Bio 3201 UNIT 1A pt 2

Spermatogenesis vs oogenesis.

Sperm cell: male gamete

Ovum (egg): female gamete

Spermatogenesis: process of male gamete production

Oogenesis: process of female gamete production

The process of spermatogenesis starts with a diploid germ cell called a spermatogonium.

Beginning at puberty, spermatogonia are stimulated to divide by mitosis to form two daughter

cells. One of these cells replenishes the spermatogonia cell population, and the other develops

into a primary spermatocyte. The primary spermatocyte undergoes meiosis I to form two

secondary spermatocytes. The secondary spermatocytes then undergo meiosis II to form four

spermatids. Following meiosis II, the spermatids go through a final set of developmental stages

in order to develop into mature sperm.

Spermatogonia primary spermatocyte 2 secondary spermatocytes  4 spermatids  4

sperm.

Each sperm cell has three parts:

 oval head,

 cylindrical middle piece,

 extended tail.

The head contains the nucleus. It is covered by a cap like structure called the acrosome. The

acrosome stores enzymes that are needed to penetrate the protective layer surrounding a

female egg.

The middle piece contains 50 to 100 mitochondria, which provide energy for the movement of

the tail.

The tail propels the sperm with a lashing motion.

About 300 to 500 million sperm are produced each day in a male’s lifetime.

Some animals, including most mammals, produce sperm throughout the year. Other animals

produce sperm only during a specific breeding season.

Since hundreds of millions of sperm are released in a single ejaculation, meiosis is constantly

occurring. Mitosis is also occurring regularly to keep a supply of germ cells for gamete

production.

Oogenesis

In female animals, meiosis takes place in the ovaries.

Oogenesis starts with a diploid germ cell called an oogonium. Each oogonium undergoes

mitosis to form two primary oocytes. They are arrested in prophase I and remain that way until

puberty.

Every month after puberty, one primary oocyte undergoes meiosis. In contrast to

spermatogenesis, however, oogenesis involves an unequal division of cytoplasm, known as

asymmetrical cytokinesis. At the end of meiosis I, the cytoplasm is not equally divided between

the two daughter cells. The cell that receives most of the cytoplasm is called the secondary

oocyte. The other cell is called the first polar body.

The first polar body may or may not go through a second division to produce a pair of

second polar bodies. In either case, the polar bodies are not functional and soon

degenerate.

When the secondary oocyte undergoes meiosis II, the cytoplasm is again unequally divided. The

cell that contains most of the cytoplasm will eventually become a mature egg, or ovum. The

other cell, another second polar body, is not a viable gamete.

The unequal division of cytoplasm means that only one egg cell is produced from the division of

the secondary oocyte.

Oogonium  2 primary oocytes 1 secondary oocyte  1 egg

Why this unequal division of cytoplasm?

The unequal division of cytoplasm means that only one egg cell is produced from the division of

the secondary oocyte. This egg cell, however, contains a large quantity of nutrients that the

zygote can use prior to implantation. (Roughly 4-5 days)

A mature ovum is a non-motile, sphere-shaped cell approximately 0.1 mm in diameter (that is,

over 20 times larger than the head of a sperm cell). The ovum contains a large quantity of

cytoplasm, which contains nutrients for the first days of development after fertilization. The

cytoplasm contains about 140 000 mitochondria. The ovum is encased in a thick membrane

that must be penetrated by a sperm cell before fertilization can take place.

The processes of meiosis I and meiosis II are not always continuous.

In human females, more than a decade separates the events of meiosis I and meiosis II. The

primary oocytes begin meiosis I before birth, but cell division stalls in prophase I. The cells

remain in this suspended state until puberty. At puberty, a hormone signal triggers a single

primary oocyte to resume meiosis. The primary oocyte completes meiosis I. The secondary

oocyte is then released from the ovary and travels down the Fallopian tube.

The secondary oocyte is arrested at metaphase II until fertilization occurs.

Sperm Egg

- Small - Large

- mobile - not mobile

- have a cap called an acrosome which contains

enzymes used to enter the egg cell

- covered by a thick outer coating. After one

sperm penetrates the egg, no more can enter

- millions produced continuously (300 million-

500 million)

- one egg matures per month from puberty to

menopause

- 50-100 mitochondria per cell - about 140, 000 mitochondria per cell

- before ejaculation: uses fat for energy- after

ejaculation: uses sugar (fructose) for energy

- can only live for about a day or so with its

food supply if unfertilized.

Reproductive Strategies

Sexual reproduction involves the production of gametes by meiosis, followed by fertilization

between genetically distinct parental gametes to produce genetically distinct offspring.

In general, mitosis is the key mechanism involved in asexual reproduction, the reproductive

process in which a parent organism produces genetically identical offspring.

Humans can only reproduce sexually, and the diploid individual is the only life stage that has

the capacity for independent existence.

While most animals have a similar life cycle, other organisms have a wide variety of life cycles.

Reproduction in Prokaryotes (bacteria):

Bacteria and other prokaryotes have a single, circular chromosome and no nucleus. Therefore,

a bacterial cell does not undergo mitosis. Instead, prokaryotes reproduce through the asexual

process of binary fission. This results in genetically identical cells. See figure 12.18 page 474.

Some bacteria are able to undergo a process called conjugation. Conjugation involves the

transfer of genetic material from one cell to another by cell-to-cell contact through a bridging

structure called a pilus. Conjugation creates cells with new genetic combinations, and thereby

provides a chance that some cells may be better adapted to changing conditions.

Other forms of asexual reproduction:

Budding: is a form of asexual reproduction in which a complete but miniature version of the

parent grows out from the parent’s body. The new organism then separates to become an

independent organism. An example can be found in Hydra.

Vegetative reproduction: growth of a new plant from a modified stem. For example,

strawberry plants can spread across a garden by extending thin creeping stems. A new

strawberry plant develops at the end of each stem.

Fragmentation: a new organism forms from a part of a parent. In the cultivation of potatoes,

for example, entire new plants are grown from a fragment, or tuber, of a parent plant. Some

animals, such as sea stars, can reproduce by fragmentation.

Parthenogenesis: a form of asexual reproduction in which an unfertilized egg develops into an

adult. In honeybees, for example, the queen bee lays both fertilized and unfertilized eggs. The

fertilized eggs develop into female worker bees, while the unfertilized eggs develop into male

drones. The whiptail lizard is another animal that reproduces by parthenogenesis.

Spores: Several different species have evolved a mechanism to reproduce asexually and

disperse their offspring long distances. A spore is a structure that contains genetic material and

cytoplasm surrounded by a protective sheath or wall. The wall protects the contents until

conditions are favourable, at which point the spore wall opens and the organism begins to

develop. Because spores tend to be very small, they are readily dispersed in water and by the

wind. (Bread mold, mushrooms, etc.)

Spores may be haploid or diploid, and not all spores are the product of asexual reproduction.

Some organisms produce spores by meiosis, resulting in an alternation of generations.

The life cycle of these plants consists of two generations: a haploid generation and a diploid

generation that alternate.

The diploid generation of a plant is called the sporophyte (spore-making body). Through the

process of meiosis, the sporophyte produces one or more haploid spores. These spores develop

without fertilization. Each haploid spore grows into a plant body called the gametophyte

(gamete-making body). Gametophytes produce male and female gametes, which fuse at

fertilization and develop into another sporophyte. The cycle then repeats.

Reproduction in Mosses – Alternation of Generations

1. Within the antheridium of a male moss, sperm (n) are created.

2. Sperm swim from the antheridium to the archegonia of a female moss containing an egg(n).

3. The egg is fertilized. A zygote (2n) forms.

4. The generation now becomes the sporophyte generation (2n).

5. On top of a fertilized female plant grows a long stalk called a sporangiophore.

6. At the top of the sporangiophore grows a capsule called a sporangium.

7. Inside the sporangium are contained spores. The spores undergo meiosis to become haploid

(n) spores.

8. When conditions are right, the sporangium breaks open and releases the spores. The

gametophyte generation begins again.

9. The spores are carried back to the ground by wind, water where they become new moss

plants.

Note: Some species have a dominant gametophyte generation and some have a dominant

sporophyte generation.

Life cycle of a typical Cnidarian

Polyps reproduce asexually by

budding, forming colonies. They

may also reproduce sexually by

the formation of specialized

buds that give rise to medusae,

in which gametes are produced.

These gametes fuse, producing

zygotes that develop into

planulae, which, in turn, settle