1. Sexual Reproduction

2. Sexual Reproduction We know all about asexual reproduction
1. Only one parent required.
2. Offspring are identical to parents.
3. The cells that produce the offspring are not usually specialized for reproduction
Sexual Reproduction is different in almost every way:
1. Sexual reproduction almost always requires 2 parents
2. Offspring are not genetically identical to parents
3. Always requires the formation of specialized cells

3. Before we begin. . .
How many chromosomes does a normal human cell have?
46!!
If a man and woman mate, each having 46 chromosomes, how many chromosomes will the offspring have?
How?
By meiosis (gamete production) and fertilization

4. MEIOSIS Sexual Reproduction
The specialized cells that are required for sexual reproduction are known as
GAMETES
And come from the process of:
MEIOSIS

5. Meiosis refers to the production of gametes, or sex cells
Meiosis is another form of cell division and is the basis of sexual reproduction
Meiosis refers to the production of gametes, or sex cells
Gametes contain half the number of chromosomes found in body cells (skin cells, muscle cells, etc.)
Gametes carry genetic info from one generation to another
Meiosis ensures variation within a species
-more on this later

6. Before we get to meiosis. . .
To the right is a set of human chromosomes
Each chromosome is paired with another chromosome.
Each normal human cell has 46 chromosomes, each of these is in a pair with another similar chromosome

7. Before we get to meiosis. . .(cont’d)
In humans, body cells contain 46 chromosomes
Half are from your mother, half from your father
These cells are said to be diploid (2n), which means there are 2 sets of chromosomes
The diploid number can be expressed as 2n = 46
All normal cells in our bodies are diploid

8. Sex chromosomes
Of the 23 pairs of human chromosomes, 22 have a matching chromosome
A pair of matching chromosomes is called a pair of homologous chromosomes
The 2 chromosomes that form the 23rd pair are called sex chromosomes

9. Sex chromosomes
For females, the sex chromosomes are an identical “X” shape
This pair is referred to as XX
For the male, the chromosomes are not homologous and don’t look identical
One chromosome has an X shape, and the other smaller chromosome a Y shape
This pair is referred to as XY

10. Quick Check – Answer the following as True or False:
There are 2 sets of chromosomes in humans
One set of human chromosomes has 46 chromosomes
Two sets of human chromosomes has 46 chromosomes
Humans have 46 pairs of homologous chromosomes
For females, the homologous pair of sex chromosomes is XY

11. Gametes
Male and female gametes contain half the number of chromosomes that other body cells have
Therefore, these cells are said to be haploid (n)

12. Gametes The haploid number in a human sex cell is n=23
In males, the sex cell is the sperm cell
In females, the sex cell is the egg cell (ovum)
Both of these specialized cells are produced through the process of meiosis

13. As mentioned above 2 parents are needed in sexual reproduction,
If each parents reproductive cell had 46 chromosomes the offspring would have 92 chromosomes
These extra chromosomes would disturb the cells functioning
To avoid extra chromosomes MEIOSIS occurs

14. What have we learned so far?
Reduces the number of chromosomes in a cell by 1/2 (46 to 23)
The cells that meiosis produces are known as “gametes”
Gametes have 1/2 the number of chromosomes as a regular cell and are known as “HAPLOID”
Meiosis changes cells from diploid to haploid

15. Let’s get to meiosis already!
Meiosis has 2 main phases: meiosis I and meiosis II
The phase before meiosis begins is called interphase
Each homologous pair of chromosomes is replicated
In meiosis I, the homologous chromosomes separate into 2 cells
In meiosis II, sister chromatids separate into 4 cells
Outcome = 4 cells with one haploid set of chromosomes in each cell

16. Easy Breezy!
Meiosis can be thought of as two cell divisions in series (Meiosis I and II)
Have similar sub-phases just like mitosis (PMAT I and PMAT II)
If you can remember what happens during mitosis, then meiosis should pose no problem at all!

17. MEIOSIS I Prophase I Metaphase I
Pairs of chromosomes move to the midline of cell
Chromosomes coil
Nuclear membrane disappears
Chromosome pairs move toward each other

18. MEIOSIS I (cont’d) Anaphase I Telophase I
Paired chromosomes move apart, one to each end of cell
Paired chromatids remain attached
Each end of the cell has half the number of chromosomes the parent cells had
Paired chromatids uncoil
Nuclear membrane reforms
Spindles disappear
Cytokinesis occurs

19. Meiosis I
Prophase I
Metaphase I
Anaphase I
Telophase I

20. MEIOSIS II Prophase II Metaphase II Anaphase II
Paired chromatids line up in middle of cell
Paired chromatids coil
Spindle forms
Nuclear membrane disappears
Each pair of chromosomes split to form 2 independent chromosomes
New chromosomes move to opposite ends of cell

21. MEIOSIS II (cont’d) Telophase II Chromosomes uncoil
Nuclear membranes form
around 4 new nuclei
Spindles disappear

22. Meiosis II
Prophase II
Metaphase II
Anaphase II
Telophase II

23. Result of Meiosis
Meiosis results in four cells with ½ the full number of chromosomes (haploid cells)
Cells are genetically unique in comparison to the parents
23 chromosomes in each new cell
So, when fertilization occurs 23 chromosomes (sperm) + 23 chromosomes (egg) = 46 chromosomes (embryo)

25. Where does Meiosis Occur?
Mitosis is needed for growth and repair and occurs in normal cells
Meiosis only occurs in reproductive cells (i.e. in the testes and ovaries)

26. 24 23 Chimpanzee 48 Fruit Fly 8 Black Bear 38 Peanut 10 Organism
Number of chromosomes in daughter cell
Diploid number
Haploid number
# of homologous pairs of chromosomes
Number of chromosomes present in the following stages
Start of
Meiosis I
Meiosis II
Chimpanzee 48 24 23
Fruit Fly 8
Black Bear 38
Peanut 10

27. Comparison of meiosis and mitosis
Keeps the number of chromosomes from doubling each generation by producing haploid sperm or egg cells
Responsible for growth, tissue repair, and some forms of asexual reproduction
Ensures variation in a species because chromosomes from each parent are combined
Ensures that all cells produced are identical
Has 2 cell divisions
Has 1 cell division
Makes the diploid sex cell become a haploid egg or sperm cell
Makes 2 exact copies (daughter cells) of a parent cell

28. Quick Check Haploid is the diploid as n is to ________.
Mitosis is to meiosis as identical daughter cell is to _________________.
Meiosis is to mitosis as n is to ___________.
Male is to female as sperm cell is to ________________.
A pair of shoes is to diploid as one shoe is to ______________.

29. Comparison of asexual and sexual reproduction
Asexual reproduction
Sexual reproduction
Requires 1 parent
Requires 2 parents
Produces offspring identical to parent
Produces offspring different from parent
Can produce many offspring quickly
Requires more time and energy to produce offspring
Results in limited variability within a species
Results in extensive variation within a species

32. end

33. Review of Reproduction
Asexual Reproduction
one parent
no special reproductive cells are involved
offspring identical to parents and each other
Sexual Reproduction
two parents
involves special reproductive cells
offspring not identical to parents or each other

34. Sexual Reproduction two parents are required
there is an exchange of DNA between the parents
there are two kinds of sexual reproduction:
conjugation – union of similar cells
fertilization – union of different cells (gametes)

35. Three Basic Stages of Sexual Reproduction
Gamete Production (Meiosis)
Fertilization
Embryo Development

36. Fertilization Female gamete the egg (or ovum)
usually larger than the male gamete
Male gamete
the sperm (or spermatozoon)
usually smaller than the female gamete

37. Stages in fertilization
1. Sperm meets the egg!!!
2. The sperm breaks through the outer covering of the egg
3. Nuclei of the sperm and egg combine, to form a diploid zygote

38. Kinds of sexual reproduction
Self-fertilization
one animal produces both gametes
Internal fertilization
fertilization occurs within the body
External fertilization
fertilization occurs outside the body

39. 1.Self-fertilization one parent produces both gametes
common in plants, also possible for animals
Not very good due to inbreeding of DNA

40. 2.Internal-fertilization
Sperm is placed in the female body
Sperm joins with the egg(s)
The zygote is nourished immediately by the body

41. 3. External-fertilization
Eggs are laid unfertilized
Sperm are placed on afterwards.
If conditions are not ideal many eggs are not fertilized

42. Sexual Reproduction: Good or Bad?
Advantages:
Recombination of DNA in the gametes results in genetic variation among the offspring.
In an environment which changes, this allows the process of natural selection to occur.
Disadvantages:
Two parents are required.

43. Last Question of the Day…
If a new species of fish has 84 chromosomes in their skin cells, how many chromosomes will their sperm have?
84/2 = 42!!

44. Now What? Sexual Reproduction Review 16.3 (3, 4, 5, 6, 7, 8)
This should be you, busy like a beaver.
Sexual Reproduction Review 16.3 (3, 4, 5, 6, 7, 8)
Worksheet comparing mitosis and meiosis pg 367

45. Twins?
Identical twins are also called monozygotic meaning one egg. Their genes are identical. They are always of the same sex.
Fraternal twins are called dizygotic or two egg. They share approximately 25 percent of their genes, as do any two brothers or sisters. One-half of fraternal twin pairs are boy-girl, one-quarter are boy-boy, and one-quarter are girl-girl.