Genomic Comparison: Study with Video Lessons, Practice Problems & Examples

Genomic comparisons reveal evolutionary relationships by analyzing genomic sequences across species. Key concepts include purifying selection, which preserves critical gene functions, leading to conserved sequences that remain similar despite evolutionary divergence. The size of a genome does not equate to organismal complexity, as seen in flowering plants versus mammals. Phylogenetic trees illustrate these relationships, highlighting that introns mutate rapidly while essential genes change slowly. Understanding these dynamics is crucial for grasping evolutionary biology and genetics.

Genomic Comparison

Hi, in this video, we're going to be talking about genomic comparisons. So, by comparing genomic sequences of different organisms, we can really provide insight into evolutionary changes that have led to the creation of life present on Earth today. And so, we do this by just comparing the genomic sequences. So, what are we looking for when we compare genomic sequences? Well, the genomic sequence of two species differs by the length of time they have separately evolved. So, the longer there has been since two species evolved from a similar ancestor, the more changes that are going to be. And so, when knowing this, we can sort of assign some stretches of DNA different terms based on, how different they are. So, one of these things is purifying selection. And so, what this is is that organisms with mutations that affect genes are really important genes die. So, those mutations never accumulate. And so, these genes can actually look very similar between very distantly evolved organisms because anytime there was a mutation in those genes they died. And so, these sequences are called conserved sequences because they're so common between these really different organisms, they must have some form of critical function. This is actually about 5% of genes. And so, we think of conserved sequences generally in terms of regions of a gene or an entire gene itself. But this can actually happen really on an entire chromosome or on chromosome segments. So, we term this as synteny which is actually stretches of genes that the order of them is conserved on chromosomes between distant organisms. So, what this means is if you were to have gene one gene two gene three gene four gene five, well that would remain that in that particular order in very different organisms because that order is really important for function. So, that's really how some we just sort of an overview of how we begin looking at the evolution of genomic sequences and how we can compare them. And we start comparing them by looking at conserved sequences. And so, there's this idea I think that a lot of people have that genomic size is really important. And of course, it is because it reflects the rates of DNA addition or loss. And when you're studying the genomic evolution of course you want to know what are the rates of DNA addition? What are the rates of DNA loss? But what it doesn't tell you. And what I think people really think it tells you is that a larger genomic size means that the organism has a larger number of genes and is more complex. But that's not true because it doesn't provide information on that. You can have really simple organisms with a lot of genes and a big genome and you can have really small genomes with fairly complex organisms. So, genomic size does provide us information when we're comparing organisms but it's not a be all end all. It doesn't say okay well the largest genome is the most complex organism because it's not true. And so, how we do this which we've already talked about this term is phylogenetic tree. And these are constructed using DNA sequences which trace relationships between organisms. Generally, when we're looking at overall genome and comparing for phylogenetic trees, we can see more of that changes in intron occur more rapidly and slower change concern occurs in conserved genes with critical functions. Which makes sense because introns we don't need. So, they're free to mutate as much as they want. Whereas genes that we really rely on for life can't change as much because we rely on them for life. So, I remember back out of the way for a second and let's compare. I don't know if I can fit the title here. The title is comparing the genome size of various organisms. But let's just look at this graph. So, down here on the X-axis, you have base pairs. It's also here this is the number of base pairs obviously increasing. So, there's larger genomes over here and smaller genomes over here and these are different organisms. And you can see that you know, bacteria and things that are generally smaller algae, they do have a smaller genome size. You know there's here it varies but when you start moving across this way you can see that this doesn't necessarily hold true and the fact that here are mammals. So, this includes humans here. And you can see that that's our size of the genome. But you also have flowering plants which have a huge range and goes much higher and much larger genomes than humans have or other mammals have. Now, flowering plants aren't necessarily more complex than humans, they are simpler in a lot of ways, but they have or can have much larger genomic sizes than other organisms. So, I think this is really important when comparing genomes between organisms to understand that genomic size doesn't equal genomic complexity. So, now let's move on.

Stretches of chromosomes where the gene order is conserved among different species is called what?

• A. Conserved Sequences
• B. Synteny
• C. Purifying Selection
• D. Conserved Selection

True or False:The larger the genomic DNA sequence is, the more complex the organism is.

• A. True
• B. False

Here’s what students ask on this topic:

What is the significance of conserved sequences in genomic comparisons?

Conserved sequences are crucial in genomic comparisons because they indicate regions of DNA that have remained relatively unchanged throughout evolution. These sequences are often associated with essential genes that perform critical functions necessary for survival. Due to purifying selection, mutations in these genes are typically detrimental, leading to the death of the organism, and thus, these mutations do not accumulate. As a result, conserved sequences can be found in very distantly related organisms, providing insights into fundamental biological processes and evolutionary relationships. Understanding conserved sequences helps researchers identify important genetic elements and trace the evolutionary history of different species.

How do phylogenetic trees help in understanding evolutionary relationships?

Phylogenetic trees are diagrams that depict the evolutionary relationships among various species based on their genetic information. By comparing DNA sequences, scientists can construct these trees to trace the lineage and divergence of species from common ancestors. Phylogenetic trees illustrate how species are related and the relative time since they diverged. They highlight that introns, which are non-coding regions, mutate more rapidly, while essential genes with critical functions change slowly. This helps in understanding the evolutionary processes and genetic changes that have occurred over time, providing a visual representation of the evolutionary history and connections between different organisms.

Why does genome size not correlate with organismal complexity?

Genome size does not correlate with organismal complexity because the number of base pairs in an organism's DNA does not necessarily reflect the number of genes or the complexity of its biological functions. For example, flowering plants can have much larger genomes than humans, yet they are generally less complex. This discrepancy arises because genome size can be influenced by factors such as the presence of non-coding DNA, repetitive sequences, and the rates of DNA addition and loss. Therefore, a larger genome does not imply a more complex organism, and genome size alone is not a reliable indicator of biological complexity.

What is purifying selection and how does it affect genomic sequences?

Purifying selection is a process in evolutionary biology where deleterious mutations in essential genes are eliminated because they negatively impact the organism's survival and reproduction. This selection pressure ensures that only beneficial or neutral mutations are retained in the population. As a result, genes under purifying selection tend to remain highly conserved across different species, as harmful mutations are not passed on. This leads to the preservation of critical gene functions over long evolutionary periods, making these conserved sequences valuable for studying evolutionary relationships and understanding the genetic basis of essential biological processes.

How do introns and essential genes differ in their mutation rates?

Introns and essential genes differ significantly in their mutation rates due to their roles and selective pressures. Introns are non-coding regions of DNA that do not directly affect the organism's phenotype, allowing them to accumulate mutations more rapidly without detrimental effects. In contrast, essential genes encode critical functions necessary for survival and reproduction. Mutations in these genes are often harmful and are removed by purifying selection, resulting in slower mutation rates. This difference in mutation rates helps researchers identify conserved sequences and understand the evolutionary pressures acting on different parts of the genome.