Sexual Reproduction

Sexual ReproductionWe know all about asexual reproduction1. 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 parents2. 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?

46!! How?

By meiosis (gamete production) and fertilization

Sexual Reproduction

The specialized cells that are required for sexual reproduction are known asGAMETES

And come from the process of:

MEIOSIS

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

•Chromosomes coil

•Nuclear membrane disappears

•Chromosome pairs move toward each other

•Pairs of chromosomes move to the midline of cell

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 coil

•Spindle forms

•Nuclear membrane disappears

•Paired chromatids line up in middle of cell

•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)

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 ofMeiosis I

Start ofMeiosis

II

Chimpanzee 48 48 24 23 48 24Fruit Fly 8

Black Bear 38

Peanut 10

Comparison of meiosis and mitosisMeiosis 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

end

Review of Reproduction

1. Asexual Reproduction• one parent• no special reproductive cells are involved• offspring identical to parents and each other

2. 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:

1. conjugation – union of similar cells

2. fertilization – union of different cells (gametes)

Three Basic Stages of Sexual Reproduction

1. Gamete Production (Meiosis)

2. Fertilization

3. Embryo Development

Fertilization

Male gamete– the sperm (or spermatozoon)– usually smaller than the female

gamete

Female gamete the egg (or ovum) usually larger than the

male gamete

Stages in fertilization1. 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

1. Self-fertilization one animal produces both gametes

2. Internal fertilization fertilization occurs within the body

3. 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?

1. Sexual Reproduction Review 16.3 (3, 4, 5, 6, 7, 8)

2. Worksheet comparing mitosis and meiosis pg 367

This should be you, busy like a beaver.

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.