1. Meiosis and Sexual Life Cycles

2. Meiosis and Sexual Life Cycles
Living organisms are distinguished by their ability to reproduce their own kind
Heredity
Is the transmission of traits from one generation to the next
Variation
Shows that offspring differ somewhat in appearance from parents and siblings

3. Inheritance of Genes Genes are segments of DNA, units of heredity
Offspring acquire genes from parents by inheriting chromosomes
Genetics is the scientific study of heredity and hereditary variation

4. Inheritance of Genes
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
Two parents give rise to offspring that have unique combinations of genes inherited from the two parents - sexual reproduction

5. Asexual Reproduction
In asexual reproduction, one parent produces genetically identical offspring by mitosis
Figure 13.2
Parent
Bud
0.5 mm

6. Sexual Reproduction - The Human Life Cycle
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)
A life cycle is the generation-to-generation sequence of stages in the reproductive history of an organism
Fertilization and meiosis alternate in sexual life cycles
At sexual maturity the ovaries and testes produce haploid gametes by meiosis
Unlike somatic cells, sperm and egg cells are haploid cells, containing only one set of chromosomes
During fertilization, sperm and ovum fuse forming a diploid zygote
The zygote develops into an adult organism

7. Meiosis Reduces the chromosome number such that each daughter
Cell has a haploid set of chromosomes
Ensures that the next generation will have:
Diploid number of chromosome
Exchange of genetic information (combination of traits
that differs from that of either parent)

8. Meiosis Only diploid cells can divide by meiosis.
Prior to meiosis I, DNA replication occurs.
During meiosis, there will be two nuclear divisions, and the result will be four haploid nuclei.
No replication of DNA occurs between meiosis I and meiosis II.

9. 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
Meiosis reduces the number of chromosome sets from diploid to haploid
Meiosis takes place in two sets of divisions
Meiosis I reduces the number of chromosomes from diploid to haploid
Meiosis II produces four haploid daughter cells

10. Meiosis Phases Meiosis involves the same four phases seen in mitosis
prophase
metaphase
anaphase
telophase
They are repeated during both meiosis I and meiosis II.
The period of time between meiosis I and meiosis II is called interkinesis.
No replication of DNA occurs during interkinesis because the DNA is already duplicated.

11. Prophase I
Prophase I occupies more than 90% of the time required for meiosis
Chromosomes begin to condense
In synapsis, the 2 members of each homologous pair of chromosomes line up side-by-side, aligned gene by gene, to form a tetrad consisting of 4 chromatids
During synapsis, sometimes there is an exchange of homologous parts between non-sister chromatids. This exchange is called crossing over
Each tetrad usually has one or more chiasmata, X-shaped regions where crossing over occurred
Prophase I
of meiosis
Tetrad
Nonsister
chromatids
Chiasma,
site of
crossing
over

12. Metaphase I
At 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
Sister
chromatids
Chiasmata
Spindle
Centromere
(with kinetochore)
Metaphase
plate
Homologous
chromosomes
separate
Sister chromatids
remain attached
Microtubule
attached to
kinetochore
Tetrad
PROPHASE I
METAPHASE I
ANAPHASE I
Homologous chromosomes
(red and blue) pair and
exchange segments; 2n = 6
in this example
Pairs of homologous
chromosomes split up
Tetrads line up

13. 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
Sister
chromatids
Chiasmata
Spindle
Centromere
(with kinetochore)
Metaphase
plate
Homologous
chromosomes
separate
Sister chromatids
remain attached
Microtubule
attached to
kinetochore
Tetrad
PROPHASE I
METAPHASE I
ANAPHASE I
Homologous chromosomes
(red and blue) pair and
exchange segments; 2n = 6
in this example
Pairs of homologous
chromosomes split up
Tetrads line up

14. Telophase I and 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

15. Prophase II Meiosis II is very similar to mitosis
In prophase II, a spindle apparatus forms
In late prophase II, chromosomes (each still composed of two chromatids) move toward the metaphase plate
Cleavage
furrow
PROPHASE II
METAPHASE II
ANAPHASE II
TELOPHASE I AND
CYTOKINESIS
TELOPHASE II AND
Sister chromatids
separate
Haploid daughter cells
forming

16. Metaphase II
At metaphase II, the sister chromatids are 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
Cleavage
furrow
PROPHASE II
METAPHASE II
ANAPHASE II
TELOPHASE I AND
CYTOKINESIS
TELOPHASE II AND
Sister chromatids
separate
Haploid daughter cells
forming

17. Anaphase II At anaphase II, the sister chromatids separate
The sister chromatids of each chromosome now move as two newly individual chromosomes toward opposite poles
Cleavage
furrow
PROPHASE II
METAPHASE II
ANAPHASE II
TELOPHASE I AND
CYTOKINESIS
TELOPHASE II AND
Sister chromatids
separate
Haploid daughter cells
forming

18. Telophase II and Cytokinesis
In telophase II, the chromosomes arrive at opposite poles
Nuclei form, and the chromosomes begin decondensing
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
Cleavage
furrow
PROPHASE II
METAPHASE II
ANAPHASE II
TELOPHASE I AND
CYTOKINESIS
TELOPHASE II AND
Sister chromatids
separate
Haploid daughter cells
forming

19. A Comparison of Mitosis and Meiosis
Mitosis conserves the number of chromosome sets, producing cells that are genetically identical to the parent cell
Meiosis reduces the number of chromosomes sets from two (diploid) to one (haploid), producing cells that differ genetically from each other and from the parent cell
The mechanism for separating sister chromatids is virtually identical in meiosis II and mitosis

20. A Comparison of Mitosis and Meiosis
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 of meiosis, homologous pairs move toward opposite poles of the cell. In anaphase II of meiosis, the sister chromatids separate

21. A Comparison Of Mitosis And 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

22. Comparison Mitosis Meiosis Homologous chromosomes do not pair up
No genetic exchange between homologous chromosomes
One diploid cell produces 2 diploid cells or one haploid cell produces 2 haploid cells
New cells are genetically identical to original cell (except for mutation)
Meiosis
DNA duplication followed by 2 cell divisions
Sysnapsis
Crossing-over
One diploid cell produces 4 haploid cells
Each new cell has a unique combination of genes

23. Sexual Reproduction - The Human Life Cycle
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)
During fertilization, sperm and ovum fuse forming a diploid zygote
The zygote develops into an adult organism

24. Spermatocytes to Spermatids
Primary spermatocytes undergo meiosis I, forming two haploid cells called secondary spermatocytes
Secondary spermatocytes undergo meiosis II and their daughter cells are called spermatids
Spermatids are small round cells seen close to the lumen of the tubule
Late in spermatogenesis, spermatids are nonmotile
Spermiogenesis – spermatids lose excess cytoplasm and form a tail, becoming motile sperm

25. Spermatogenesis
Figure 27.8b, c

26. Oogenesis Production of female sex cells by meiosis
In the fetal period, oogonia (2n ovarian stem cells) multiply by mitosis and store nutrients
Primordial follicles appear as oogonia are transformed into primary oocytes
Primary oocytes begin meiosis but stall in prophase I
From puberty, each month one activated primary oocyte completes meiosis one to produce two haploid cells
The first polar body
The secondary oocyte
The secondary oocyte arrests in metaphase II and is ovulated
If penetrated by sperm the second oocyte completes meiosis II, yielding:
One large ovum (the functional gamete)
A tiny second polar body

27. Oogenesis