1. EW Title Meiosis Define the term gene.
What is the difference between sexual and asexual reproduction?

2. Chapter 13~
Meiosis and Sexual Life Cycles

3. Heredity
Heredity: the transmission of traits from one generation to the next
Variation: Inherited differences among individuals of the same species.
Genetics: The study of heredity and hereditary variations.
Gene: A segment of DNA that codes for a particular trait
Locus: Location on a chromosome for a particular gene (It is consistent throughout a species).

4. Karyotype
An ordered image of chromosomes starting with the longest chromosome.

5. Key Maternal set of chromosomes (n = 3) 2n = 6 Paternal set of
LE 13-4
Key
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)

6. Heredity Asexual reproduction: clones Sexual reproduction: variation
Human life cycle: • 23 pairs of homologous chromosomes (46); • 1 pair of sex and 22 pairs of autosomes ]
• gametes are haploid (1N)/ all other cells are diploid (2N);
•fertilization (syngamy) results in a zygote
Meiosis: cell division to produce haploid gametes

7. Important Terms to Know
Autosomes:
Chromosomes that do not determine the sex of an organism (1-22).
Diploid Cells:
Cells that have 2 sets of chromosomes
Haploid Cells:
Cells that have 1 set of chromosomes

8. Animal Life Cycle

9. Plants/some algae
Alternation of generations: 2N sporophyte, by meiosis, produces 1N spores; spore divides by mitosis to generate a 1N gametophyte; gametes then made by mitosis which then fertilize into 2N sporophyte
Plants and some algae have a second type of life cycle called alternation of generations.
This life cycle includes two multicellular stages, one haploid and one diploid.
The multicellular diploid stage is called the sporophyte.
Either haploid or diploid cells can divide by mitosis, depending on the type of life cycle.
Only diploid cells can undergo meiosis because haploid cells have a single set of chromosomes that cannot be further reduced.
Although the three types of sexual life cycles differ in the timing of meiosis and fertilization, they share a fundamental feature: Each cycle of chromosome halving and doubling contributes to genetic variation among offspring.
Meiosis in the sporophyte produces haploid spores.
Unlike a gamete, a haploid spore doesn’t fuse with another cell but rather divides by mitosis to form a multicellular haploid gametophyte stage.
Gametes produced via mitosis by the gametophyte fuse to form the zygote, which grows into the sporophyte by mitosis.
In this type of life cycle, the sporophyte generation produces a gametophyte as its offspring, and the gametophyte generation produces the next sporophyte generation.

10. Fungi/some algae
meiosis produces 1N cells that divide by mitosis to produce 1N adults (gametes by mitosis)
Most fungi and some protists have a third type of life cycle.
Gametes fuse to form a zygote, which is the only diploid phase.
The zygote undergoes meiosis to produce haploid cells.
These haploid cells divide by mitosis to form either unicellular daughter cells or a haploid multicellular adult organism.
The haploid adult produces gametes by mitosis.
The only diploid stage in these species is a single-celled zygote.

11. Meiosis
Preceded by chromosome replication, but is followed by 2 cell divisions (Meiosis I & Meiosis II)
4 daughter cells; 1/2 chromosome number (1N); variation

12. Meiosis
Preceded by chromosome replication, but is followed by 2 cell divisions (Meiosis I & Meiosis II)
4 daughter cells; 1/2 chromosome number (1N); variation

13. Prophase I
Prophase I typically occupies more than 90% of the time required for meiosis
Chromosomes begin to condense
In synapsis, homologous chromosomes loosely pair up, aligned gene by gene
In crossing over, nonsister chromatids exchange DNA segments
Each pair of chromosomes forms a tetrad, a group of four chromatids
Each tetrad usually has one or more chiasmata, X- shaped regions where crossing over occurred

14. 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

15. 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

16. 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

17. 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

18. 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

19. 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

20. MEIOSIS II: Separates sister chromatids
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

21. Prophase II Meiosis II is very similar to mitosis
In prophase II, a spindle apparatus forms
In late prophase II (not shown in the art), chromosomes (each still composed of two chromatids) move toward the metaphase plate

22. MEIOSIS II: Separates sister chromatids
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

23. Animation: Metaphase II
At metaphase II, the sister chromatids are arranged 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
Animation: Metaphase II

24. MEIOSIS II: Separates sister chromatids
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

25. 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

26. MEIOSIS II: Separates sister chromatids
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

27. 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

28. MEIOSIS II: Separates sister chromatids
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

29. 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

30. Meiosis vs. Mitosis Synapsis/tetrad/chiasmata (prophase I)
Homologous vs. individual chromosomes (metaphase I)
Sister chromatids do not separate (anaphase I)
Meiosis I separates homologous pairs of chromosomes, not sister chromatids of individual chromosomes.

31. Origins of Genetic Variation, I
Independent assortment: homologous pair of chromosomes position and orient randomly (metaphase I) and nonidentical sister chromatids during meiosis II
Combinations possible: ; with n the haploid number of the organism n

32. Origins of Genetic Variation, II
Crossing over (prophase I): • the reciprocal exchange of genetic material between nonsister chromatids during synapsis of meiosis I (recombinant chromosomes)
Random fertilization: • 1 sperm (1 of 8 million possible chromosome combinations) x 1 ovum (1 of 8 million different possibilities) = 64 trillion diploid combinations!