Sexual Reproduction

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

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

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

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

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

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

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

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

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

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)

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

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

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

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

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!

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

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

Meiosis I Prophase I Metaphase I Anaphase I Telophase I

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

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

Meiosis II Prophase II Metaphase II Anaphase II Telophase II

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)

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)

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

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

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 ______________.

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

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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

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)

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

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

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

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

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

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

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

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.

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!!

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

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.