Science With Mr. Milstid

Introduction to DNA
All animals and plants on Earth share something in common: they all use the same code to grow and function.
This code can be found in bacteria, elephants, oak trees, and humans. It’s the “master plan” for all organisms, like the source code for a computer program. It’s the reason your toes are on the ends of your feet (where they belong) instead of on your hands or hanging off the edges of your ears.

This code is called DNA, which is shorthand for Deoxyribose Nucleic Acid. DNA exists as long molecules within the nucleus of almost every cell in an organism.

DNA is in the form of a twisted ladder that scientists call a double helix.

The rungs of the ladder make up the four-letter DNA code or alphabet: A, T, C, and G.
These alphabet pieces are called bases and they bond together (in base pairs) according to special rules: A always pairs with T, and C always pairs with G [A–T C–G].

When the DNA code is read by the cell, only one strand of the code is used. The string of letters (or bases) make 3-letter long “words” and the words make “sentences,” just like the letters, words, and sentences in your science textbook. In DNA, the “sentences” are called genes. One strand of DNA may contain many genes.

Genes tell the cells how to make other complex molecules called proteins. Cells produce thousands of different proteins that work together to allow organisms to do all the functions necessary for life.

Click here for an in-depth tour of DNA from University of Utah Genetics

Fun Fact #1:
Each cell in our body contains a lot of DNA (about 1.7 meters). If our cells were enlarged to the size of an aspirin pill, the DNA in that cell would be 10,000 meters long. That’s as long as 109 football fields.

Fun Fact #2:
If you opened up all your cells and laid out the DNA end to end, the strands would stretch from the Earth to the Moon about 6,000 times.

Chromosomes and Mitosis
The DNA in each cell is coiled up tightly and packed into compact units called chromosomes.

Each body cell of an organism has the same number of chromosomes.
The number of chromosomes in each cell is different for different species of organisms. In humans, every cell contains 46 chromosomes. Dogs have 78 chromosomes in every cell. Cats have 38 chromosomes, horses have 64, and fruit flies have 8.

Within the nucleus of each cell, the chromosomes are found in homologous pairs, with half coming from each parent. So for humans, 23, or half of the 46 chromosomes in each of your cells came from your father and half (23) came from your mother.

Fun Fact #3:
The least number of chromosomes is found in a species of ant that has only 2 chromosomes. The record for the most chromosomes goes to a species of fern, which has 1,260 chromosomes in each cell.

Remember that within the nucleus of every cell in the body, the chromosomes contain the DNA, which carries the genetic code that directs how every cell functions. When the body makes new cells as part of growth, or to repair damaged tissues, the cells divide in a process called mitosis.
During mitosis, each parent cell divides to form two daughter cells.
The chromosomes in each parent cell also divide, so that each daughter cell has an exact copy of the chromosomes found in the parent cell. This way skin cells, for example, will all have the same genetic information so they will all function alike.

Asexual and Sexual Reproduction
Some organisms can produce “offspring” by making exact copies of the parent.
This is called asexual reproduction, and is found in single-celled organisms like the Paramecium and Amoeba, and also in simple organ-isms like sponges. Many plants can also reproduce asexually. Very few complex organisms use this method of reproduction (see an exception below.)

The offspring produced by asexual reproduction are always exact copies of their parents — in effect they are all clones of the parent organism. Asexual reproduction allows an organism to make lots of copies of itself, but all the copies are identical.

 
So, how do we get so much variation in nature? Why don’t we all look just like our brothers and sisters and parents?

 
When most organisms produce offspring, they use a different method, called sexual reproduction.
In sexual reproduction, offspring are not exact copies of their parents. Instead, they receive half their genes from their mothers and half from their fathers, so offspring may have characteristics in common with both parents, but they also have many unique traits—characteristics that may not be seen in either parent.

During sexual reproduction, special cells called gametes are produced by each parent.
Because these cells undergo a special type of cell division called meiosis, each gamete contains only half the number of chromosome found in the parent’s body cells.

So for humans, each body cell has 46 chromosomes (or 23 pairs–half from mom and half from dad), but each gamete only has 23 chromosomes, or one chromosome from each pair. During sexual reproduction, the male and female gametes unite to form a zygote, which then has the 46 chromosomes characteristic of human body cells.

Zygotes undergo cell division (mitosis) and grow and eventually develop into adults. However, offspring produced by sexual reproduction are not identical to their parents or to their brothers and sisters, because they each received different combinations of the chromosomes from each parent. This is one way that variation is introduced into each generation.

Another way that variation can be achieved is through mutation.
Sometimes when chromosomes are making copies of themselves, a mistake in the DNA code can occur. Also, there are factors in the environment that can cause errors or changes in the DNA code, such as UV radiation from sunlight, ionizing radiation from natural or man-made radioactive substances, and harsh chemicals introduced into the environment. All of these can damage DNA and may result in a mutation in a gene.

Tags: Biological Science, Cell Division, Genetics
Posted in Biological Science, Cells, Genetics