DNA REVIEW

Let's revisit what we know about DNA from your work in the distance lessons.

A DNA molecule is made up of subunits called nucleotides. All nucleotides are made up of three different groups/arrangements of atoms - a sugar, a phosphate group, a nitrogenous (contains nitrogen) base.

• One group of atoms is the sugar molecule called ribose: (sugars have the ending -ose in their names, like glucose), we find it hidden in the DNA name DeoxiriboNucleic Acid. 

• Another group of atoms is the phosphate group (it comes from phosphoric acid, so it it the A part of the DNA:  DeoxiriboNucleic Acid.)

• A third group of atoms is called a nitrogenous base (usually just called a base). There are four bases, but each nucelotide only has one of the four: Adenine (A),Thiamine (T), Guanine (G) and Cytosine (C).

In the model below with lollies, the long chains each have squares (sugar) and circle (phosphate) forming the backbone, and in between are the bases. The yellow and orange bases always go together, the red and green bases always go together. So Adenine and Thymine go together A-T or T-A, and Cytosine and Guanine go together C-G or G-C. Track those on the picture. 

(A gene is a chain of particular nucleotides.)

When cells divide to make new cells - in mitosis when an organism is growing or when it needs to replace damaged cells; when special cells divide in meiosis to form sex cells - the cell must make exact copies of the DNA to go into the new cell. The process of exactly copying DNA is called replication. 

There are 4 steps:

1. The enzyme helicase unzips the double helix into two strands (like the strands in the image of the model above)

2. The separation now forms a Y shape with separate strands that will be templates for the new DNA.

3 and 4  One of the strands is oriented in the 3’ to 5’ direction (towards the replication fork), this is the leading strand. The other strand is oriented in the 5’ to 3’ direction (away from the replication fork), this is the lagging strand. As a result of their different orientations, the two strands are replicated differently.

The end result of the four processes is identical DNA, with the two new strands each having one strand of the original DNA and one strand that is newly replicated. (The picture below has the dark orange as the original DNA and the light colour is new.) 

MORE COMPLEX DISCUSSION

A gene mutation is a permanent alteration in the DNA sequence that makes up a gene, so that the sequence differs from what is found in most people. Mutations range in size; they can affect anywhere from a single DNA building block (base pair A-T or G-C) to a large segment of a chromosome that includes multiple genes.

Gene mutations can be classified in two major ways:

• Hereditary mutations are inherited from a parent and are present throughout a person’s life in virtually every cell in the body. These mutations are also called germline mutations because they are present in the parent’s egg or sperm cells, which are also called germ cells. When an egg and a sperm cell unite, the resulting fertilised egg cell receives DNA from both parents. If this DNA has a mutation, the child that grows from the fertilised egg will have the mutation in each of his or her cells.

• Acquired (or somatic) mutations occur at some time during a person’s life and are present only in certain cells, not in every cell in the body. These changes can be caused by environmental factors such as ultraviolet radiation from the sun, or can occur if an error is made as DNA copies itself during cell division. Acquired mutations in somatic cells (body cells - cells other than sperm and egg cells) cannot be passed to the next generation.

Genetic changes that are described as de novo (new) mutations can be either hereditary or somatic. In some cases, the mutation occurs in a person’s egg or sperm cell but is not present in any of the person’s other cells. In other cases, the mutation occurs in the fertilised egg shortly after the egg and sperm cells unite. (It is often impossible to tell exactly when a de novo mutation happened.) As the fertilised egg divides, each resulting cell in the growing embryo will have the mutation. De novo mutations may explain genetic disorders in which an affected child has a mutation in every cell in the body but the parents do not, and there is no family history of the disorder.

Somatic mutations that happen in a single cell early in embryonic development can lead to a situation called mosaicism. These genetic changes are not present in a parent’s egg or sperm cells, or in the fertilized egg, but happen a bit later when the embryo includes several cells. As all the cells divide during growth and development, cells that arise from the cell with the altered gene will have the mutation, while other cells will not. Depending on the mutation and how many cells are affected, mosaicism may or may not cause health problems.

Most disease-causing gene mutations are uncommon in the general population. However, other genetic changes occur more frequently. Genetic alterations that occur in more than 1% of the population are called polymorphisms. They are common enough to be considered a normal variation in the DNA. Polymorphisms are responsible for many of the normal differences between people such as eye color, hair color, and blood type. Although many polymorphisms have no negative effects on a person’s health, some of these variations may influence the risk of developing certain disorders.

1. View video: Clickview Mutations - Changing the Code (Canvas) I can't find the link for this video but I will ask the other Year 10 teachers.

2. Complete Worksheet