1. Chapter 13: Meiosis and Sexual Life Cycles

2. Genetics is the scientific study of heredity and variation
Heredity is the transmission of traits from one generation to the next
Variation is demonstrated by the differences in appearance that offspring show from parents and siblings.
Figure 13.1

3. Inheritance of Genes Genes Locus Are the units of heredity
Are segments of DNA
Recall that DNA is made up of repeating subunits called nucleotides (A C T G).
Locus
Each gene in an organism’s DNA has a specific locus (location) on a certain chromosome

4. Gametes
Are the reproductive cells that are the vehicles that transmit genes from one generation to the next
Male Gamete=sperm, Female Gamete=egg
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 (usually results in cloned closed individuals)
One parent produces genetically identical offspring by mitosis
Occurs in single celled and simple multi-cellular organisms such as the hydra
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. Sets of Chromosomes in Human Cells
In humans
Each somatic cell has 46 chromosomes, made up of two sets (one set from each parent)
A chromosome
Is DNA that has been condensed and coiled

7. A somatic cell
Is any cell in a multi-cellular organism except for sperm or egg cells (e.g. liver and muscle cells)
Have 23 pairs of chromosomes, one set from the father, the other from the mother, for a total of 46 chromosomes (2n=46)
Each set of 23 consists of 22 sets of autosomes (a chromosome not directly involved in determining sex) and a single set of sex chromosomes.
Males  XY, Females  XX

8. Gametes (e.g. sperm and egg cells)
Have only 23 chromosomes (n=23)
22 are autosomes and 1 is a sex chromosome.
Male sperm cell  can have X or Y
Female egg cell  can only have X

9. What type of cell?
Male or female?

10. X Y Explanation # of Sets in Human Cells Sex Chromosome
Chromosome directly involved in determining sex 1
Autosome
Chromosome not directly involved in determining sex 22 X Y

11. 5 µm
Pair of homologous
chromosomes
Centromere
Sister
chromatids
A karyotype
Is an ordered, visual representation of the chromosomes in a cell
Prepared from isolated somatic cells that are treated with drugs that stimulate mitosis
Cells are then arrested in metaphase and stained
Photographs are taken and scanned into the computer, and then chromosomes are electronically arranged according to size, shape and centromere position

12. A karyotype shows:
The two chromosomes in each pair (called homologous chromosomes or homologs)
Chromosomes in a homologous pair are the same length and carry genes controlling the same inherited haracters
The sex chromosomes called X and Y

13. Homologous chromosomes
Are the two chromosomes that have the same staining pattern, centromere position and length
Have the same characteristics, one from mother, one from father
Autosomes are considered to be homologous, but sex chromosomes are not
Only parts of the X and Y are homologous
Most genes on X do not have counterparts on Y and vice versa

14. A haploid cell (haploid number, n)
Has only one homologous pair
Human: n=23
A diploid cell (diploid number, 2n)
Has two sets of each homologous pair
A human has 46 chromosomes (2n = 46)

15. In a cell in which DNA synthesis (Interphase) 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)

16. Behavior of Chromosome Sets in the Human Life Cycle
At sexual maturity in animals
The ovaries (eggs) and testes (sperm) produce haploid gametes by meiosis
Gametes are the only types of human cells produced by meiosis, rather than mitosis
Meiosis results in one set of chromosomes in each gamete

17. During fertilization The zygote
These gametes, sperm and ovum, fuse, forming a diploid zygote (2n)
The zygote
Develops into an adult organism by mitosis
At differing points during development, cells will specialize
Those cells that are destined for the testicles or eggs will undergo meiosis to develop gametes

19. Interesting…

20. The purpose of meiosis:
The Human Life Cycle
The purpose of meiosis:
To produce 1 set of chromosomes in each gamete instead of two to compensate for the doubling that occurs at fertilization
Produce genetic variability
Figure 13.5
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)

21. In animals Life cycle includes only diploid multi-cellular stages
Gametes are the only haploid cells (unicellular)
Meiosis occurs only during gamete formation
Mitosis occurs only in multi-cellular organisms
Gametes
Figure 13.6 A
Diploid
multicellular
organism
Key
MEIOSIS
FERTILIZATION n 2n
Zygote
Haploid
Mitosis
(a) Animals

22. The Variety of Sexual Life Cycles
Plants and some algae exhibit an alternation of generations
The life cycle includes both diploid and haploid multicellular stages
The two generations are: gametophyte (n-haploid) and sporophyte (2n-diploid)
MEIOSIS
FERTILIZATION n 2n
Haploid multicellular
organism (gametophyte)
Mitosis
Spores
Gametes
Zygote
Diploid
multicellular
organism
(sporophyte)
(b) Plants and some algae

23. Polytrichum Moss
Sporophyte
Gametophyte

24. Mitosis Mitosis Mitosis Spore making a gametophyte Gametophyte making
gametes
Mitosis
Sporophyte development

25. Alternation of generations
Plants and some algae
Plants have a life cycle that involves spores, which form as a result of meiosis. These spores are haploid. Notice that both haploid and diploid cells can divide by mitosis.
Meiosis always begins with cells that are diploid (2n), and a result, daughter cells formed are always haploid (n).

26. The Stages of Meiosis An overview of meiosis Meiosis I
Reduces the number of chromosomes from diploid to haploid
Meiosis II
Produces four haploid daughter cells
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. 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

28. 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

29. Prophase I
Synapsis: Duplicated homologous chromosomes line up and are held together by a protein called the synaptonemal complex. This eventually dissassembles in late prophase.
Crossing Over: Non-sister chromatids exchange DNA segments
Each pair of chromosomes forms a tetrad, a group of four chromatids.
Each tetrad usually has one more more chiasmata or X shaped criss-cross regions where crossing over has occurred.

30. Metaphase – Mitosis vs. Meiosis
In metaphase of mitosis, the sister chromatids are aligned at the metaphase plate, whereas in metaphase I of meiosis, the homologous chromosomes (tetrads) are lined up.

31. Separation of homologues
At anaphase I of meiosis, homologous pairs move toward opposite poles of the cell
Sister chromatids remain attached
Resultant daughter cells (after anaphase I and telophase I) are haploid (n)
In anaphase II of meiosis, the sister chromatids separate
Chromosomes never duplicate in meiosis! (Interphase is not technically part of meiosis…or mitosis!)

32. (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

33. Mitosis
Meiosis
Role in Animal Body
Enables multi-cellular adult to arise from a zygote
Produces cells for growth and repair
Asexual reproduction (in some species)
Produces gametes
Reduces number of chromosomes in half
Introduces variability among gametes
Number of DNA replications
Number of divisions 1 2
Number of Daughter Cells 4*
Chromosome Number of Daughter Cells
Diploid (2n)
Haploid (n)

34. Spermatogenesis vs. Oogenesis

36. The original source of genetic variation in individuals are mutations
Most of these are repaired
The vast majority of genetic variability is as a result of the “reshuffling of genetic material” that occurs in meiosis
Independent Assortment
Crossing Over
Random Fertilization

37. Independent Assortment of Chromosomes
Homologous pairs of chromosomes orient randomly at metaphase I of meiosis
Each pair of chromosomes sorts its maternal and paternal homologues into daughter cells independently of the other pairs

38. Number of chromosomes possible when chromosomes assort independently into gametes is 2n, where n is the haploid number
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
n=2
22=4 combos of gametes

39. 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

40. Fertilization: the fusion of gametes
Random Fertilization
Random Fertilization
Fertilization: the fusion of gametes
Any sperm can fuse with any ovum (egg), each containing different combination of genes

41. In humans…
Number of possible different combinations of chromosomes in sperm:
223 = approx 8 million combos
Number of possible different combinations of chromosomes in egg:
Number of possible diploid combinations as a result of fertilization:
8 million x 8 million = 70 trillion
WE ARE SO UNIQUE AND SPECIAL!

42. Mutations!