Life Science: Session 3
Sex Cell Production
What are sex cells?
Sex cells, or gametes, are unique to organisms that reproduce sexually. In animals and plants (fungi are somewhat different in this regard) there are two types of sex cells: male and female. The male sex cells are sperm, while the female sex cells are eggs. Sex cells are formed from special body cells that are typically located in sex organs. In most animals, sperm are formed in the testes of males, and eggs are formed in the ovaries of females.
Sex cells contain only half of the hereditary material present in the body cells that form them. This is important because male and female sex cells ultimately join to become a fertilized egg, which gives rise to a new organism, or offspring. In order for the offspring to resemble its parents, its first cell must receive the entire genome from its two parents. For humans, we know there are 46 chromosomes in body cells existing as 23 pairs. A fertilized egg must therefore contain this same number and arrangement. In an elegant process called meiosis, each sex cell receives one member of each chromosome pair—23 total. When sperm fertilizes egg, these singles unite to reform pairs, with half the genome coming from each parent. With a few exceptions, this pattern holds true for all sexually reproducing organisms.
How are sex cells produced?
Sex cells are produced from special body cells that contain the entire genome. The process by which the genome is halved is very precise — it’s not just a matter of randomly dividing the chromosomes into two sets. The process involves two cell divisions. Before the first occurs, all of the chromosomes are duplicated just as they are in body cell reproduction, but what happens next is different: the two duplicated strands remain attached to each other as the members of each chromosome pair move alongside each other. During the cell division that follows, only one member of each pair is transferred to each daughter cell—this is where the number of chromosomes is halved. The two strands of each chromosome are then separated during the second cell division, still maintaining half the number that existed in the parent cell. This results in four daughter cells — sperm or egg — that contain one member of each chromosome pair. This process is called meiosis.
What is the role of sex cell production in an animal life cycle?
Sex cell production ensures that the genome is maintained between parent and offspring generations. Occasionally, this process goes awry with chromosome pairs not lining up or not separating. The consequences are almost always harmful, and frequently lethal to potential offspring. A successful animal life cycle therefore depends on successful sex cell production.
There is another consequence to sex cell production that has a profound impact on the populations involved. Unlike body cell production, where the daughter cells are identical to parent cells, fertilized eggs result from genetic material from two different parents. Furthermore, each of these parents is only able to pass on half of its genome. The mixing and matching of half sets of chromosomes results in the astounding diversity we see in the living world. For example, we can see “parts” of both our parents when we look in the mirror. Similarly, a litter of puppies will reflect the size and coloration of both parents. The significance of this is explored in Session Five: Variation, Adaptation, and Natural Selection.
Compare body cell reproduction with sex cell production:
|Body cell reproduction||Sex cell production|
|Role in life cycle||Growth and maintenance||Reproduction|
|Where process occurs||Cells in all parts of body||Sex organs or tissues|
|Number of cell divisions||One||Two|
|What happens to chromosomes||All chromosomes line up singly, each chromosome duplicates, the two copies separate, and one copy of each chromosome is distributed to each daughter cell.||First division: chromosomes duplicate and copies remain attached, chromosome pairs line up alongside each other, the members of each pair separate, one member of each pair goes to each daughter cell. Second division: all chromosomes line up singly, the two copies separate, one copy of each chromosome is distributed to each daughter cell.|
|Number of cells that result||Two||Four|
|Number of chromosomes in resulting cells||Same number as in parent cell||Half the number as in parent cell|
|Significance||Genome is maintained; all information is passed along||Genome is halved; will be restored at fertilization|
|prev: body cell reproduction||next: cloning|