MEIOSIS
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If mitosis is responsible for growth and regeneration, meiosis is responsible for all sexual reproduction. That's a lot of responsibility!
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In sexual reproduction, two cells that are haploid (have half the DNA of the species, or one of each chromosome) get together to form a diploid cell (the beginning of a new organism, which is diploid, or has a pair of each chromosome, just like you and me).
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AM I A JOKE TO YOU??
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This is why we say 2n for diploid and n for haploid -- it lets us know that if an organism has a certain number of chromosomes on a haploid sex cell, it will have twice as much on the actual final organism. So, it works like a formula. For example, a fungus-farming ant has 4 pairs of chromosomes (a total of 8). This means an ant haploid gamete will have ____________ chromosomes.
An animal has 57 pairs of chromosomes on their cells. How many chromosomes will the gamete have?
Each human has 23 pairs of homologous chromosomes. This means that every pair that is a match will be the same size, have genes coded for the same thing, on the exact same place, even if one says blue eye and the other says brown eye! Without meiosis, our gametes would have 23 pairs each, which means our offspring would turn out to have 46 pairs, and this would continue multiplying.
The consequences would be terrifying!
So, meiosis is not needed on our whole body; that's why we said that all the cells in our body reproduce asexually. But it is crucial for sexual reproduction.
Meiosis happens only in an area that is designed to create a gamete. In humans, these areas are the testicles for the male, and the ovaries for the female. The male parent contributes one gamete to the offspring, which is the sperm cell; and the female parent contributes one gamete, which is the ovum, or egg cell. We will look at these in more detail on the chapter "sexual reproduction".
Meiosis happens only in an area that is designed to create a gamete. In humans, these areas are the testicles for the male, and the ovaries for the female. The male parent contributes one gamete to the offspring, which is the sperm cell; and the female parent contributes one gamete, which is the ovum, or egg cell. We will look at these in more detail on the chapter "sexual reproduction".
Activity - Gamete Creation
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Our visible characteristics, or phenotype, are defined by our genotype, which is our genetic makeup.
Every individual gamete (so, every egg and every sperm) get random genes for each characteristic. We will see how this happens later when we study the meiosis process.
Some genes are dominant; this means when they show up on a parent, all the children will have that visible characteristic even if the other parent did not have it. Dominant genes are represented by an uppercase letter.
Some genes will be recessive; these genes will only affect the phenotype of the offspring if the other parent also gave the child the same recessive gene. Recessive genes are represented by a lowercase letter.
Every individual gamete (so, every egg and every sperm) get random genes for each characteristic. We will see how this happens later when we study the meiosis process.
Some genes are dominant; this means when they show up on a parent, all the children will have that visible characteristic even if the other parent did not have it. Dominant genes are represented by an uppercase letter.
Some genes will be recessive; these genes will only affect the phenotype of the offspring if the other parent also gave the child the same recessive gene. Recessive genes are represented by a lowercase letter.
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For this activity, you will create a bracelet with your individual phenotype.
If you have a dominant gene, add a RED bead to your pipecleaner. If that trait shows a recessive gene, add a BLUE bead. Look at every one of the traits carefully; ask a friend for help if needed. Once your bracelet is done, follow the instructions on your worksheet! |
If each gamete has half the DNA, and that's where the story ended, then siblings should look pretty similar. But they often look very different unless they are identical twins.
Your siblings and even parents are different than you are.
How does this happen?
Your siblings and even parents are different than you are.
How does this happen?
For instance, curly hair is a dominant gene; this means even if only one of your parents has curly hair, the children will all have some sort of curl.
This is why sometimes a child has blue eyes when both parents and even grandparents had brown eyes; as brown eyes are dominant, they could be hiding the blue eye gene for many generations, and only finally appearing when the other parent also provides a gamete with the same recessive gene!
This is why sometimes a child has blue eyes when both parents and even grandparents had brown eyes; as brown eyes are dominant, they could be hiding the blue eye gene for many generations, and only finally appearing when the other parent also provides a gamete with the same recessive gene!
Homozygous and Heterozygous
When a parent has two of the same gene of a certain trait, for instance -- two genes for curly hair, then we say they are homozygous. This would look like "AA" or "aa" for instance.
A parent can be homozygous recessive ("aa") or homozygous dominant ("AA"). If a parent has one dominant and one recessive gene, "Aa", we say they are heterozygous.
One thing is not clear: When a father provides the offspring with a gamete that came from his mother, how the heck does the genetic material gets mixed with the genes that came from his father? How come we can have all these possibilities? Do the genes jump from one chromosome to the other??
WHAT IS GOING ON!!!
Well, ACTUALLY....
A parent can be homozygous recessive ("aa") or homozygous dominant ("AA"). If a parent has one dominant and one recessive gene, "Aa", we say they are heterozygous.
One thing is not clear: When a father provides the offspring with a gamete that came from his mother, how the heck does the genetic material gets mixed with the genes that came from his father? How come we can have all these possibilities? Do the genes jump from one chromosome to the other??
WHAT IS GOING ON!!!
Well, ACTUALLY....
BEHOLD! The answer to all your questions!!!
The Opposite of Cloning: The Meiosis Process
The meiosis process may seem familiar to the mitosis at first hand. The phases are named the same way, for instance. But the meiosis actually divides twice!
The first division of meiosis looks like this:
The first division of meiosis looks like this:
Meiosis I
You probably noticed that the homologous chromosomes this time separate from their pair and go to each cell intact, with both their chromatids, even though they divided just like mitosis.
They never separate their "hands" -- those are called centromeres, by the way!
This means that each one of the daughter cells gets only one chromosome from each homologous pair. Read more about it below, under Independent Assortment.
In the end of this first cell division, we are left with two cells, just like mitosis; but then, each of them divides again.
They never separate their "hands" -- those are called centromeres, by the way!
This means that each one of the daughter cells gets only one chromosome from each homologous pair. Read more about it below, under Independent Assortment.
In the end of this first cell division, we are left with two cells, just like mitosis; but then, each of them divides again.
Meiosis II
Before the second cell division, there is no replication of DNA. This time, the sister chromatids from each chromosome finally separate, forming 4 very different haploid gametes.
Big events that contribute to the variety of life
1. Crossing Over
This is a super interesting event that happens during Prophase 1 of Meiosis.
During this process, parts of non-sister chromatids (the ones not holding "hands" with you) give each other parts of their genes, trading them. This is done gene-to-gene, so if one gives a gene that controls hair, the other will also do the same.
This process can happen multiple times per homologous pair, so the gametes have a mind-blowing number of possibilities.
This also makes it easier for mutations to happen.
During this process, parts of non-sister chromatids (the ones not holding "hands" with you) give each other parts of their genes, trading them. This is done gene-to-gene, so if one gives a gene that controls hair, the other will also do the same.
This process can happen multiple times per homologous pair, so the gametes have a mind-blowing number of possibilities.
This also makes it easier for mutations to happen.
2. Independent Assortment
This is also done during Meiosis 1; at the end, the chromosomes are fished to each side. This is done randomly, so each daughter cell will have some chromosomes that came from a grandparent, and some from the other, randomly.
This would already bring variety to the gametes, but before they separate, the crossing over process (above) makes sure grandma's chromosomes trades parts with grandpa's.
This would already bring variety to the gametes, but before they separate, the crossing over process (above) makes sure grandma's chromosomes trades parts with grandpa's.
3. Mutations
Some small mutations in genes may have no effect at all in an organism. But big changes in the organization of DNA can have unexpected outcomes. This happens when pieces of chromosomes are lost, duplicated or moved within a chromosome, or moved to another chromosome where it should not be.
The 5 types of mutations that can happen in individual chrmosomes are listed below.
The 5 types of mutations that can happen in individual chrmosomes are listed below.
DELETION
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In this case, a piece of the chromosome gets lost when it is crossing over to its homologous, resulting in a chromosome that is missing genes.
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DUPLICATION
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When trying to trade two pieces of DNA, one of the homologous chromosomes makes a mistake and ends up copying instead of trading, so that it ends up with the piece it received from the oposite chromosome and his old piece -- now they have two of the same section of gene.
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INVERSION
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When trading the DNA, the homologous chromosome inverts the gene piece, which could cause a completely different protein to be generated.
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INSERTION
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One of the chromosomes ends up giving a piece to a totally random chromosome that is not even its homologous. That's what happens when you are swimming in soup.
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TRANSLOCATION
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The same as above, but the confused chromosome actually convinces the other rando chromosome to trade pieces with it. And now you have blue eyes on your butt.
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This not only makes the DNA different, it also alters the protein produced by that portion of gene, which in turn will alter the organism, how it looks and functions. This can happen when the parent organism is exposed to environmental things such as chemicals and radiation during the formation of the gamete.
Whole chromosome mutations can also cause chromosomes not to divide, or one gamete to get an extra copy of one of the chromosomes. This often causes the offspring to fail to develop.
Whole chromosome mutations can also cause chromosomes not to divide, or one gamete to get an extra copy of one of the chromosomes. This often causes the offspring to fail to develop.
Mitosis versus Meiosis
There are many similarities between the processes of mitosis and meiosis, but also important differences. A summary can be seen below.
Now that you know what the process of cell division looks like during asexual and sexual reproduction, let's dig even deeper and see when and where meiosis happens!
Vocabulary:
The "child you better know" list
- Haploid
- Diploid
- Gamete
- Chromatid
- Centromere
- Homologous
- Allele
- Testicles
- Ovaries
- Ovum
- Sperm
- Genotype
- Phenotype
- Homozygous
- Heterozygous
- Dominant gene
- Recessive Gene
- Meiosis I
- Meiosis II
- Crossing Over during Prophase I
- Independent assortment
- Mutations/types
