Give an example of a variation that could occur at the molecular/cellular level.

Understandings:

•  Crossing over and random orientation promotes genetic variation

•  Fusion of gametes from different parents promotes genetic variation

    
The advantage of meiotic division and sexual reproduction is that it promotes genetic variation in offspring

The three main sources of genetic variation arising from sexual reproduction are:

• Crossing over (in prophase I)
• Random assortment of chromosomes (in metaphase I)
• Random fusion of gametes from different parents 

Crossing Over

Crossing over involves the exchange of segments of DNA between homologous chromosomes during prophase I

• The exchange of genetic material occurs between non-sister chromatids at points called chiasmata

As a consequence of this recombination, all four chromatids that comprise the bivalent will be genetically different

• Chromatids that consist of a combination of DNA derived from both homologous chromosomes are called recombinants
• Offspring with recombinant chromosomes will have unique gene combinations that are not present in either parent 

Random Orientation

When homologous chromosomes line up in metaphase I, their orientation towards the opposing poles is random

The orientation of each bivalent occurs independently, meaning different combinations of maternal / paternal chromosomes can be inherited when bivalents separate in anaphase I

• The total number of combinations that can occur in gametes is 2n – where n = haploid number of chromosomes
• Humans have 46 chromosomes (n = 23) and thus can produce 8,388,608 different gametes (223) by random orientation 
• If crossing over also occurs, the number of different gamete combinations becomes immeasurable

Random Fertilisation

The fusion of two haploid gametes results in the formation of a diploid zygote

• This zygote can then divide by mitosis and differentiate to form a developing embryo

As meiosis results in genetically distinct gametes, random fertilisation by egg and sperm will always generate different zygotes

• Identical twins are formed after fertilisation, by the complete fission of the zygote into two separate cell masses

Video transcript

- [Instructor] We are now going to discuss molecular variation in cells. You are probably familiar with the idea that you have a variation of genetic makeups in a population. But even within an organism you have variation in the types of molecules that an organism can produce and when they produce them. So, for example, we know that we all have DNA, all organisms, living organisms that we know about, they have DNA. I'll just do this as a quick drawing of DNA. We know that we have genes in our DNA that code eventually, they go from DNA to messenger RNA, and then they go to the ribosomes to be translated into proteins. And these proteins are a major way of expressing what is encoded in our DNA. Now, it turns out that our DNA will encode for not only multiple proteins but multiple types of the same protein, and it can encode for some of these proteins more under certain circumstances and other proteins more in other circumstances based on environmental factors. Those environmental factors might influence what part of the DNA is being transcribed into mRNA, which then is translated into proteins at different times. And there's several very interesting examples of this. It turns out that hemoglobin, which you might recognize as the protein complex that binds to oxygen in our red blood cells, that the type of predominant hemoglobin changes from when we are inside our mothers' wombs to when we become independent beings. So, this right over here is a picture of a hemoglobin molecule. You see here four heme groups that each bind two oxygen, and when you're a fetus the primary type of hemoglobin is hemoglobin F, and then once we come out of our mothers' wombs the hemoglobin F stops getting produced and we go to hemoglobin A. Now you might say, well, why do we have this variation in the type of hemoglobin? And the answer is, that those are two different environments. When a fetus is in the mother's womb, it's not directly breathing. It's getting its oxygen from the mother's blood. The mother's blood does not mix directly with the baby's blood, but there's a boundary where you have the mother's blood here, and I'll say this is the baby's blood right over here, and you have the gas exchange of the oxygen going through that boundary, and then of course the release of the carbon dioxide going the other way. And this environment where the baby's red blood cells have to bind to the oxygen is a relatively low oxygen environment compared to, say, our lungs, because it has oxygenated and deoxygenated blood mixing in that same place, and it does not have direct access to, say, the lungs. And so in this low oxygen environment, the hemoglobin molecules have to be really really really good at binding to oxygen. And we can see that from this diagram right over here, where the horizontal axis is the partial pressure of oxygen, and the vertical axis is how saturated with oxygen these different hemoglobin molecules can become. And you can see that the fetal hemoglobin, which is depicted by this blue curve, it gets 50% saturated at a lower partial pressure of oxygen than the adult hemoglobin. So one way to think about it, it is stickier, it binds with that oxygen. It can pull that oxygen out of the blood far better, which makes sense for the environment that the fetus is in. But once it comes out of the mother's womb, it doesn't need that stickiness and there're some drawbacks of that stickiness as well because it makes it hard for that oxygen to go into as many of the body's tissues, and so that's why you have this transition from hemoglobin F to hemoglobin A. It's not just hemoglobin where we see this molecular variation. Plants and other organisms that conduct photosynthesis contain multiple types of chlorophyll. Remember chlorophyll is a very important molecule in capturing light energy which can then be used to help synthesize carbohydrates in things like plants. And here we see how two different chlorophyll molecules, both that would be found in plants, how well they absorb light of different frequencies. So you can see chlorophyll a is really good at absorbing the violet bordering on blue light, while chlorophyll b is better at the blue-green type of light. And then you have another peak here where chlorophyll b is better at absorbing an orangish red, while chlorophyll a is better at absorbing, I guess you could say, a red bordering on infrared wavelength. And the reason why it is valuable is that the light that the plant gets, especially at different times of day, at different times of year, is going to have different wavelengths, and so this just lets the plant capture more energy that it can use in photosynthesis. And these were just two examples of molecular variation. In our cellular membranes, there's multiple types of phospholipids that are forming the phospholipid bilayer, and those multiple types are, they have different levels of how fluid they are at different temperatures. And there're animal studies that show that the variations change depending on the conditions. For example, a cold-blooded animal might have more of the fluid phospholipids when it is very cold so that the membranes don't become overly rigid. But I will leave you there. This is just to appreciate this idea that we have all sorts of molecular variation inside organism cells, and it allows those organisms to better adapt to their environment or different stages of their development.

What causes genetic variation at the molecular level?

What is an example of molecular evolution?

What are some examples of genetic variation?

What are the 4 types of genetic variation?