What is a gene?

A gene is the basic physical and functional unit of heredity. Genes, which are made up of DNA, act as instructions to make molecules called proteins. In humans, genes vary in size from a few hundred DNA bases to more than 2 million bases. The Human Genome Project has estimated that humans have between 20,000 and 25,000 genes.

Every person has two copies of each gene, one inherited from each parent. Most genes are the same in all people, but a small number of genes (less than 1 percent of the total) are slightly different between people. Alleles are forms of the same gene with small differences in their sequence of DNA bases. These small differences contribute to each person’s unique physical features.

What is a cell?

Cells are the basic building blocks of all living things. The human body is composed of trillions of cells. They provide structure for the body, take in nutrients from food, convert those nutrients into energy, and carry out specialized functions. Cells also contain the body’s hereditary material and can make copies of themselves.

Cells have many parts, each with a different function. Some of these parts, called organelles, are specialized structures that perform certain tasks within the cell. Human cells contain the following major parts, listed in alphabetical order:

Cytoplasm (illustration)

Within cells, the cytoplasm is made up of a jelly-like fluid (called the cytosol) and other structures that surround the nucleus.
Cytoskeleton

The cytoskeleton is a network of long fibers that make up the cell’s structural framework. The cytoskeleton has several critical functions, including determining cell shape, participating in cell division, and allowing cells to move. It also provides a track-like system that directs the movement of organelles and other substances within cells.
Endoplasmic reticulum (ER) (illustration)

This organelle helps process molecules created by the cell. The endoplasmic reticulum also transports these molecules to their specific destinations either inside or outside the cell.
Golgi apparatus (illustration)

The Golgi apparatus packages molecules processed by the endoplasmic reticulum to be transported out of the cell.
Lysosomes and peroxisomes (illustration)

These organelles are the recycling center of the cell. They digest foreign bacteria that invade the cell, rid the cell of toxic substances, and recycle worn-out cell components.
Mitochondria (illustration)

Mitochondria are complex organelles that convert energy from food into a form that the cell can use. They have their own genetic material, separate from the DNA in the nucleus, and can make copies of themselves.
Nucleus (illustration)

The nucleus serves as the cell’s command center, sending directions to the cell to grow, mature, divide, or die. It also houses DNA (deoxyribonucleic acid), the cell’s hereditary material. The nucleus is surrounded by a membrane called the nuclear envelope, which protects the DNA and separates the nucleus from the rest of the cell.
Plasma membrane (illustration)

The plasma membrane is the outer lining of the cell. It separates the cell from its environment and allows materials to enter and leave the cell.
Ribosomes (illustration)

Ribosomes are organelles that process the cell’s genetic instructions to create proteins. These organelles can float freely in the cytoplasm or be connected to the endoplasmic reticulum

Mitochondrial DNA

Although most DNA is packaged in chromosomes within the nucleus, mitochondria also have a small amount of their own DNA. This genetic material is known as mitochondrial DNA or mtDNA.

Mitochondria (illustration) are structures within cells that convert the energy from food into a form that cells can use. Each cell contains hundreds to thousands of mitochondria, which are located in the fluid that surrounds the nucleus (the cytoplasm).

Mitochondria produce energy through a process called oxidative phosphorylation. This process uses oxygen and simple sugars to create adenosine triphosphate (ATP), the cell’s main energy source. A set of enzyme complexes, designated as complexes I-V, carry out oxidative phosphorylation within mitochondria.

In addition to energy production, mitochondria play a role in several other cellular activities. For example, mitochondria help regulate the self-destruction of cells (apoptosis). They are also necessary for the production of substances such as cholesterol and heme (a component of hemoglobin, the molecule that carries oxygen in the blood).

Mitochondrial DNA contains 37 genes, all of which are essential for normal mitochondrial function. Thirteen of these genes provide instructions for making enzymes involved in oxidative phosphorylation. The remaining genes provide instructions for making molecules called transfer RNAs (tRNAs) and ribosomal RNAs (rRNAs), which are chemical cousins of DNA. These types of RNA help assemble protein building blocks (amino acids) into functioning proteins.

DNA INFROMATION

DNA, or deoxyribonucleic acid, is the hereditary material in humans and almost all other organisms. Nearly every cell in a person’s body has the same DNA. Most DNA is located in the cell nucleus (where it is called nuclear DNA), but a small amount of DNA can also be found in the mitochondria (where it is called mitochondrial DNA or mtDNA).

The information in DNA is stored as a code made up of four chemical bases: adenine (A), guanine (G), cytosine (C), and thymine (T). Human DNA consists of about 3 billion bases, and more than 99 percent of those bases are the same in all people. The order, or sequence, of these bases determines the information available for building and maintaining an organism, similar to the way in which letters of the alphabet appear ihttp://www.blogger.com/img/blank.gifn a certain order to form words and sentences.

DNA bases pair up with each other, A with T and C with G, to form units called base pairs. Each base is also attached to a sugar molecule and a phosphate molecule. Together, a base, sugar, and phosphate are called a nucleotide. Nucleotides are arranged in two long strands that form a spiral called a double helix. The structure of the double helix is somewhat like a ladder, with the base pairs forming the ladder’s rungs and the sugar and phosphate molecules forming the vertical sidepieces of the ladder.

An important property of DNA is that it can replicate, or make copies of itself. Each strand of DNA in the double helix can serve as a pattern for duplicating the sequence of bases. This is critical when cells divide because each new cell needs to have an exact copy of the DNA present in the old cell.

Nucleic acids deoxyribonucleic acid (DNA) and ribonucleic acid (RNA) are the genetic material of cells. Their names are derived from the type of sugar, ribose, contained within these molecules.

Phosphate-Sugar Backbone

Nucleotides linked together by covalent bonds between the phosphate of one nucleotide and the sugar of next. These linked monomers become the phosphate-sugar backbone of nucleic acids. Nitrogenous bases extending from this phosphate-sugar backbone like teeth of a comb.

The Twisted “Ladder” of Nucleic Acid

Hydrogen bonds form between specific bases of two nucleic acid chains, forming a stable, double-stranded DNA molecule, which looks like a ladder. Three H bonds form between bases cytosine (C) and guanine (G), which always pair up together between two nucleic acid chains. Two H bonds form between adenine (A) and thymine (T) in DNA or adenine and uracil (U) in RNA molecules.

The structure is analogous to a ladder, with the two deoxyribose-phosphate chains as side rails and the base pairs, linked by hydrogen bonds, forming the rungs. Hydrogen bonding also twists the phosphate-deoxyribose backbones into a helix, thus typical DNA is a double helix.
Additional Organic Chemistry Resources

To learn more about cell biology and organic chemisty and molecules, see the Suite101 articles What Is a Lipid, Amino Acids & Proteins, What Is a Carbohydrate and What Are Organic Molecules. Other excellent sites for information on organic chemistry include Science Prof Online and the Organic Chemistry Help site.

The British scientists first created normal embryos from the sperm of a man and egg of a woman (as per usual). However, the woman’s egg contained defective mitochondrial DNA.

Humans inherit their mitochondria only from our mother. So, in order to “fix” the defective mtDNA, the researchers transplanted the existing embryo into an egg with no nuclear DNA (no genome) that had been donated from a second woman who had healthy mitochondria (BBC 2008).
Inherited Mitochondrial Mutations

Inherited changes in mitochondrial DNA (those passed on through the egg of the mother) can cause problems relating to growth, development, and function of the body's systems. These mutations disrupt the mitochondria’s ability to generate ATP energy, and often involve multiple organ systems, particularly organs and tissues that require more energy (such as the heart, brain, and muscles). This new research may be a way of circumventing some of these devastating genetic disorders (National Library of Medicine).

It may sound like a rather simple question or even bizarre riddle, but the recent announcement that British scientist have created an embryo with DNA from 3 people has caused a bit of a stir.

How is it even possible? Why was DNA from 3 people necessary, when the DNA from 2 people has, for the most part, worked well in generating Homos sapiens and their predecessors for millions of years (Leakey 1978)? Well, this new twist on fertilization has to do with mitochondrial DNA (mtDNA) and mutations that can lead to serious health problems in humans.
Mitochondria: The Cell’s Powerhouse

The mitochondrion is a type of cellular organelle, and each cell of the body typically has hundreds to thousands of them. These microscopic organelles are the powerhouses of our cells.

Mitochondria are the sites of cellular respiration, a series of reactions that turn food energy into ATP (adenosine triphosphate) energy. ATP is like the "cellular Euro" of energy currency; a molecule that can be used to drive many types of reactions within the cell. Since most of our ATP is generated by the mitochondria, the role of this organelle is absolutely vital to the viability of every cell in our body (Campbell & Reece 2005).

What Does The Three-Parent Embryo Have to Do With Mitochondria?The DNA-from-three technique is a procedure that researchers hope may be used in the future, to produce embryos free of certain inherited diseases related to mitochondrial DNA. Mutations, or mistakes, in the mitochondria's genetic code can contribute to serious genetic disorders such as muscular dystrophy, epilepsy, strokes and mental retardation.

Compared with Traditional nuclear nDNA analysis, Mitochondrial mtDNA offers three primary benefits to forensic scientists:

* Its structure and location in the cell make mtDNA more stable, enabling investigators to test older or degraded samples
* mtDNA is available in larger quantities per cell – smaller samples can be tested
* mtDNA can be extracted from samples in which nDNA cannot, especially hair shafts and bone fragments.

MtDNA – Maternal Lineage Test

mtDNA sequence analysis is a valuable tool for determining whether individuals are biologically related through their mothers’ side of the family. This is commonly referred to as a maternal lineage test. An mtDNA maternal lineage test works by comparing the mitochondrial DNA (mtDNA) sequences of two or more individuals.

People who are biologically related in this way will have similar mtDNA sequences, while individuals who are not will have dissimilar mtDNA sequences.
Mitochondrial DNA- mtDNA - Testing for Cold Cases

The rise of mtDNA testing in the field of forensics means that cases that were previously thought hopeless, may now be resolved. Mitochondrial DNA in human cells is often more robust and more plentiful than nuclear DNA. MtDNA typing can be performed on hair shafts, bone, and teeth. As a result, mtDNA testing has been widely utilized by investigators in "cold case" police units