1. Sexual Cell Reproduction
Meiosis
Sexual Cell Reproduction

2. Sexual Cell Reproduction
All living cells arise from pre-existing cells, tracing the lineage of every living things back to some primordial ancestor. If mitosis were the only process involved the production of new cells, then all cells would be exactly the same.
Instead, there exists an incredible variety among organisms

3. Gametes and the Life Cycle of a Sexual Organism
Egg cell
The life cycle of a multicellular organism is the sequence of stages leading from the adults of one generation to the adults of the next
Sperm cell
Meiosis
Fertilization
Multicellular
diploid adults
(2n = 46)
Diploid
zygote
(2n = 46)
Mitosis and
development
Figure 8.13

4. DNA Sequence
It is the sequence of the bases in a gene that holds the genetic code
And therefore holds the code for all cellular processes and the variety of inherited traits we see in organisms

5. The cellular bases for sexual reproduction
Meiosis
The cellular bases for sexual reproduction
-the mixing of genetic traits produces offspring different from each parent
- Fertilization requires 2 gametes (egg and sperm)
These gametes contain ½ the genetic information
Each sperm or egg produced carries 1 of over 8 million possible combinations of parental chromosomes 2n
46 chrom 2n
(46) 2n
(46) n 23 n 23 n 23 n 23

6. Meiosis versus Mitosis
3 Important Differences:
1. Takes place in 2 stages involving 2 successive divisions
2. The chromosomes arrange themselves in homologous pairs (pair up with another chromosome of the same size and shape)
3. Therefore, the four daughter cells are not necessarily identical and have only ½ the number of chromosomes present in the original parent cell.

7. Meiosis
The production of gametes with half the number of chromosomes as the original “parent” cell
During this process specialized cells in the gonads (ovary & testes), produce sex cells that contain only one set of chromosomes.
A human germ cell with 46 chromosomes will undergo meiosis and produce gametes that have 23 chromosomes

8. Meiosis
The 46 chromosomes number is referred to as diploid and is written as 2n
The 23 chromosomes number is referred to as haploid and is written as n
Fertilization occurs when 2 gametes (sperm & egg) fuse, forming a diploid zygote (46 chromosomes)
Zygote
46 chromosomes (diploid)
Egg
23 chromosomes (haploid)
Sperm
23 Chromosomes (haploid)

9. Each chromosome has a partner
Keep in mind that the 23 chromosomes are not just any 23, but one member from each pair.
Each of the 23 chromosomes that you receive from your father is matched by 23 chromosomes from your mother
Example: Your father gives you a chromosome with genes that code for eye colour and so does your mother

10. Homologous Chromosomes
The paired chromosomes are called homologous chromosomes
Homologus Pairs = a pair of chromosomes that have similar lengths, shapes and carry genes controlling the same traits
However each chromosome does not necessarily carry the same genetic information
For example: both chromosomes in a homologous pair may carry a gene for eye colour but one may code for blue and the other for brown eyes
Each cell produced during meiosis contains one member from each pair of homologous chromosomes
After DNA replication

11. Phases of Meiosis Meiosis I Interphase Prophase I Metaphase I
Anaphase I
Telophase I
Meiosis II
Prophase II
Metaphase II
Anaphase II
Telophase II

12. Meiosis
Meiosis involves 2 cell divisions that produces 4 haploid cells
To keep things simple, in our example we will use cells that contain 2n=4 chromosomes.
Therefore, the cell will contain two pairs of homologous chromosomes.

13. Interphase
As in mitosis, interphase (cell growth and DNA replication), must occur before cell can replicate
Very important = interphase occurs before prophase I but not before prophase II
Chromatin
Centrioles

14. Interphase: DNA replication of homologous pairs

15. Prophase 1
Chromatin coils tightly to form chromosomes - DNA has already replicated
Homologous chromosomes pair up side by side in a process called synapsis.
- When 2 homologous chromosomes are paired, the structure is called a bivalent ( = 2 chromosomes)
Paternal Homolog
Maternal homolog

16. Prophase 1 3. Chromosomes shorten & thicken
- the homologous chromosomes (bivalents) pair up to form tetrads.
Each tetrad contains 2 homologous chromosomes and 4 chromatids
Tetrad

17. Crossing Over
As the homologous chromosomes come close together, they often intertwine
Sometimes chromatids break & exchange segments. This process is called crossing over
The area(s) where the chromatids overlap is called a chiasma
Here homologous chromosomes exchange genetic material

18. Crossing Over
This leads to enormous genetic variation even between siblings
There are over 8 million possible combinations of parental chromosomes
5. The chiasma begins to disappear
The nuclear membrane disintegrates
**In females this is where meiosis stops until puberty (Prophase I)

19. Prophase I : 2n = 6 or n=3 Draw a germ cell in prophase I Label:
Homologous chromosomes & tetrad
Nuclear membrane
Centrioles, astral rays
Homologous pair
Tetrad

20. Metaphase I The tetrads line up on the equatorial plate
Metaphase plate
The tetrads line up on the equatorial plate
The homologous pairs line up randomly
There is a chance for the daughter cells to get either pair from each chromosome.
Centriole
You could get this one or that one

22. Spindle fibres attach to the centromeres of the chromosomes
Each pair of sister chromatids from each homologous chromosome is ready to move to opposite poles of the cell
Draw a diagram

23. Anaphase I Homologous chromosomes separate and move to opposite poles
This process is known as segregation – the separation of paired genes
At this point, reduction division has occurred!
Each chromosome remains double stranded

24. Draw a diagram

25. Telophase I Cytoplasm divides = Cytokinesis
A nuclear membrane begins to form
Chromosomes unwind into chromatin
The two new daughter cells contain 2 chromosomes each.
Two haploid cells (cells with half the chromosome number) start to form. Chromosomes are half the number, but still double stranded.

26. Draw a diagram Label the following: 1. Cleavage furrow
2. Daughter cells
3. Nuclear membrane
4. Chromatin

27. Meiosis I versus Mitosis
The first division is different from mitosis because the daughter cells are not identical
Each daughter cell contains 1 member of the chromosome pair
Remember homologous pairs are similar but they are not identical

28. Meiosis II
****The short phase between Meiosis I and II is referred to as interkinesis
There may or may not be an interphase II depending on species
The next set of cell divisions will separate the chromatids
Begin with the 2 daughter cells from meiosis I
DNA replication does NOT occur
Nuclear membrane dissolves, spindle fibres form
Prophase II

29. Draw a diagram

30. Metaphase II
The chromosomes , each with two sister chromatids, align on the equatorial plate
The centromeres attach to spindle fibres

31. Anaphase II
The centromeres separate and chromatids move towards opposite poles

32. Telophase II 4 new cells will be formed
Each of the new cells will contain only one member from each homologous pair = haploid (n)
The parent cell had 6 chromosomes; the daughter cells have 3 chromosomes each

33. Meiosis Summary
The haploid cells complete the meiotic cycle and differentiate to produce gametes (egg & sperm)
Important to remember:
-Meiosis I is referred to as reductional division because the chromosome number is reduced by half
- Meiosis II is called equational division and is similar to mitosis as centromeres on sister chromatids separate and chromosome number remains unchanged

34. MEIOSIS I: Homologous chromosomes separate
INTERPHASE
PROPHASE I
METAPHASE I
ANAPHASE I
Centrosomes (with centriole pairs)
Sites of crossing over
Metaphase plate
Sister chromatids remain attached
Spindle
Nuclear envelope
Chromatin
Sister chromatids
Tetrad
Homologous chromosomes separate

35. Meiosis II TELOPHASE I & CYTOKINESIS TELOPHASE II & CYTOKINESIS
MEIOSIS II: Sister chromatids separate
TELOPHASE I & CYTOKINESIS
TELOPHASE II & CYTOKINESIS
PROPHASE II
METAPHASE II
ANAPHASE II
Sister chromatids separate
Haploid daughter cells forming