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Genetic Variance

An image of two identical white sheep facing each other. The image is a side view of the heads only. The sheep have their mouth open and their tongue is protruding slightly over their teeth. On the ear of each sheep is a green tag. The background is grey.

In the Structures Unit, you explored various reproductive strategies that resulted in either genetic continuity or genetic variance. 

This is the question/answer icon. Challenge!

  1. Which reproductive strategy leads to genetic variance in offspring, rather than simply maintaining genetic continuity?
    Answer

    Sexual reproduction.

 
  1. What is the difference between a diploid and haploid cell?
    Answer

    The main difference between haploid and diploid cells is the number of chromosome sets found in the nucleus. Diploid cells contain two complete sets (2n) of chromosomes. Haploid cells have half the number of chromosomes (n) as diploid.

 Important!

You've had a chance to review mitosis and how that process created daughter cells genetically identical to the parent cell. Each new generation of cells would therefore be a clone of the last generation. This type of cell division can used for growth, repair, maintenance, and asexual reproduction.

When looking at sexual reproductive strategies, you saw how combining genetic material from two parents can result in different traits. It was also mentioned that each parent can pass on different genetic material to each gamete, and this is why siblings with the same parents can display different traits from one another. This genetic variance plays an important role in natural selection and evolution, as you will see later on in this unit.

What Is Meiosis?

Meiosis(definition:A type of cell division that results in four genetically diverse daughter cells with half the number of chromosomes as the parent.) is the process that produces gametes that contain different genetic material from each other. In this activity, you will look at the phases of meiosis and investigate how it differs from mitosis. You will also explore how meiosis is a factor behind genetic variance.  

Although the results are quite different, there are a number of similarities between mitosis and meiosis. In fact, many of the phase names are shared between the two processes. Some of the steps are virtually identical. It is therefore a good idea to compare the two prior to looking at the nuts and bolts of meiotic division.

This is the Portfolio icon. Mitosis versus Meiosis

The following video will play both processes side by side to allow you to compare and contrast each phase. 

 
 

There are a lot of similarities between the two processes. The differences, however, have a profound impact on life and the variance we see between individuals of the same species.

This is an image of a group of twenty children with a field of yellow flowers in the foreground, and blue sky with wispy clouds in the background. The children show a number of different traits, including skin colour and facial features.
Although we are all the same species, there can be great variation in individuals.

You will now have a chance to explore the process of meiosis and the events within that process that result in gametes that are genetically diverse from one another. When a gamete combines with another gamete, it produces a daughter cell whose gender is determined by the combination of chromosomes. You will see how geneticists(definition:One who studies genes and the traits that they produce.) can map the chromosomes in a cell to determine information such as the sex of the organism.

Action.

ACTION

Genes and a T-Shirt

This is an image of a cartoon showing a set of steps leading to a door with a sign saying ‘Night Club’ above it. In front of the stairs are two bouncers. Both are large men wearing an overcoat, sun glasses, and a bow-tie. To the lower left of the image is a velvet rope across two stands. The bouncer on the left of the image has his arms crossed and is looking to the left of the image. The bouncer in the centre of the image is pointing to the right of the image at a piece of DNA in the form of a double helix, which looks like a spiral staircase. The caption at the bottom of the image reads, ‘Sorry mate...no genes.’
by Steve Ward

Before going much further, it is a good idea to refresh your memory about some of the basics of chromosome structure. A chromosome,(definition:Thread-like structure of composed of DNA that carries the genetic information of the organism.) which contains the DNA, holds the genetic information needed to maintain and grow the cell. A section of a chromosome that holds the information for one specific trait(definition:A distinguishing quality or characteristic.) is called a gene.(definition:A section of DNA that codes for a specific trait and that can be passed on to the next generation.) Multiple genes are located on each chromosome.

Here is the basic structure of a eukaryotic chromosome at metaphase:

This is a diagram of two X shaped chromosomes beside each other. The chromosomes look identical to one another, and are drawn with a series of coloured bands running horizontally across the chromosomes. The bands, from top to bottom, are dark green, baby blue, light green, orange, light green, pink, dark green, baby blue, pink, and peach. The chromosome on the left has lines pointing to each lower ‘arm’ of the X labelled ‘Sister chromatids.’ The chromosome on the right has the label ‘One duplicated chromosome’ below it, with a bracket to indicate that the label applies to the entire chromosome. The right chromosome has an additional label on the right that says, ‘Centromere’ with a line pointing to the centre of the chromosome where all four arms meet at the centre of the X shape. At the top of the image is the label, ‘Homologous pair of chromosomes’, which a bracket that indicates that the label applies to both X-shaped chromosomes together.
The structure of chromosomes in a homologous pair.
by Wikimedia

The ‘X’ shaped chromosome is actually made up of two chromatids(definition:Either of the two daughter strands of a duplicated chromosome.) connected together at the centromere.(definition:The region of a chromosome that attaches the two sister chromatids together.) Each sister, or single chromatid, is a genetic copy or duplicate of the other. The coloured sections on the image can represent the different genes. As you can see, each sister chromatid would have a copy of each gene.

You may be familiar with all of those terms, but homologous pair(definition:A pair of chromosomes, one from each parent, which are similar in length, gene location, and centromere location.) may be new to you. Homologous simply means similar in structure and location. Each cell has a set of chromosomes from each parent, and the genes on those chromosomes are found in the same location or loci.(definition:A fixed position on a chromosome.) Offspring receive the same set of genes from each parent, but the way those genes are expressed may be different. For example, you will receive genes for hair colour and eye colour from each parent, but the genes from your mother may code for straight hair and blue eyes, and the genes from your father may code for curly hair and brown eyes. You can think of homologous chromosomes like a pair of shoes. Both shoes are the same in basic structure including colour, markings, type of laces, etc., but one is left and one is right. They may not be exactly the same, but together they make an obvious pair.

Why Do We Need Meiosis?

There is a clue in the name. The word meiosis is based upon a Greek word that means 'to diminish.' You know that each organism has a specific number of chromosomes in its body or somatic cells. For humans, that chromosome number is 46. Looking back at the reproduction strategies from Unit 1, you will remember that a gamete from each parent comes together to form the zygote. If the chromosome number in each gamete isn’t reduced or ‘diminished,’ then the zygote would have double the chromosomes that it should! A process is therefore needed to reduce the chromosome number. Gametogenesis is that process. A specialized diploid cell undergoes meiosis resulting in haploid cells. 

Remember that a normal body cell has the diploid number of chromosomes (46 for humans), and the gametes have the haploid number (23 for humans).

Did You Know?

Although the process of meiosis reduces the chromosome number to produce zygotes with the normal diploid chromosome number, some organisms have a polyploid(definition:Having multiple sets of chromosomes.) condition, meaning that they have multiple sets of the chromosomes in their cells. This is a situation often induced in agriculture, as the polyploid condition can produce desirable traits.

For example, polyploid fruit are often sterile, and therefore do not produce seeds. Next time you eat a seedless grape or watermelon, you have the polyploid condition to thank!

This is an image of three watermelons on a wooden table arranged in a row. The melons on the left and right of the image are whole, laying on their sides, and are green with darker green stripes running along them from top to bottom. The melon in the centre has been cut in half, with the cut, pink surface facing the viewer. There are blurred leaves in the background.
Seedless watermelon is a common sight during the summer.

This may sound like a broken record (uh, for those people who are younger, this is a reference to a scratched vinyl record which skips and keeps playing the same section over and over again), but remember that meiosis is also the process that allows for variation between gametes. This is a big deal in terms of diversity.

Challenge!

We often see variation of characteristics within species in our everyday lives. Think of all the different types of dogs you see when you take a walk. How about all the different varieties of the same type of vegetable at the market? If you are a gardener, do you have choice of colour and height of the same type of plant? As you work through this activity and the others in this unit, take some time to think about all the ways that humans have used variation in species to their advantage. In general, do you feel that these human interactions have been beneficial to life on Earth? 
 

How it All Works

The process of meiosis is a good example of interactions in Biology. There are many videos and simulations that go through the steps of meiosis and give a good overview of the process. Having a good understanding of the process as a whole will make following the individual steps a bit easier. Before you look at each phase in detail, take a few minutes to go over this video. As you watch, try to keep track of the chromosome number. At what point does the cell go from diploid to haploid?

 

Homologous chromosome tetrads,(definition:A pair of homologous chromosomes, containing four chromatids.) crossing over,(definition:A process where homologous chromosomes exchange genetic material.) two rounds of cell division, and random, independent assortment(definition:Formation of random combinations of chromosomes during meiosis.) are all aspects of the meiotic process that are quite different from mitosis. Let’s take a look at the main phases of meiosis and then how those aspects listed above create that variation I just can’t seem to stop talking about.

Set Phasers to Stun!

As you have seen, the basic phases of meiosis are quite similar to mitosis, only there are two rounds of cell division. In addition, a few of the events that occur are unique to meiosis.

 

Scoping Out the Phases

This is a closeup image of the nose piece of a compound light microscope. At the bottom of the image is the microscope stage, blurred in the foreground by in focus in the background. On the stage you can just see a glass microscope slide. The the top left of the image, there are four silver, cylindrical objective lenses attached to a round, silver convex dome. The lenses are arranged like the legs of a chair. The two lenses in the foreground are blurred, and the two lenses in the background are in focus. The background is grey.

It’s relatively easy to identify the various meiotic stages from diagrams, but how about the images from a microscope? The following interactive will test your skill at determining which phase is which.

This is the dropbox icon. Scoping it Out

Using the microscope interactive below, view the slide that contains developing pollen cells from a Lilium plant. Using your good independent work(definition:I will be complete the meiosis microscope activity and identify the different stages.) skills, you will create a biological diagram of two separate cells, each showing a different phase of meiosis. One should be from meiosis I and the other should be from meiosis II. Refer to the Resource Room for the biological diagram rules. Be sure to label the chromosomes, cytoplasm, and the cell membrane. The title of the diagram should indicate the phase of meiosis it depicts. In addition, add a short paragraph to indicate how you know which phase the diagram is depicting in your diagrams, and indicate the main events of that stage.

Phases

 

Whoops! On your way out of the microscope lab, you mistakenly knock over a box of prepared microscope slides. Each slide contains a single cell in a specific phase of meiosis. The issue is that the lab technician didn’t attach the labels to the slides yet! You don’t want the technician to lose his job, so hopefully you can help him determine the phase depicted on each slide.

In the following interactive, you will attach the appropriate label to each slide. Be sure to check your work after you have arranged everything to make sure you have labelled the slides properly.

Slides

Long Description

 

Maybe you should be more careful in the lab next time, but thanks for making it right!

Major Events

Now that you have a good understanding of the phases of meiosis, it is time to look closer at the unique events that occur in the process.

Crossing Over

This is an image of a narrow, paved road starting from the bottom-left of the image and going off into the distance in the centre of the image. Both sides of the road is lined with thick brush and trees. There is a round, white sign with a black diagonal line through it on both sides of the road. The horizon is blue, sunny sky with wispy clouds. Further over to the centre-right of the image is a white sign with an image of a chicken with the wording ‘Chicken Crossing’ on it.
by Michael Trolove used under ShareAlike CC BY-SA

The most important event in terms of creating variation occurs in prophase I. Earlier you learned about homologous chromosomes. This is the point at which those chromosomes pair up in a process called synapsis.(definition:The pairing of two homologous chromosomes to form a tetrad.) Each chromosome is made up of two identical sister chromatids. This tetrad consists of two homologous chromosomes (one from each parent) for a total of four chromatids.

This is an image of three pair of chromosomes in a row. Each chromosome is in the shape of an X, and they are the same shape and size. In the centre of each X, there are two small yellow circles, depicting the location of the centromere. In each pair, the chromosome on the left is blue, while the chromosome on the right is red. Over the first set of chromosomes on the right, a label above the pair states, ‘Homologous chromosomes aligned.’ A grey, right pointing arrow is located to the right of the first pair and points to the next pair in the row. This second pair of chromosomes looks similar to the first pair, only the right-lower arm of the left chromosome and the left-lower arm of the chromosome on the right are stretched toward each other and cross over. The label about this central set of chromosomes states, ‘Chromosome crossover.’ Another grey, right pointing arrow is located to the right of the central pair and points to the last pair in the row to the right. Again, these two chromosomes look similar to the other chromosomes. The lower arms on this pair are not crossed over like in the central pair, but the right-lower arm of the left chromosome now has a red tip and the left-lower arm of the chromosome on the right has a blue tip, indicating that sections were exchanged between the two. The label on top of the rightmost chromosome pair states, ‘Recombinant chromatids’ and points to the half of each chromosome X-shape that now has a different coloured tip. A label below the rightmost chromosome pair states, ‘Non-recombinant chromatids’ and points to the half of each chromosome that has not been modified.
Homologous chromosomes can exchange portions of genetic material through an event called ‘crossing over.’
by Bio Ninja.com

Once synapsis has occurred, non-sister chromatids will ‘crossover’ and exchange genes. Remember, the genes are all the same on homologous chromosomes, so a gene for hair colour from one chromatid will exchange for the gene for hair colour on the other chromatid.

Although you will look at the interaction of genes that make the characteristics of an individual in the next activity, now is a good time to clear the air about a few terms. You may have heard the word ‘allele’ used in place of ‘gene’ a number of times. Can these words be used interchangeably? Let’s take a look!

 

So, an allele(definition:One of the possible forms of a gene.) is one of the possible forms of a gene. Now you know!

Once the chromatids have completed the crossover, they are called recombinant chromatids(definition:Chromatids that are comprised of exchanged genetic material from crossing over in meiosis.), since they are now made up of genes combined from two different chromatids. The recombinant chromatids now contain genes from both original chromosomes. As you can imagine, this random exchange of genes is a major source of variation among the chromatids.  

On average, it is estimated that two or three crossovers occur for each pair of homologous chromosomes. These crossovers can occur anywhere on the chromosomes, and the multiple crossovers can happen simultaneously. Think of all the possible combinations that can occur!

This quick video will help you visualize the process:

 

Independent Assortment

After the crossover in prophase I, the tetrads align their centromeres along the middle of the cell during metaphase I. Spindle fibres from one pole attach to one of the homologous chromosomes, and fibres from the other pole attach to the other homologous chromosome. The homologous chromosomes are then pulled apart during anaphase I and start to migrate to opposite poles.  

In the process detailed above, there is another event that adds to the genetic variance. When the tetrads align in the centre, it is a random arrangement. A homologous chromosome could go to either pole depending on how it was arranged in metaphase I. This means that the chromosomes in each homologous pair are assorted independently from the other pairs. The result is that one pole could receive chromosomes from both the female and the male parent. The process is random.

The number of combinations that can result from this process by the end of meiosis II depends on the number of chromosomes pairs that species has. Using the relation 2n, where ‘n’ is the number of chromosomes pairs, you can determine the number of possible outcomes.  

For example, if there were only two pairs of homologous chromosomes, you would have 22 = 4 possible combinations after the homologous chromosomes are split during anaphase II.

This is a diagram of two pyramids of round cells beside each other. Each pyramid is made of three levels. The top level is a single circle, the next level is two circles, and the third level is made up of four smaller circles. Arrows below the first two levels indicate that each circle has split into the cells seen in the next level down. The pyramid on the left has the label ‘Possibility 1’ above it. The top circle contains four X-shaped chromosomes arranged into two pairs, one above the other. The chromosome on the left of the top pair is blue, and the chromosome to the left of it is red. The pair below that looks identical to the one above it, blue on the right and red on the left. In the next level down, the circle on the left contains the two blue chromosomes from the level above, one above the other. The circle on the right contains the two red chromosomes from the level above, one above the other. On the next level down, the two leftmost circles each contain the two blue halves of a single X -shaped chromosome, arranged with one above the other. These two circles have the label ‘Combination 1’ below them, with a bracket indicating that the label applies to both circles. The two rightmost circles each contain the two red halves of a single X -shaped chromosome, arranged with one above the other. These two circles have the label ‘Combination 2’ below them, with a bracket indicating that the label applies to both circles. The pyramid to the right of the image has the same basic arrangement but with different colour combinations and the label above it reads, ‘Possibility 2.’ The top circle contains four X-shaped chromosomes arranged into two pairs, one above the other. The chromosome on the left of the top pair is blue, and the chromosome to the left of it is red. The pair below that looks identical to the one above it, but the colours are reversed. Red on the right and blue on the left. In the next level down, the circle on the left contains two chromosomes from the level above, blue above the red. The circle on the right contains two chromosomes as well, only red above blue. On the next level down, the two leftmost circles each contain two halves of a single X -shaped chromosome, arranged with a blue half above a red half. These two circles have the label ‘Combination 3’ below them, with a bracket indicating that the label applies to both circles. The two rightmost circles each contain two halves of a single X -shaped chromosome, arranged with a red half above a blue half. These two circles have the label ‘Combination 4’ below them, with a bracket indicating that the label applies to both circles. In the space between the two pyramids, a label at the top reads, ‘Two equally probable arrangements of chromosomes at metaphase I.’ Near the bottom of the space, and beside the lowest level of each pyramid, is a label that reads, ‘Daughter cells.’
Independent assortment can produce different chromosome combinations in the daughter cells.
by Pearson Education Inc.

Since humans have 23 chromosome pairs, that would be 223 = 8,388,608 possible combinations! Now, that number is just based on independent assortment. If you include all the variation that occurs during crossing over in prophase I, you will have an almost limitless number of possible combinations!  You can now see why we all look different from one another, even when we have the same parents.

One parent cell becomes four daughter cells, each being genetically different from the others and the original parent cell.

And THAT is why sexual reproduction and the process of meiosis is a main driver of genetic variance within a population. You will see that another process, mutation, also plays a large role in genetic variation later in this unit.

Diploid to Haploid

Although the idea of meiosis being a reductive process has been mentioned, it is very important that you remember that the end result of meiosis is four haploid cells. The diploid number drops to the haploid number of chromosomes at the end of meiosis I, and then continues in the haploid configuration all the way through meiosis II.

This is a diagram of a pyramid of circles with the label ‘meiosis’ above it. The top level is a single circle, the next level is two circles, and the third level is made up of four circles. Arrows below the first two levels indicate that each circle has split into the cells seen in the next level down. The circle at the top level is light green and has the number ‘46’ written in it. To the right, a label states, ‘diploid (2n) parent cell.’ The two circles in the next level down are yellow, and each has the number ‘23’ written in them. To the right of this level is a label that states ‘meiosis I’ and then ‘two haploid (n) cells’ below that. The four circles in the third and lowest level are red, and each have the number ‘23’ written in them. To the right of this level is the label ‘meiosis II’ and ‘four haploid (n) cells’ below that.
Meiosis is a reductive division, in that the chromosome number goes from diploid to haploid.
by Nelson Biology 11

When the two gametes combine during fertilization, the zygote will be back to the diploid number of chromosomes.  

Unfortunately, the process doesn’t always go as planned. The goal is to have each of the four daughter cells contain the haploid number of chromosomes, but this isn’t always the case. In rare cases, the chromosomes do not separate equally during anaphase I or anaphase II. You will take a detailed look at this process in the next activity.

Gametogenesis

This is an image of a black Sega-Genesis video game console on a white background. The game console is square, with a slot near the top with ‘Genesis’ written above it in white. Below that slot is the word ‘Sega’ written in white above an ‘on’ and ‘reset’ button. On the front edge of the console are two ports for game controllers. The left port has a controller plugged into it. The controller is also black with the word ‘Sega’ written in white on it. The controller is kidney-shaped and has a thumb pad on the left, a ‘start’ button in the centre, and six buttons to the right, with the top three being being grey, and the bottom three black.
The classic Sega-Genesis video game console.

Nope, gametogenesis(definition:The meiotic process that results in the creation of male or female gametes.) is not a video-game console from the late 80s! Instead, it is the name given to the formation of sex cells through meiosis.

Now that you know the phases of meiosis, it is easy to make the jump to gametogenesis and understand the differences between the male sperm and the female eggs produced through the process.

Explore this video to see the main differences between the sperm cells and eggs cells produced through the process of meiosis. Pay close attention to the end result of each process and the names associated with each stage.

 

The formation of sperm cells in the male testes is called spermatogenesis,(definition:The production of four haploid male sperm cells through the process of meiosis.) while the formation of eggs in the female ovaries is called oogenesis.(definition:The production of one viable haploid female egg cell through the process of meiosis.) As previously mentioned, the big difference is found between the final form of the male and female sex cells.

The sperm cells need to be highly motile, so they form whip-like tails to allow them to swim to the female egg. In addition, they are quite small, as this is an advantage when it comes to their ability to move quickly.

In the formation of egg cells, the majority of the cytoplasm and the organelles contained within the cytoplasm, goes to one of the sex cells. This one large sex cell now has the nutrients and cellular make-up available to support a developing zygote after fertilization. The three smaller cells, called polar bodies,(definition:Small cells produced during oogenesis that do not develop into a viable egg.) die. This leaves just one egg or ovum(definition:A female gamete.) available for fertilization.

Another major difference is the number produced. Females will release only one egg per month, while males will produce 1500 sperm cells a second! Why such a big difference in number?  Only one egg and sperm is needed to create a zygote, but the race for the egg is very competitive, and very few sperm make it to the egg. If there were fewer sperm released, the probability of fertilization would drop dramatically.

Using your good self regulation(definition:I am able to think critically about the terms in the gametogenesis interactive.) skills, test your understanding of gametogenesis by attempting the following interactive. Fill each box with the appropriate term or number. Some options can be used more than once. Be sure to check your work when you think you are finished!

Gametogenesis

Long Description

 

Karyotypes

This is an image of a micrograph showing nine red circles of varying sizes along the top and left edges of the image and down the centre. The circles are reddish-orange and are on a yellow background. At the top left, there are 46 X-shaped chromosomes of various sizes arranged in a bunch. There is a second bunch of X-shaped chromosomes at the lower right of the image. Each chromosome is reddish-orange in colour.
Human chromosomes at 4000X magnification using the bright field microscopy technique.
by By Doc. RNDr. Josef Reischig, CSc. - Author's archive used under ShareAlike CC BY-SA

Do chromosomes really look like the letter ‘X?’ For the most part, yes! Obviously the X shaped image used to depict a chromosome has been simplified, but the basic structure is similar to the real deal.

The chromosomes in a cell are many different sizes, and some have shorter arms on one end. In fact, the chromosome that determines if you are a male looks more like a Y than an X, since the arms on one end are very short.  

From your study of meiosis, you know that there are homologous pairs of chromosomes; one from the female and one from the male. Cytogeneticists(definition:A geneticist that studies the structure and transfer of chromosomes and genes.) can look at the chromosomes from a cell, pair the chromatids into homologous pairs, and determine if the sample tissue comes from a male or female and if it has any chromosomal abnormalities. You will learn more about these abnormalities and how they occur in the next activity.

An image of the chromosomes in their homologous pairings and arranged based according to shape and size is called a karyotype.(definition:An arrangement of the the complete set of homologous chromosomes found in a cell.) The scientist will use special chemicals to ‘freeze’ and stain the chromosomes of white blood cells while the cell is in metaphase of mitosis.

This is the question/answer icon. Challenge!

Why would scientists use cells that are in metaphase to create a karyotype?

Answer

While in metaphase, the chromosomes are condensed, and sister chromatids are still attached at the centromere.

The Puzzle of Life

This is an image of two featureless white human figures pushing two large puzzle pieces together. The puzzle pieces are as tall as the figures. The image is set on a white background.

As you can imagine, it takes lots of experience and a trained eye to be able to match homologous chromosomes. The banding patterns and the location of the centromeres on the chromosomes help the scientist arrange them accurately. It really is like a jigsaw puzzle. Thankfully, computers are able to do this job quickly and accurately and make the process much more efficient.

The process allows you to go from a metaphase smear(definition:An image of the full set of chromosomes from a cell in the metaphase stage of mitosis.) of somatic cells to a fully arranged karyotype.

From this:

This is an image of a micrograph with a lime-green background. There is a reddish-brown circle at the bottom left of the image, with darker spots in the centre. In the middle of the image is 46 X-shaped chromosomes of different sizes spread out in a region that takes up half of the image. The chromosomes are yellow in colour.
Metaphase smear of human male chromosomes at 4000X magnification.
by By Doc. RNDr. Josef Reischig, CSc. - Author's archive used under ShareAlike CC BY-SA

To this!

This is an image of a karyotype showing chromosomes that have been paired up with matching chromosomes. The chromosomes appear as rod-shaped strands of different lengths with light and dark grey banding along their length. The chromosome pairs are arranged neatly into four rows. Below each pair is a number. The top row have pairs numbered 1 to 5, the second row has pairs numbered 6 to 12, the third row has pairs labelled 13 to 18, and the last row has pairs labelled 19 to 22. The last pair on the right has the label ‘X/Y’ below it. The grey coloured chromosomes are on a white background.
Karyotype of human male chromosomes at 4000X magnification.

You can see that human karyotypes are arranged into the 23 pairs of chromosomes. The first 22 pairs are called autosomes,(definition:Chromosomes that are not sex chromosomes.) or non-sex chromosomes, while the chromosomes of the 23rd pair are called sex chromosomes. The sex chromosomes are what distinguish male from female. In humans, a female has two X chromosomes in that location, while a male has an X and a Y chromosome.

In actual fact, both chromosomes are still in the X configuration, but the Y chromosome has the centromere close to one end. This makes it look more like the letter Y.

This is a diagram of two purple X-shaped chromosomes side by side on a white background. A dashed line runs horizontally across the image and goes through the position where all four arms of the X meet. On the chromosome to the left, the arms on the top and the bottom are the same length. On the chromosome to the right, the bottom arms are the same length as the arms on the other chromosome, but the top arms are much shorter.
The centromere location, depicted by the dashed line, can make the arms on one end much smaller than the arms on the other.

A Source of Confusion

Often people see the chromosomes in the karyotype and think that they depict single chromatids as compared to the X shape shown above. The reason that many of the chromosomes look like single strands is simply due to the two sister chromatids being very close to one another. The resolution of the microscope can’t distinguish between the two sister chromatids.

On the metaphase smear at the top of this section, the lighting and contrast used enables you to see the classic X configuration. The distinction between the two chromatids is simply lost in many karyotypes due to the settings used.

In this example, you can just make out the X shape on a number of the chromosomes.

This is an image of a section of a karyotype showing four pairs of grey chromosomes arranged neatly in two columns. The chromosomes in each pair are side by side. In each row, there is a solid black line that runs horizontally through the chromosomes where all four arms of the X shape meet. Each chromosome pair has a number written below it. The top pairs are labelled 10 and 11 from left to right, and the lower pairs are labelled 16 and 17 left to right. Although each chromosome appears to be rod-shaped, contrast in the grey colouring shows a faint X shape in each chromosome.
by Imad Fadl-Elmula - Chromosomal changes in uroepithelial carcinomas. Cell & Chromosome 2005, 4:1 doi:10.1186/1475-9268-4-1 used under CC BY

 

 

Reproduction Equals Life...Or Does It?

In Unit 1, you saw how organisms can reproduce using asexual or sexual mechanisms...or both! Reproduction has always been the mainstay of life, and is in fact one of the factors that are necessary for us to consider something to be living.

But what happens if that reproduction relies on others to make it happen? What happens if that organism contains genetic material - the molecules of life - in the form of DNA or RNA? Should that organism be considered life? You be the judge!

Viruses

This is an image of a micrograph showing two bacteriophages viruses at very high magnification. Each virus has a geometrical head and a tube-like tail with long strands like legs protruding from the end.
by AFADadcADSasd - Own work used under CC BY

In the last unit, you looked at all the various kingdoms, but what about viruses? You hear about viruses all the time, in that they are responsible for many serious infections such as HIV and rabies, so why are they not listed as one of the kingdoms?

The reason is simple: viruses are not living organisms. They do not adhere to the seven basic characteristics of life, but more about that soon. Even though they are not considered to be living, they are intimately connected to life. As previously mentioned, they are the cause of numerous diseases in plants, animals, and other organisms. In fact, they actually contain genetic material in the form of DNA or RNA!(definition:Ribonucleic acid, genetic material present in all living cells. Its principal role is to act as a messenger carrying instructions from DNA for controlling the synthesis of proteins.) The DNA viruses are responsible for diseases such as hepatitis B, cold sores, and respiratory infections, while the RNA viruses cause diseases such as measles, polio, and the common cold.

Viruses are very abundant. It is said that there can be millions of viruses in a single mL of ocean water! Thankfully, most viruses have specific host cells that they infect. Some will only infect plant cells, while others, only bacteria.

Viral Structure

This is a diagram of two seperate viruses titled ‘Virus Structures.’ The image on the left is labelled ‘Animal Virus Structure’ and is a purple icosahedron labelled ‘Capsid’ with protrusions from each vertex. A square cut out of the forward face reveals a number of purple, squiggly lines labelled ‘DNA.’ The image on the right is labelled ‘Bacteriophage Structure’ and is blue in colour. At the top of the structure is an icosahedron labelled ‘head’ and a square cut out of the forward face reveals a number of purple, squiggly lines labelled ‘DNA.’ The head sits on top of a cylinder labelled ‘tail sheath’, connected to the head at a point labelled ‘Neck.’ At the bottom of the cylinder is a section labelled as ‘End plate’ with short protrusions labelled ‘Pins’ coming from the bottom. Equally spaced around the outer edge of the end plate are six leg-like structures with a joint in the centre of each. These legs are labelled as ‘tail fibers.’
The structure of animal and bacterial viruses.
by Biology Teks

There are many different viruses (scientists believe that there may be millions), and although they can look quite different, they have some basic structural similarities. Basically, a virus has some form of genetic material surrounded by a protein shell called a capsid. Some viruses have additional structures such as elongated tails and fibres attached to the main capsid.

Viral Reproduction

One of the main reasons that viruses are considered non-living is the fact that they cannot reproduce on their own. They first need to infect a host cell and transfer their genetic material into the host cell’s genome in a process called transduction.(definition:The transfer of genetic material from one organism or virus to another organism.)

Once the viral genetic material is part of the host cell, the replication process of that cell will not only make new copies of itself, but copies of the virus, as well. Essentially, the virus hijacks the host cell and uses it to reproduce.

This is the Portfolio icon. Viral Infectious Cycle

This video does a nice job of describing the infectious cycle of a virus. While you watch, pay particular attention to the differences between the lytic cycle(definition:A phase of the virus life cycle during which the virus replicates within the host cell, releasing a new generation of viruses when the infected cell lyses.) and the lysogenic cycle.(definition:A phase where the viral genetic material fuses with the host cell’s genetic material, allowing the virus genetic information to be replicated into the daughter cells.) Also, listen carefully to the information about the use of viruses as pesticides. Think about how this can have an impact on other organisms. 

 
 

Living or Not?

As you can see, viruses are closely interconnected with other life forms. The fact that they have genetic material seems to be the bridge between living and nonliving. Remember, the idea of what is alive and what is not is based upon rules and conditions created by humans. In fact, some people do consider viruses to be another life form. What do you think?

What Does it All Mean?

Throughout this activity, you have learned about genetic diversity and how meiosis plays a large role in the variance that leads to this diversity. Events such as crossing over and independent assortment create random arrangements of the genes that are passed on to the gametes and ultimately, to a new individual.

 

Consolidation

CONSOLIDATION

Variance Leads to Diversity

This is an image of three sheep in a row. Only the head and shoulders of each sheep can be seen in the image. The background shows a blurred wooden fence and leaves on trees. The two sheep on the left are white, while the sheep on the right is black with a brown face and a tuft of white at the top of the head. The black sheep has small and turned down black horns.

Ultimately, the random variance resulting from the meiotic process is a major driver of diversity in populations. As you move through this unit, you will see how this variance allows for the evolution of a species and the diversity of life you learned about in the previous unit.  

The random nature of crossing over and independent assortment can together create an almost unlimited number of combinations when dealing with all 46 chromosomes in the human genome. To make life a little easier for you, the number of chromosomes involved in the following task will be reduced. Even with only a few chromosomes involved, you will still find that the meiotic processes can create a surprising amount of variance.

That's So Random!

This is an image of the upper half of a pineapple. The pineapple is at the lower-centre of the image, and in the background is a blurred wooden fence and snow is on the ground. The intention of the image is to depict randomness.
Now that IS random!

Crossing over in prophase I and independent assortment at metaphase I are the events that are responsible for the variation in the gametes produced during meiosis.

Starting with three pairs of paper homologous chromosomes (organisms such as the yellow fever mosquito and the Indian muntjac have only three pairs of chromosomes), you will simulate random crossing over and independent assortment events using the roll of a die and a flip of a coin. You will then share the resulting gametes with your classmates to see if any two are alike.

 

Similar Yet Different

In this activity, you were introduced to meiosis and the events that make it different from mitosis. Ultimately, each process is about creating new daughter cells from a parent cell, but the end result is quite different in each process.

Check your understanding of the two processes using the following interactive.

This is the Portfolio icon. Which is Which?

For each statement below, determine if the event applies to mitosis, meiosis, or both by clicking the appropriate box. Repeat the exercise until you can complete the chart without making a mistake.

Comparison

Long Description

 
 

Meiosis is a pretty impressive process! The ability to create gametes with a virtually limitless amount of variance is a main driver behind genetic diversity in populations. As you go through this unit, you will see how this diversity is a major player behind the evolution of a species. You will also gain a better understanding of how the different genes found in the gametes impact the traits of an organism. This is just starting to get good!

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