
	Reproduction

ï¿½ Asexual Reproduction ï¿½ another term used by biologists is cloning. Asexual reproduction is when there is a production of new organisms without the fusion of nuclei (only one cell and one nucleus are involved). There is a production of genetically identical offspring from a single parent cell. The process of mitosis is associated with asexual reproduction and the growth and repair of cells in sexually reproducing organisms.

Mitosis ï¿½ the purpose of mitosis is growth, repair and reproduction, depending on the given organism. There is also an orderly series of changes that yields two identical sets of chromosomes.
Nuclear and cytoplasmic division ï¿½ all cells arise from other cells by cell division. Cell division is complex changes in the nucleus that yield identical sets of chromosomes. When mitosis occurs the nucleus of a cell divides into 2.
Cell Cycle ï¿½ interphase (living without cell division) and then mitosis occur. Interphase is when the cell appears to be resting but is really biochemically very active.
Cell division ï¿½ when the cells divide, the chromosomes shorten and thicken and become visible under the microscope. A single stranded chromosome is one chromosome connected at the middle by a centromere and is called a chromatid. A double stranded chromosome is two chromosomes (have the appearance of a scissor) and is also known as a chromatid pair. Chromosomes replicate during interphase and are double stranded at the beginning of mitosis.
Stages of Mitosis:

a. Prophase ï¿½ nuclear membrane disintegrates, centrioles migrate to opposite poles, chromosomes shorten and thicken and appear as double stranded, spindles form (fiber that are attached to chromosomes).

b. Metaphase ï¿½ spindle is formed, centrioles at opposite poles, chromosomes are lines up at the equilateral plate (middle of the cell where spindles attach to each chromosome at the centromere)

c. Anaphase ï¿½ centromeres divide, chromosomes begin to be pulled to opposite poles

d. Telaphase ï¿½ chromosomes located in two cells, spindles and nuclear membranes reappears.

End Result: cells identical to each other and the original cell but they have single stranded chromosomes.
Plants vs. Animals ï¿½ plants have cell walls, which donï¿½t allow the cell to pinch into two cells. Therefore the cell plate (wall) grows between the cells to separate them. Also plant cells donï¿½t have centrioles but the spindles still form.
Cancer ï¿½ abnormal cell division that occurs when mitosis is out of control and abnormal cells are formed.
Types of Asexual reproduction are binary fission, sporulation (fungi reproduction), regeneration, vegetative propagation.

Back to Top

ï¿½ What is sexual reproduction? ï¿½ a type of reproduction in which new organisms are formed as a result of the fusion of gametes from two parent organisms. In sexual reproduction there are 2 organisms that unite, the two cells are called sperm and egg. They are found in the Male and female reproductive organs, the ovaries and testis.

1. Sperm and egg unite in fertilization. The cell that results is called a zygote. The zygote will then divide because of mitosis to form an embryo.

2. Zygote ï¿½ the single diploid cell that results from the fusion of gametes in sexual reproduction, a fertilized egg. The zygote contains all the information necessary for growth, development, and eventual reproduction of the organism.

3. The sperm and egg unite in a process called fertilization. This process forms a single celled structure called a zygote which contains the complete genetic information to develop into a complete new organism having characteristics of both parents. The zygote will then divide by mitosis and form the specialized cells, tissues, and organs of the organism. This development of specialized structures from the zygote is called differentiation.

4. Cells have nuclei which contain chromosomes. The chromosomes number is unique to each species (pea plants have 14, humans have 46). The chromosome number is constant from generation to generation.

5. Problem: If the sperm cell has 46 and if the egg cell has 46 then the baby would have 92 chromosomes in each cell. How does the body stop this from occurring?

6. There must be a reduction in the chromosome number in sperm and egg cells in order to maintain the constant chromosome number = 46 (to prevent the doubling of chromosomes in each generation). The process by which this occurs is a reduction division called meiosis.

7. Body cells are called somatic cells which contain 46 chromosomes. Sex cells are called gametes contain 23 chromosomes.

8. Sex chromosomes are pairs of homologous chromosomes carrying genes that determine the sex of an individual. These chromosomes are designed as X and Y chromosomes. Sex determination is a pattern of inheritance in which the conditions of maleness and femaleness are determined by the inheritance of a pair of sex chromosomes (XX = female, XY = male).

9. The chromosome number in each somatic cell is called a diploid. ï¿½ 2n ï¿½ in humans 46 chromosomes.

10. Diploid chromosome number ï¿½ represents the number of chromosomes found characteristically in the cells (except gametes) of sexually reproducing species.

11. Each chromosome has a homologous one that carries information for the same traits. Humans have 23 homologous pair of chromosomes. Humans have 23 pairs of chromosomes. 22 pairs ï¿½ autosomes, 1 pair ï¿½ sex chromosomes

12. Sex linkage ï¿½ a pattern of inheritance in which certain non-sex genes are located on the X sex chromosome, but have no corresponding alleles on the Y sex chromosome.

13. sex linked trait ï¿½ a genetic trait whose inheritance is controlled by the genetic patter of sex linkage (ex: color blindness)

14. Meiosis ï¿½ gametogenesis ï¿½ the creation of gametes. Gametes are formed in the sex organs are also called gonads (♂testes, ♀ ovaries)

15. Sperm + Egg ï¿½(fertilization)ï¿½ zygote

(Human) 23 + 23 ï¿½ 46

(Pea plant) 7 + 7 ï¿½ 14

monoploid + monoploid ï¿½ diploid

ï¿½ï¿½ï¿½ï¿½ï¿½ n + n ï¿½ 2n

ï¿½ How is the chromosome number maintained in sexual reproduction?

1. Meiosis ï¿½ The process of meiosis produces gametes or sex cells. While some parts of this cell division process are similar to the asexual cell division process of mitosis, there are several key differences. Meiosis produces gametes, while mitosis produces other cell types. The process of meiosis halves the chromosome number from the original parent cell in the four cells it forms. It does this by having two cell divisions forming four cells, where mitosis has only one cell division forming two cells. Both processes start out with one doubling or replication of the chromosome material. The graphic below will help to visually illustrate some of the key events of meiosis. Another important way that meiosis differs from mitosis is the exchange of chromosome pieces which occurs in the first division of this process. This exchange of chromosome pieces is called crossing over. Crossing over assures that the cells produced as a result of meiosis will be different from and exhibit variations from the parent cell that produced them. This process is chiefly responsible for the variations seen in members of the same species of sexually reproducing organisms. The process of meiosis is a two step process.

ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½

Meiosis I ï¿½ reduction division
Meiosis II ï¿½ similar to mitosis (involves 2 cell divisions)

Steps of Meiosis

Interphase ï¿½ replication of chromosomes (when the chromosome first appears in Prophase II, they are double stranded)
Prophase I ï¿½ pairing homologous chromosomes into tetrads (four chromatids on each side) in a process called synapsis ï¿½ crossing over occurs here (crossing genes ï¿½ brown hair, blue eyes)
Anaphase I ï¿½ disjunction of chromosome (when the centromeres divide and chromosomes begin to move to opposite poles)

Results of Meiosis

A 2n primary sex cell will divide into four specialized ï¿½nï¿½ cells (for reproduction)
The distribution of homologous chromosomes between nuclei is random.

Interphaseï¿½ ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ Prophase Iï¿½ ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ Metaphase Iï¿½ ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ Anaphaseï¿½ ï¿½ï¿½ï¿½ï¿½ï¿½ Telaphase ï¿½ (continues to next cell reproduction)

Result ï¿½ ï¿½ the number of chromosomes, but theyï¿½re double stranded

Prophase IIï¿½ ï¿½ Metaphase IIï¿½ ï¿½ï¿½Anaphaseï¿½ ï¿½ï¿½ Telaphase II ï¿½ sex cells

Result ï¿½ 4 cells ï¿½ each cell is single stranded and homologous chromosomes distributed.

Final result of meiosis ï¿½ The process of crossing over and how it produces variation when these chromosomes are recombined in the process of fertilization.

ï¿½ Comparison of Mitosis and Meiosis

	Mitosis	Meiosis
Purpose:	In asexual reproduction ï¿½ growth	Reproduction in chromosome number for sexual reproduction
Outcome:	Daughter cells ï¿½ identical to original cells	Daughter cells have ï¿½ the number of chromosomes
Overall:	One 2n cell yield two 2n cells	One 2n cell yield four n cells
Results:	Clones	Variations

ï¿½ What is Gametogenesis? ï¿½ the process of cell division by which gametes are produced.

spermatogenesis ï¿½ occurs in the male gonad (testes). A type of meiotic cell division in which four sperm cells are produced for each primary sex cell. The primary sex cell is spermocyte. During the division, they lose baggage and gain a tail because they need to swim to the egg.
Oogenesis ï¿½ a type of meiotic division in which one ovum and three polar bodies are produced from each primary sex cell. Occurs in the female gonad (ovary). The primary sex cell is the oocyte. The result is one viable egg cell and 3 non-functioning polar bodies. There is an unequal cell division ï¿½ ovum has a large cytoplasm because it contains yolk (nutrients for embryo).

ï¿½ Fertilization ï¿½ Is located in vertebrates. Depending on the vertebrate, the fertilization occurs inside or outside the female body.

External Fertilization ï¿½ fertilization that occurs outside the female body. There is external fertilization in fish and amphibians. Gametes fuse outside of the female body from the large number of eggs that are required to be released to ensure the survival of the species.
Internal Fertilization ï¿½ fertilization that occurs inside the female body and in terrestrial vertebrates. Because of that there is no need for hundreds and thousands of eggs to be produced because the fertilization is occurring inside the body, which is a safe environment. The gametes fuse in the moist reproductive tract of the female.

ï¿½ Embryonic Development ï¿½ cells divide by mitosis

zygote develops into the embryo (process of mitosis). This is called cleavage and occurs through the process of mitosis.
Gasturation ï¿½ at this point differentiation ï¿½ cells specialized into 3 embryonic layers (ectoderm, mesoderm, endoderm) ï¿½ occurs.
The embryo will eventually develop into a three cell layered structure. This structure is called a gastrula and will eventually differentiate to form the specialized cells. Differentiation means that the cells will develop specific jobs and develop into specific tissues in the maturing organism. An example of this is that the outer cell layer of the developing gastrula will develop into the skin and nervous system of a mature human organism. Most multicellular animals undergo a similar pattern of development and differentiation. Stem cells are cells that are not yet differentiated and havenï¿½t become a specific type ï¿½ undefined beginning of cleavage.

ï¿½ Where does the embryo develop?

External Development ï¿½ the development of the embryo that occurs outside of the female body.

a. Fish and amphibians ï¿½ fertilization and development occur in the aquatic environment. Food for the embryo is found in the yolk of the egg.

b. Birds and reptiles ï¿½ (on land) the external development occurs after the internal fertilization. Yolk is the food source. The adaptations are the shell that is used as a protection and the membranes provide a favorable environment.

i. The yolk sac ï¿½ its purpose is to feed the embryo

ii. Amnion ï¿½ contains watery fluid that is a cushioning shock observer for the embryo

iii. Shell ï¿½ the outermost protection that separates the embryo and the outside world

iv. Chorion ï¿½ outer most membrane

v. Allantois ï¿½ respiration membrane, wastes are deposited here

Internal Development ï¿½ in placental mammals ï¿½ humans ï¿½ growth of the embryo within the body of the parent.

embryo develops in a structure called the uterus (womb). Eggs of mammals have relatively little yolk and they are relatively small.
The placenta is a combination of maternal and fetal tissue which allows for the exchange of materials with the fetus and mother. Needed materials such as food and oxygen diffuse through the placenta to the fetus, while wastes from the fetus diffuse to the mother (exchange of gasses and nutrients). It is formed from embryonic and maternal tissue. There is no direct connection of blood streams.
The umbilical cord is a fetal structure containing blood vessels which allows materials to be carried between the fetus and placenta in both directions.
The amniotic fluid surrounds the fetus and helps to provide a shock absorber to protect the fetus against mechanical injury in the event the mother is shaken or injured in some manner.

Development is a highly regulated process. After this small cluster of cells called the gastrula forms in humans, tissues begin to form. In humans, the embryonic development of essential organs occurs in early stages of pregnancy. During the first three months of human development, organs begin to form. The human embryo is usually referred to as a fetus when human like features become visible in its structure. All organs and body features are developed by the end of the sixth month. During the last three months of pregnancy, organs and features develop well enough to function after birth.

Contents:

Back to Top

Back to Top