Sunday, March 5, 2017

What is a Genome?

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What is a genome?

"A genome is the complete set of genetic information in an organism."(1)  It is the entirety of our DNA—the molecule that contains our biological code. (2)  In my favorite analogy, the genome is like an instruction manual containing the information for how to put your body together and then maintain it. Every living organism—from single-celled to human—has a genome, but your genome is unique to you. (3)

Image 1: Map of a Cell (2)
Where is my genome?

Copies of your genome exist inside your cells. Most of the genomic information will be inside the nucleus of a cell, clustered into 46 structures called chromosomes. A very small part of the genomic information is in the mitochondria of the cell— structures outside of the nucleus that give energy to the cell and help it to function properly. (4)

Cells with no nucleus, like red blood cells, will not have a copy of the genome inside of them. Cells with only 23 chromosomes, like egg and sperm, will also not have a complete copy of the genome in them. (3)

What is my genome made out of?

Our genome is the complete set of our DNA. (2)  Each chromosome inside of a nucleus is made of a single strand of coiled DNA. (4)  DNA is shaped like a twisted ladder; the "sides" of the ladder are made of sugar and phosphate, and the "rungs" are made of four chemical bases (adenine, guanine, cytosine, and thymine) that form pairs called base pairs. (3)
Image 2: DNA in the Nucleus (5)

There are about 3 billion base pairs in human DNA. (2) Adenine and thymine are always paired together, and cytosine and guanine are always paired together.

Codons are groups of three base pairs that have the instructions to make a specific amino acid—the building blocks of proteins. Genes are groups of codons that provide the instructions to make a protein. (3)

As mentioned in the introduction, the genome is like an instruction manual. Our DNA can be thought of as the string of all letters in the instruction manual from beginning to end, and chromosomes can be thought of as chapters that divide the DNA. (3)

Instead of 26 letters of the alphabet, our imaginary genomic book is written with only 4 letters (A, G, C, and T) that represent DNA's four chemical bases. Codons are like words that are built from three chemical letters, and genes are like sentences built from a stretch of codons that provide the instructions for how to make a protein. (3)

What are proteins? 

"Proteins are large, complex molecules that play many critical roles in the body. They do most of the work in cells and are required for the structure, function, and regulation of the body's tissues and organs." (6)

Proteins can function as antibodies (that protect the body from foreign particles), enzymes (that perform different chemical reactions), and messengers (such as hormones, that transmit signals). (6) They can also provide structure for cells, or store and transport atoms and small molecules throughout the body. (6)

Proteins are essential for creating a functional organism, and their production is the central purpose of DNA. (3)

How does my genome make proteins?

Ribosomes are structures in a cell where proteins are made. They are outside of the nucleus and assemble the protein when they receive the instructions for making it. (In Image 1, the ribosomes are the small red dots that surround the nucleus and are scattered throughout the cell.)

DNA—and its genes that provide the instructions for building proteins—cannot leave the nucleus of cells. DNA in the nucleus can be thought of like a reference book that cannot be checked out of a library. (3)  To get information from a reference book out of a library, it needs to be photocopied. Similarly, to get the genetic information to the ribosomes to assemble a protein, the information inside the nucleus needs to be copied. (3)
Image 3: Transcription (Building messenger RNA) (2)

The copying process is called transcription and it uses molecules called RNA. RNA is very similar to DNA, but it is single-stranded, contains a different sugar, and has the chemical uracil (U) instead of thymine (T) as a base. There are different types of RNA. (3)

Transcription begins with RNA polymerase; an enzyme that attaches to the start of the gene and "unzips" the DNA strand. Using the base chemicals of the unzipped DNA strand as a template, free-floating base chemicals that match each base on the DNA strand are brought together to form an RNA strand. (3)(7)

Image 4: Translation (Building the Protein) (8)
This RNA is called "messenger RNA" (mRNA) and carries a copy of the gene that can leave the nucleus. But before mRNA can be "read" in the ribosome, it must keep only the "coding" information that directs production of amino acids (the building blocks of proteins) and remove any "noncoding" information that does not direct production of amino acids. (3)(7)

When mRNA leaves the nucleus, a ribosome binds to it for the step known as translation. Translation is when the mRNA is "read" to assemble the chain of amino acids that will create the protein. Every codon (three bases) on the mRNA strand corresponds to a single amino acid. Another type of RNA, called transfer RNA (tRNA), carries an amino acid to the ribosome by matching its chemical bases to the corresponding chemical bases on the mRNA strand. (3)(7)

As the tRNA brings amino acids to the ribosome, the amino acids bind together to form a chain. Once the last amino acid is added, the chain folds together and makes the final product; a protein. (7)  And from these proteins, we get the building blocks of life that put our bodies together and allow us to function.

(An excellent 3D explanation of transcription, translation, and how a protein is built can be found at (7)

Have anything to add? Additional sources, helpful and relevant resources, and professional insights are welcome! Share in the comments below.


(1) Scitable. (c2014). Genome. Nature [Online]. [Accessed Mar 05, 2017]. Available at:

(2) National Human Genome Research Institute. (2010). The Human Genome Project Completion: Frequently Asked Questions. [Online]. [Accessed Mar 05, 2017]. Available at:

(3) Health Education England. (2016). Week 1: The Genome and How We Explore It. Whole Genome Sequencing: Decoding the Language of Life and Health. Sept 19, 2016, FutureLearn Online course.

(4) Wellcome Trust Centre for Mitochondrial Research. (n.d.) What is Mitochondrial DNA? [Online]. [Accessed Mar 05, 2017]. Available at:

(5) Image from: Murphy, E. (2013). The Hunt for a Diagnosis: The Science Bit. Little Mama Murphy. [Online]. [Accessed Mar 05, 2017]. Available at:

(6) Genetics Home Reference. (2017). What are Proteins and What Do They Do? U.S. National Library of Medicine. [Online]. [Accessed Mar 05, 2017]. Available at:

(7) yourgenome. (2015). From DNA to Protein - 3D. [Online]. [Accessed Mar 05, 2017]. Available at:

(8) Image from: Norman, H. (n.d.) What Function do Ribosomes Serve in Polypeptide Synthesis? Quora. [Online]. [Accessed Mar 05, 2017]. Available at:

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