D.2a: Origin of Species: Microevolution

28/08/2013 § 1 Comment

What even is an allele? I’ll tell you what it is, it’s a form of a gene, (thanks study guide). And see here, sometimes there are populations where there are two alleles in a gene pool, and that’s what we call polymorphic, kids. More specifically, if one allele is slowly replacing the other, then that particular population is showing transient polymorphism. One example of this is the peppered moth, Biston betularia, where dominant alleles control the characteristics of the moths’ wing colour.

But wait, wait, there’s something even better, oh my god. An even more intriguing – and possibly more well known – example would be sickle cell anaemia combined with malaria. Okay, okay, so we’re going to call it balanced polymorphism because it totally follows the law of equivalent exchange. What happens during balanced polymorphism is that the two alleles continually persist against each other in the gene pool of a population and transient polymorphism doesn’t occur as both traits are being expressed at the same time, and is therefore balanced.

With sickle cell anaemia, if an individual is homozygous with allele HbA, they won’t actually get sickle cell anaemia but may get malaria. In contrast, if an individual is homozygous with allele HbS, they won’t get malaria, but they’ll develop a severe case of sickle cell anaemia. And the lucky dogs who are heterozygous with both alleles will not develop sickle cell anaemia and are also resistant to malaria. They are therefore… superior.

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D.1c Origin of Life: Endosymbiosis

26/08/2013 § Leave a comment

So prokaryotes were the first living organisms on Earth – before eukaryotes – and because they started using water a source of hydrogen in photosynthesis, they also started releasing oxygen into the atmosphere as a waste product. As photosynthetic prokaryotes continued to produce oxygen as a waste product, the amount of oxygen in the atmosphere accumulated to a point where certain prokaryotic organisms could start using it for aerobic cell respiration. The evidence of oxygen in the atmosphere at that time is found through banded iron formation on old rocks found in Greenland.

One of the biggest differences between a prokaryote and a eukaryote (as we should know, hahahaha) is the content of each cell; while a eukaryote has both mitochondria and chloroplasts, a prokaryote doesn’t. However, since we assume that eukaryotes arose from prokaryotes, then the mitochondria and chloroplasts must’ve gotten there somehow and this can be explained through the endosymbiotic theory. What happened was mitochondria and chloroplasts probably evolved from independent prokaryotic cells which were then quite literally sucked into larger heterotrophic cells through endocytosis. The independent prokaryotic cells (early mitochondria and chloroplasts) were allowed to live and so continued their lives from within a larger cell.

Mitochondria and chloroplasts hold evidence that supports this theory, such as:

  • they can both grow and divide like cells
  • they have their own naked loop of DNA, just as prokaryotes do
  • they synthesise their own proteins using 70S ribosomes
  • they both have double membranes, necessary for endocytosis

(Some scientists also say flagella and cilia have an endosymbiotic origin, too, but let’s not get ahead of ourselves now.)

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D.1b: Origin of Life: RNA World

25/08/2013 § 1 Comment

I know we talked about this in class a little bit – RNA. We assume that the mechanism of life had to start somewhere and one possibility is RNA itself because of two specific properties: self-replication and catalysis. We know that RNA molecules can perform self-replication because they act as the templates that replicate DNA strands during protein synthesis or simple replication. The RNA molecules’ ability to build these templates also gives them the ability to replicate themselves. Also during protein synthesis, RNA molecules can create the peptide bond formation in the ribosome, which is a catalysis of a reaction, meaning that RNA’s ability to catalyse chemical reactions could definitely have assisted in the evolution of life.

Also, since membranes were needed to form the first cells, phospholipids naturally grouped together to form the bilayers we know of now. These phospholipids, called protobionts, allowed for an internal environment to develop.

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D.1a: Origin of Life: Organic molecules

10/08/2013 § Leave a comment

Hi. It’s me. Let’s skip the formalities and cut to the chase. Seven units to go, and our first one is Evolution. As review of what we should already know, cells can only be formed from other cells. However, obviously at some point in the history of the Earth, the first living cells must have come from somewhere and there are four necessary processes for that to happen:

  1. simple organic molecules were produced by chemical reactions (e.g. amino acids, ammonia, and water)
  2. the simple organic molecules would need to assemble into polymers, e.g. polypeptides
  3. the polymers could self-replicate (allowing for inheritance)
  4. membranes were developed to package the molecules

There are three major possible locations where organic compounds could have been synthesized to begin the origin of life. The first was in 1953 in an experiment by scientists Stanley Miller and Harold Urey wherein they recreated the conditions of the Earth (back then) to synthesize the chemical reactions of the atmosphere, water, and on the surface of the earth. This involved electrical discharges, boiling and condensing water, and mixing gases like ammonia, methane and hydrogen.

Secondly, hydrothermal deep-sea vents were said to create the chemical reactions through the gushing hot water. The chemicals here held all the necessary energy and raw material needed to create monomer organic chemicals.

Finally, some scientists think aliens were involved.

(NASA thinks they might have come to Earth via meteorite, comets, or interplanetary dust seeing as plenty of meteorites collided into the Earth those 4000 million years ago.)

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