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Sexual Reproduction

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By meiosis (gamete production) and fertilization. Sexual Reproduction ... Meiosis refers to the production of gametes, or sex cells ... – PowerPoint PPT presentation

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Title: Sexual Reproduction


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?
  • 46!!
  • How?
  • By meiosis (gamete production) and fertilization

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

5
  • 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. . .(contd)
  • 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 dont 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 n23
  • 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
Lets 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 (contd)
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
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 (contd)
  • Telophase II
  • Chromosomes uncoil
  • Nuclear membranes form
  • around 4 new nuclei
  • Spindles disappear

22
Meiosis 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)

24
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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
Organism Number of chromosomes in daughter cell Diploid number Haploid number of homologous pairs of chromosomes Number of chromosomes present in the following stages Number of chromosomes present in the following stages
Start of Meiosis I Start of Meiosis II
Chimpanzee 48 48 24 23 48 24
Fruit Fly 8
Black Bear 38
Peanut 10
27
Comparison of meiosis and mitosis
Meiosis 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
30
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31
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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
  1. Gamete Production (Meiosis)
  2. Fertilization
  3. Embryo Development

36
Fertilization
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?
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.
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