Understanding Growth & Development

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FORMATION OF GAMETES

Meiosis2

Reproduction Strategies

• One characteristic of all living things is the ability to reproduce

• This is important because it allows genes for common traits to pass from one generation to the next

• This ensures the survival of the species

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Reproduction Strategies, cont.

• Two main categories of reproductive strategies:

1. Asexual

2. Sexual• Many species use only one strategy• Some species are able to use both

strategies depending on availability of partners or environmental conditions

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Reproduction Strategies, cont.

• Methods of asexual reproduction:

1. Binary fission – bacteria

2. Budding – yeast & hydra

3. Mitosis – single celled eukaryotes• Advantages:

1. One organism can increase a population

2. Energy is not used making sex cells

3. No need to search for mates• Disadvantage:

No genetic diversity; one dies, they all die!!!

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Reproduction Strategies, cont.

• Methods of sexual reproduction:

1. External fertilization (sperm and egg have to make contact in body of water) – fish & frogs

2. Internal fertilization (implantation of male gamete in moist environment of female) – plants, birds, reptiles & most mammals

• Advantage: genetic diversity• Disadvantage: requires a mate

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Formation of Gametes

• Gametes – sex cells; sperm or egg (ovum)• Diploid (2n) cell – cell with complete set of

chromosomes

- All body cells are diploid• Haploid (n) – cell with half the genetic

material of parent cell

- Gametes are haploid

- Creation of haploid cells is called meiosis

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Meiosis

• The creation of haploid cells requires 2 cycles of division:

1. Meiosis I – genetic recombination

2. Meiosis II – reduction division• Somatic (body) cells do not undergo meiosis• As in mitosis, meiosis is preceded by

interphase and DNA replication

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Meiosis I

• Prophase 1:

1. Matching (homologous) chromosomes pair up with each other to form a tetrad

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Meiosis I, cont.

• Prophase I:

2. Homologous chromosomes exchange genetic information with each other – called “crossing over”

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Meiosis I, cont.

• The cell goes through Metaphase, Anaphase and Telophase I

• At the end of Telophase I/Cytokinesis I, there are two new cells that are still diploid (2n)

• The genetic material has been recombined, so the new cells are no longer identical to the parent cell

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Meiosis II

• From Cytokinesis I, the cells immediately go through P M A T (II)

• At the end of Cytokinesis II, there are four new daughter cells that are haploid (n)

• These are the gametes

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Importance of Meiosis

• Allows for sexual reproduction which is the basis of genetic variation in offspring

• This variation allows the offspring a diversity that could be the difference between living or dying

• This gives the species as a whole a greater chance of surviving in a changing ecosystem

• Genetic variation is the foundation of evolution by natural selection

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The Human Reproductive System

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• Species that reproduce sexually have specialized structures that produce gametes

• In humans, this process is influenced by the endocrine system

• This system produces hormones that play an important role in the development and management of reproductive capacity

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Male Reproduction

• During childhood the body grows but does not change much

• Puberty begins between the ages of 11 & 16

• This process is triggered when the brain sends signals to the hypothalamus that it is time

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Male Reproduction, cont.• Steps of puberty:

1. Hypothalamus signals pituitary gland to release FSH (follicle stimulating hormone)

2. FSH travels through circulatory system to testes

3. FSH triggers luteinizing hormone (LH) to trigger specialized cells to make testosterone

4. Testosterone triggers:a. Sperm production

b. Development of external organs

c. Facial/body hair

d. Deepening of voice

e. Increase in muscle/bone mass

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Male Reproduction, cont.

• In human males (and many other organisms) gametes are called spermatozoa or sperm

• Formation of sperm from stem cells (called follicles) is called spermatogenesis

• 1 follicle produces 4 sperm• Length of meiosis: 65 to 75 days

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Male Reproduction, cont.

• Approximate number of stem cells that go through process each day: 3 million

• There are 300-500 million sperm that exit the body upon ejaculation

• Sperm are able to remain viable up to 48 hours inside the female reproductive system

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Did You Know?• The shape of the head of

a sperm varies from rounded to more oblong or pointed depending on the species?

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Rat

Human

Fly

Female Reproduction• Oogenesis – production of female gametes

(ovum/ova)• Begins as female fetus develops within the

uterus of her mother• As the fetus develops, follicles go through

Prophase I of Meiosis and then lie dormant within the ovaries

• Each follicle will produce one egg and 3 non-viable polar bodies

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Female Reproduction, cont.• Steps of puberty (10-14 years old):

1. Hypothalamus signals pituitary gland to release FSH (follicle stimulating hormone)

2. FSH travels through circulatory system to ovaries

3. FSH triggers luteinizing hormone (LH) to trigger the rest of meiosis in one follicle

4. Estrogen and progesterone are also secreted to support growth of uterine lining

5. If the ovum is not fertilized, it and the uterine lining are shed; this is menstruation

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Female Reproduction, cont.

• Estrogen production is the beginning of puberty; also triggers:a. Breast development

b. Broadening of hips

c. Growth of body hair

d. Slight deepening of voice

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Female Reproduction, cont.• The zygote:- Once the egg is fertilized, it becomes a

zygote- The zygote secretes a hormone that allows it

to implant in the uterine wall- Estrogen and progesterone keep the zygote

implanted and the uterine wall becomes a structure called the placenta

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Female Reproduction, cont.

• Function of placenta – trading post between circulatory systems of mother and fetus

• Umbilical cord – attaches placenta to fetus (~ 60 cm long)

• Amniotic sac -a. Provides a place for fetus to grow and develop that is safe

from outside pressures

b. Filled with fluid

c. Amniotic sac and fluid maintain a slightly higher temperature than the mother’s body, usually 99.7 F

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Cell Differentiation• Zygote goes through rapid mitosis and

becomes an embryo• This is possible because of cell

differentiation• During process, genes switch on and off

that control production of chemicals• These chemicals initiate differentiation in

cells

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