Mitosis | Learn Science at Scitable Mitosis Mitosis is the process in which a eukaryotic cell nucleus splits in two, followed by division of the parent cell into two daughter cells. The word "mitosis" means "threads," and it refers to the threadlike appearance of chromosomes as the cell prepares to divide. Early microscopists were the first to observe these structures, and they also noted the appearance of a specialized network of microtubules during mitosis. These tubules, collectively known as the spindle, extend from structures called centrosomes — with one centrosome located at each of the opposite ends, or poles, of a cell. As mitosis progresses, the microtubules attach to the chromosomes, which have already duplicated their DNA and aligned across the center of the cell. The spindle tubules then shorten and move toward the poles of the cell. As they move, they pull the one copy of each chromosome with them to opposite poles of the cell. This process ensures that each daughter cell will contain one exact copy of the parent cell DNA. What Are the Phases of Mitosis?

Mitosis consists of five morphologically distinct phases: prophase, prometaphase, metaphase, anaphase, and telophase. Each phase involves characteristic steps in the process of chromosome alignment and separation. Once mitosis is complete, the entire cell divides in two by way of the process called cytokinesis (Figure 1). Figure 1: Drawing of chromosomes during mitosis by Walther Flemming, circa 1880 This illustration is one of more than one hundred drawings from Flemming's \"Cell Substance, Nucleus, and Cell Division.\" Flemming repeatedly observed the different forms of chromosomes leading up to and during cytokinesis, the ultimate division of one cell into two during the last stage of mitosis. © 2001 Nature Publishing Group Paweletz, N. Walther Flemming: pioneer of mitosis research. Nature Reviews Molecular Cell Biology 2, 72 (2001). All rights reserved. What Happens during Prophase? Prophase is the first stage in mitosis, occurring after the conclusion of the G2 portion of interphase.

During prophase, the parent cell chromosomes — which were duplicated during S phase — condense and become thousands of times more compact than they were during interphase. Because each duplicated chromosome consists of two identical sister chromatids joined at a point called the centromere, these structures now appear as X-shaped bodies when viewed under a microscope. Several DNA binding proteins catalyze the condensation process, including cohesin and condensin. Cohesin forms rings that hold the sister chromatids together, whereas condensin forms rings that coil the chromosomes into highly compact forms. The mitotic spindle also begins to develop during prophase. As the cell's two centrosomes move toward opposite poles, microtubules gradually assemble between them, forming the network that will later pull the duplicated chromosomes apart. What Happens during Prometaphase? When prophase is complete, the cell enters prometaphase — the second stage of mitosis. During prometaphase, phosphorylation of nuclear lamins by M-CDK causes the nuclear membrane to break down into numerous small vesicles. As a result, the spindle microtubules now have direct access to the genetic material of the cell.

Each microtubule is highly dynamic, growing outward from the centrosome and collapsing backward as it tries to locate a chromosome. Eventually, the microtubules find their targets and connect to each chromosome at its kinetochore, a complex of proteins positioned at the centromere. The actual number of microtubules that attach to a kinetochore varies between species, but at least one microtubule from each pole attaches to the kinetochore of each chromosome. A tug-of-war then ensues as the chromosomes move back and forth toward the two poles. What Happens during Metaphase and Anaphase? As prometaphase ends and metaphase begins, the chromosomes align along the cell equator. Every chromosome has at least two microtubules extending from its kinetochore — with at least one microtubule connected to each pole. At this point, the tension within the cell becomes balanced, and the chromosomes no longer move back and forth. In addition, the spindle is now complete, and three groups of spindle microtubules are apparent.

Kinetochore microtubules attach the chromosomes to the spindle pole; interpolar microtubules extend from the spindle pole across the equator, almost to the opposite spindle pole; and astral microtubules extend from the spindle pole to the cell membrane. Metaphase leads to anaphase, during which each chromosome's sister chromatids separate and move to opposite poles of the cell. Enzymatic breakdown of cohesin — which linked the sister chromatids together during prophase — causes this separation to occur. Upon separation, every chromatid becomes an independent chromosome. Meanwhile, changes in microtubule length provide the mechanism for chromosome movement. More specifically, in the first part of anaphase — sometimes called anaphase A — the kinetochore microtubules shorten and draw the chromosomes toward the spindle poles. Then, in the second part of anaphase — sometimes called anaphase B — the astral microtubules that are anchored to the cell membrane pull the poles further apart and the interpolar microtubules slide past each other, exerting additional pull on the chromosomes (Figure 2).

Figure 2: Types of microtubules involved in mitosis During mitosis, several types of microtubules are active. The motor proteins associated with the interpolar microtubules drive the assembly of the spindle. Note the other types of microtubules involved in anchoring the spindle pole and pulling apart the sister chromatids. Cell Division - Mitosis and Meiosis | ASU - Ask A Biologist Cell Division show/hide words to know Cell: a tiny building block that contains all the information necessary for the survival of any plant or animal. It is also the smallest unit of life. ... more Chromosome: a long, thread-like molecule made of the chemical called DNA (deoxyribonucleic acid) that is held together with special proteins and is visible (with strong microscopes) during cell division... more DNA: deoxyribonucleic acid is the information "blue-print" of the cell. It is a nucleic acid and is made from building blocks called nucleotides. This genetic information is passed from parent to child... more Diploid cell: a cell with two sets of chromosomes (46 chromosomes total)... more Haploid cell: a cell with only one set of chromosomes...

more Organelle: "little organ". An internal organ of a cell... more Unique: one of a kind. Where Do Cells Come From? Sometimes you accidentally bite your lip or skin your knee, but in a matter of days the wound heals. Is it magic? Or, is there another explanation? Every day, every hour, every second one of the most important events in life is going on in your body—cells are dividing. When cells divide, they make new cells. A single cell divides to make two cells and these two cells then divide to make four cells, and so on. We call this process "cell division" and "cell reproduction," because new cells are formed when old cells divide. The ability of cells to divide is unique for living organisms. Why Do Cells Divide? Cells divide for many reasons. For example, when you skin your knee, cells divide to replace old, dead, or damaged cells. Cells also divide so living things can grow. When organisms grow, it isn't because cells are getting larger. Organisms grow because cells are dividing to produce more and more cells. In human bodies, nearly two trillion cells divide every day. Watch cells divide in this time lapse video of an animal cell (top) and an E.

coli bacteria cell (bottom). The video compresses 30 hours of mitotic cell division into a few seconds. (Video by the National Institute of Genetics) How Many Cells Are in Your Body? You and I began as a single cell, or what you would call an egg. By the time you are an adult, you will have trillions of cells. That number depends on the size of the person, but biologists put that number around 37 trillion cells. Yes, that is trillion with a "T." How Do Cells Know When to Divide? In cell division, the cell that is dividing is called the "parent" cell. The parent cell divides into two "daughter" cells. The process then repeats in what is called the cell cycle. Cells regulate their division by communicating with each other using chemical signals from special proteins called cyclins. These signals act like switches to tell cells when to start dividing and later when to stop dividing. It is important for cells to divide so you can grow and so your cuts heal. It is also important for cells to stop dividing at the right time. If a cell can not stop dividing when it is supposed to stop, this can lead to a disease called cancer.

Some cells, like skin cells, are constantly dividing. We need to continuously make new skin cells to replace the skin cells we lose. Did you know we lose 30,000 to 40,000 dead skin cells every minute? That means we lose around 50 million cells every day. This is a lot of skin cells to replace, making cell division in skin cells is so important. Other cells, like nerve and brain cells, divide much less often. How Cells Divide Depending on the type of cell, there are two ways cells divide—mitosis and meiosis. Each of these methods of cell division has special characteristics. One of the key differences in mitosis is a single cell divides into two cells that are replicas of each other and have the same number of chromosomes. This type of cell division is good for basic growth, repair, and maintenance. In meiosis a cell divides into two cells that have half the number of chromosomes. Reducing the number of chromosomes by half is important for sexual reproduction and provides for genetic diversity. Mitosis Cell Division Mitosis is how somatic—or non-reproductive cells—divide.

Somatic cells make up most of your body's tissues and organs, including skin, muscles, lungs, gut, and hair cells. Reproductive cells (like eggs) are not somatic cells. In mitosis, the important thing to remember is that the daughter cells each have the same chromosomes and DNA as the parent cell. The daughter cells from mitosis are called diploid cells. Diploid cells have two complete sets of chromosomes. Since the daughter cells have exact copies of their parent cell's DNA, no genetic diversity is created through mitosis in normal healthy cells. The Mitosis Cell Cycle Before a cell starts dividing, it is in the "Interphase." It seems that cells must be constantly dividing (remember there are 2 trillion cell divisions in your body every day), but each cell actually spends most of its time in the interphase. Interphase is the period when a cell is getting ready to divide and start the cell cycle. During this time, cells are gathering nutrients and energy. The parent cell is also making a copy of its DNA to share equally between the two daughter cells.

The mitosis division process has several steps or phases of the cell cycle—interphase, prophase, prometaphase, metaphase, anaphase, telophase, and cytokinesis—to successfully make the new diploid cells. When a cell divides during mitosis, some organelles are divided between the two daughter cells. For example, mitochondria are capable of growing and dividing during the interphase, so the daughter cells each have enough mitochondria. The Golgi apparatus, however, breaks down before mitosis and reassembles in each of the new daughter cells. Many of the specifics about what happens to organelles before, during and after cell division are currently being researched. (You can read more about cell parts and organelles by clicking here .) Meiosis Cell Division Meiosis is the other main way cells divide. Meiosis is cell division that creates sex cells, like female egg cells or male sperm cells. What is important to remember about meiosis? In meiosis, each new cell contains a unique set of genetic information. After meiosis, the sperm and egg cells can join to create a new organism. Meiosis is why we have genetic diversity in all sexually reproducing organisms.

During meiosis, a small portion of each chromosome breaks off and reattaches to another chromosome. This process is called "crossing over" or "genetic recombination." Genetic recombination is the reason full siblings made from egg and sperm cells from the same two parents can look very different from one another. The Meiosis Cell Cycle Meiosis has two cycles of cell division, conveniently called Meiosis I and Meiosis II. Meiosis I halves the number of chromosomes and is also when crossing over happens. Meiosis II halves the amount of genetic information in each chromosome of each cell. The end result is four daughter cells called haploid cells. Haploid cells only have one set of chromosomes - half the number of chromosomes as the parent cell. Before meiosis I starts, the cell goes through interphase. Just like in mitosis, the parent cell uses this time to prepare for cell division by gathering nutrients and energy and making a copy of its DNA. During the next stages of meiosis, this DNA will be switched around during genetic recombination and then divided between four haploid cells. So remember, Mitosis is what helps us grow and Meiosis is why we are all unique!

References: Bianconi E, Piovesan A, Facchin F, Beraudi A, Casadei R, Frabetti F, Vitale L, Pelleri MC, Tassani S, Piva F, Perez-Amodio S, Strippoli P, Canaider S. Ann. An estimation of the number of cells in the human body. Retrieved March 14, 2014 from The cell cycle, mitosis and meiosis — University of Leicester Cells divide into two different ways to make new cells. Mitosis Mitosis is used to produce daughter cells that are genetically identical to the parent cells. The cell copies - or 'replicates' - its chromosomes, and then splits the copied chromosomes equally to make sure that each daughter cell has a full set. Meiosis Meiosis is used to make special cells - sperm cells and egg cells - that have half the normal number of chromosomes. It reduces the number from 23 pairs of chromosomes to 23 single chromosomes. The cell copies its chromosomes, but then separates the 23 pairs to ensure that each daughter cell has only one copy of each chromosome. A second division that divides each daughter cell again to produce four daughter cells.

See below for more detailed descriptions of mitosis and meiosis. Mitosis Your body contains trillions of cells (thousands of millions). But you started life as a single cell - a fertilised egg cell. This cell then divided and divided to make more cells through a process called mitosis. Mitosis is a way of making more cells that are genetically the same as the parent cell. It plays an important part in the development of embryos, and it is important for the growth and development of our bodies as well. Mitosis produces new cells, and replaces cells that are old, lost or damaged. In mitosis a cell divides to form two identical daughter cells. It is important that the daughter cells have a copy of every chromosome, so the process involves copying the chromosomes first and then carefully separating the copies to give each new cell a full set. Before mitosis, the chromosomes are copied. They then coil up, and each chromosome looks like a letter X in the nucleus of the cell. The chromosomes now consist of two sister chromatids. Mitosis separates these chromatids, so that each new cell has a copy of every chromosome. A diagram of a cell ready for mitosis.

The copied chromosomes consist of two chromatids joined at the centromere The process of mitosis involves a number of different stages. The following diagram sets out the stages, and the main events that occur in each stage. Meiosis Some simple organisms - such as bacteria - can reproduce by simply dividing into two new individuals. Other organisms, including human beings, reproduce through sexual reproduction. New individuals are formed by the joining together of two special cells: a sperm cell and an egg cell. The cells in our bodies contain 23 pairs of chromosomes - giving us 46 chromosomes in total. Sperm cells and egg cells contain 23 single chromosomes, half the normal number, and are made by a special form of cell division called meiosis. Meiosis separates the pairs of matching (or 'homologous') chromosomes, so that sperm cells and egg cells have only one copy of each. That way, when an egg cell fuses with a sperm cell, the fertilised egg has a full set: that is, two copies of every chromosome. Meiosis involves two cell divisions: Meiosis I and Meiosis II. Meiosis I separates the matching - or 'homologous' - pairs of chromosomes.

Meiosis II divides each chromosome into two copies (much like mitosis). In Meiosis I, each daughter cell receives a mix of chromosomes from the two sets in the parent cell. In addition, the chromosomes in each matching pair swap some genetic material before they are parted in a process called crossing over. These processes produce new combinations of genes in the sperm cells and egg cells. The following diagrams set out the main stages of Meiosis I and Meiosis II in males. (A similar process in females produces egg cells rather than sperm cells.) NOVA - Official Website | How Cells Divide NOVA Posted 10.09.01 NOVA Most of the time, when a cell in our bodies divides, each new cell carries a complete set of chromosomes. The cells involved with human reproduction, however, carry only half after division occurs. In this step-by-step explanation, learn about mitosis and meiosis, the two types of cell division. This feature originally appeared on the site for the NOVA program 18 Ways to Make a Baby . Background on Meiosis and Mitosis As viewed from a human perspective, nature seems to have done ingenious engineering to overcome obstacles. Take the evolution of sex, for instance.

To make the move from asexual to sexual reproduction, nature took a system by which parent cells reproduced simply by dividing (asexual reproduction) and altered it to allow two parent cells to combine to create offspring (sexual reproduction). Asexual reproduction relies on a process called mitosis, in which the nucleus of a cell divides to create two new nuclei, each containing an identical copy of DNA. Mitosis allows the cells in your body to divide and regenerate—your hair to grow, your skin to heal after being wounded. Almost all of the DNA duplication in your body is carried out through mitosis. Meiosis, on the other hand, is the process by which certain sex cells are created. If you're male, your body uses meiosis to create sperm cells; if you're female, it uses meiosis to create egg cells. While all other cells in your body contain 46 chromosomes (23 from your father and 23 from your mother), your egg (or sperm) cells contain only half that number—a total of 23 chromosomes. When an egg and sperm unite to make a fertilized egg, the chromosomes add up to equal 46. How exactly does meiosis halve and mix chromosomes?

Find out through this feature, which provides a step-by-step, side-by-side comparison of meiosis and mitosis. Related Links Biological Basis of Heredity: Cell Reproduction View in: QuickTime or Windows Media Player (length = 1 min 35 secs) Some human somatic cells are frequently replaced by new ones and other cells are rarely duplicated. Hair, skin, fingernails, taste buds, and the stomach's protective lining are replaced constantly and at a rapid rate throughout our lives. In contrast, brain and nerve cells in the central nervous system are rarely produced after we are a few months old. Subsequently, if they are destroyed later, the loss is usually permanent, as in the case of paraplegics. Liver cells usually do not reproduce after an individual has finished growing and are not replaced except when there is an injury. Red blood cells are also somewhat of an exception. While they are being constantly produced in our bone marrow, the specialized cells from which they come do not have nuclei nor do the red blood cells themselves. Meiosis Meiosis is a somewhat similar but more complex process than mitosis. This is especially true in females.

While mitosis produces 2 daughter cells from each parent cell, meiosis results in 4 sex cells, or gametes in males and 1 in females. Unlike the cells created by mitosis, gametes are not identical to the parent cells. In males, meiosis is referred to as spermatogenesis because sperm cells are produced. In females, it is called o�genesis because ova, or eggs, are the main ultimate product. The illustration below shows the 8 phases of spermatogenesis. Phases of spermatogenesis (i.e., meiosis in males) How cells divide --side by side simulations of mitosis and meiosis This link takes you to an external website. To return here, you must click the "back" button on your browser program. Conception Sperm carries the father's chromosomes to the mother's ovum where they combine with her chromosomes at the time of conception. Sperm cells are microscopic, but ova may be large enough in some species to be visible with the naked eye. Human ova are about the diameter of a hair. Human sex cells same scale) The two sequential division processes of meiosis culminate in the production of gametes with only half the number of chromosomes of somatic cells.

As a result, human sperm and ova each have only 23 single-stranded chromosomes. Summary of reduction division in meiosis Human somatic cells, with their full set of 46 chromosomes, have what geneticists refer to as a diploid number of chromosomes. Gametes have a haploid number (23). When conception occurs, a human sperm and ovum combine their chromosomes to make a zygote (fertilized egg) with 46 chromosomes. This is the same number that the parents each had in their somatic cells. In doing this, nature is acting conservatively. Each generation inherits the same number of chromosomes. Without reducing their number by half in meiosis first, each new generation would have double the number of chromosomes in their cells as the previous one. Within only 15 generations, humans would have over 1� million chromosomes per cell and would be a radically different kind of animal. In fact, when a zygote has an extra set of chromosomes, it usually is spontaneously aborted by the mother's reproductive system--it is a lethal condition. The complete meiosis process in human males takes about 74 hours.

Spermatogenesis usually begins at 12-13 years of age and continues throughout life. Several hundred million sperm cells are produced daily by healthy young adult males. Between 200 and 600 million sperm cells are normally released in each ejaculation. Since only one sperm cell is required for conception, this huge number would seem to be an extreme overkill. However, as many as 20% of sperm cells are likely to be defective and the female reproductive tract is hostile even to healthy ones--it is acidic and contains antibodies that seek out and destroy the sperm cells. Ejaculating large numbers of sperm at the same time is nature's way of overcoming these difficulties and increasing the likelihood that conception will take place. The number of sperm cells produced can be significantly diminished by psychological and physiological stress. Sperm count also progressively declines with age after reaching a peak, usually in the early 20's. In addition, the percentage of sperm that move randomly rather than in a straight line generally increases in older men. The result is a decrease in male fertility. The genes that are responsible for sperm production are in the Y sex chromosome.

Unfortunately, the mutation rate for the Y chromosome is thought to be thousands of times higher than for those in other chromosomes. This may be a major cause of male infertility. As a result, genetic testing is beginning to be used to diagnose it. Human female reproductive system Meiosis in human females is more complex. By the 5th month after conception, immature sex cells begin to develop in the fetal ovaries but stop at an early stage of meiosis (after prophase I). They remain in this precursor egg cell, or primary o�cyte , phase until puberty when hormones cause a resumption of meiosis for one to several cells each month. They proceed to the 1st and 2nd reduction divisions and once again stop developing. At this stage they are secondary o�cytes. When a secondary o�cyte is finally released from the ovaries into the fallopian tube (during ovulation ), the egg still has not completed the last stage of meiosis. That happens only at conception as a result of chemical changes that occur when the main part of a sperm cell enters the ovum.

Virtually all (99.9%) sex cells in a woman's ovaries never develop beyond the primary o�cyte stage and eventually are reabsorbed by her body. By 20 weeks after conception, there are approximately 7,000,000 primary o�cytes. All but about 1,200,000 are lost by birth. At puberty, there are only around 400,000 of them remaining. Throughout life, there is a constant decline in the number of potential eggs. Each time one is successfully ovulated, as many as 2000 are lost. Normally, women have on average 11-14 ovulations per year for 33-36 years. This means that less than 500 secondary o�cytes usually are produced out of the store of hundreds of thousands of primary o�cytes. The actual number of ovulations is highly variable and often much lower since the process is governed by hormones and ultimately other factors including psychological stress, nutrition, physical activity, and pathological conditions.

The fact that women rarely have more than a few children is evidence that only a small fraction of the successfully ovulated eggs are fertilized and become viable zygotes. Beginning about age 27, a woman's fertility progressively declines. Around 35-37 years old, the decline becomes much steeper and the chances of conception significantly lower. By the early 50's, most women begin the transition to menopause when they stop ovulating altogether. The temporary cessation of ovulation and subsequent infertility can occur much earlier in life as a result of reduced blood estrogen levels caused by excessive physical activity. This is very likely the reason that roughly a quarter of American women athletes in high school and college cease having menstrual periods. It also results in a significant decrease in their bone density. NOTE: Humans may be the only animal species in which females now normally live for many years following menopause. Freed from having more children themselves, human grandmothers are in a position to assist their own daughters and sons in rearing their offspring. This potentially increases the chances that grandchildren will survive, thereby giving our species an advantage over other animals in the competition for survival.

However, people rarely lived beyond menopause until a little over a century ago when modern medicine and other technological advances made it possible. Prior to that time, we were more often like other animals in that most of us succumbed to disease, accidents, or predators before middle age and menopause. NOTE: On-going research suggests that it may be possible within a few years to return fertility to post menopausal women by stimulating stem cells in their ovaries to produce new eggs. Study Suggests Way To Create New Eggs In Women --National Public Radio audio report on February 27, 2012. To return here, you must click the "back" button on your browser program. (4 mins, 3 secs) When human sexual intercourse occurs, it takes about 5 minutes for sperm to reach the upper end of the fallopian tubes where conception usually takes place. Of the several hundred million sperm cells that enter a vagina, rarely do more than a few hundred successfully pass through the cervix and uterus. Only about 100 reach the upper end of the fallopian tubes. Sperm cells are guided over this path mostly by heat sensing.

The upper ends of the fallopian tubes are about two degrees warmer than the lower ends. Secondary o�cytes secrete chemicals that also may guide sperm cells to them when they are in close proximity. Usually, only the most viable sperm cells reach the secondary o�cytes and play their part in conception. Those sperm that fail in this competition are often genetically abnormal. 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 . It takes about 4 days for secondary o�cytes to pass through the fallopian tube in their journey to the uterus. Conception must occur early in this process. Sperm usually can remain viable for up to 48 hours in the female reproductive tract, but secondary o�cytes remain viable for only about 24 hours after they have left the ovaries. 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. Half of the sperm cells produced normally carry the X-chromosome and half have the Y-chromosome. Subsequently, we would expect that 50% of human babies would be males and 50% females, but this usually is not the case. The ratio of male to female newborns in the U.S. and most of the world is 105-110 males to 100 females. 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. Male-Female Ratios in the United States 100 Current cultural practices in China and India result in even higher rates of male births. Since 1979, 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 119 boys born for every 100 girls in China, and in some regions the ratio is as high as 144 to 100. A consequence of this practice is the growing scarcity of marriageable women. As of the 2005 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. Life's Greatest Miracle --PBS Nova series video on conception, gestation, and birth This link takes you to an external website. To return here, you must click the "back" button on your browser program. (8 parts of 4-10 mins each) Why Twins? 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 30. 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.

Any 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. NOTE: X-chromosome inactivation in females was described at the end of the first topic section of this tutorial ("Basic Cell Structures") and mitochondrial DNA is described in the last topic section ("Molecular Level of Genetics"). 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. This double fertilization of an egg apparently occurs in about 1% of human conceptions. In most cases, the embryo is not viable and dies.

Dizygotic twins can also be produced when a woman has sexual intercourse with more than one man around the time she is ovulating. If multiple viable eggs are released from her ovaries, each can be fertilized by sperm from a different man. This is referred to as heteropaternity. Why Intersex? Normally in humans one sperm cell combines its chromosomes with those of one ovum at conception and that in turn develops into a single embryo that will become a fetus. Very rarely, however, two zygotes fuse and become a single embryo. If it survives gestation , a baby will be born who is a true chimera --it is genetically two "people" in one body. If those two "people" are not the same gender, the baby will likely be an intersex individual (also known as hermaphrodite )--it will have both male and female sex organs and other body tissues. Some researchers believe that the frequency of chimeras being born will be increasing as in vitro fertilization becomes more common since two or more embryos are usually placed in the uterus with this procedure.

NEWS: On January 16, 2005, a 66 year old Romanian woman named Adriana Iliescu gave birth to a daughter. This makes her the oldest woman to become pregnant and deliver a live baby. She was implanted with a fertilized egg from a healthy younger woman. The baby was 6 weeks premature and weighed only 1.4 kilograms (3.1 pounds), which is less than half the normal newborn weight. Two other fetuses died during the pregnancy (News at Nature.com, January 17, 2005). NEWS: In the August 23, 2009 issue of Biology Letters, Kristen Navara of the University of Georgia presented the results of a 202 nation survey of data on male to female ratios at birth. She noted that there is a skewing of sex ratios that corresponds to latitude. In temperate and subarctic latitudes, there are slightly more boys born compared to tropical latitudes. Navara said that these differences were independent of cultural practices and the socio-economic status of families.

CELL DIVISION: BINARY FISSION AND MITOSIS CELL DIVISION: BINARY FISSION AND MITOSIS Table of Contents The Cell Cycle | Back to Top Despite differences between prokaryotes and eukaryotes, there are several common features in their cell division processes. Replication of the DNA must occur. Segregation of the "original" and its "replica" follow. Cytokinesis ends the cell division process. Whether the cell was eukaryotic or prokaryotic, these basic events must occur. Cytokinesis is the process where one cell splits off from its sister cell. It usually occurs after cell division. The Cell Cycle is the sequence of growth, DNA replication, growth and cell division that all cells go through. Beginning after cytokinesis, the daughter cells are quite small and low on ATP. They acquire ATP and increase in size during the G1 phase of Interphase. Most cells are observed in Interphase, the longest part of the cell cycle. After acquiring sufficient size and ATP, the cells then undergo DNA Synthesis (replication of the original DNA molecules, making identical copies, one "new molecule" eventually destined for each new cell) which occurs during the S phase.

Since the formation of new DNA is an energy draining process, the cell undergoes a second growth and energy acquisition stage, the G2 phase. The energy acquired during G2 is used in cell division (in this case mitosis). The cell cycle. Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates ( ) and WH Freeman ( ), used with permission. Regulation of the cell cycle is accomplished in several ways. Some cells divide rapidly (beans, for example take 19 hours for the complete cycle; red blood cells must divide at a rate of 2.5 million per second). Others, such as nerve cells, lose their capability to divide once they reach maturity. Some cells, such as liver cells, retain but do not normally utilize their capacity for division. Liver cells will divide if part of the liver is removed. The division continues until the liver reaches its former size. Cancer cells are those which undergo a series of rapid divisions such that the daughter cells divide before they have reached "functional maturity". Environmental factors such as changes in temperature and pH, and declining nutrient levels lead to declining cell division rates.

When cells stop dividing, they stop usually at a point late in the G1 phase, the R point (for restriction). Prokaryotic Cell Division | Back to Top Prokaryotes are much simpler in their organization than are eukaryotes. There are a great many more organelles in eukaryotes, also more chromosomes. The usual method of prokaryote cell division is termed binary fission . The prokaryotic chromosome is a single DNA molecule that first replicates, then attaches each copy to a different part of the cell membrane. When the cell begins to pull apart, the replicate and original chromosomes are separated. Following cell splitting ( cytokinesis ), there are then two cells of identical genetic composition (except for the rare chance of a spontaneous mutation). This animated GIF of binary fission is from: #two_bact_groups The prokaryote chromosome is much easier to manipulate than the eukaryotic one. We thus know much more about the location of genes and their control in prokaryotes. One consequence of this asexual method of reproduction is that all organisms in a colony are genetic equals.

When treating a bacterial disease, a drug that kills one bacteria (of a specific type) will also kill all other members of that clone (colony) it comes in contact with. Rod-Shaped Bacterium, E. coli, dividing by binary fission (TEM x92,750). This image is copyright Dennis Kunkel at , used with permission. Rod-Shaped Bacterium, hemorrhagic E. coli, strain 0157:H7 (division) (SEM x22,810). This image is copyright Dennis Kunkel at , used with permission. Eukaryotic Cell Division | Back to Top Due to their increased numbers of chromosomes, organelles and complexity, eukaryote cell division is more complicated, although the same processes of replication, segregation, and cytokinesis still occur. Mitosis | Back to Top Mitosis is the process of forming (generally) identical daughter cells by replicating and dividing the original chromosomes, in effect making a cellular xerox. Commonly the two processes of cell division are confused. Mitosis deals only with the segregation of the chromosomes and organelles into daughter cells. Click here to view an animated GIF of mitosis from .

Eukaryotic chromosomes occur in the cell in greater numbers than prokaryotic chromosomes. The condensed replicated chromosomes have several points of interest. The kinetochore is the point where microtubules of the spindle apparatus attach. Replicated chromosomes consist of two molecules of DNA (along with their associated histone proteins ) known as chromatids . The area where both chromatids are in contact with each other is known as the centromere the kinetochores are on the outer sides of the centromere. Remember that chromosomes are condensed chromatin (DNA plus histone proteins). Structure of a eukaryotic chromosome. Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates ( ) and WH Freeman ( ), used with permission. During mitosis replicated chromosomes are positioned near the middle of the cytoplasm and then segregated so that each daughter cell receives a copy of the original DNA (if you start with 46 in the parent cell, you should end up with 46 chromosomes in each daughter cell). To do this cells utilize microtubules (referred to as the spindle apparatus ) to "pull" chromosomes into each "cell".

The microtubules have the 9+2 arrangement discussed earlier. Animal cells (except for a group of worms known as nematodes) have a centriole . Plants and most other eukaryotic organisms lack centrioles. Prokaryotes, of course, lack spindles and centrioles; the cell membrane assumes this function when it pulls the by-then replicated chromosomes apart during binary fission. Cells that contain centrioles also have a series of smaller microtubules, the aster , that extend from the centrioles to the cell membrane. The aster is thought to serve as a brace for the functioning of the spindle fibers. Structure and main features of a spindle apparatus. Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates ( ) and WH Freeman ( ), used with permission. The phases of mitosis are sometimes difficult to separate. Remember that the process is a dynamic one, not the static process displayed of necessity in a textbook. Prophase | Back to Top Prophase is the first stage of mitosis proper.

Chromatin condenses (remember that chromatin/DNA replicate during Interphase), the nuclear envelope dissolves, centrioles (if present) divide and migrate, kinetochores and kinetochore fibers form, and the spindle forms. Pea Plant Nuclear DNA, from Vicea faba (TEM x105,000). This image is copyright Dennis Kunkel at , used with permission. The events of Prophase. Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates ( ) and WH Freeman ( ), used with permission. Anaphase | Back to Top Anaphase begins with the separation of the centromeres, and the pulling of chromosomes (we call them chromosomes after the centromeres are separated) to opposite poles of the spindle. The events of Metaphase and Anaphase. Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates ( ) and WH Freeman ( ), used with permission.

Telophase | Back to Top Telophase is when the chromosomes reach the poles of their respective spindles, the nuclear envelope reforms, chromosomes uncoil into chromatin form, and the nucleolus (which had disappeared during Prophase) reform. Where there was one cell there are now two smaller cells each with exactly the same genetic information. These cells may then develop into different adult forms via the processes of development. The events of Telophase. Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates ( ) and WH Freeman ( ), used with permission. mitosis / cell division | Learn Science at Scitable mitosis / cell division Mitosis is a process of nuclear division in eukaryotic cells that occurs when a parent cell divides to produce two identical daughter cells. During cell division, mitosis refers specifically to the separation of the duplicated genetic material carried in the nucleus. Mitosis is conventionally divided into five stages known as prophase, prometaphase, metaphase, anaphase, and telophase. While mitosis is taking place, there is no cell growth and all of the cellular energy is focused on cell division.

During prophase, the replicated pairs of chromosomes condense and compact themselves. The pairs of chromosomes that have been replicated are called sister chromatids, and they remain joined at a central point called the centromere. A large structure called the mitotic spindle also forms from long proteins called microtubules on each side, or pole, of the cell. During prometaphase, the nuclear envelope that encloses the nucleus breaks down, and the nucleus is no longer separated from the cytoplasm. Protein formations called kinetochores form around the centromere. The mitotic spindle extends from the poles and attaches to the kinetochores. During metaphase, the microtubules pull the sister chromatids back and forth until they align in a plane, called the equatorial plane, along the center of the cell. During anaphase, the sister chromatids are separated simultaneously at their centromeres. The separated chromosomes are then pulled by the spindle to opposite poles of the cell. Anaphase ensures that each daughter cell receives an identical set of chromosomes. Finally, during telophase, a nuclear membrane forms around each set of chromosomes to separate the nuclear DNA from the cytoplasm.

The chromosomes begin to uncoil, which makes them diffuse and less compact. Along with telophase, the cell undergoes a separate process called cytokinesis that divides the cytoplasm of the parental cell into two daughter cells. Further Exploration Concept Links for further exploration Mitosis - The process of cell division by mitosis 15 April - Womens' death rates lower in greener neighbourhoods (USA) Mitosis: (Cell Division via Mitosis) This page describes the context of mitosis - explaining its position in the series of processes that, together, form the "cell cycle" for somatic cells (cells relating to the non-reproductive parts of the body) . The four stages of mitosis - prophase, metaphase, anaphase and telophase - are also listed and described. For an illustration of this process see the page - diagram of mitosis . Background: This follows the page about an introduction to cell division . Reminder: Mitosis is defined as the type of cell division by which a single cell divides in such a way as to produce two genetically identical "daughter cells".

This is the method by which the body produces new cells for both growth and repair of aging or damaged tissues throughout the body - as opposed to for sexual reproduction (when meiosis applies). Mitosis is the simplest of the two ways (mitosis and meiosis) in which the nucleus of cells divide - as part of a process of cell division. The context in which mitosis occurs during the "cell cycle" is explained as follows: The Stages of the "Cell Cycle" for Somatic Cells In all somatic cells (that is, all cells relating to the non-reproductive parts of the body = all cells except for those of the gametes) the "cell cycle" consists of two periods: Interphase (also known as "interkinesis") is the period in which the cell is not dividing. This does not mean that little is happening. Interphases are very active periods during which cells perform all the functions necessary for life, including the synthesis of DNA (deoxyribonucleic acid) so that both of the new cells formed by the miotic phase will contain a complete copy of the original and hence have all the necessary information. and Miotic phase (M) - when the cell is dividing.

The miotic phase of the "cell cycle" consists of two stages: Mitosis Mitosis is the division of the cell nucleus, and is followed by: Cytokinesis Eventually the miotic spindle breaks-up. ... then the cytoplasm begins to divide around the two new nuclei: Cytokinesis (Cytoplasmic Division) Cytokinesis is the process by which the cytoplasm of the original cell forms the two new ("daughter") cells around the two new ("daughter") nuclei formed by the process of mitosis (or meiosis - cytokinesis being a part of both types of processes of cell division). In the case of animal - rather than plant - cells, a cleavage furrow forms around the cell's equator then constricts as a ring until it cuts completely through the cell. When cytokinesis is complete, interphase begins (see further up this page). This begins the next "cell cycle". Now see the page illustrating a diagram of mitosis , and also read the page about cell division via meiosis .

Biology Textbooks Harnessing the healing properties of honey to help combat antimicrobial resistance - 29 Apr '16 Womens' death rates lower in greener neighbourhoods (USA) - 15 Apr '16 Combining motivational interviewing with cognitive behavioural therapy to treat anxiety - 23 Mar '16 Yellow fever mosquito larvae found in New Zealand - 8 Mar '16 Public health nursing in the UK - 3 Mar '16 Aromatherapy Book wins Botanical Literature Award (USA) - 19 Feb '16 Lassa Fever outbreak in Benin, West Africa - 11 Feb '16 AMA endorses 2015-2020 Dietary Guidelines (USA) - 8 Jan '16 More news about ... How do cells grow and divide? How do cells grow and divide? How do cells die? How do cells grow and divide? The stages in a cell's life are called the cell cycle. This works a bit like a washing machine cycle - each stage must be finished completely before the next one is allowed to start.

Instead of wash, rinse, spin and drain, the cell cycle is made up of stages in which the cell grows and rests, copies its DNA, and divides into two new cells. The cell cycle works a bit like a washing machine cycle. How long does it take? There are many different sorts of cells but all have roughly the same cell cycle. However, the time taken to complete it can vary enormously. The cell cycle of a fly embryo cell takes just eight minutes while a human liver cell cycle can last longer than a year. Some cells can withdraw from the cell cycle and rest before re-entering it. Many cells in growing embryos often skip the resting stages altogether. Cells that have stopped dividing, like brain cells, never re-enter the cell cycle.

Question: What divides in two in a process called mitosis?
Answer: Cells