DNA encodes hereditary information used in the development and functioning of all living organisms. The segments that carry this genetic information are called genes. DNA stands for deoxyribonucleic acid, an organic molecule composed of a chain of nucleotides. These nucleotides are adenine, thymine, cytosine, and guanine. They are often abbreviated as A, C, G, and T and written as a code, which then can be analyzed for changes that might, for example, carry a higher risk of disease development.
Structure
DNA molecules consist of 2 long strands made up of nucleotides bonded together. The nucleotides consist of a sugar and phosphate group and nitrogenous bases. The sugar in DNA is called deoxyribose, and together with the phosphate group, they make up the backbone of DNA - 2 long strands. [1]
Each sugar has a nitrogenous base connected to it, and they are connected to the opposite nitrogenous base, making the typical 3D double helix structure of DNA.
There are four bases: adenine (A), cytosine (C), guanine (G), and thymine (T). Guanine can only pair with cytosine and adenine to thymine.
Furthermore, DNA double helix is wrapped around histones - and then folded and coiled more. Eventually, it will make up a chromosome. Humans have 46 chromosomes that are stored in the nucleus of our cells. There are 22 pairs of autosomal chromosomes and one pair of sex chromosomes that determine biological sex. Females have XX chromosomes and males XY.
A small part of mitochondrial DNA is stored in mitochondria (special organelles in our cells). This part is inherited solely from the mother.
The complete DNA set is called a genome - it contains roughly 3 billion bases, 20,000 genes, and 23 pairs of chromosomes.
Genes are sequences that contain information on how to make proteins, and they make up only 1% of our genome; they are also called the coding regions. The non-coding regions, which comprise most of our genome, help the cells regulate protein synthesis.
Inheritance
Your DNA itself is a record of your ancestors. Half of your DNA comes from your mother, and the other comes from your father. It is contained in the oocytes and sperm cells. These cells combine their information in fertilization, leading to embryo formation.
You get 22 autosomal chromosomes and one X chromosome from your mother. The father will also give 22 autosomal chromosomes and either an X or a Y chromosome. So the sex of the baby depends solely on what chromosome will be inherited from the father. [2]
Function
DNA contains all the necessary information for your growth, development, reproduction, and proper function. These instructions are encoded in the specific sequences of the nucleotides. Our DNA is responsible for forming different proteins in our body to control all the processes happening in the body.
How do you get from DNA to a protein?
The process of making a protein from a gene is called gene expression.
First, DNA is changed to RNA (ribonucleic acid) in the process known as transcription. RNA is similar to DNA but has ribose sugar and is single-stranded. During this process, nucleotide bases are changed to the corresponding ones:
- Adenosine is transcribed to Uracil (nucleotide not present in DNA)
- Thiamine to Adenosine
- Cytosine to Guanine
- Guanine to Cytosine
After that, RNA serves as a template for the production of proteins in the process called translation. Polypeptides are created by linking various amino acids based on the mRNA template. Amino acids are the basic building blocks of proteins. Three nucleotides each correspond to a specific amino acid. These triplets are called codons. For example:
- The base pairs T-G-G specify the amino acid tryptophan.
- Some triplets (TAA, TAG, and TGA) signal the end of the protein sequence, meaning no amino acids will be added.
- Some triplets serve as start signals (AUG).
Although all the cells in our body contain the same set of DNA, they all have different structures and functions. That is caused by different expressions of various genes. We can say that the expression pattern is determined by information and factors from inside and outside the cell.
Mutations and damage
DNA in each cell is prone to damage, especially when exposed to environmental dangers like ultraviolet radiation from the Sun. Every day, tens of thousands of DNA damage happens. To counter this threat, life has evolved several systems to detect this damage, signal its presence and repair it. [3]
There are various ways DNA damage can occur. For example, it can be due to errors in replication, the effect of free radicals, or exposure to radiation, including UV radiation from sun exposure.
However, our cells have learned to deal with it, and there are at least five ways that they can repair broken DNA. In our cells, specialized proteins detect and fix most errors. If this process is impaired somehow, it leads to genetic mutations. In most cases, one mutation will not cause relevant gene expression changes, but there are some exceptions. For example, certain diseases, like cystic fibrosis or Huntington's, occur due to a mutation in a single gene. These mutations don't mean you can develop those diseases during your lifetime. Still, exposing yourself to a higher DNA damaging environment might increase your children's chances of developing many diseases. These diseases can be inherited in different patterns, such as autosomal dominant/recessive or X-linked (linked to a sex chromosome). [5]
On the other hand, if mutations in certain regions of your DNA accumulate during your life, they can eventually lead to the development of cancer. However, not all mutations lead to the development of a disease or cancer. Some of them are harmless. For example, heterochromia is a genetic mutation that causes different eye colors.
Aging
There are several ways DNA has been linked to the aging process. Accumulation of mutation is one of them. Mostly these are mutations caused by the influence of free radicals. The repair processes in our bodies become less effective as we age. After the peak of our reproductive years, they start to decline.
Another way in which DNA is connected to aging is by the shortening of telomeres. These are regions of repetitive DNA sequences at the ends of chromosomes. They protect our chromosomes from becoming frayed or tangled. However, telomeres become shorter each replication cycle (every time a cell divides). Eventually, they become so short that the cells can't divide anymore and die. Some lifestyle factors, like obesity, smoking, or psychological stress, can also contribute to their shortening, decreasing our lifespan.
Gene editing
Modern science has developed some ways in which we can change some parts of our DNA. Gene therapy holds promise for treating many diseases, such as cancer, cystic fibrosis, heart disease, diabetes, hemophilia, and AIDS. [6]
There are some gene therapies available already. For example, the most expensive drug in the world, Onasemnogene abeparvovec, is used to cure spinal muscular atrophy in children.
