1. Meiosis
AP Biology

2. What is the function of meiotic cell division
What is the function of meiotic cell division? How does this form the basis of genetics and evolution?
Outcome: 4 haploid cells (gametes)
Crossing over: prophase I
Homologous chromosomes
Sister chromatids

3. Hereditary Similarity and Variation
Heredity is the transmission of traits from one generation to the next
Variation shows that offspring differ in appearance from parents and siblings
Genetics is the scientific study of heredity and variation
Heredity is the transmission of traits from one generation to the next
Variation shows that offspring differ in appearance from parents and siblings
Genetics is the scientific study of heredity and variation

4. Genes Genes are the units of heredity Genes are segments of DNA
Each gene has a specific locus on a certain chromosome
One set of chromosomes is inherited from each parent (humans 23 from each parent)

5. Sexual/Asexual
Sexual reproduction- two parents give rise to offspring that have unique combinations of genes inherited from the two parents
Asexual reproduction- one parent produces genetically identical offspring by mitosis

6. Karyotype
karyotype -ordered display of the pairs of chromosomes from a cell
homologous chromosomes (homologues)
Both chromosomes in a pair. They carry genes controlling the same inherited characteristics
Alleles-

7. Pair of homologous 5 µm chromosomes Centromere Sister chromatids
LE 13-3
Pair of homologous
chromosomes
5 µm
Centromere
Sister
chromatids

8. “Hey, that’s my chromosome!”
LE 13-4
Key
“Hey, that’s my chromosome!”
Maternal set of
chromosomes (n = 3)
2n = 6
Paternal set of
chromosomes (n = 3)
Two sister chromatids
of one replicated
chromosomes
Centromere
Two nonsister
chromatids in
a homologous pair
Pair of homologous
chromosomes
(one from each set)

9. Homologous Chromosomes
Each pair of homologous chromosomes includes one chromosome from each parent
For humans, the diploid number is 46 (2n = 46) 23 Pairs.
Gametes are haploid cells, containing only one set of chromosomes

10. Meiosis: twice as nice Meiosis I Synapsis Tetrad Formation
Crossing over
Prophase I
Homologs split
Meiosis II same as mitosis Sister chromatids split 4 unique cells result haploid

11. LE 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
Meiosis I
Homologous
chromosomes
separate
Haploid cells with
replicated chromosomes
Meiosis II
Sister chromatids
separate
Haploid cells with unreplicated chromosomes

12. MEIOSIS I: Separates homologous chromosomes
LE 13-8ab
MEIOSIS I: Separates homologous chromosomes
PROPHASE I
METAPHASE I
ANAPHASE I
Centromere
(with kinetochore)
Sister chromatids
remain attached
Sister
chromatids
Chiasmata
Metaphase
plate
Spindle
Tetrad
Microtubule
attached to
kinetochore
Homologous
chromosomes
separate
Homologous chromosomes
(red and blue) pair and
exchange segments; 2n = 6
in this example
Tetrads line up
Pairs of homologous
chromosomes split up

13. 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

14. LE 13-8b MEIOSIS II: Separates sister chromatids TELOPHASE I AND
CYTOKINESIS
TELOPHASE II AND
CYTOKINESIS
PROPHASE II
METAPHASE II
ANAPHASE II
Cleavage
furrow
Haploid daughter cells
forming
Sister chromatids
separate
Two haploid cells
form; chromosomes
are still double
During another round of cell division, the sister chromatids finally separate;
four haploid daughter cells result, containing single chromosomes

16. Crossing Over
Because of crossing over in Prophase I, the two sister chromatids of each chromosome are no longer genetically identical
Anaphase I: Homologous chromosomes separate. sister chromatids stay together
Anaphase II: sister chromatids separate
The sister chromatids of each chromosome are now two newly individual chromosomes.

17. Four for the Price of One
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

18. (before chromosome replication)
LE 13-9
MITOSIS
MEIOSIS
Parent cell
(before chromosome replication)
Chiasma (site of
crossing over)
MEIOSIS I
Propase
Prophase I
Chromosome
replication
Chromosome
replication
Tetrad formed by
synapsis of homologous
chromosomes
Duplicated chromosome
(two sister chromatids)
2n = 6
Chromosomes
positioned at the
metaphase plate
Tetrads
positioned at the
metaphase plate
Metaphase
Metaphase I
Anaphase
Sister chromatids
separate during
anaphase
Homologues
separate
during
anaphase I;
sister
chromatids
remain together
Anaphase I
Telophase
Telophase I
Haploid
n = 3
Daughter
cells of
meiosis I 2n 2n
MEIOSIS II
Daughter cells
of mitosis n n n n
Daughter cells of meiosis II
Sister chromatids separate during anaphase II

19. Genetic variation contributes to Evolution
Mutations create different versions of genes
Mechanisms for variation in Sexual:
Independent Segregation of alleles
Independent assortment of chromosomes
Crossing over (prophase I)
Random fertilization

20. Independent Assortment
Independent assortment-each pair of chromosomes sorts maternal and paternal homologues into daughter cells independently of the other pairs (not all dad’s chroms to 1 daughter! And mom’s to the other)
For humans (n = 23), there are more than 8 million (223) possible combinations of chromosomes

21. Crossing over to the other side
Crossing over produces recombinant chromosomes, which combine genes inherited from each parent
In crossing over, homologous portions of two nonsister chromatids trade places
Prophase I

22. Recombinant chromosomes
LE 13-11
Prophase I
of meiosis
Nonsister
chromatids
Tetrad
Chiasma,
site of
crossing
over
Metaphase I
Metaphase II
Daughter
cells
Recombinant
chromosomes

23. Match.cell
Random fertilization adds to genetic variation because any sperm can fuse with any ovum
The fusion of gametes produces a zygote with any of about 64 trillion diploid combinations
Crossing over adds even more variation