1. Chapter 13
Meiosis

2. Heredity Genes Are the units of heredity Are segments of DNA
Each gene in an organism’s DNA has a specific locus on a certain chromosome

3. Pairs of Chromosomes A diploid cell
Has 2 copies of each of its chromosomes—therefore 2 copies of each gene
In a human, diploid cells (somatic) have 46 chromosomes (2n = 46)

4. Pairs of Chromosomes A haploid cell
Has only 1 copy of each of its chromosomes— therefore 1 copy of each gene
In a human, a haploid cell (sex cells: egg & sperm) has 23 chromosomes (n = 23)

5. Sexual Reproduction In sexual reproduction Fertilization and meiosis
Two parents give rise to offspring that have unique combinations of genes inherited from the two parents
Fertilization and meiosis
Alternate in sexual life cycles
A life cycle
Is the generation-to-generation sequence of stages in the reproductive history of an organism

6. Multicellular diploid
Human Life Cycle
Key
Haploid (n)
Diploid (2n)
Haploid gametes (n = 23)
Ovum (n)
Sperm
Cell (n)
MEIOSIS
FERTILIZATION
Ovary
Testis
Diploid
zygote
(2n = 46)
Mitosis and
development
Multicellular diploid
adults (2n = 46)

7. Pairs of Chromosomes Homologous chromosomes
Are the two chromosomes composing a pair
Have the same genes
Pair of
homologous
chromosomes

8. Pairs of Chromosomes Sex chromosomes
Are distinct from each other in their characteristics
Are represented as X and Y
Determine the sex of the individual, XX being female, XY being male

9. Pairs of Chromosomes A karyotype
Is an ordered, visual representation of the chromosomes in a cell

10. Chromosomes in Dividing Cell
In a cell in which DNA synthesis has occurred
All the chromosomes are duplicated and thus each consists of two identical sister chromatids
Key
Maternal set of
chromosomes (n = 3)
2n = 6
Paternal set of
chromosomes (n = 3)
Two sister chromatids
of one replicated
chromosome
Centromere
Two nonsister
chromatids in
a homologous pair
Pair of homologous
chromosomes
(one from each set)

11. Meiosis Meiosis is a form of cell division Meiosis
That reduces the number of chromosome sets from diploid to haploid
Meiosis
Takes place in two sets of divisions, meiosis I and meiosis II

12. Overview of Meiosis Interphase Meiosis I Meiosis II
Pair of homologous chromosomes in diploid parent cell
Overview
of Meiosis
Duplicated pair of homologous chromosomes
Chromosomes duplicate
Sister chromatids
Diploid cell with duplicated chromosomes
Meiosis I 1
Homologous chromosomes separate
Figure 13.7 Overview of meiosis: how meiosis reduces chromosome number.
Haploid cells with duplicated chromosomes
Meiosis II 2
Sister chromatids separate
Haploid cells with unduplicated chromosomes

13. Stages of Meiosis Meiosis I Meiosis II
Reduces the number of chromosomes from diploid to haploid
Meiosis II
Produces four haploid daughter cells

14. Telophase I and Cytokinesis
Prophase I
Metaphase I
Anaphase I
Centrosome (with centriole pair)
Sister chromatids remain attached
Sister chromatids
Chiasmata
Centromere (with kinetochore)
Spindle
Metaphase plate
Cleavage furrow
Homologous chromosomes separate
Homologous chromosomes
Fragments of nuclear envelope
Figure 13.8 Exploring: Meiosis in an Animal Cell
Microtubule attached to kinetochore
Each pair of homologous chromosomes separates.
Two haploid cells form; each chromosome still consists of two sister chromatids.
Duplicated homologous chromosomes (red and blue) pair and exchange segments; 2n  6 in this example.
Chromosomes line up by homologous pairs.

15. Telophase II and Cytokinesis
Prophase II
Metaphase II
Anaphase II
During another round of cell division, the sister chromatids finally separate; four haploid daughter cells result, containing unduplicated chromosomes.
Sister chromatids separate
Haploid daughter cells forming
Figure 13.8 Exploring: Meiosis in an Animal Cell

16. Prophase I
Prophase I typically occupies more than 90% of the time required for meiosis
Chromosomes begin to condense
In synapsis, homologous chromosomes loosely pair up, aligned gene by gene
In crossing over, nonsister chromatids exchange DNA segments
Each pair of chromosomes forms a tetrad, a group of four chromatids
Each tetrad usually has one or more chiasmata, X- shaped regions where crossing over occurred

17. Metaphase I
In Metaphase I, tetrads line up at the metaphase plate, with one chromosome facing each pole
Microtubules from one pole are attached to the kinetochore of one chromosome of each tetrad
Microtubules from the other pole are attached to the kinetochore of the other chromosome

18. Anaphase I In Anaphase I, pairs of homologous chromosomes separate
One chromosome moves toward each pole, guided by the spindle apparatus
Sister chromatids remain attached at the centromere and move as one unit toward the pole

19. Telophase I & Cytokinesis
In the beginning of Telophase I, each half of the cell has a haploid set of chromosomes; each chromosome still consists of two sister chromatids
Cytokinesis usually occurs simultaneously, forming two haploid daughter cells
In animal cells, a cleavage furrow forms; in plant cells, a cell plate forms
No chromosome replication occurs between the end of meiosis I and the beginning of meiosis II because the chromosomes are already replicated

20. Prophase II In Prophase II, a spindle apparatus forms
In late Prophase II, chromosomes (each still composed of two chromatids) move toward the metaphase plate

21. Metaphase II
In Metaphase II, the sister chromatids are arranged at the metaphase plate
Because of crossing over in Meiosis I, the two sister chromatids of each chromosome are no longer genetically identical
The kinetochores of sister chromatids attach to microtubules extending from opposite poles

22. Anaphase II In Anaphase II, the sister chromatids separate
The sister chromatids of each chromosome now move as two newly individual chromosomes toward opposite poles

23. Telophase II & Cytokinesis
In Telophase II, the chromosomes arrive at opposite poles
Nuclei form, and the chromosomes begin de- condensing
Cytokinesis separates the cytoplasm
At the end of meiosis, there are four daughter cells, each with a haploid set of unreplicated chromosomes
Each daughter cell is genetically distinct from the others and from the parent cell

24. Genetic Variation Genetic variation
Is the raw material for evolution by natural selection
Natural selection results in the accumulation of genetic variations favored by the environment

25. Mechanisms of Variation
Meiosis provides various mechanisms for variation by:
Crossing over
Independent assortment
Random fertilization
Mutations

26. Crossing Over Crossing over produces recombinant chromosomes
Combines DNA from each parent
Homologous portions of two nonsister chromatids trade places
Crossing over increases genetic variation by combining DNA from two parents into a single chromosome

27. Crossing Over Prophase I of meiosis
Nonsister chromatids held together during synapsis
Pair of homologs
Chiasma
Crossing Over
Centromere
TEM
Anaphase I
Figure The results of crossing over during meiosis.
Anaphase II
Daughter cells
Recombinant chromosomes

28. Independent Assortment
In independent assortment
Each pair of chromosomes sorts its maternal and paternal homologues into daughter cells independently of the other pairs
There are 2n possibilities of arrangements (i.e. 8.4 million in humans)

29. Independent Assortment 2n = 4 so 2n is 22 = 4 possible combinations
Possibility 1
Possibility 2
Two equally probable arrangements of chromosomes at metaphase I
Metaphase II
Figure The independent assortment of homologous chromosomes in meiosis.
Daughter cells
Combination 1
Combination 2
Combination 3
Combination 4
2n = 4 so 2n is 22 = 4 possible combinations

30. Random Fertilization Don’t forget random fertilization!
Adds to genetic variation: any sperm can fuse with any ovum (unfertilized egg)
Each gamete has 8.4 million possible chromosome combinations
Produces a zygote with any of about 70 trillion diploid combinations

31. Review of Mitosis vs. Meiosis
Prophase
Duplicated chromosome
(two sister chromatids)
Chromosome
replication
Parent cell
(before chromosome replication)
Chiasma (site of
crossing over)
MEIOSIS I
Prophase I
Tetrads formed by
synapsis of homologous
chromosomes
Metaphase
Chromosomes
positioned at the
metaphase plate
Tetrads
Metaphase I
Anaphase I
Telophase I
Haploid
n = 3
MEIOSIS II
Daughter
cells of
meiosis I
Homologues
separate
during
anaphase I;
sister
chromatids
remain together
Haploid Daughter cells of meiosis II n
Sister chromatids separate during
anaphase II
Anaphase
Telophase
Sister chromatids
separate during
anaphase 2n
Diploid daughter cells
of mitosis
2n = 6
Diploid
2n = 6