1. Meiosis and Sexual Life Cycles
Chapter 13
Meiosis and Sexual Life Cycles

2. Living organisms
Are distinguished by their ability to reproduce their own kind

3. Heredity
Is the transmission of traits from one generation to the next
Variation
Shows that offspring differ somewhat in appearance from parents and siblings
Figure 13.1

4. Offspring acquire genes from parents by
Genetics
Is the scientific study of heredity and hereditary variation
Offspring acquire genes from parents by
inheriting chromosomes

5. Inheritance of Genes Genes Are the units of heredity
Are segments of DNA
Their locations along the length of the DNA are called:
Loci “locus for singular”

6. Each gene in an organism’s DNA
Has a specific locus on a certain chromosome
We inherit
One set of chromosomes from our mother, and
One set from our father

7. Comparison of Asexual and Sexual Reproduction
In asexual reproduction
One parent produces genetically identical offspring by mitosis
Figure 13.2
Parent
Bud
0.5 mm

8. In sexual reproduction
Two parents give rise to offspring
Offspring have:
Unique combinations of genes
These genes are inherited from the two parents

9. Fertilization (diploid cells) and meiosis (haploid cells) alternate in sexual life cycles
A life cycle
Is the generation-to-generation sequence of stages in the reproductive history of an organism

10. Sets of Chromosomes in Human Cells
In humans
Each somatic (body) cell has 46 chromosomes, made up of two sets
One set of chromosomes comes from each parent
Chromosomes differ in:
Size
Positions of their centromeres
Staining patterns (colored bands)

11. Distinguishing Chromosomes Using Staining
A karyotype
Is an ordered, visual representation of the chromosomes in a cell using staining.
5 µm
Pair of homologous
chromosomes
Centromere
Sister
chromatids
Figure 13.3

12. Homologous chromosomes
Are the two chromosomes composing a pair
Have the same characteristics (size, centromere position, staining pattern)
Each member is contributed by one parent
May also be called autosomes
People normally have 22 pairs of autosomes

13. Sex chromosomes
Are distinct from autosomes and from each other in their characteristics
Are represented as X and Y
Determine the sex of the individual,
XX being female & XY being male
Homologous in females (XX)
Only partially homologous in males (XY)

14. A diploid cell Has two sets of each of its chromosomes
a human has 46 chromosomes (2n = 46)
All somatic cells are diploid cells

15. In a cell in which DNA synthesis has occurred:
All chromosomes are duplicated
Each chromosome consists of two identical sister chromatids
Figure 13.4
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)

16. Unlike somatic (body) cells, reproductive cells or gametes (sperm and egg) are:
Haploid cells containing only one set (n) of chromosomes
Produced only in the gonads (ovaries & testes)
At sexual maturity,
The ovaries and testes produce haploid gametes by meiosis

17. During fertilization The zygote These gametes, sperm and ovum, fuse
Fusion results in forming a diploid zygote
The zygote
Develops into an adult organism

18. The human life cycle Key Figure 13.5 Haploid gametes (n = 23)
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)

19. The Variety of Sexual Life Cycles
There are three main types of sexual life cycles
They are all common in the alternation of meiosis and fertilization
But differ in the timing of meiosis, and fertilization

20. Examples of types of Life Cycles:
In animals (including humans)
Meiosis occurs during gamete formation
Gametes are the only haploid cells
Gametes
Figure 13.6 A
Diploid
multicellular
organism
Key
MEIOSIS
FERTILIZATION n 2n
Zygote
Haploid
Mitosis
(a) Animals

21. (b) Plants and some algae
Exhibit a 2nd type of life cycle called alternation of generations
This life cycle includes both diploid and haploid multicellular stages
MEIOSIS
FERTILIZATION n 2n
Haploid multicellular
organism (gametophyte)
Mitosis
Spores
Gametes
Zygote
Diploid
multicellular
organism
(sporophyte)
(b) Plants and some algae
Figure 13.6 B

22. In most fungi and some protists (3rd life cycle)
Meiosis produces haploid cells that give rise to a haploid multicellular adult organism
The haploid adult carries out mitosis, producing cells that will become gametes
No multicellular diploid organism produced

23. (c) Most fungi and some protists
MEIOSIS
FERTILIZATION n 2n
Haploid multicellular
organism
Mitosis
Gametes
Zygote
(c) Most fungi and some protists
Figure 13.6 C

24. Meiosis reduces the number of chromosome
from diploid (2n) to haploid (n)
Meiosis takes place in two sets of divisions
meiosis I and meiosis II
Meiosis I is also known as
reductional division
Like mitosis, meiosis is
Preceded by chromosome replication

25. Break Slide
Wed, , 2015

26. The Stages of Meiosis
Chromosome replication is followed by two consecutive cell divisions (meiosis I & meiosis II)
These divisions result in 4 daughter cells (2 in mitosis)
Each cell with half the parent-al number of chromosomes
Figure 13.7
Interphase
Homologous pair
of chromosomes
in diploid parent cell
Chromosomes
replicate
Homologous pair of replicated chromosomes
Sister
chromatids
Diploid cell with
replicated
chromosomes 1 2
Homologous
separate
Haploid cells with
replicated chromosomes
Sister chromatids
Haploid cells with unreplicated chromosomes
Meiosis I
Meiosis II

27. Meiosis I
Reduces the number of chromosomes from diploid to haploid, therefore:
known as reductional division
Meiosis II
Produces four haploid daughter cells

28. Interphase and meiosis I
Centrosomes
(with centriole pairs)
Sister
chromatids
Chiasmata
Spindle
Tetrad
Nuclear
envelope
Chromatin
Centromere
(with kinetochore)
Microtubule
attached to
kinetochore
Tertads line up
Metaphase
plate
Homologous
chromosomes
separate
Sister chromatids
remain attached
Pairs of homologous
chromosomes split up
Chromosomes duplicate
Homologous chromosomes
(red and blue) pair and exchange
segments; 2n = 6 in this example
INTERPHASE
MEIOSIS I: Separates homologous chromosomes
PROPHASE I
METAPHASE I
ANAPHASE I
Figure 13.8

29. MEIOSIS II: Separates sister chromatids
Telophase I, cytokinesis, and meiosis II
TELOPHASE I AND
CYTOKINESIS
PROPHASE II
METAPHASE II
ANAPHASE II
TELOPHASE II AND
MEIOSIS II: Separates sister chromatids
Cleavage
furrow
Sister chromatids
separate
Haploid daughter cells
forming
During another round of cell division, the sister chromatids finally separate;
four haploid daughter cells result, containing single chromosomes
Two haploid cells
form; chromosomes
are still double
Figure 13.8

30. A Comparison of Mitosis and Meiosis
Meiosis and mitosis can be distinguished from each other by:
Three events that are unique to meiosis
All of these events occur during meiosis l

31. Events Unique to Meiosis
1. Synapsis and crossing over
During prophase I of meiosis, homologous chromosomes physically connect and exchange genetic information
2. Tetrads on the metaphase plate
At metaphase I of meiosis, paired homologous chromosomes (tetrads) are positioned on the metaphase plates

32. 3. Separation of homologues
At anaphase I of meiosis, homologous pairs move toward opposite poles of the cell
In anaphase II of meiosis, the sister chromatids separate

33. (before chromosome replication)
A comparison of mitosis and meiosis
Figure 13.9
MITOSIS
MEIOSIS
Prophase
Duplicated chromosome
(two sister chromatids)
Chromosome
replication
Parent cell
(before chromosome replication)
Chiasma (site of
crossing over)
MEIOSIS I
Prophase I
Tetrad 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
Daughter cells of meiosis II n
Sister chromatids separate during anaphase II
Anaphase
Telophase
Sister chromatids
separate during
anaphase 2n
Daughter cells
of mitosis
2n = 6

34. Origins of Genetic Variation Among Offspring
In species that reproduce sexually
The behavior of chromosomes during meiosis and fertilization is responsible for most of the variation that arises each generation

35. Three mechanisms that contribute to genetic variation arising from sexual reproduction:
Independent Assortment of chromosomes
Crossing Over
Random Fertilization

36. Independent Assortment of Chromosomes
Homologous pairs of chromosomes
Orient randomly at metaphase I of meiosis

37. In independent assortment
Each pair of chromosomes sorts its maternal and paternal homologues into daughter cells independently of the other pairs
Key
Maternal set of
chromosomes
Paternal set of
Possibility 1
Two equally probable
arrangements of
chromosomes at
metaphase I
Possibility 2
Metaphase II
Daughter
cells
Combination 1
Combination 2
Combination 3
Combination 4
Figure 13.10

38. Crossing Over Crossing over
Produces recombinant chromosomes that carry genes derived from two different parents
Figure 13.11
Prophase I
of meiosis
Nonsister
chromatids
Tetrad
Chiasma,
site of
crossing
over
Metaphase I
Metaphase II
Daughter
cells
Recombinant
chromosomes

39. Random Fertilization The fusion of gametes
Will produce a zygote with any of about 64 trillion diploid combinations

40. Evolutionary Significance of Genetic Variation Within Populations
Is the raw material for evolution by natural selection

41. Mutations Sexual reproduction
Are the original source of genetic variation
Sexual reproduction
Produces new combinations of variant genes, adding more genetic diversity