Embedded Files
Diploid vs. Haploid
Diploid vs. Haploid
Unlike in mitosis, meiosis is going to involve numerous changes to the number of chromosomes in a cell. Not only that, but there will specifically be changes in the number of copies of each chromosome. What does that mean? Well, humans (including you, of course) have 46 chromosomes... but more specifically, humans have two copies of 23 different chromosomes (excluding atypical chromosomal phenomena such as Down Syndrome which we will discuss later). That means that we have 23 unique strands of DNA that have different genes on them. However, we have 2 copies of each of those genes because of the the two copies. These copies of each chromosome are referred to as homologous chromosomes.
This gets a little confusing for now, but this means that you have 2 copies of every gene in your cells. So if there was a single gene for hair color (there are actually many that contribute to this trait, as with most traits), you could have one allele, or version of a gene, that codes for black hair and another allele that codes for blonde. The only typical exception to this would be for a male individual, as they do not have two copies of the X chromosome, but, instead, have one copy of an X and one of a Y chromosome.
Having two copies of each chromosome (as you can see in the shown karyotype above) classifies our cells as diploid (sometimes represented with a 2n - 'n' being the number of chromosomes). Ploidy is the number of copies of chromosomes you have. 'Di-' means two. So, we have two copies of each - hence, diploid. In reality, there are theoretically no limits to the ploidy level possible. Many organisms even have hundreds of copies of each chromosome! Does this mean they're more complex than humans? No, not really, there isn't a significant link between ploidy level and complexity.
Having one copy of your chromosomes is referred to as being haploid. This is represented by n. Remember, 'n' represents the number of chromosomes. So, the haploid number of a human would be 23 because if we have one copy of each chromosome, and we have 23 different chromosomes, then 1x23=23. So, humans have a diploid number of 46 chromosomes and a haploid number of 23 chromosomes.
But if all of human cells are diploid, why do we have a 'haploid number'? Well, not all of your cells are diploid. Some don't even have DNA, for one thing. But those are the unusual cases, so we won't get into that here. The only body cells you need to know that are haploid are the gametes, or sex cells. There are, of course, the sperm and the egg in humans. These cells are super important to this unit and meiosis is the process by which we go from our typical diploid cells to form haploid gametes. We will also soon discuss some of the benefits of doing this strange activity.
Overview of Meiosis - It Happens Twice?!
Overview of Meiosis - It Happens Twice?!
Meiosis is the process by which we take our diploid cells and create haploid gametes. Meiosis sounds an awful lot like mitosis, and they actually have a lot of similarities, so you already did the hard part of learning about meiosis. Meiosis involves some key differences you must keep in mind, however. This diagram is a nice summary of some of the basic differences for an overview of meiosis.
Firstly, you will notice that there is a meiosis I and a meiosis II... yes - meiosis kind of occurs twice. You will also notice that the ploidy of the cells change. You can see that the initial cells are diploid, but meiosis I creates two haploid cells from that one diploid cell. This is very important, and the reasons why will only be clear as we go through the steps of meiosis I and II and track the chromosomes.
Keep in mind that this is all in the interest of sexual reproduction. Mitosis was the mechanism by which asexual reproduction occurs and meiosis is the mechanism by which sexual reproduction occurs.
Let's take a closer look at meiosis I and meiosis II, paying particularly attention to when the ploidy of the cells change. Try to figure out the ploidy just by looking at the diagrams as you read about each stage. This is an excellent studying method for this particular process, so I recommend you give a shot multiple times throughout the chapter.
Stages of Meiosis I
Stages of Meiosis I
Meiosis I, as mentioned previously, is comprised of several steps. Luckily, these steps have almost the same names as those we learned in mitosis: prophase I, metaphase I, anaphase I, and telophase I.
A. Prophase I
A. Prophase I
Chromosomes condense and become visible under a microscope
Nucleolus breaks down & spindle fibers begin to form
Homologous chromosomes pair up
Sections of DNA can be exchanged between homologous chromosomes in a process called crossing over
B. Metaphase I
B. Metaphase I
Spindle fibers attach to the kinetochores (centromere) on each sister chromatid
The pairs of homologous chromosomes align at the cell's equator
The orientation of the chromosomes is random. Members of each pair can face either pole
C. Anaphase I
C. Anaphase I
Spindle fibers contract and separate the homologous chromosome pairs
This halves the chromosome count of each cell!!!
So we have officially (once these cells close off into two) gone from diploid to haploid
D. Telophase I
D. Telophase I
Homologous chromosome pairs reach opposite poles
Nuclear envelopes reform around each haploid set
A cleavage furrow forms which eventually results in two distinct haploid cells
Note that meiosis I resembles mitosis in a lot of ways, as discussed previously. However, be sure that you are able to identify the differences by looking at how the chromosomes align, crossing over occurs, and ploidy levels change.
Stages of Meiosis II
Stages of Meiosis II
Meiosis II is also comprised of steps with similar names: prophase II, metaphase II, anaphase II, and telophase II. Now, of course, the cells are haploid, having one copy (albeit copies that contain two identical sister chromatids) of each chromosome.
A. Prophase II
A. Prophase II
Nuclear membrane breaks down and the spindle apparatus forms in each cell
Centrioles move to the poles of the haploid cells
Spindle fibers attach to each set of sister chromatids at their kinetochores
B. Metaphase II (not shown)
B. Metaphase II (not shown)
Chromosomes align along the equator of the cell
Which pole each sister chromatid enters is random
C. Anaphase II (shown)
C. Anaphase II (shown)
Spindle fibers contract and separate the sister chromatids at the centromere
Chromatids (now considered chromosomes just like in mitosis!) are pulled to opposite poles
D. Telophase II
D. Telophase II
Nuclear membranes begin to form around the four haploid nuclei
Cleavage furrows form which results in four distinct haploid cells
Note that this whole process has formed 4 haploid daughter cells. These cells are all unique and are not duplicates, unlike what we saw in mitosis.
Meiosis I vs. Meiosis II - Zooming in on Chromosomes
Meiosis I vs. Meiosis II - Zooming in on Chromosomes
What Was It All For?
What Was It All For?
As mentioned, meiosis produces gametes, specifically 4 haploid daughter cells. In reality, meiosis does create 4 sperm cells in males, but only produces one egg in females. This is due to the immense energy and resources that must be devoted to each egg cell. Sperm basically just carry DNA - eggs do all the hard work.
Two haploid gametes will, of course, come together via fertilization and form a zygote (2n) which will eventually develop into a multicellular organism. Sexual reproduction is incredibly costly in terms of time and energy when compared to asexual reproduction, so let's dive in an examine what the benefits are: namely the higher genetic diversity created via sexual reproduction.
How Meiosis Creates Genetic Diversity
How Meiosis Creates Genetic Diversity
You've probably noticed that, unless you have an identical twin, no one else on Earth appears to look like you. That's amazing - just think about it, every human (even identical twins if you look beyond just the DNA) has a unique combination of traits and characteristics unlike those found in anyone else. This is not only a really interesting consequence of sexual reproduction, but is also incredibly helpful for the population. Genetic diversity is crucial - think about it this way... if there was a disease that appeared and wiped out everyone that had brown hair, that would be disastrous for our species. However, imagine if we had no genetic diversity and everyone was a clone of everyone else... Hope we don't all have brown hair! Genetic diversity helps protect a population from extinction and a wide variety of threats
So how does meiosis result in such genetic variation? Well, in 3 ways that were modeled in all of those images of chromosomes but warrant special attention here: crossing over, independent assortment, and random fertilization.
A. Crossing Over
A. Crossing Over
During prophase I, sections of homologous chromosomes are swapped. This results in new allele combinations that wouldn't be possible without crossing over.
B. Independent Assortment
B. Independent Assortment
Random assortment (or random orientation) during metaphase I and metaphase II results in nearly infinite possible combinations of chromosomes. Even without crossing over, there are 223 possibilities in humans! That is 8,388,608. This means that there are over 8 million unique sperm or egg cells that each human can produce. But in reality, there are even more because of the aforementioned crossing over. So it is possible for your parents to have another child that is identical to you... But, it would require both parents making the same gamete as with you. So that is a possibility of (1/8,388,608)(1/8,388,608). As a percentage, that is 0.000000000001421%… I don't see that happening any time soon. (Note: this calculation is assuming you have no crossing over possible)
C. Random Fertilization
C. Random Fertilization
The fusion of sperm and egg cells is random - there is not much choice that goes into which sperm or which egg reach one another and fuse. So not only would each parent have to make those identical gametes to recreate you, but they would also have to be the ones that just happened to fuse! Just think about what an amazing phenomenon you are... out of all of the infinite possibilities of DNA combinations, your parents got you. You're one in a million... well actually one in an infinity.
"Sexual reproduction demands the collapse of an organism into a single cell, but then requires that single cell to expand back into an organism. ... A single cell must be capable of carrying the entire set of instructions to build an organism from scratch - hence genes." (The Gene: An Intimate History by Siddhartha Mukherjee)
Google Sites
Report abuse