Table of Contents
Learning Objective
By the end of this lecture, you should be able to identify and explain every stages of meiosis, you must also be able to point out differences between mitosis and meiosis. Remember, you are to go nowhere if you are not able to score upto 70% in the quiz.
Introduction
In the previous blog post, we fully discussed the Cell cycle, Cell growth and Cell division. It was made known that all cells basically divide by method of Mitosis except the sex gamete cells.
Why?
When a cell divides by mitosis, two identical cells that contain the same number of chromosomes (structure that contain the Cell's DNA) as the original cell are formed. If this same thing occur for the formation of gamete cells, when the male and the female gametes eventually fuse together (fertilised), the product (offspring) will be having a total number of Chromosome that is twice that of its parent (a set from the father and another set from the mother).
When the offspring also gets mature and reproduce, its child or offspring will have double of its chromosomes too (a number coming from itself and the others from its partner), the same thing repeats itself in the next generation, making the chromosomes number to be growing geometrically in every new generation.
If that was the case, by now our body cells should contain thousands of trillions of chromosomes.
However, nature has it that gamete cells should not follow the method of Mitosis for its own division or formation.
What is a gamete cell (Sex gamete cell)?
A gamete cell is simply a reproductive cell. They are also called germ cells or non-somatic cells.
A male and a female animal have different Gametes.
The gamete in Male animals is called Sperm.
The gamete in Female animals is called Ovum or Egg cell.
Formation of Gamete cells
Gamete cells are formed by what we call Gametogenesis.
Gameto - Gamete
Genesis - Formation or creation
For an offspring to be produced, the sex gametes from both the male and female have to fuse together to give rise to the offspring (basically called a zygote).
Male Gamete + Female gamete = Zygote.
However, this zygote must have the same number of chromosomes as its parent's. Let me use humans as example.
As a human, we have 46 chromosomes you know that right?
A male has 46 chromosomes and a female also has 46 chromosomes. A zygote must also have 46 chromosomes.
Now this is looking a little bit complex right?
How did nature solve this issue?
Since 46 chromosomes from male and 46 chromosomes from female will birth a total of 92 chromosomes which is not ideal, nature halves the number of chromosomes from the male and the number of chromosomes from the female.
This makes it that when a male and female come together to produce an offspring, instead of 46 chromosomes coming from both of their cells, 23 is gotten from each, these fuse together to make a total of 46 chromosomes again which will be contained in the new offspring.
I believe you are getting me right?
While other cells that have 46 chromosomes divide to give rise to a new cell with 46 chromosomes (mitosis), gamete cells divide to give rise to a new cell with only 23 chromosomes which will be stored (in males) or disposed after sometime (in females).
Since Gamete cells do not divide by mitosis, how do they divide then?
Gamete or reproductive cells divide by what we call Meiosis.
What basically make Mitosis different from Meiosis?
Mitosis is used for almost all of your body’s cell division needs. It adds new cells during development and replaces old and worn-out cells throughout your life. The goal of mitosis is to produce daughter cells that are genetically identical to their mother cells, with not a single chromosome more or less.
Meiosis, on the other hand, is used for just one purpose in the animal's body: the production of gametes—sex cells (sperm and eggs). Its goal is to make daughter cells with exactly half as many chromosomes as the starting cell.
If you wish to read more on Mitosis, enter here.
Meiosis
Meiosis is a type of cell division in which the number of chromosomes per cell is cut in half.
Although Meiosis may appear complicated, it really is not, but you need to follow closely:
Meiosis involves two cell divisions and each of this division looks just like mitosis, just there's a small upgrade.
Before we delve into how meiosis occur, I want you to get familiar with some terms I will be using along the way
Diploid cell:
Every cell in an animal's body is considered Diploid except of course the sex cells. A diploid cell contains chromosomes from both the organism's father and its mother. In humans, a diploid cell contain 46 chromosomes. It is a normal cell with two sets of chromosomes.
Haploid cell:
This is a type of cell which the chromosome number has been reduced by half. Example of an haploid cell is the gamete sex cells.
Here, the cell does not contain both the mother and father chromosomes anymore. Instead, the father and mother chromosomes have mixed up and divided to form the chromosomes that the organism will give to its own offspring.
While diploid cells remain constant all over, haploid cells are not the same and they change, they are random. [which is why you're one type of a luckiest person for you to have been created].
Hit your chest and say "I am Lucky!" Yeah, you most certainly are.
Daughter cells:
The resulting cells after a particular cell has been divided.
Sister chromatids:
When a chromosome replicates or clones itself, this lead to two exact and identical chromosomes, but they're are joined together at a point (called centromere) and are referred to as sister chromatids. Here, although the chromosome is already in two, it is still referred as being one Chromosome.
Homologous chromosomes:
It is very important for you to understand this concept else meiosis may confuse you.
Homologous chromosomes are two exact types of chromosomes (in shapes and in sizes and in what type of gene they carry), but they are not identical (because they do not carry the same genes).
[genes from the father and mother can be different, the father can be tall and the mother can be short as women prefer taller guys, the long s shaped chromosome that carries the tallness gene of the father and the long s shaped chromosome that carries the shortness gene of the mother are referred to as homologous chromosomes in the child. You can see they are both long and s shaped, but they're different. One carries gene for tallness and the other carries gene for shortness].
Let me put this in other words, when a human father mates with a human mother, he has 23 chromosomes in his sex cell (sperm) which fuses with the 23 chromosomes in the mother's cell (Ovum).
In these 23, 23 chromosomes, they come in various types.
What I mean is that in 23 chromosomes from a gamete cell, each chromosomes are different from one another. Their sizes and shapes are different.
The same thing happens in the 23 chromosomes from the partner's gamete, each chromosomes are different from one another in shapes and sizes.
But one thing is, as the 23 chromosomes in a sex cell (let's say the father's cell) are different from each other, they have their exact same type in the opposite sex's gamete.
The exact same type of chromosome in two different gametes are referred to as homologous chromosomes. They appear the same, perform the same function, but it is what is inside them that is different.
I believe you get me now! If you don't get it please read it again! Do not proceed with this lecture if you do not get it!
Synapsis:
Synapsis is the coming together of homologous chromosomes.
Crossing over:
Crossing over is the exchange of genetic material between homologous chromosomes when they come extremely close to each other.
Recombination:
Recombination is the exchange of genetic material either between homologous chromosomes or between different regions of the same chromosome (sister chromatids are referred to as the same chromosome).
Synaptonemal complex:
Synaptonemal complex is a protein structure that connects two homologous chromosomes promoting crossing over.
Chiasmata:
Chiasmata (chiasma) is the point where genetic materials have been exchanged by non-sister chromatids of a homologous chromosome during crossing over.
Tetrad or Bivalent:
When two different sister chromatids of a homologous chromosomes come extremely close together (synapsis), they are joined by a protein structure (synaptonemal complex) making them a total of four chromatids. These four chromatids is referred to as a tetrad or Bivalent.
Stages of Meiosis
Like I said earlier, Meiosis involves two Divisions.
That is Meiosis 1 and Meiosis 2
Meiosis 1
Just prior to meiosis 1, the cell undergoes a round of chromosome replication called INTERPHASE.
During Interphase, each chromosome is replicated (cloned) and forms sister chromatids (two chromatids which are joined together by the centromere).
After Interphase is complete, meiosis 1 can now begin.
Meiosis cell division is divided into four phases (like mitosis) which I shorten as PMAT. PMAT is pronounced as pee mat and it is short for:
- Prophase.
- Metaphase.
- Anaphase.
- Telophase.
Prophase 1
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After the chromosomes have condensed, sister chromatids of same homologous chromosomes come very close together and one aligns on top of the other. This is called synapsis.
During this synapsis, the two different sister chromatids aligns exactly together (like when you place your two palms exactly on top of each other) and this is aided by a protein structure called synaptonemal complex.
When these two sister chromatids of the same homologous chromosomes come together, they form four chromatids called a tetrad or a Bivalent.
Remember that homologous chromosomes are 2 (same type, size and shape) in every animal cell, these 2 chromosomes have replicated in interphase (forming sister chromatids), when all now come together they form a total of 4 identical chromatids called a tetrad.
Please I hope you are following me!
When these four chromatids are still aligned, the synaptonemal complex will tell the two different sister chromatids that “you guys should better exchange something, else, I'm not going to separate you!”
This will make one section of one sister chromatids to cut out, the same exact thing happens to the other sister chromatids.
When these two sections have cut out, they now substitute each other's places.
This process is called Crossing Over.
The places that have been cut out and replaced in the two sister chromatids are called Chiasmata (chiasma).
Crossing-over is important because it produces new combinations of alleles in the cell.
It's common for multiple crossovers (up to 25!) to take place for each homologue pair.
The spots where crossovers happen are more or less random.
After crossing over is carried out, the synaptonemal complex will ease its grip on the both sister chromatids, but the chiasmata still keeps them together and this marks the end of Prophase 1.
Metaphase 1
As prophase 1 ends, centrioles (from the centrosome matrix of the cell) start heading towards opposite sides of the cell. When they have reached opposite sides of the cell, they start forming spindle (which is like a thin wire or microtubule).
Spindle from opposite poles of the cell attach to each tetrads and align them across the center of the cell (metaphase plate).
Anaphase 1
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Once spindle fibers from opposite ends of the cell have attached to every tetrads, anaphase begins.
Here, the spindle pulls each sister chromatids of every homologous pair towards the end of the two opposite poles.
The difference between the Anaphase 1 in meiosis and Anaphase in mitosis is that in mitosis, the spindle fiber separate each chromosomes from sister chromatids and the centromere splits, but here, the spindle fiber separate each sister chromatids from tetrads and the centromere still attaches the sister chromatids together.
The spindle does not separate each chromosome here, but it separates each sister chromatids from the tetrads.
In every tetrad, if a set of sister chromatids are pulled to one pole of the cell, the other set of sister chromatids will be pulled towards the other pole.
I Hope you get that? Let it sink in!
Telophase 1
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When the non-identical sister chromatids have reached opposite ends of the cell, the chromosomes decondense (become less visible), the spindle dissolves and a nuclear membrane forms around the two new clusters — this does not happen in every cell since Meiosis 2 is still going to occur.
Cytokinesis follows telophase 1, forming two new cells.
Meiosis 1 overview
- Meiosis 1 results in two cells, called daughter cells.
- The two cells produced in meiosis 1 contain sister chromatids (two joined chromosomes) but they are not identical.
- The two cells produced by meiosis 1 have sets of chromosomes and alleles that are different from each other and from the parent cell.
Meiosis 2
The two cells produced by meiosis 1 now enter a second meiotic division.
What essentially happens in meiosis 2 is now the separation of the non-identical sister chromatids from each other.
Unlike the first division, neither cell goes through a round of chromosome replication before entering meiosis 2.
Meiosis 2 follows same stages as meiosis 1,
- Prophase 2.
- Metaphase 2.
- Anaphase 2.
- Telophase 2.
Prophase 2
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As the cells enter prophase 2, the chromosomes recondense and become visible again. The nuclear envelope also breaks down, if needed.
The centrosomes (centrioles) move apart, the spindle forms between them, and the spindle microtubules begin to capture chromosomes at the centromere.
The two non-identical sister chromatids of each chromosome are captured by microtubules from opposite spindle poles.
Metaphase 2
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In metaphase 2, the chromosomes line up individually along the metaphase plate.
Anaphase 2
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In anaphase 2, the non-identical sister chromatids separate (centromere splits) and are pulled towards opposite poles of the cell.
Telophase 2
In telophase 2, nuclear membranes form around each set of chromosomes, and the chromosomes decondense.
Cytokinesis splits the chromosome sets into new cells, forming the final products of meiosis: four haploid cells in which each chromosome has just one chromatid. In humans, the products of meiosis are sperm or egg cells.
Rounding up of Gamete formation in Animals
Meiosis is responsible for the formation of sex cells or gametes that are responsible for sexual reproduction.
It maintains the constant number of chromosomes in offsprings by halving it in parents. This is important because the chromosome number doubles after fertilization.
The genetic mutation occurs due to irregularities in cell division by meiosis. The mutations that are beneficial are carried on by natural selection.
Crossing over produces a new combination of traits and variations.
In human male meiosis, all four daughter cells of meiosis will go through a complicated cellular differentiation process called spermiogenesis to become mature functional sperm.
In contrast, oogenesis results in only one of the four products of meiosis becoming an egg. The other three products donate their cytoplasm to the chosen oocyte, and then die. The oocyte then completes the cellular differentiation process to become a mature egg.
Fertilization doubles the number of chromosomes from 1 set to 2 sets.
Error in Meiosis
The failure of two chromosomes to cross over or recombine properly is called NONDISJUNCTION.
Nondisjunction occurs either because two homologs failed to pair and∕or recombine or because of a failure of the cell to properly move the chromosomes on the meiotic spindle. The result of nondisjunction is the production of gametes that are aneuploid (carrying the wrong number of chromosomes). When such a gamete is involved in a fertilization event, the resulting zygote is also aneuploid.
Those cases where the embryo carries an extra copy of a given chromosome are said to be trisomic, while those that carry only one copy are said to be monosomic for that chromosome.
Most aneuploid zygotes are not viable and result in early spontaneous abortion or miscarriages.
There are no viable monosomies for the human autosomes; however, a few types of trisomic zygotes are capable of survival. These are trisomies for the sex chromosomes (XXX, XXY, XYY ), trisomy 21 (Down syndrome), trisomy 18, and trisomy 13.
Differences between mitosis and meiosis
| Mitosis | Meiosis |
|---|---|
Homologous chromosomes do not pair up. | Homologous Chromosome pair up (synapsis). |
No genetic exchange between homologous chromosomes. |
There's genetic exchange between homologous chromosomes (Crossing Over). |
|
DNA duplication followed by one cell Division. |
DNA duplication followed by two cell divisions. |
|
One diploid cell produces 2 diploid cells OR one haploid cell produces two haploid cells. |
One diploid cell produces 4 haploid cells. |
|
New cells are genetically identical to original cells except in time of Mutation. |
Each new cell has a unique combination of genes. |
Summary
- A gamete cell is simply a reproductive cell. They are also called germ cells or non-somatic cells.
- The gamete (sex cell) in Male animals is called Sperm.
- The gamete (sex cell) in female animals is called Ovum or Egg cell.
- Gamete cells are formed by what we call Gametogenesis.
- Male Gamete + Female gamete = Zygote.
- Gamete or reproductive cells divide by what we call Meiosis.
- Diploid cells that divide by meiosis to give rise to a gamete are called germ line cells.
- Meiosis is a type of cell division in which the number of chromosomes per cell is cut in half.
- Homologous chromosomes are two exact types of chromosomes (in shapes and in sizes and in the which type of gene they carry), but they are not identical (because they do not carry the same genes).
- Synapsis: Synapsis is the coming together of two homologous chromosomes.
- Crossing over: Crossing over is the exchange of genetic material between homologous chromosomes when they come extremely close to each other.
- Synaptonemal complex: synaptonemal complex is a protein structure that connects two homologous chromosomes promoting crossing over.
- Chiasmata: Chiasmata is the point where genetic materials have been exchanged by non-sister chromatids during crossing over.
- Meiosis involves 2 cell division: Meiosis 1 and 2.
- Meiosis 1 cell division is divided into four stages:
- Prophase.
- Metaphase.
- Anaphase.
- Telophase.
- Just prior to meiosis 1, the cell undergoes a round of chromosome replication called interphase.
- Meiosis 1 results in two haploid daughter cells, each containing a pair of sister chromatids.
- What essentially happens in meiosis 2 is the separation of the sister chromatids from each other.
- In human male meiosis, all four daughter cells of meiosis will go through a complicated cellular differentiation process called spermiogenesis to become mature functional sperm.
- Oogenesis results in only one of the four products of meiosis becoming an egg. The other three products donate their cytoplasm to the chosen oocyte, and then die. The oocyte then completes the cellular differentiation process to become a mature egg.
- The failure of two chromosomes to cross over or recombine properly is called nondisjunction.
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Test Questions
1.
The end product of meiosis is?
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