Sex Cells | Asexual vs Sexual | Fertilization
Ovaries vs Testes | Diploid vs Haploid | Haploid Cell Fusion
Meiosis I and II | Sets of DNA | Synapsis | Tetrads
Crossing Over | Meiosis Phases | Prophase I | Metaphase I
Anaphase I | Telophase I | Cytokinesis I | Prophase II
Metaphase II | Anaphase II | Telophase II | Cytokinesis II
Egg Production | Sperm Production | Sperm Meiosis Animated
Egg Meiosis Animated | Genetic Recombination | Quiz
Copyright © Steve Kuensting, 2004, All Rights Reserved.
One of the fundamental and truly unique characteristics of all organisms is their ability to reproduce. All life forms are able to reproduce only because their cells that make them up are capable of this feature. Most life forms on earth sexually reproduce -- that is they fuse sex cells to make offspring.
(To fully understand this program, you must have completed the program of MITOSIS, go there if you have not done so already.)
Asexual vs Sexual
A simpler way to reproduce is through asexual reproduction, where one organism produces an offspring by itself. This method of reproduction is not very successful and most organisms on earth do not reproduce this way. Most organisms sexually reproduce -- meaning that they use another organism to produce an offspring. Sexual reproduction is more successful because the offspring have more genetic variety, getting DNA from 2 sources instead of 1.
Sexual reproduction involves the fusion of two living cells which will then grow into a new offspring. The fusion cells are called GAMETES , or sex cells. Males of a species produce sperm - cells that can swim to another cell, and females produce eggs - cells that contain all of the nutrients to start a new organism. They are unique in that they (sperm and egg) contain the developmental potential to produce an entire organism composed of trillions of cells. No ordinary cell (skin or bone) is capable of such a feat.
Ovaries vs Testes
Gametes are produced in special organs only. Eggs are produced in OVARIES, an organ located in the lower abdominal region of all mammalian females. Sperms are produced in TESTES, organs usually located outside the lower abdominal region of mammalian males.
Diploid vs Haploid
Chromosomes usually exist in pairs in sexually reproducing organisms and for this reason most sexually reproducing organisms are termed DIPLOID (2N) because they have chromosomes in pairs. Some cells do not have chromosomes in pairs but rather singly, or as individuals - one of each. They are said to be HAPLOID (1N). Sperms and eggs are haploid cells.
Cells are usually created by normal cell division involving mitosis and cytokinesis. Normal cell division is used by unicellular organisms for reproduction, and in multicellular organisms for growth, maintenance, and repair.
Haploid Cell Fusion
Gametes cannot be made by normal cell division. Since gametes must fuse, they must each have one-half the normal number of chromosomes so that when they fuse they have exactly the right amount. Since mitosis produces daughter cells with the diploid number of chromatids, it cannot be used for sperm or egg production. If a 2N (diploid) sperm united with a 2N egg, the results would be disastrous (4N). The fertilized egg would have twice the amount of genetic material it must have.
Sperms and eggs are produced by a unique cell division process. The nucleus is not split by mitosis as in normal cell division. All sperms and eggs are made by MEIOSIS AND CYTOKINESIS. Meiosis splits the nucleus so that the daughter cells have one-half the normal number of chromosomes - the haploid number. Because of this, sperms and eggs will not survive for long. If they do not fuse (sperm with egg) they will die.
Since sperms and eggs contain only the 1N (haploid) number of chromosomes they can safely fuse (sperm with egg), and produce a fertilized egg with the correct number of chromosomes that can then develop normally.
Meiosis I and II
Meiosis cannot split a 2N nucleus into a 1N nucleus in a single division. That is why meiosis occurs in two stages or divisions. The first meiotic cell division involves a nuclear division and then a cytokinesis (cytoplasm division). The second meiotic cell division involves another nuclear division and another cytokinesis. The first meiotic division is called MEIOSIS I and the second is called MEIOSIS II.
In meiosis I, the homologous chromosomes are separated from one another and placed into separate cells after cytokinesis. In meiosis II, the homologs are separated into individual chromatids which are then placed into separate cells after cytokinesis. The result is that the daughter cells (sperms or eggs) have only one set of instructions - a haploid set of chromatids.
A human cell in an ovary or teste about to begin meiotic cell division has 46 chromosomes. At the end of meiosis I each of the two daughter cells will have only 23 chromosomes (each double-stranded) - the original 46 will have been separated into 2 sets of 23 each. Then at the end of meiosis II each of the 4 daughter cells produced will have 23 CHROMATIDS, one-fourth of the DNA that the original cell possessed.
Sets of DNA
Twenty-three different chromatids possessed by a human sperm or egg is exactly one set of human instructions - instruction on how to make a human being. Any cell with 46 double-stranded chromosomes actually has 4 sets of instructions, but only two different sets, the other 2 sets are copies. Replication is the process that makes those copies.
Thus meiosis splits 4 sets of instructions in a diploid cell through 2 cell divisions into 4 haploid cells each with 1 set of instructions in chromatids. Replication does not occur between the two cell divisions and the cells do not pass through the events of the cell cycle - G1, S, G2.
In order to ensure that the sperm or egg are not missing any chromatids or have doubles of chromatids, meiosis is a very exact process. A critical event that occurs in meiosis I is that the HOMOLOGOUS CHROMOSOMES find one another and bond to one another. This never happens except in a cell undergoing meiosis. Then meiosis I splits the homologous chromosomes apart thus ensuring that no sperm or egg will receive a homologous pair of chromosomes which would be disastrous.
The process of the homologous chromosomes finding one another is called SYNAPSIS. Synapsis results in two chromosomes bonded together - or 4 chromatids bonded together. The bonded chromosomes are called TETRADS.
When the tetrads form by synapsis in meiosis I, the chromosomes exchange homologous pieces of DNA. This is called Crossing Over and results in each chromosome receiving as much genetic variety as possible. Genes are exchanged between the chromatids ensuring the best chance of survival for the possible offspring.
The exchanged genes in meiosis must code for the same characteristic - hair color, eye color, etc. The reason crossing over is needed is because there is more than one gene for hair and eye color. Since chromosomes are passed down from generation to generation, without crossing over, genetic instructions would be locked in place forever - this lowers variety and chances for offspring survival.
Thus, with crossing over, there is almost no chance that the offspring will look exactly like either parent or any brother and sister. Since crossing over is random, there are many different combinations of the different genes for traits. And there are many possibilities for the appearance of the offspring.
We will now discuss meiotic cell division in greater detail. Each of the two divisions consists of 5 phases. Division I consists of:
1) Prophase I, 2) Metaphase I, 3) Anaphase I, 4) Telophase I, and
5) Cytokinesis. Division II consists of: 1) Prophase II,
2) Metaphase II, 3) Anaphase II, 4) Telophase II, and
As a human cell in an ovary or teste enters meiotic cell division, it first enters Prophase I. Most events of prophase I are much like prophase of mitosis. Nucleolus and nuclear membrane disappear, while chromosomes and spindle fibers appear.
The unusual event making Prophase I different than prophase of mitosis is SYNAPSIS, the coming together of the homologous chromosomes. Once the tetrads form from synapsis, crossing over occurs between the chromatids within each tetrad.
Then, once crossing over is complete, metaphase I begins where the TETRADS line up at the center of the cell. The difference between metaphase I of meiosis and metaphase of mitosis is that individual chromosomes (each double stranded) line up in mitosis whereas tetrads (homologous pairs) line up in meiosis.
After the chromosome pairs are lined up, Anaphase I begins. The
tetrads are pulled apart into homologs which then move toward opposite ends of the cell. The difference between anaphase I of meiosis and anaphase of mitosis is that in mitosis chromatids are pulled to opposite ends, whereas in meiosis chromosomes (each double-stranded) are pulled to opposite ends.
Once the chromosomes (homologs) are at opposite ends, telophase I may begin. In sperm production, telophase I is skipped and the cell goes straight into cytokinesis. In eggs, the cell enters telophase I and then goes to cytokinesis. Telophase is mostly a reverse of prophase I, nuclear membrane and nucleolus reappear, chromatin forms, spindle fibers disappear.
Cytokinesis then occurs. In sperms production, the cytoplasm is divided evenly but in egg production it is divided unevenly. The large cell formed in meiosis I of egg production will live, the small cell is called a POLAR BODY and it will die. The significance of this event will be discussed later.
The first phase of meiosis II is prophase II. No DNA replication occurs between meiosis I and II. That would defeat the purpose of the process. In sperm production, Prophase II is skipped, in egg production it is not. Prophase II is exactly like prophase of mitosis except there is only a haploid number of chromosomes that will form. Two cells are shown but only one proceeds in egg production.
After the chromosomes are formed (each is double stranded), they line up to be split. This is Metaphase II. The haploid number of chromosomes line up at the center of the cell. Metaphase II is exactly like metaphase of mitosis except for the haploid number of chromosomes in metaphase II.
The chromosomes are then pulled apart into chromatids which move to opposite ends of the cell. This is Anaphase II. Anaphase II is exactly like anaphase of mitosis except for the haploid number of chromosomes in Anaphase II.
Telophase II occurs in both sperm and egg production. It is essentially the reverse of prophase II. Chromatin, nuclear membrane, and nucleolus form, spindle fibers disappear. Telophase II is exactly like telophase of mitosis.
Finally cytokinesis occurs. In sperm production, it is even, producing equally sized cells. In egg production it is uneven, producing a polar body which will die, and the true egg. Note that in sperm production, 4 sperm are formed from the meiotic division of one cell, in egg production only 1 egg forms.
DNA replication DOES NOT OCCUR in the sperm or egg. This would defeat the process of meiosis. Each sperm or egg only carries one complete set of genetic instructions, a haploid number of chromatids (in chromatin form). When a sperm and egg fuse, then they will replicate their DNA and begin normal mitotic cell division for growth.
Only one egg is formed because it is important for the egg to be as large as possible. The cytokinesis is uneven in egg production so that all possible nutrients can be put into one cell only. Since the egg carries all of the nutrients for survival after fertilization, this is very important. The egg will enlarge even more before fertilization.
Then sperm grow a flagellar tail for swimming and the cell elongates to become shaped like an ellipse. This shape will pass through the water better during swimming.
Plants use meiosis to produce sex cells also. Meiosis in plant occurs in cones, flowers, or other structures. In flowers and cones, meiosis produces special cells called SPORES. Some spores grow into pollen, cells that fly through the air to fertilized eggs. Other spores grow into ovules. Pollen are tiny structures that produce sperm, ovules produce eggs. In this way plants also participate in meiotic cell division.
Sperm Meiosis Animated/Egg Meiosis Animated
Sperm Meiosis Animation || Egg Meiosis Animation
Download/Play Simple Gif Animations
Sperm Meiosis: slow | regular | fast
Egg Meiosis: slow | regular | fast
The animations are Copyright © 1989, Steve Kuensting, All Rights Reserved.
In both animations the final cells produced had the haploid number (1N#) of chromatids, 4. The original diploid cell had 8 double-stranded chromosomes or 16 chromatids (in chromatin form). The 2 meiotic divisions produced cells that had 4 (one-fourth of 16) that number of chromatids (in chromatin form). The sperm and egg, when fused, will have 8 (4+4) chromatids (in chromatin form) and will then replicate their DNA and prepare for their first cell division.
The sperm production process skips telophase I and prophase II because sperm production is a continuous process. Because the DNA is already in chromosome form from anaphase I, the chromosome only need to line up at the cell center and prepare to be split again. For efficiency, the sperm go straight from anaphase I to metaphase II.
Egg production, on the other hand, does not occur continuously. Meiosis begins in mammalian female ovaries before a female is born, but meiosis stops at meiosis I. The cells do not go on to meiosis II. These cells are called ootids. The ootids wait in interphase until puberty, whereupon one or a few of them complete meiosis seasonally or monthly to produce eggs.
Thus, in men, sperm are always produced constantly, usually throughout life. In females, ootids are made before birth, and age as the woman gets older. No more are ever made throughout life. Women eventually menopause, meaning they stop producing eggs. What actually happens is that the ootids they carry stop completing meiosis II. Since the ootids are over 50 years old in a woman age 50, menopause stops egg production to prevent an aged and defective eggs from being fertilized which could produce abnormal embryos.
Meiosis also ensures GENETIC RECOMBINATION, which is where genes are combined in new ways. Meiosis does this in two important ways. First, crossing over ensures that the chromosomes can exchange genes and maximize the genetic variety they have. This allows the chromosomes we received to shuffle their genes somewhat so that our offspring will have as much genetic chance for survival as possible.
Secondly, organisms do not create their chromosomes, they receive them from their parents. The chromosomes we carry are our parent's that were given to us. Also, the chromosomes we pass on to our children are not ours, they are our parents'. Chromosomes are passed down from the generations, from grandparent to parent to offspring.
To ensure that either of our parent's set of chromosomes don't pass directly into our sex cells unshuffled, the homologs in
meiosis I line up randomly so that always, some of mom's and some of dad's will end up in each sex cell. Each sex cell made by an organisms is unique in this way, each carries different genetic instructions.
Because no two sperm or eggs made by an organism are alike, no two brothers or sisters will ever be identical (unless identical twins occur), even though they had the same parents. Since the genes on the chromosomes are shuffled and the chromosomes do not pass down in parental sets (mom or dad's chromosomes all in one of our gametes), our offspring will always be slightly different.
By ensuring genetic recombination generation after generation, meiosis ensures that all offspring have the most shuffled set of genes possible so that they have the best chance of survival. In a changing environment it is important to have genetic variety. If all organisms were the same and the environment changed, they could all easily die. Meiosis prevents that from ever happening. Those organisms with the correct features will live, and there will almost always be some organism with correct features that will survive any environmental change, thanks to meiosis.
Diagram - Name the phaes represented by the following cell diagrams.