1. Meiosis and Sexual Life Cycles
Chapter 13

2. Overview
Sexual reproduction depends on the cellular processes of meiosis and fertilization.
Hereditary, Similarity, and Variation
Living organisms are distinguished by their ability to reproduce their own kind.

3. Key Terms
Heredity: Is the transmission of traits from one generation to the next
Variation: Shows that offspring differ somewhat in appearance from parents and siblings
Genetics: The scientific study of heredity and hereditary variation

4. Inheritance of Genes
Offspring acquire genes from parents by inheriting chromosomes.
Genes are the units of heredity found on segments of DNA
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.

5. Comparison of Asexual and Sexual Reproduction
In asexual reproduction
One parent produces genetically identical offspring by mitosis
Gives rise to clones
In sexual reproduction
Two parents give rise to offspring that have unique combinations of genes inherited from the two parents
Parent
Bud
0.5 mm

6. 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
A number of characteristics need to be considered when discussing life cycles:
Chromosome numbers in somatic cells and gametes
Chromosome behavior through sexual life cycles

7. 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
A karyotype is an ordered, visual representation of the chromosomes in a cell
5 µm
Pair of homologous
chromosomes
Centromere
Sister
chromatids

8. Chromosomes
Homologous chromosomes are the two chromosomes composing a pair
They have same the characteristics and may also be called autosomes.
Sex chromosomes are distinct from each other in their characteristics
Are represented as X and Y and determine the sex of the individual, XX being female, XY being male
Only small parts of the X and Y are homologous. Most of the genes carried on the X chromosome do not have counterparts on the tiny Y, and the Y chromosome has genes lacking on the X.

9. Comparison of Chromosome Number
A diploid cell has two sets of each of its chromosomes
In a cell in which DNA synthesis has occurred all the chromosomes are duplicated and thus each consists of two identical sister chromatids
Unlike somatic cells gametes, sperm and egg cells are haploid cells, containing only one set of chromosomes

10. Behavior of Chromosome Sets in the Human Life Cycle
At sexual maturity, the ovaries and testes produce haploid gametes by meiosis.
During fertilization these gametes, sperm and ovum, fuse, forming a diploid zygote.
The zygote develops into an adult organism.
As a human develops into a sexually mature adult, mitosis of the zygote generates all of the somatic cells of the body.

11. The Human Life Cycle Key Haploid gametes (n = 23) Haploid (n) Ovum (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)

12. The Variety of Sexual Life Cycles
There are three main types of sexual life cycles
The common feature of all three cycles is the alternation of meiosis and fertilization
Contributes to the genetic variation among offspring
They differ in the timing of meiosis and fertilization

13. Life Cycle in Animals
In animals, meiosis occurs during gamete formation.
Gametes are the only haploid cells.

14. Haploid multicellular organism (gametophyte) (b) Plants and some algae
Life Cycle in Plants
MEIOSIS
FERTILIZATION n 2n
Haploid multicellular
organism (gametophyte)
Mitosis
Spores
Gametes
Zygote
Diploid
multicellular
organism
(sporophyte)
(b) Plants and some algae
Plants and some algae
Exhibit an alternation of generations
The life cycle includes both diploid and haploid multicellular stages
Meiosis in the sporophyte produces haploid cells called spores. Unlike a gamete, a spore gives rise to a multicellular individual without fusing with another cell. A spore divides mitotically to generate a multicellular haploid stage called a gametophyte. The haploid gametophyte makes gametes by mitosis. Fertilization among the haploid gametes results in a diploid zygote, which develops into the next sporophyte generation.
The sporophyte generation produces a gametophyte as it offspring, and the gametophyte generation produces the next sporophyte generation.

15. Life Cycle of Fungi & 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

16. Reduction Division
Meiosis reduces the number of chromosome sets from diploid to haploid
Takes place in two sets of divisions, meiosis I and meiosis II

17. Interphase 1 2 Meiosis I Meiosis II
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

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

19. BioFlix Animation

20. (with centriole pairs)
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

21. MEIOSIS II: Separates sister chromatids
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

22. Prophase I
1. Synapsis occurs, forming tetrads. 2. Crossing over occurs between homologous chromosomes in the tetrads. 3. Crossing over increases genetic variation. 4. Areas of crossing over form chiasmata. 5. The nuclear envelope disintegrates, allowing the spindle to attach to the homologs.
Synapsis is the joining of homologous chromosomes along their length. This newly formed structure is called a tetrad and precisely aligns the homologous chromosomes gene by gene.
Crossing over the DNA from one homolog is cut and exchanged with an exact portion of DNA from the other homolog.
4. Where crossing over has occurred (two or three times per homologous pair), crisscrossed regions termed chiasmata form, which hold the homologs together until anaphase.
5. After crossing over, the centrioles move away from each other, the nuclear envelope disintegrates, and the spindle microtubules attach to the kinetochores forming on the chromosomes that begin to move to the metaphase plate.

23. Metaphase I
Homologous pairs of chromosomes line up at the metaphase plate.
The microtubules from each pole attach to each member of the homologous pairs.
Homologous pairs of chromosomes line up at the metaphase plate
The microtubules from each pole attach to each member of the homologous pairs
This is in preparation for pulling them to opposite sides of the cell.

24. Anaphase I The spindle apparatus helps to move the chromosomes.
Sister chromatids stay connected and move together toward the poles.
The spindle apparatus helps to move the chromosomes toward opposite ends of the cell
Sister chromatids stay connected and move together toward the poles.

25. Telophase I and Cytokinesis
The homologous chromosomes move until they reach the opposite poles.
Each pole, then, contains a haploid set of chromosomes, with each chromosome still containing two sister chromatids.
Cytokinesis is the division of the cytoplasm and occurs during telophase. A cleavage furrow occurs in animal cells, and cell plates (the forming new cell wall) occur in plant cells. Both result in the formation of two haploid cells.

26. Prophase II A spindle apparatus forms.
Sister chromatids move toward the metaphase plate.

27. Metaphase II The chromosomes are lined up on the metaphase plate.
The kinetochores of each sister chromatid prepare to move to the opposite poles.

28. Anaphase II The centromeres of the sister chromatids separate.
Individual chromosomes move to opposite ends of the cell.

29. Telophase II and Cytokinesis
The chromatids have moved all the way to opposite ends of the cell.
Nuclei reappear.
Cytokinesis occurs.
Each of the four daughter cells has the haploid number of chromosomes and is genetically different from the other daughter cells and from the parent cell.

30. A Comparison of Mitosis and Meiosis
Meiosis and mitosis can be distinguished from mitosis by three events in Meiosis l
1. Synapsis and crossing over
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.
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.
How do sister chromatids stay together through meiosis I but separate from each other in meiosis II and mitosis?
Sister chromatids are attached along their lengths by protein complexes called cohesions.
In meiosis, sister chromatid cohesion is released in two steps. In metaphase I, homologs are held together by cohesion between sister chromatid arms in regions where DNA has been exchanged. At anaphase I, cohesions are cleaved along the arms, allowing homologs to separate. At anaphase II, cohesions are cleaved at the centromeres, allowing chromatids to separate.

31. (before chromosome replication)
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

32. Origins of Genetic Variation Among Offspring
Three mechanisms that contribute to the genetic variation arising from sexual reproduction:
Independent Assortment of Chromosomes
Crossing Over
Random Fertilization
Genetic variation produced in sexual life cycles contributes to evolution
Reshuffling of genetic material in meiosis produces genetic variation
In species that produce sexually the behavior of chromosomes during meiosis and fertilization is responsible for most of the variation that arises each generation

33. Independent Assortment of Chromosomes
Homologous pairs of chromosomes orient randomly at metaphase I of meiosis.
In independent assortment, each pair of chromosomes sorts its maternal and paternal homologues into daughter cells independently of the other pairs.
In humans, the number of possible combinations is 8.4 million. Each gamete that you produce in your lifetime contains one of roughly 8.4 million possible combinations of chromosomes.

34. 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

35. Crossing Over
During prophase I, the exchange of genetic material on homologous chromosomes between nonsister chromatids occurs.
In metaphase II when sister chromatids separate, each chromatid is unique, thus increasing variation.
Produces recombinant chromosomes that carry genes derived from two different parents.
Prophase I
of meiosis
Nonsister
chromatids
Tetrad
Chiasma,
site of
crossing
over
Metaphase I
Metaphase II
Daughter
cells
Recombinant
chromosomes
Recombinant Chromosomes: individual chromosomes that carry genes (DNA) derive from two different parents.
In meiosis in humans, an average of one to three crossover events occur per chromosome pair, depending on the size of the chromosomes and the position of their centromeres.

36. Random Fertilization
The random nature of fertilization adds to the genetic variation arising from meiosis.
Each male and female gamete represents just one of 8 million possible chromosome combinations (independent assortment).
The fusion of gametes will produce a zygote with any of about 64 trillion diploid combinations

37. Evolutionary Significance of Genetic Variation Within Populations
Genetic variation is the raw material for evolution by natural selection
Individuals best suited to the local environment leave the most offspring; transmitting their genes
But… the environment can change!!!