The information on this site should not be used as a substitute for professional medical care or advice. Contact a health care provider if you have questions about your health. How do genes control the growth and division of cells? From Genetics Home Reference. Topics in the How Genes Work chapter What are proteins and what do they do? How do genes direct the production of proteins? Can genes be turned on and off in cells?
The endurance test that they must go through in the female reproductive tract is nature's way of eliminating these poorer specimens. Among humans, fertilization usually occurs within a day after ovulation. Conception must occur early in this process.
This means that sexual intercourse must occur from a few days before to one day after ovulation if conception is desired. In most non-human mammals, birds, reptiles, fish, and insects, fertilization is made more likely by the fact that females are sexually receptive only around the time of ovulation. This period of female sexual receptivity is called estrus. In most species, it is common for all females to have their ovulations around the same time of year.
This reproductive synchronization results in a common mating season. In humans and some other primates, sexuality is far less related to the timing of ovulation. Among these species, there is no mating season. More precisely, the entire year is a mating season since they have a more or less chronic interest in sex. This is another way in which nature has chosen to increase the likelihood of conception. H alf of the sperm cells produced normally carry the X-chromosome and half have the Y-chromosome.
The ratio of male to female newborns in the U. The ratio of males to females at conception is generally even higher than it is at birth. This is fortunate in the long run for society because male spontaneous abortions and infant mortality rates are higher.
In addition, males are more likely to die from accidents and combat as teenagers and young men. Males in developed nations who survive to adulthood also can expect to die at a younger age than women.
Since , China has had a national policy of allowing parents to have only one child in order to reduce population growth. A traditional preference for male children and the ready availability of ultrasound technology has led to large numbers of abortions of female fetuses.
This occurs despite the fact that abortions for the purpose of preventing the birth of female children is illegal. There are now boys born for every girls in China, and in some regions the ratio is as high as to A consequence of this practice is the growing scarcity of marriageable women. As of the census, there were 32 million more young men than women in China. While India does not have a one child policy, it is facing a similar problem because of the selective abortion of female fetuses, especially in the more prosperous states of North India.
This is driven by the economic difficulty of raising large families and the preference for male children. The huge excess of marriageable men in China and India could be socially and politically explosive in the near future. In societies that encourage a form of marriage in which one man has more than one wife at the same time polygyny , higher numbers of female children are usually born than in the predominantly monogamous nations. Why this reverse birth ratio pattern occurs is not entirely understood.
However, it is very likely connected with the fact that each wife has less frequent sexual intercourse. Girls are more likely to be conceived when conception is close to the time of ovulation.
When there is intercourse at other times as well, the sperm cells are more likely to be waiting for ovulation at the upper end of the fallopian tubes. They do not have as far to go to reach the egg. Multiple births at the same time are rare for humans and most other primate species.
Having fraternal, but not identical, twins apparently runs in family lines, and is also somewhat more likely for women over age Fraternal twins may look similar but are not genetically identical. In fact they are no more identical than any brother or sister. They share their mother's uterus during gestation but come from two different eggs fertilized with different sperm.
Subsequently, they are called dizygotic twins. In contrast, identical twins are mostly identical genetically because they result from one zygote splitting into two or more separate ones within a few days after conception.
As a consequence, they are called monozygotic twins. If the division of the original zygote does not occur until the 9th to the 12th day after conception, the monozygotic twins are likely to be mirror twins. That is, they will have small mirror image differences internally and externally. For instance, one may be left handed and the other right handed. Likewise, the cowlick in their hair at the back of the head will be on opposite sides.
If the division of the zygote occurs after day 13, the monozygotic twins are likely to be born conjoined. A ny differences between monozygotic twins later in life are mostly the result of environmental influences rather than genetic inheritance. However, monozygotic twins may not share all of the same sequences of mitochondrial DNA.
This is due to the fact that the mitochondria in a cell may have somewhat different versions of DNA, and the mitochondria can be dispersed unequally when a zygote fissions. Female monozygotic twins can also differ because of differences between them in X-chromosome inactivation. Subsequently, one female twin can have an X-linked condition such as muscular dystrophy and the other twin can be free of it. There has been at least one recorded instance of twins who are identical on their mother's side but share only half of their father's genes.
These "semi-identical" twins result from two sperm cells fertilizing the same egg. Mutations in specific genes can alter the behavior of cells in a manner that leads to increased tumor growth or development. Providing reliable information about cancer biology and treatment. Cell Division During a lifetime, many of the cells that make up the body age and die. Reasons that cells are lost and must be replaced include the following: Sloughing off of epithelial cells such as those lining the skin and intestines.
The old, worn out cells on the surface of the tissues are constantly replaced. A special case of this is the monthly replacement of the cells lining the uterus in pre-menopausal women. Wound healing requires that cells in the area of the damage multiply to replace those lost.
Viral diseases such as hepatitis may also cause damage to organs that then need to replace lost cells. Replacement of the cells that make up blood. Red blood cells carry oxygen to tissues. White blood cells such as B and T lymphocytes are part of the body's immune system and help to ward off infections. Most of these cells have very short lifespans and must be constantly replaced.
The precursors of these cells are located in bone marrow. These precursors, or stem cells, must reproduce at a very high rate to maintain adequate amounts of the blood cells. In order for this to happen, the following must occur: The genetic material, the DNA in chromosomes, must be faithfully copied.
This occurs via a process known as replication. The organelles, such as mitochondria , must be distributed so that each daughter cell receives an adequate amount to function. The cytoplasm of the cell must be physically separated into two different cells. Sections included on this page: Normal Cell Division Cancer Cell Division Cell Division Summary Normal Cell Division There are several safeguards built into the cell division process to assure that cells do not divide unless they have completed the replication process correctly and that the environmental conditions in which the cells exist are favorable for cell division.
Among others, there are systems to determine the following: Is the DNA fully replicated? Is the DNA damaged? Are there enough nutrients to support cell growth?
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