Science 9

Life Science: Reproduction

A Common Thread Are the following situations permanent? What eventually happens?

a) Your hairdresser misunderstands and you leave the shop with a style that is much too short.

b) You stub your toe hard and soon have a black toe nail that falls off.

c) Reaching into the package of looseleaf, you get a paper cut.

d) Sewing on a loose button, you stick yourself with the needle - a big drop of blood on your finger.

e) Water on the floor, you slip and get a large bruise on your hip.

f) Another season and your favorite autographed team jersey is still too large.

g) Your grandfather worries the types of plants so important in his youth favorite are disappearing.

h) Dirt gets into a cut and it soon feels hot and looks red.

Did you know each of these situations is affected by the same process called cell division? It is how one cell can become millions. But, why is cell division the common thread among all these situations?

Cell Theory Today, most of us accept the idea our body contains cells and they are important for us to live. But, even just a hundred years ago, many people did not know about cells nor their huge importance in most aspects of daily life. However, after a sequence of investigations, the idea gained credibility and the Cell Theory was developed. It states that: all living organisms contain at least one cell; cells are the basic units of structure and function in organisms; all cells come from previously existing cells; the activity of the whole organism depends on the total activity of its independent cells.

Cell Structure Before we begin to look at cell division, we must renew our understanding of what cells are. A cell is a miniature factory: it takes in raw materials from which it creates products used by itself and other cells. Waste materials are created during the manufacturing process and these must be eliminated. And all the while, it uses energy.

The two basic cell types are plant cells and animal cells and, there are certain similarities between them. Animal cells tend to be spherical while plant cells are boxy and their contents are called organelles.

Note: many students, even college students, do not realize just how extremely small cells are. We look at a microscope slide and see a smear of some colored suff on it. We put it under the microscope, focus and suddenly see some amazing things but often fail to realize these are enormously magnified images. If we could really grasp just how many cells we contain (hundreds of trillions!!) and how wonderfully tiny and complex they are, we would be continually amazed when we look at ourselves.

Animal Cell Organelles

cell membrane: a flexible, porous, double layered covering around the cell

mitochondrion: the power supply for the cell

nucleus: a sphere inside the cell; it is enclosed by a double layered porous nuclear membrane

nucleolus: an area inside the nucleus that makes ribosomes

chromatin: long strands of DNA, the cell's genetic material, that control cell behavior; each strand contains part of the DNA and so controls part of the cell's behavior

ribosome: makes materials needed by the cell to keep it functioning

endoplasmic reticulum: a maze of canals that moves materials to various areas in the cell

Golgi body: creates packages containing materials sent out of the cell to other parts of the organism

vacuole: a tiny blob storing useful materials or wastes

cytoplasm: "jelly" in which all the organelles float

lysosome: contains chemicals that break apart materials

Plant Cell Organelles

A plant cell contain most of the same organelles as an animal cell because the two types of cells do most of the same things. There are a few differences.

cell wall: a rigid wall outside the cell membrane; it gives plant the strength to stand up

chloroplast: contains chemicals that allow the plant cell to use sunlight to make its own food

vacuole: huge because plant cells store much more material than animal cells

Who's In Charge? In any complex situation, e.g., cooking for a large party, building a car, there must be an organized sequence in which things are done so the final product is created in an efficient and economical way. Your cells are the world's tiniest and most efficient factories. What controls their processes? Experiments have revealed that the nucleus is the "boss" because it contains the chromatin strands, also called chromosomes, with instructions that make the cell's organelles behave as they should. This chapter looks at how the nucleus and cell behave during cell division.

The Cell Cycle With a sharp knife we can cut a strand of spaghetti into smaller and smaller pieces. Use a magnifying glass and we can get even tinier pieces. The amazing thing about nature is that if the original piece of spaghetti were a cell, each time it divided, the halves would grow back to full size! Cell division takes place through a process called mitosis. It has four steps or phases called prophase, metaphase, anaphase and telophase (PMAT.) But, even before cell division begins, the nucleus makes copies of its chromatin strands so it contains two identical sets of genetic instructions. A chromatin strand looks like a short length of fuzzy dark yarn. Imagine two identical pieces joined together in the middle by a centromere to form an "X"which is called a double stranded chromosome. Every type of organism has a specific number of chromosomes; we have 46. The steps of cell division ensure that, as the parent cell splits into two "daughter" cells, each one receives a full set of genetic instructions so it will behave like the parent cell.

Prophase: the chromatin strand pairs (chromosomes) enlarge, become darker and more visible; the nuclear membrane disappears.

Metaphase: the double stranded chromosomes line up along the cell's equator.

Anaphase: small ropes (spindle fibres) grow out from centrioles at the cell's poles and grab onto the centromeres. The spindle fibres contract, pulling apart each double stranded chromosome. Now, each side of the parent cell has a complete set of single stranded chromosomes, a complete set of genetic information.

Telophase: the parent cell pinches in half, forming the daughter cells, each with its complete set of DNA information. At the end of telophase, the single stranded chromosomes become less visible chromatin strands again. Nuclear membranes form around each set of chromatin strands - each daughter cell has its own nucleus. As the two daughter cells grow to full size (still super microscopic,) they begin to behave as factories. In plant cells, a plate forms between the daughter cells; it develops into a rigid cell wall. Recall cell walls are an important plant cell organelles.

The time spent by the cell during division is tiny compared with that spent behaving as a factory. This active period of protein production is called Interphase.

I Am Cell Division! Cell division was responsible for your development from conception until birth.

Now, as you grow, it increases the number of cells in your body and replaces dead cells lost during your body's normal functioning. Cells have a wide variety of lifespans: brain cells are designed to last up to fifty years (take care of them!!) while intestine lining cells last just three days.

Regeneration is the process of repairing injured tissues or, in the more extreme situations, the replacement of missing body parts. We can not do this because our limbs are too complex but some lizards can regrow a chewed off tail.

The aging process occurs because the rate of cell replacement falls behind the rate of cell loss - tissues begin to cease functioning. Research suggests growth hormones may stimulate the rate of cell division and so help in slowing or perhaps reversing some symptoms of aging.

One of modern society's detested diseases is cancer. It occurs when a group of cells begins to divide in an out of control fashion - the cells take over and disrupt the structure and behavior of the surrounding tissue. The cancerous cells may move to other parts of the body and form internal or external lumps called tumors. Fortunately, treatments are becoming more effective but prevention through proper diet and environment is the key to diminishing the impact of this disease.

Asexual Reproduction Asexual reproduction is the creation of offspring with genetic information identical to that of the parent. Two parents are not required and the offspring carries the DNA from just the one organism. Many simple organisms increase their numbers through asexual reproduction. There are five common types of asexual reproduction.

Binary fission: bacteria multiply asexually by binary fission. First, as the bacterium copies its DNA ring, the cell begins to elongate. Then, when the cell is long enough, one DNA ring goes into each end and the cell pinches in half: two new bacteria with the same DNA.

Mitosis: the amoeba, a common type of Protist, uses regular mitotic cell division to multiply.

Fragmentation: a small piece of a fungi can break away from the main mass and grow into a new individual.

Budding: Did you know the yeasts used to make beer and bread are living single celled organisms? A yeast cell copies its DNA and then forms a tiny bump that contains the DNA copy. The bump enlarges until it breaks away and becomes a second yeast cell. Even some multi-celled organisms may use budding to increase their population.

Spores: moulds appear as coatings on surfaces, e.g., bread, oranges, and may send up tiny stalks supporting hollow spore cases containing millions of tiny spores. When conditions are right, the spore cases rupture and release the spores. If a spore lands on a suitable surface, it develops into another mould colony.

What about asexual reproduction in plants? Gardeners know many plants can be grown asexually, i.e., without using seeds. In most cases, a piece (a cutting) of the desirable plant is placed in a suitable environment, roots soon develop and the young plant is ready for the garden or lawn. Sections of stems from one plant can be tied into cuts in the stems of another suitable plant, the cuts heal and the grafted stem is then part of the larger plant. Some farmers will grow three or four types of apples on the same trunk. Strawberry plants can spread by layering - bury a section of a runner (thin, flexible stem) and it develops into another plant. Want to get rid of dandelions? Don't try to dig them up because you just break apart the root system and each fragment becomes another plant. By the way, dandelions on your lawn are not a pest: they are telling you to add calcium to the soil. Certain plants, e.g., pines, orchids, can be multiplied through tissue culture in which bits of tissue taken from a plant are grown in special solutions, often in test tubes, until they reach transplant size.

Sexual Reproduction Sexual reproduction is the creation of offspring containing a combination of

DNA from two parents. This allows for variations and similarities in the offspring. Look at the members of a family - the children show some features of the parents and the other children but each child also has some distinctive features. Each child has similar but not exactly identical DNA - their DNA is a mixture of the parents' DNA.

The first step in sexual reproduction is the creation of sex cells (gametes) in the gonads (reproductive structures) of the parents. Males create sperm in their reproductive structures called testes. Females create eggs in their ovaries. During fertilization, a gamete is supplied by each parent and these combine to create a zygote. The zygote then undergoes thousands and thousands of cell divisions as it grows and develops into the offspring. Each gamete has just a half set (haploid) of chromosomes so that when the two gametes combine during fertilization, the zygote will contain a full set (diploid) of DNA, a combination of DNA from each parent..

The gametes are created in an eight step process called meiosis. The first four steps, called

Meiosis I, create two daughter cells each with a different half set of double stranded chromosomes. The second four steps, called Meiosis II, look like mitosis and divide each daughter cell into two sex cells. So, we get four sex cells, but now each one has a different half set of single stranded chromosomes. The key to the whole process is how the chromosomes line up and interact along the equator each time a cell divides. If an organism requires many sex cells (gametes,) many meiosis processes will occur.

Mating Patterns If a species is to thrive, a certain number of the offspring must survive and reproduce. Nature has determined the best time and conditions for mating to give the offspring the best chance for survival. Mating may be influenced by such things as season, time of month, precipitation, or tides. Some creatures mate monthly, some once a year, others once in a lifetime.

Animal Fertilization Patterns Moisture is an important requirement for fertilization so that the sperm can move to the egg which must have a flexible coat. Many water-dwelling creatures, e.g., corals, just release eggs and sperm into the water (external fertilization) and chance determine how many gametes meet and fertilize. Female fish may deposit eggs in a cluster and the male releases sperm into the water nearby; this increases the chance of fertilization. When the female frog releases her eggs, the male releases his sperm, again increasing the chance of fertilization.

Land dwellers tend to use internal fertilization in which the male uses a specialized structure to deposit sperm in the female where the eggs are located. Once fertilization has occurred, the embryo may develop inside the mother (mammals,) develop in an egg released outside the mother (reptiles, birds,) or develop inside an abdominal pouch (marsupials.)

Plant Fertilization Patterns Angiosperms are flowering plants. The flower contains the male stamens that make pollen grains and the female pistil containing the ovules. Some mechanism, e.g, wind, insects, carries the pollen grains from the stamens of one blossom to the pistil of another (cross-pollination.) Usually, a flower cannot pollinate itself (self-pollination); cross-pollination increases the variation among the offspring and this increases the chances for the plant's survival.

Once the pollen grains reach the sticky end (stigma) of the pistil, they send tubes into the ovary so the sperm can unite with the ovules there. The fertilized ovules becomes seeds. As they mature, the flower changes into a shell or pod, e.g., an apple.

The plants spread when the seed pods are carried away by wind, rain, by sticking to the coat of passing animals, or by animals eating the pods and defecating elsewhere.

Gymnosperms are cone bearing plants, e.g., pines, spruce. Male cones create and release winged pollen grains from between the scales. They are carried on the wind and, by chance, reach the female cones containing ovules between their scales. Inside the ovules are eggs. Fertilization produces winged seeds that are released from the female cone.

What about plants that make no seeds? Ferns and mosses form attractive layers or coverings on rocks and logs in damp, low light forest environments. They develop spore containers that, when conditions are right, break open to release the spores. The spores germinate into tiny, male and female tree like structures that create sperm and eggs. Wind or water moves the sperm to the eggs so fertilization can occur and a new moss or fern plant grows up. Spores are lightweight, tough and can become dormant to survive periods of unfavorable temperatures or drought. When appropriate conditions return, the spores can produce healthy plants.

Variation Why is it so important to have some genetic variety? Survival. In any situation, certain characteristics increase an organisms chance to live and reproduce. And, if conditions turn bad, the stresses on a population worsen and certain members will die, perhaps many. But, by chance, some will have a genetic makeup that gives them a better chance of surviving until favorable conditions return and the population can increase again. In a protected environment, we may never see why a particular skill or color or size is an advantage. But, would we, for example, want to live like cave people to find out what characteristics would lead to our death? Animals and plants don't have this choice but we do pamper them and so may not recognize merely cute characteristics from crucial ones.