Tutorial: Meiosis
Index
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.
This web tutorial may not be distributed by any means
without the expressed permission of the author!




Sex Cells

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.)

Beginning meiosis study



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 vs asexual



Fertilization

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.

Fertilization



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.

2N vs N


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.

Haploid



Normal Cell Division

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.

Normal cell division



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.

Need for chromosome reduction



Meiosis and Cytokinesis

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.


Meiosis and cytokinesis



N + N -> 2N

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.
N + N = 2N



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.

Meiosis I and Meiosis II



MI and Homolog Separation

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.

Meiosis I



Chromosome Reduction

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.

Meiosis I result



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.

Sets of DNA



Chromosome Set Reduction

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.

Meiosis splits instructions 4 ways



Synapsis

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.

Synapsis


Tetrads

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.

Synapsis



Crossing Over

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.

Crossing over



Crossing Over

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.

Meiosis and variety



Crossing Over

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.
Crossing over randomness



Meiosis Phases

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
5) Cytokinesis.

Meiosis phases



Prophase I

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.

Prophase I



Prophase I and Synapsis

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.

Synapsis in Prophase I



Metaphase I

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.

Metaphase I
Metaphase I



Anaphase I

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.

Anaphase I
Anaphase I



Telophase I

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.

Telophase I



Cytokinesis I

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.

Cytokinesis I



Prophase II

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.

Prophase II



Metaphase II

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.

Metaphase II



Anaphase 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.

Anaphase II



Telophase 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.

Telophase II



Cytokinesis II

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.

Cytokinesis II



No DNA Replication During Meiosis

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.

NO DNA replication between MI and MII



Polar Body Formation

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.

Polar bodies



Sperm Tail Formation

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.


Sperm function



Plant Meiosis

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.

Pollen and ovules



Sperm Meiosis Animated

Sperm meiosis animated
The animations are Copyright © 1989, Steve Kuensting, All Rights Reserved.
Speed = | Delay = milliseconds | Frame # =
Loop


Egg Meiosis Animated

Egg meiosis animated
The animations are Copyright © 1989, Steve Kuensting, All Rights Reserved.
Speed = | Delay = milliseconds | Frame # =
Loop

Explanation

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.

Animation explanations



Sperm Production

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.

Sperm production in life



Egg Production

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.

Egg production in life



Male vs Female Gamete Production

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.

Egg vs sperm production in life



Genetic Recombination

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.

Genetic recombination



Hand-Me-Down Chromosomes

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.

Chromosomes hand-me-down



Random Chromosome Assortment

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.

Shuffling instructions in meiosis



Meiosis is Anti-Cloning

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.

Meiosis ensures variety



Meiosis and Variety

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.



Quiz

  1. What is the name of the process where homologs find one another in meiosis I?

  2. What is the name of the process where chromosomes exchange genes in meiosis I?

  3. Does DNA replication occur during meiosis?

  4. During what phase of meiosis are homologs pulled to opposite poles of the cell?

  5. Is telophase I skipped in sperm production?

  6. During what phase of meiosis are chromosomes split into chromatids?

  7. As the homologs twist around one another to cross over n meiosis I, what is the name of the structure they form?

  8. Is cytokinesis 'even' in sperm production?

  9. How many eggs are formed from the meiotic division of a cell?

  10. During what phase of meiosis do tetrads line up at the center of the cell?

  11. What are the small cells formed, which die, in egg production?

  12. How many sperm are formed from the meiotic division of a cell?

  13. If a cell with a 2N# of 12 chromosomes underwent meiotic division, how many chromatids would be in the sex cells?

  14. In what animal organ are eggs made?

  15. Do sperms or eggs undergo DNA replication just after they are formed?

  16. If a sperm contains 26 chromatids, how many chromosomes are in the body cells of the organism that formed it?

  17. Do cells in egg formation skip prophase II?

  18. Is mitosis more similar to meiosis I or II?

  19. During what stage of meiosis does a diploid set of chromosomes form from chromatin?

  20. Is a fertilized egg formed from the union of 2 haploid sex cells always diploid?

  21. During what stage of meiosis would a haploid set of chromatids unravel into chromatin?

  22. Do polar bodies form in sperm and egg production?

  23. Does a polar body and the true egg at the end of meiosis have the same number of chromatids?

  24. In what organ are sperm produced?

  25. In what stage of meiosis does a haploid set of chromosomes unravel into chromatin?

  26. During what stage of meiosis does synapsis occur?

  27. What type of structure made by a plant spore produces sperm?

  28. Are all sperms made by one organism identical to one another?

  29. How many chromatids are in a tetrad?

  30. How many chromosomes/chromatids will a cell have at the end of meiosis when the original cell that made it had 46?


Diagram - Name the phases represented by the following cell diagrams.
Quiz
Answers