous chromosomes line up and then sister chromatids
line up)
Mitosis maintains the chromosome number (meiosis cuts the
chromosome number in half
Mitosis ends with two diploid cells (meiosis ends with four haploid
cells)
Lesson Overview
Meiosis
Question #10
Thomas Hunt Morgan’s Conclusions
Each chromosome is actually a group of linked genes
Chromosomes assort independently (not the individual genes)
Alleles for different genes tend to be inherited together from
one generation to the next when those genes are located
on the same chromosome
Lesson Overview
Meiosis
Question #11
Alfred Sturtevant’s Conclusion
The farther apart two genes were on a chromosome, the more
likely it would be that crossing-over would occur between
them
If two genes are close together, then crossovers between them should
be rare
If two genes are far apart, then crossovers between them should be
more common
He used the frequency of crossing-over between genes to
determine their distances from each other (and he also
created gene maps from that data)
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Meiosis
Question #12
Curved and dumpy OR Curved and vestigial (small)
Curved wing and dumpy wing are more likely to cross over
The farther apart, the more likely crossing-over will occur
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Meiosis
Question #13
Least likely to cross over
Purple eye and Light eye are the least likely to cross over
The closer together the genes are, the less likely they are to
cross over
Meiosis
Lesson Overview
Question #14
Gene Map
In which gene map is the probability of crossing-over between
A and D greatest?
Number 3
1
2
3
4
Meiosis
Lesson Overview
Question #15
Gene Map
In which gene map is the probability of crossing-over between
A and D the least?
Number 4
1
2
3
4
Meiosis
Lesson Overview
Question #16
Gene Map
In which map are genes C and D most closely linked?
Number 2
1
2
3
4
Meiosis
Lesson Overview
Question #17
Gene Map
In map 4, which genes are least likely to cross over?
C and B
1
2
3
4