1. Background information for Sexual Reproduction
Mr. Shrout

2. Mitosis
Mitosis produces genetically identical cells and is involved in growth and asexual reproduction.
This form of cell division can quickly produce new cells (organisms), but does not produce any variations.
Variations are produced by the mechanisms involved in Sexual Reproduction

3. Chromosome Numbers All organisms have a specific number of chromosomes
Homologous pairs: In organisms that reproduce sexually, those chromosomes occur in pairs, one member of each pair originating with each parent
Examples:
Humans have 46 chromosomes, 23 homologous pairs
Fruit flies have 8 chromosomes, 4 homologous pairs

4. Homologous chromosomes
Each pair of homologous chromosomes has genes for the same traits.
Homologous Chromosomes
eye color
(brown?)
eye color
(blue?)
Sister chromatids
homologous chromosomes
double stranded homologous chromosomes

5. karyotype
A karyotype is a photographic inventory of an individual’s chromosomes
Human female karyotype- 46 total, 23 pairs
Notice the chromosomes in this picture aren’t duplicated

6. Human female karyotype
46 chromosomes
23 pairs XX

7. Human male karyotype
46 chromosomes
23 pairs XY

8. Cell Types Organisms have 2 main cell types:
Somatic cells: cells found in normal body tissues that aren’t associated with reproduction
These cells have the full number of chromosomes
Germ Cells: cells that produce cells that function in sexual reproduction
These cells produce gametes: sperm (or pollen) and egg
Gametes don’t have the full number of chromosomes of an organism
Why?

9. How about the rest of us? + 46 46 92
What if a complex multicellular organism (like us) wants to reproduce?
joining of egg + sperm
Do we make egg & sperm by mitosis?
What if we did, then…. 46 + 46 92
egg
sperm
zygote
Doesn’t work!

10. Some cells have different numbers of chromosomes
Normal body cells are Diploid- they have the full amount of chromosomes abbreviated 2n
But all gametes (sperm and egg cells) are Haploid- they have half the normal amount of genetic information abbreviated 1n
One copy of each chromosome
This is so that when the gametes combine the offspring has the correct amount of genetic information

11. How do we make sperm & eggs?
Must reduce 46 chromosomes  23
Sperm and eggs must be haploid: having half the number of chromosomes of the organism 23 46
zygote 23 46
egg 23
meiosis 46 23
fertilization
sperm
gametes

12. Stages of Meiosis

13. Interphase the cell replicates its chromosomes.
The Phases of Meiosis
Interphase
Interphase
the cell replicates its chromosomes.
After replication, each chromosome consists of two identical sister chromatids, held together by a centromere.

14. Prophase I The chromosomes coil up and a spindle forms.
The Phases of Meiosis
Prophase I
Prophase I
The chromosomes coil up and a spindle forms.
As the chromosomes coil, homologous chromosomes line up with each other gene by gene along their length, to form a four-part structure called a tetrad.

15. Prophase I The Phases of Meiosis
The chromatids in a tetrad pair tightly
In fact, they pair so tightly that non-sister chromatids from homologous chromosomes can actually break and exchange genetic material in a process known as crossing over.

16. The Phases of Meiosis
Metaphase I
Metaphase I
During metaphase I, the centromere of each chromosome becomes attached to a spindle fiber.
The spindle fibers pull the tetrads (homologous pairs) into the middle

17. Prophase I + Metaphase 1 The Phases of Meiosis
Crossing over can occur at any location on a chromosome, and it can occur at several locations at the same time.
Genetic recombination results from crossing over during prophase I and metaphase 1 of meiosis
This increases variation further

18. Anaphase I The Phases of Meiosis
begins as homologous chromosomes, each with its two chromatids, separate and move to opposite ends of the cell.
This critical step ensures that each new cell will receive only one chromosome from each homologous pair.
*Random assortment*

19. The Phases of Meiosis Telophase I
Nucleus reforms, chromosomes may or may not unwind
The spindle is broken down and the cytoplasm divides to yield two new cells
Cells are now HAPLOID- half the amount of genetic information

20. Meiosis 1 overview 1st division of meiosis 4 chromosomes diploid 2n
Divide 1
Line Up in
tetrads
Copy DNA during interphase
prophase 1
metaphase 1
4 chromosomes
diploid 2n
telophase 1
2 chromosomes
haploid 1n

21. The Phases of Meiosis Meiosis II
Prophase II
Meiosis II
The second division in meiosis is simply a mitotic division of the products of meiosis I.
During prophase II, a spindle forms in each of the two new cells and the spindle fibers attach to the chromosomes.
Nucleus dissolves

22. The Phases of Meiosis Metaphase II
The chromosomes, still made up of sister chromatids, are pulled to the center of each cell and line up randomly at the equator.

23. The Phases of Meiosis Anaphase II
the centromere of each chromosome splits, allowing the sister chromatids to separate and move to opposite poles of each cell.

24. The Phases of Meiosis Telophase II
nuclei reform, the spindles break down, and the cytoplasm divides.
At the end of meiosis II, four haploid cells have been formed from one diploid cell.
These haploid cells will become gametes, transmitting the genes they contain to offspring.

25. Meiosis 2 overview 2nd division of meiosis gametes looks like mitosis
telophase 2
telophase 1
Line Up 2
2nd division of meiosis
looks like mitosis
metaphase 2
2 chromosomes
haploid 1n
gametes

26. The Phases of Meiosis
At the end of meiosis II, four haploid cells have been formed from one diploid cell.
These haploid cells will become gametes, transmitting the genes they contain to offspring.

27. Gamete Formation
Formation of Gametes (egg and sperm- sexual reproductive cells)
Meiosis in males occurs in the testes and produces 4 sperm cells. The process is called spermatogenesis.
Meiosis in females occurs in the ovaries and produces 1 egg and 3 polar bodies, which disintegrate. The process is called oogenesis.
Variation: Meiosis allows for crossover and genetic recombination (gene mixing), which creates the differences between parents and offspring.

29. Meiosis to make gametes
Meiosis & mitosis
Meiosis to make gametes
sperm & egg
Mitosis to make copies of cells
growth
repair
development

30. Meiosis 1 (diploid -> haploid)

31. Meiosis 2 (haploid 2 cells -> haploid 4 cells)

32. Comparison of Mitosis & Meiosis

33. Key Features of Meiosis
Genetic Recombination- a set of processes that produce genetic variation during sexual reproduction (producing variation is the whole point of sexual reproduction!!)
Random Assortment- during anaphase 1, homologous chromosomes move randomly to opposite ends
not all of the chromosomes from one parent go in one direction

34. Key Features cont..
Crossing Over- During prophase 1 and metaphase 1, sections from homologous chromosomes can be exchanged
genes from one parent can be exchanged with genes from the other parent
Fertilization- combination of gametes from the two parents leading to the formation of a zygote (fertilized egg)
This combines genetic information from two individuals

35. Crossing Over

36. Random Assortment

37. Fertilization