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

2. Heredity
Is the transmission of traits from one generation to the next
Variation
Shows that offspring differ somewhat in appearance from parents and siblings
Two parents give rise to offspring that have unique combinations of genes inherited from the two parents
Figure 13.1

3. Oppose to asexual reproduction
One parent produces genetically identical offspring by mitosis
Ex: Binary Fission
Asexual reproduction results in clones
Clone: a group of genetically identical individuals
Figure 13.2
Parent
Bud
0.5 mm

4. Meiosis and Fertilization
Genetics
Is the scientific study of heredity and hereditary variation
Studies the passing of chromosomes from parents to offspring
Meiosis creates gametes, sex cells containing half the species chromosome number
Meiosis and Fertilization
maintain a species chromosome number during a sexual life cycle
Increase variation of individuals within a species

5. Offspring acquire genes from parents by inheriting chromosomes
Cell
Nucleus
Chromosomes
Genes

6. Genes
Are the units of heredity
Are segments of DNA
Contain specific sequence of nucleotides
Genes program cells to synthesize specific enzymes and other proteins which create an organism’s inherited traits

7. We inherit
One set of chromosomes from our mother and one set from our father
gametes transmit genes from on generation to the next
Specific location of a gene
one chromosome contains hundreds to a few thousand genes
each gene has a specific sequence of nucleotides

8. 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
All 46 are visible during mitosis
There are 23 “types” of chromosomes and every cell has a maternal version and a paternal version of each one
The two chromosomes composing a pair of each type are called homologous chromosomes

9. Visualizing Homologous Chromosomes
A karyotype
Is an ordered, visual representation of the chromosomes in a cell
5 µm
Pair of homologous
chromosomes
Centromere
Sister
chromatids
Figure 13.3

10. Homologous chromosomes
Are the two chromosomes composing a pair
Have the same characteristics or gene loci
Therefore, every cell has two copies of every gene
Allele: version of a gene
Ex: Hitchhikers thumb, Tongue rolling
All cells have two alleles for every gene – one maternal and one paternal
May also be called autosomes
Autosomes: do not control an individuals sex – sex is controlled by sex chromosomes

11. Sex chromosomes Are distinct from each other in their characteristics
Are represented as X and Y
Only small sections of X and Y are homologous
Determine the sex of the individual, XX being female, XY being male

12. The Human Life Cycle At sexual maturity
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)
At sexual maturity
The ovaries and testes produce haploid gametes by meiosis
During fertilization
These gametes, sperm and ovum, fuse, forming a diploid zygote
This starts the human life cycle
The zygote (fertilized egg)
Develops into an adult organism
Generates all of the somatic cells of the organism

13. Meiosis reduces the number of chromosome sets from diploid to haploid
Takes place in two sets of divisions, meiosis I and meiosis II
Results in 4 daughter cells with half the chromosome number of the parent

14. Overview of Meiosis Meiosis I Meiosis II
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 I
Reduces the number of chromosomes from one diploid cell to two haploid cells
Meiosis II
Two haploid daughter cells become four haploid daughter cells

15. 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

16. 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

17. A Comparison of Mitosis and Meiosis
Meiosis and mitosis can be distinguished from mitosis
By three events in Meiosis l that are unique to meiosis

18. Synapsis and crossing over
Crossing Over: Homologous chromosomes physically connect and exchange genetic information (sections of DNA)
Synapsis: nonsister chromatids connection preceding crossing over
Occurs at the end of Prophase I
Nonsister chromatids remain connected via chiasmata
Chiasmata: location on nonsister chromatids where DNA exchange occurred

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

20. Separation of homologues
At anaphase I of meiosis, cohesins between nonsister chromatids are cleaved and homologous pairs move toward opposite poles of the cell
Sister chromatids remain attached
In anaphase II of meiosis, cohesins between sister chromatids are cleaved and the sister chromatids separate
Shugoshin – protein that keeps the sister chromatids together during anaphase I

21. (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

22. Origins of Genetic Variation Among Offspring
Three mechanisms that contribute to genetic variation from sexual reproduction:
Independent assortment of chromosomes
Crossing over
Random Fertilization

23. Independent Assortment of Chromosomes
Homologous pairs of chromosomes
Orient randomly at metaphase I of meiosis
Maternal and paternal sister chromatids can be closer to either pole
This results in 2n possibilities
In humans, this means there are 223 or 8.4 million possible combinations of maternal and paternal chromosomes

24. Independent assortment
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

25. Homologous pair DNA is not exclusively maternal or paternal though; due to crossing over
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
Prophase I: synapsis and crossing over occur, homologs separate slightly
Chiasmata and cohesin between nonsister chromatids hold homologs together; they move to the metaphase plate
Break down of proteins holding between nonsister chromatid arms together allow homologs with recombinant chromosomes to separate

26. Random Fertilization The fusion of gametes
Will produce a zygote with any of about 64 trillion diploid combinations
This does not even account for variations produced by crossing over

27. Evolutionary Significance of Genetic Variation Within Populations
Is the raw material for evolution by natural selection
Darwin: a population evolves through the differential reproductive success of its variant members
Individuals best suited to their environment reproduce more – leaving offspring behind to continue transmitting beneficial genes
Mutations
Are the original source of genetic variation