1. Meiosis and Chromosome Assortment
Introduction to Biology

2. Chromosomes in Human Cells
Somatic cells include all cells in the human body except sperm and eggs.
Gametes are human sperm and egg cells.
Each human somatic cell has 23 pairs of chromosomes, 46 total.
Each pair of chromosomes are called homologous chromosomes.
Each homologous chromosome carries a copy of the same genes, either from the father or mother.

3. LE 13-3
Pair of homologous
chromosomes
5 µm
Centromere
Sister
chromatids
This is called a karyotype. All 23 pairs of homologous chromosomes are lined up.

4. Key Maternal set of chromosomes (n = 3) 2n = 6 Paternal set of
LE 13-4
Key
Maternal set of
chromosomes (n = 3)
2n = 6
Paternal set of
chromosomes (n = 3)
Two sister chromatids
of one replicated
chromosomes
Centromere
Two nonsister
chromatids in
a homologous pair
Pair of homologous
chromosomes
(one from each set)

5. The sex chromosomes are called X and Y
Human females have two X chromosomes.
Human males have one X and one Y chromosome
The 22 pairs of chromosomes that do not determine sex are called autosomes.

6. Inheritance of Genes
A gene is a unit of heredity that carries the information for a specific trait or body function.
A gene is made of a segment of DNA.
Each gene is located on a specific chromosome.
Everyone has two copies of each gene (one on each homologous chromosome).

7. A cell with a full pair of each chromosome is called diploid.
Diploid is written shorthand as 2n.
All somatic cells are diploid (46 chromosomes).
A cell with only one of each homologous chromosome is called haploid.
Haploid is written shorthand as n.
All gametes are haploid and have 23 total chromosomes.

8. Gametes are haploid cells, containing only one set of chromosomes
For humans, this means 23 total chromosomes (no pairs)
This includes 22 autosomes and a single sex chromosome
In an unfertilized egg (ovum), the sex chromosome is always X
In a sperm cell, the sex chromosome may be either X or Y

9. Chromosomes and the Human Sex Cycle
At sexual maturity, the ovaries and testes begin producing sperm and eggs through meiosis.
Gametes are the only types of human cells produced by meiosis, rather than mitosis
Meiosis is a form of cell division that results in one set of chromosomes in each gamete instead of two.
The resulting daughter cells are haploid.
When fertilization occurs, the haploid sperm and haploid egg fuse together to form a diploid embryo.

10. Interphase
At the end of interphase, each cell has grown into its full size, produced a full set of organelles, and duplicated its DNA.
The cell is diploid at this point.
The nucleus contains 23 homologous chromosome pairs.
Each chromosome is made of two sister chromatids (copies).

11. Prophase I
The cells begin to divide, and the chromosomes pair up, forming a structure called a tetrad, which contains four chromatids.

12. Prophase I
As homologous chromosomes pair up and form tetrads, they undergo a process called crossing-over.
First, the chromatids of the homologous chromosomes overlap each other.
Then, the crossed sections of the chromatids are exchanged.
Crossing-over is important because it produces new combinations of genes in the cell.

13. Metaphase I
As prophase I ends, a spindle forms and attaches to each tetrad.
During metaphase I of meiosis, paired homologous chromosomes line up across the center of the cell.

14. Anaphase I
During anaphase I, spindle fibers pull each homologous chromosome pair toward opposite ends of the cell.
When anaphase I is complete, the separated chromosomes cluster at opposite ends of the cell.

15. Telophase I and Cytokinesis
During telophase I, a nuclear membrane forms around each cluster of chromosomes.
Cytokinesis follows telophase I, forming two new cells.

16. Summary of Meiosis I Two new haploid cells have been produced.
Each haploid cell contains one chromosome out of the original pair.
Each chromosome still contains two sister chromatids.

17. Prophase II
As the cells enter prophase II, their chromosomes—each consisting of two chromatids—become visible.
The chromosomes do not pair to form tetrads, because the homologous pairs were already separated during meiosis I.

18. Metaphase II
During metaphase of meiosis II, chromosomes line up in the center of each cell.

19. Anaphase II
As the cell enters anaphase, the paired chromatids separate.

20. Telophase II and Cytokinesis
The two daughter cells from Meiosis I divide, resulting in four daughter cells, each with two chromatids.
These four daughter cells now contain the haploid number (N)—just two chromosomes each.

21. Summary of Meiosis II A total of four cells have been produced.
Each cell is haploid and only contains one out of the original pairs of homologous chromosomes.
Each chromosome only contains a single chromatid.

22. A Comparison of Mitosis and Meiosis
Mitosis produces cells that are genetically identical to the parent cell.
Meiosis reduces the number of chromosomes sets from two (diploid) to one (haploid).
Meiosis allows crossing over of chromosomes.
This produces cells that are genetically different from the parents and each other.

23. Three events are unique to meiosis, and all three occur in meiosis l:
Synapsis and crossing over in prophase I: Homologous chromosomes physically connect and exchange genetic information
At the metaphase plate, there are paired homologous chromosomes (tetrads), instead of individual replicated chromosomes
At anaphase I, it is homologous chromosomes, instead of sister chromatids that separate and are carried to opposite poles of the cell

24. (before chromosome replication)
LE 13-9
MITOSIS
MEIOSIS
Parent cell
(before chromosome replication)
Chiasma (site of
crossing over)
MEIOSIS I
Propase
Prophase I
Chromosome
replication
Chromosome
replication
Tetrad formed by
synapsis of homologous
chromosomes
Duplicated chromosome
(two sister chromatids)
2n = 6
Chromosomes
positioned at the
metaphase plate
Tetrads
positioned at the
metaphase plate
Metaphase
Metaphase I
Anaphase
Sister chromatids
separate during
anaphase
Homologues
separate
during
anaphase I;
sister
chromatids
remain together
Anaphase I
Telophase
Telophase I
Haploid
n = 3
Daughter
cells of
meiosis I 2n 2n
MEIOSIS II
Daughter cells
of mitosis n n n n
Daughter cells of meiosis II
Sister chromatids separate during anaphase II

25. Synapsis and crossing over Daughter cells, genetic composition
Mitosis
Meiosis
DNA replication
During interphase
Divisions
One
Two
Synapsis and crossing over
Do not occur
Form tetrads in prophase I
Daughter cells, genetic composition
Two diploid, identical to parent cell
Four haploid, different from parent cell and each other
Role in animal body
Produces cells for growth and tissue repair
Produces gametes

26. Genetic Variation Among Offspring
The behavior of chromosomes during meiosis and fertilization is responsible for most of the variation that arises in each generation
Three mechanisms contribute to genetic variation:
Independent assortment of chromosomes
Crossing over
Random fertilization

27. Independent Assortment of Chromosomes
In independent assortment, each pair of chromosomes sorts maternal and paternal homologous chromosomes into daughter cells independently of the other pairs.
Example:
One human sperm cell could contain 15 chromosomes from his father, and 8 from his mother
Another contains 20 from the mother, 3 from the father.

28. LE 13-10
Key
Maternal set of
chromosomes
Possibility 1
Possibility 2
Paternal set of
chromosomes
Two equally probable
arrangements of
chromosomes at
metaphase I
Metaphase II
Daughter
cells
Combination 1
Combination 2
Combination 3
Combination 4

29. Crossing Over
Crossing over produces new chromosomes with a mixture of genes from each parent.
Instead of a chromosome that is 100% from the person’s father or mother, it might now be 95% from the father, 5% from the mother.

30. LE 13-11 Prophase I Nonsister of meiosis chromatids Tetrad Chiasma,
site of
crossing
over
Metaphase I
Metaphase II
Daughter
cells
Recombinant
chromosomes

31. Random Fertilization
Random fertilization adds to genetic variation because any sperm can fuse with any egg.