General ---------Biology----------------------------------------------------- ------------------Online--------------------------------------- Ch. 13 Meiosis and Sexual Life Cycles

Organisms reproduce in two ways: asexually and sexually. 

A citrus nursery selects the tree that produces the best tasting orange and grows new plants from cuttings.  They cut a small branch off the tree, put it in a pot and keep it well watered and out of direct sunlight until it has rooted.  The new plant is, obviously, genetically identical to the parent.

Marine sponges and some worms can be cut in two, and each half will regrow into a new individual, genetically identical to the "parent.".

A single cell bacterium undergoes mitosis, and two genetically identical individuals result.

Asexual reproduction produces clones, individuals that are genetically identical to the parent;  asexual reproduction does not produce offspring with new genetic variations.

Sexual reproduction involves the production of gametes (sperm cells and egg cells) followed by fertilization.  In the production of gametes, the chromosomes of each parent are shuffled and sorted into gametes.  This shuffling and sorting process produces gametes with tremendous genetic variation.  In humans with 23 pair of chromosomes, there are over 8 million different genetic possibilities, i.e. 2²³,each time a gamete is produced.  The shuffled chromosomes of the male parent are combined with those of the female parent when the sperm penetrates the egg and discharges the male chromosomes into the egg cell.  The fertilized egg is called a zygote, and there are 64 trillion different genetic possibilities each time fertilization produces a zygote, i.e. 8 million X 8 million. 

What advantage does all this genetic variation bring to a population? (Hint:  evolution and natural selection) ____________________________________________________.

Meiosis is the cell division process that produces gametes. It is this process that "shuffles" the chromosomes of the parent cell and sorts them into gametes.  Meiosis occurs only in tissues that give rise to gametes, e.g. ovaries and testes.  This specialized tissue is called germ tissue.  In animals, gametes are called sperm cells and egg cells. 

Gametes contain half the DNA of the parent cell.  More specifically, a gamete contains only one chromosome from each chromosome pair.  Upon fertilization, the pairs are united and the resulting cell has a mix of genes from the male parent and the female parent. 

The fertilized egg, called a zygote, has two genes for every inherited characteristic.  For example, every person carries two genes for eye color, one received from their father and one from their mother.   A person might inherit a blue eyed gene from their father and a brown eyed gene from their mother. Since brown eye pigment is dominant over blue, that person's eyes would be brown, but they would be a "carrier" of the blue eyed gene.

Chapters 14, 15, 16 and 17 are all about the cellular mechanisms, such as meiosis, that control  inheritance.  You will learn how certain diseases such as sickle cell anemia, Down syndrome, hemophilia and Huntington's disease are inherited and how to calculate the probability of their occurrence in certain offspring. 

Cancer has genetic causes, some of which are inherited.  Understanding how these diseases can be treated with new advances in biotechnology, requires an understanding of cellular mechanisms such as meiosis.

Each of us is the result of  reproduction involving both meiosis and mitosis.  The egg was fertilized in our mother's womb (meiosis produced the gametes) and the single celled zygote grew into the baby and then the adult (mitosis, mitosis, mitosis...).