DNA Replication: The Three-act Play

Overview
(from http://www.teacherweb.com/CA/NogalesHighSchool/mespinoza/h4.aspx)

ACT 1: Initiation

• Single-strand binding proteins enter to stabilize the template strands, keeping them apart during replication.
• Helicases unwind the template DNA strands. They keep working at the replication fork to continuously open up the bubbles. 
• To release the tension, as the bubbles opening up, gyrases cut the DNA, afterwards, put it back.

ACT 2: Elongation

• RNA primase enzymes begin the process by building a small complementary RNA segment called RNA primers to the origin of the strand, therefore, DNA polymerase III can add DNA nucleotides to the RNA primer. 
• Nucleotides on the daughter strand can only elongate in 5' to 3' direction. This strand is called leading strand.
• On the opposite strand, which is called lagging strand, DNA polymerase III is moving away from the replication fork.
• On lagging strand, okazaki fragments are formed with series of RNA primers and short DNA fragments added by DNA polymerase III.

ACT 3: Termination

• DNA polymerase I enter. They remove the RNA nucleotides at the primers with DNA nucleotides.
• DNA ligase catalyzes the reaction of  forming bonds between short DNA strands on the lagging strand.
• DNA polymerase I proofread the daughter DNA.

Photosynthesis and Cellular Respiration

On Friday's class we were divided into groups to present the processes of photosynthesis and cellular respiration by creating some 3D models on chart papers.

Photosynthesis
Here is what happens during photosynthesis.

Cyclic Light Dependent Reaction

Non-cyclic Light Dependent Reaction

Calvin Cycle

From photosynthesis, glucose is synthesized and energy is stored. This kind of reactions are called anabolism.

Cellular Respiration

Glycolysis

Crebs Cycle

Electron Transportation Chain

The above shows the reactions which break down the glucose and release the energy. This kind of reactions is called catabolism.

To sum up, these two kinds of reactions, which is anabolism and catabolism, combine and form the overall metabolic processes in organisms.

A Story from ROM

On Friday, we went to the Royal Ontario Museum(ROM) for a field trip in the afternoon, after the lectures in the morning by two great and price-winning biologists in U of T.

Here is one of the stories I heard and found significant in ROM.

Passenger Pigeons

The three birds in the picture above cannot be seen flying in the sky any more. They are pigeons, but they are called passenger pigeons and are extinct.

The passenger pigeon was once the most abundant bird in North America. They are migratory and gregarious birds. People  used to see them flying with enormous amount of companions during migrating seasons. There is a record of people in Ontario seeing about 3 billion of passenger pigeons passing in the sky in spring, and they were described to "darken the sun", causing a lot of panic within folks.

Passenger pigeons live and can only live in large groups with at least thousands of companions. This is because they cannot protect themselves from the enemies. Therefore, in order to survive the predation, they hide in flocks to reduce the chance of being attacked.

There are two main factors contributing to the extinction of passenger pigeons, but they are all caused by humans. 

The first factor was over hunting. Because there were once billions of them flying by, people can't avoid noticing the appearance of an excellent source of food. They are always in large groups, so people can easily find them and either capture or kill them. It is on the record that the amount of hunting passenger pigeons in one specific forest was 5 thousand a day, and it kept going in the same way everyday within 5 months. Because they can only survive in large groups, once the great reduction of population occurs, the rest of them will die rapidly.

The second factor contributing to extinction was reduction of woods which they nested. During that time(around the beginning of the 20th century), a mass of forests was cut down  by people to become the farmlands. This is terrible because the passenger pigeons therefore lose both their shelter and food source(although they fertilize the land, no one wants pigeons to eat all his corps). 

The two factors worked together, and only in the 1914, the last passenger pigeon known died in a zoo. This is a sad and warning story to human------ a species once had an enormous amount of population can die out within a few years. Now that the fact is dreadful, only few people known about this story. This probably makes the story even sadder.