Understanding genetic diseases

Understanding Genetics

To understand gene therapy, it may help to first understand some basics about genetics. This will help you build a foundation for your genehome experience. This page provides important context and introduces key terms for you to use throughout genehome.

Start with a foundation

Image of a cell

Cells

Every living thing is made up of one or many building blocks called Cellsthe basic building blocks of living things; they are enclosed by a wall or membrane and have structures inside them that perform a variety of functions that help them, and/or the organism they are part, to survive

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. The human body has trillions of cells, and they are divided into many different types. For example, there are muscle cells, bone cells, kidney cells, Red blood cellsthe blood cells that carry oxygen

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 and White blood cellscells the body makes to help fight infections

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. All cells work together to keep us alive. They carry out functions like converting food to energy. Some of them provide structural support, giving our organs and our body shape and strength. Nearly every cell contains the same DNA (deoxyribonucleic acid)the substance (or molecule) that carries genetic information in a human cell

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in a cell structure called the Nucleusa membrane-bound organelle that contains the cell’s chromosomes

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.1 A small amount of DNA may also be found in a part of the cell called the Mitochondriamembrane-bound cell organelles that produce energy

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, which is the energy production center of a cell.1,2
Image of a DNA strand Image of a dividing cell

DNA (deoxyribonucleic acid)

DNA provides instruction for development and function of living things. This means that DNA helps determine our physical characteristics, such as skin coloring, eye color, and height.3,4 

You can picture DNA as a winding ladder. It has two support columns connected by rungs. The columns are made of sugar and phosphate molecules. The rungs are made of a pair of chemical bases: adenine (A), cytosine (C), guanine (G), and thymine (T). A always pairs with T and C with G to form units called Base pairs2 complementary DNA bases that are bonded together forming one step on the DNA ladder

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. The order or sequence of these base pairs is a code that gives instructions for building and maintaining all cells.1
Moreover, DNA is the carrier of Genetic informationthe hereditary information coded in a person’s DNA or RNA

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, which means it gets passed down from generation to generation. This is possible because DNA has the ability to replicate—or to make copies of itself—which it does when a cell divides. If this process goes as it should, the new cell will have an exact copy of the DNA of the original cell. Replication of DNA allows cells to divide and become new cells.1
Image of a DNA strand Image of a dividing cell
Moreover, DNA is the carrier of Genetic informationthe hereditary information coded in a person’s DNA or RNA

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, which means it gets passed down from generation to generation. This is possible because DNA has the ability to replicate—or to make copies of itself—which it does when a cell divides. If this process goes as it should, the new cell will have an exact copy of the DNA of the original cell. Replication of DNA allows cells to divide and become new cells.1
Image of gene expression

Genes

All of your DNA is called a Genomethe entire set of genetic instructions found in a cell nucleus

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, not to be confused with genehome, where you are now! Each genome contains about 3 billion base pairs. Within the genome are many smaller sections of DNA called Genesinstructions made of DNA used to create the proteins the body needs to function

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. Each gene has a unique sequence of base pairs. Genes can contain from a few hundred to over 2 million base pairs.1 

Some genes provide instructions to make Proteinsclass of molecules composed of one or more chains of amino acids that perform different functions the body needs including structure, function, and regulation of tissues and organs

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. Proteins do the work of the cell. They perform all sorts of different tasks in your cells, such as making eye pigments, powering muscles, and attacking invading bacteria.1

Proteins are made of Amino acidsa set of 20 different molecules used to build proteins

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. There are 20 different amino acids. Proteins can consist of 50 to 2000 of these amino acids. It takes 3 base pairs of DNA, called a Codona sequence of DNA that is tied to a specific amino acid

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, to code for 1 amino acid.1,5
Image of a chromosome

Chromosomes

DNA is packaged into structures called Chromosomesan organized package of DNA found in the nucleus of the cell. Humans have 23 pairs of chromosomes, and they receive half from their mother and half from their father

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, in which the DNA is tightly wound around proteins called Histonea protein that provides structural support to a chromosome

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.1 Except for sperm and egg cells, every cell in a human body normally has 23 pairs of chromosomes (for a total of 46). One set is inherited from your mother and the other from your father. Twenty-two pairs of chromosomes, called Autosomesany of the numbered chromosomes, not including sex chromosomes

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and numbered 1 to 22, look alike in males and females. The 23rd pair, called Sex chromosomesa type of chromosome that helps determine the sex

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, are different in each gender. Males have an X and a Y chromosome. Females have two X chromosomes. Sperm and egg cells each contain 23 chromosomes. Sperm cells carry an X or Y chromosome and egg cells carry an X chromosome.1,6
Gene-ius Questions
Genes are made of DNA. Your genes are found in your DNA—genes are actually just smaller sections of longer strands of DNA. In order to tie all of the concepts in this section together, it may be helpful to think about how they relate to each other.1

Here we go: numerous base pairs make up DNA. Genes are sections within long strands of DNA. DNA winds around histones to make up chromosomes. Chromosomes are located inside of cells.1
Genes are sections of DNA, which winds around histones to make chromosomes that are located inside of cells

Coding and noncoding DNA

The genome (all the DNA in the body) includes regions that are coding and noncoding.1

Coding DNArefers to regions within a gene that provide the instructions to produce a protein

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refers to regions within a gene that provide the instructions to produce a protein. There are an estimated 20,000 genes that code for proteins in the human genome.7

Noncoding DNADNA sequences that do not code for amino acids; some noncoding DNA has no known function, while some noncoding DNA plays a role in regulating gene expression

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 does not provide instructions to produce proteins but may serve other functions, such as1:

  • Giving a cell the instructions to regulate when and where a gene is turned on and off1
  • Telling a cell how to produce different types of Ribonucleic acid (RNA)a nucleic acid that is present in all human cells. It is the product of transcription and acts as an intermediary between DNA and proteins

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    , another nucleic acid closely related to DNA that is involved in making proteins (protein synthesis)1
  • Showing where the beginning and end of a gene is1
  • Providing structural elements to chromosomes, such as Telomeresrepetitive sequence of noncoding DNA at the end of a chromosome, which protects the chromosome from damage during cell division

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    , which protect the ends of chromosomes from degrading when the DNA is replicating1
  • Approximately 99% of the human genome is noncoding DNA1
Gene-ius Questions
Genetic codes are the instructions that DNA gives to the body’s cells. These instructions tell the cell how to make a specific protein. The code is made up of the letters A, C, G, and T, which represent adenine, cytosine, guanine, and thymine. Three-letter combinations of these letters (eg, GCA) encode amino acids needed to make proteins.1

How do you get your genes?
Genes are the basic units of heredity. Each gene consists of 2 copies, one on each chromosome inherited from each parent. 99.9% of the genes in the human genome are identical from person to person.6 The rest are variants, meaning they can vary a small amount from person to person. These variants are called alleles. Alleles account for many of the differences among people, such as physical traits, or whether you are at higher or lower risk for certain diseases.6,8 Just like all genes, you inherit alleles—one from your mother and one from your father. So you can have 2 identical alleles, or they can be different. If they are different, 1 allele will be dominant 1 allele will be recessive. In these cases, the dominant allele will determine which trait will be expressed.

Genes are the basic units of heredity. Each gene consists of 2 copies, one on each chromosome inherited from each parent. 99.9% of the genes in the human genome are identical from person to person.6 The rest are variants, meaning they can vary a small amount from person to person. These variants are called Allelesany variant of a gene that can be found in the same place on a specific chromosome

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. Alleles account for many of the differences among people, such as physical traits, or whether you are at higher or lower risk for certain diseases.6,8

Just like all genes, you inherit alleles—one from your mother and one from your father. So you can have 2 identical alleles, or they can be different. If they are different, 1 allele will be Dominanta genetic trait that appears when there is only one copy of that gene

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 1 allele will be Recessivea trait that appears only when a person has two copies of a gene

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. In these cases, the dominant allele will determine which trait will be expressed.8

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References

1. National Institutes of Health. Genetics Home Reference. Help me understand genetics. Accessed March 4, 2020. https://ghr.nlm.nih.gov/primer 2. Benz EJ. Learning about genomics and disease from the anucleate human red blood cell. J Clin Invest. 2010;120(12):4204-4206. 3. National Institutes of Health. National Cancer Institute. NCI dictionary of cancer terms. Accessed March 16, 2020. https://www.cancer.gov/publications/dictionaries/cancer-terms/def/dna 4. Scientific American. How are traits passed on through DNA? Accessed March 16, 2020. https://www.scientificamerican.com/article/how-are-traits-passed-on/ 5. Alberts B, Johnson A, Lewis J, et al. The shape and structure of proteins. Molecular Biology of the Cell. 4th edition. New York: Garland Science; 2002. 6. Genetic Alliance; The New York–Mid-Atlantic Consortium for Genetic and Newborn Screening Services. Understanding Genetics: A New York, Mid-Atlantic Guide for Patients and Health Professionals. Genetic Alliance Monographs and Guides; 2009. https://www.ncbi.nlm.nih.gov/books/NBK115563/pdf/Bookshelf_NBK115563.pdf 7. Salzberg SL. Open questions: how many genes do we have? BMC Biol. 2018;16(94). https://doi.org/10.1186/s12915-018-0564-x 8. Jackson M, Marks L, May GHW, Wilson JB. The genetic basis of disease. Essays Biochem. 2018;62:643-723.

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