General Biology – Unit Two

Objective Six

Let’s begin this area of study with a look at some triviaJ

Here’s a little more trivia.

When would the majority of mistakes in the DNA happen? Well . . .when is the most DNA being produced? . . .During DNA Replication.

In theory, these bases should match up A=T, C=G, but that doesn’t always happen – MUTATIONS OCCUR!

For example: Let’s say you have a strand of DNA with hundreds of nucleotide pairs. We are going to focus on what happens to a base pair triplet in the middle of our strand of DNA.

C:G

T:A

C:G

C                                                   G

T                                                    A

C                                                   G

C:G

T:A

C:G

C:G

A:A*

C:G

This would represent the base triplet in our original DNA.

This would represent the two sides of the DNA separating.

The diagram above would indicate the two strands of DNA that formed as a result of replication.

*NOTICE THE MISTAKE IN THE STRAND ON THE RIGHT!

I am going to give you an example of a genetic disorder caused by a point mutation that is quite serious. Hemoglobin is a large protein molecule found in red blood cells. Its job is to carry oxygen. The hemoglobin protein is made of 573 amino acids. There are different forms of hemoglobin that have resulted from different point mutations. Most of these forms of the hemoglobin still function fine, but one form that has a VALINE instead of GLUTAMIC ACID in the 6th position does not function correctly. Apparently this is a critical location for determining hemoglobin shape. Individuals that have this mutation have the condition known as Sickle Cell Anemia.

In Sickle Cell Anemia, the hemoglobin is sickled. This causes the red blood cells to be pulled out of shape. This results in a host of problems; ranging from anemia and lassitude, to problems caused by blood cells clogging various body organs. We will look at sickle cell anemia in more depth in the 3rd unit. For now, just realize that Sickle Cell Anemia is an example of a serious genetic disorder that is caused by a point mutation.

Now to examine some other types of mutations, let’s look at how the code is read. The code is read in sets of three nucleotides so in a way, it’s like reading sentences of three letter words.

Let’s look at an example of a frameshift mutation that is very common in humans. Do you ever go outside? Some folks enjoy walking in the sunshine, playing sports, gardening or doing other activities outside. One of the causes of mutations is radiation, and sunshine has a type of radiation known as Ultraviolet radiation or U.V. radiation. When U.V. light strikes your skin, it penetrates to the cells in the skin and anytime this radiation encounters two thymine nucleotides that are side by side in the DNA, it causes them to stick together.

The answer is because we have an enzyme repair system. A wonderful thing! One set of enzymes travels up and down the DNA strand searching for bulges – mistakes. When a mistake is located, the enzyme cuts the faulty DNA away. Another enzyme brings the correct nucleotides in the attach to the DNA strand and another enzyme "sews" the new DNA in place. This happens constantly along the DNA in all of your cells, and so – for the most part, these dimers don’t cause us problems. Of course, too many dimers (too much U.V.) can overtake our repair system and still lead to skin cancer.

http://www.nih.gov/sigs/dna-rep/whatis.html

There is a genetic disorder called Xeroderma pigmentosum in which the individuals do not have this enzyme repair system. It is very serious and the folks that have this often die from skin cancer at a young age. You may have read or seen on TV television shows about these little kids that cannot go out into the sunlight. They even have to keep the lights dimmed in their homes. It makes one appreciate how effectively our body’s work, when they do work correctly.

·       http://www.xps.org/  

Our final topic in looking at mutations is chromosomal mutations. All of the previous topics have dealt with single gene mutations. But how many genes are on a chromosome? Lots. So if a chromosome is broken – there is the potential for very serious mutations. When chromosomes break, a piece may become lost, a piece may reattach to another chromosome, a piece may become inverted and reattach. Any of these can result in lethal problems.