1. Meiosis and Sexual Life Cycles
Chapter 13
Meiosis and Sexual Life Cycles
Modified by Maria Morlin

2. Theories on Inheritance
It was clear for millennia that offspring resembled their parents, but how this came about was unclear.
Do males and females harbor homunculi?
Do the components of sperm and egg mix like paint?
What role do gametes and chromosomes play?

3. Inheritance and genetics
Genetics is the science of heredity

4. The discovery of the gene as the unit of heredity
Using fruit flies (Drosophila melanogaster) as a model organism, Thomas Hunt Morgan and his group at Columbia University showed that genes, strung on chromosomes, are the units of heredity.
In 1933, Morgan received the Nobel Prize in Physiology or Medicine for helping establish the chromosome theory of inheritance.

5. Heredity & variation Living organisms reproduce their own kind
Is the transmission of traits from one generation to the next
Variation
Shows that offspring differ somewhat in appearance from parents and siblings

7. Inheritance of Genes Genes
Are the units of heredity
Are segments of DNA
Program traits
Offspring acquire genes from parents by inheriting chromosomes

9. Genes program traits
Most genes program cells to synthesize specific enzymes and other proteins whose cumulative action produces an organism’s inherited traits such as:
Freckles
Eye colour
Curly hair
Attached earlobes
Skin colour

10. 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

13. 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

14. Daphnia (water fleas)
Parthenogenesis
Produces female clones
Differences due to mutation

15. In sexual reproduction
Two parents give rise to offspring that have unique combinations of genes inherited from the two parents

16. 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

17. Sets of Chromosomes in Human Cells
In humans
Each somatic cell has 46 chromosomes, made up of two sets
One set of chromosomes comes from each parent

18. A karyotype
Is an ordered, visual representation of the chromosomes in a cell

19. Homologous chromosomes
Are the two chromosomes composing a pair
Have the same characteristics but may have different versions of it

20. 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

21. A diploid cell Has two sets of each of its chromosomes
In a human has 46 chromosomes (2n = 46)

22. In a cell in which DNA synthesis has occurred
All the chromosomes are duplicated and thus each 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)

23. Behavior of Chromosome Sets in the Human Life Cycle
At sexual maturity
The ovaries and testes produce haploid gametes by meiosis
Unlike somatic cells
Gametes, sperm and egg cells are haploid cells, containing only one set of chromosomes

24. During fertilization gametes, sperm and ovum, fuse, forming a diploid zygote
The zygote develops into an adult organism

26. 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)

27. The Variety of Sexual Life Cycles
The three main types of sexual life cycles
Differ in the timing of meiosis and fertilization

28. In animals 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

29. (b) Plants and some algae
Exhibit an alternation of generations
The 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

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

31. Reduces the number of chromosome sets from diploid to haploid Meiosis
Takes place in two sets of divisions, meiosis I and meiosis II

32. The Stages of Meiosis An overview of meiosis 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

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

34. 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

35. 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

36. A Comparison of Mitosis and Meiosis
Meiosis and mitosis can be distinguished from mitosis
By three events in Meiosis l

37. Synapsis and crossing over
Homologous chromosomes physically connect and exchange genetic information

38. Tetrads on the metaphase plate
At metaphase I of meiosis, paired homologous chromosomes (tetrads) are positioned on the metaphase plates

39. 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

40. (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

41. Reshuffling of genetic material in meiosis
Genetic variation produced in sexual life cycles contributes to evolution
Reshuffling of genetic material in meiosis
Produces genetic variation

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

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

44. 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

45. 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

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

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

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