Core Concepts
In this tutorial, you will learn all about deoxyribonucleic acid (DNA). We begin with an introduction to DNA and an analysis of its structure. Then, we discuss the role DNA plays in protein synthesis and consider the differences between DNA and RNA. Lastly, some other interesting facts about DNA are shared!
Topics Covered in Other Articles
- The DNA Tautomer: B-DNA, A-DNA, and Z-DNA
- Proteins and Amino Acids
- Protein Structure and Stability
- Enzymes – Function and Types
- Catalysts & Activation Energy
- RNA Interference
Introduction to DNA
Deoxyribonucleic acid (DNA) is a molecule that contains the genetic material (i.e., biological instructions) used for the growth, development, reproduction, and functioning of all living things.
The complete set of DNA in an organism is known as a genome. The human genome consists of 23 pairs of chromosomes (i.e., thread-like structures made up of DNA). Each chromosome contains hundreds or thousands of genes, which are the basic units of heredity. Genes are passed from parent to offspring and can help determine certain qualities or traits of an individual.
DNA Structure
DNA has a double-helix structure, meaning it consists of two linked strands that are intertwined with one another. The DNA double helix (shown below) resembles the shape of a spiraling staircase or twisting ladder.
Strands of DNA are composed of chemical building blocks known as nucleotides. Each nucleotide contains a phosphate group, five-carbon sugar molecule, and nitrogenous base. Nucleotides are joined together by phosphodiester bonds (shown below) to form the sugar-phosphate backbone of DNA.
Hydrogen bonds between complementary base pairs connect the two strands of the DNA double helix. In other words, each of the four different nitrogenous bases in a molecule of DNA (adenine, thymine, guanine, and cytosine) links with another to form the double strand. In Watson and Crick base pairing, thymine always pairs with adenine, while cytosine always pairs with guanine.
Each strand of DNA has two different ends: the 5′ (five prime) end and the 3′ (three prime) end. These designations refer to the number of the carbon in the sugar molecule that has either a free phosphate group (5′ carbon) or a free hydroxyl group (3′ carbon). The two strands of DNA in a double helix are antiparallel (i.e., they run in opposite directions); the 5′ end of one strand aligns with the 3′ end of the other strand.
How Does DNA Work?
DNA stores the instructions used to synthesize complex molecules known as proteins. Proteins consist of long chains of amino acids, linked together by peptide bonds. They perform a wide range of functions in the cell (such as catalyzing chemical reactions, providing structural support, and serving as chemical messengers).
The process by which DNA is used to produce proteins occurs through two key steps: transcription and translation.
- Transcription: A segment of DNA is copied into a new molecule of ribonucleic acid (RNA) during transcription. This process is carried out by an enzyme known as RNA polymerase, which synthesizes messenger RNA (mRNA) or non-coding RNAs (such as transfer RNA or ribosomal RNA) by using DNA as a template.
- Translation: A molecule of mRNA (synthesized during transcription) carries information to a ribosome, located in the cytoplasm of the cell, during translation. The ribosome decodes the mRNA sequence (using the genetic code) into a long chain of amino acids, forming a protein.
DNA vs. RNA
The two main types of nucleic acids are deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). Listed below are some of the key differences between DNA and RNA.
- Sugar: DNA contains the sugar deoxyribose, while RNA contains the sugar ribose. Deoxyribose and ribose are both five-carbon (pentose) sugar molecules; however, ribose contains an extra hydroxyl group on the 2′ carbon.
- Base Composition: Both DNA and RNA contain the following nitrogenous bases: adenine, guanine, and cytosine. However, DNA uses the base thymine, while RNA uses the base uracil. Adenine pairs with thymine in DNA and uracil in RNA.
- Number of Strands: DNA is most often double-stranded, while RNA is generally single-stranded.
- Location: The majority of DNA is located in the nucleus of eukaryotic cells or the nucleoid of prokaryotic cells. However, a small amount of DNA is also contained in the mitochondria and chloroplasts. RNA is mostly found in the cytoplasm of both eukaryotic and prokaryotic cells.
- Length: DNA is a much longer molecule than RNA. A human chromosome (containing a single DNA molecule) ranges from about 50 million to 250 million base pairs. On the other hand, RNA molecules are generally only a few thousand base pairs long (at most).
Interesting Facts About Deoxyribonucleic Acid (DNA)
- A Swiss biochemist named Johannes Friedrich Miescher discovered deoxyribonucleic acid (DNA) in 1869 while conducting research with white blood cells.
- James Watson and Francis Crick discovered the double-helix structure of DNA in 1953 (with the help of key contributions from other researchers such as Rosalind Franklin and Maurice Wilkins).
- All human beings are roughly 99.9% identical in their genetic makeup. The 0.1% of DNA content that differs across individuals can determine certain traits (eye color, blood type, etc.) or affect one’s likelihood of developing a particular disease.
- Nearly all of the cells in a person’s body contain DNA. One major exception is red blood cells, which lose their nucleus (and DNA) as they mature.
- Only 1-2% of the human genome codes for proteins. The other 98-99% is non-coding DNA sequences, which serve a number of different functions (e.g., regulating gene expression and providing instructions for the formation of non-coding RNAs).
- Before cell division, a double-stranded DNA molecule makes a copy of itself, producing two identical molecules of DNA through a process known as DNA replication. This process is semiconservative because both of the DNA molecules formed contain one original strand and one newly synthesized strand.
- Mutations (i.e., changes in DNA sequence) can occur due to errors during DNA replication or as a result of environmental factors (such as an exposure to radiation or certain chemicals).
- Genome editing is an exciting (but somewhat controversial) area of research in which scientists use cutting-edge technology to add, remove, or alter DNA in a living organism. Gene-editing tools (such as CRISPR-Cas9) have numerous applications, including the potential to prevent and treat a wide range of genetic diseases.