Every living thing on earth uses DNA to store genetic information and transmits that info from one generation to the next. A copy of some (or all) of every creature’s DNA is passed on to its offspring.

Side Note: Viruses contain DNA but they are not considered living things. To reproduce, a virus must attach itself to a living cell. As soon as the virus finds a host cell, the virus injects its DNA into the cell and forces that cell to reproduce the virus. A virus can not grow without stealing energy from a living cell, and it can not move from one organism to another on its own. In general, a virus is just a DNA surrounded by a protein shell. So, a protein is not alive but its not quite dead either. Damn virus (Especially Covid one).

Chemical Ingredients of DNA

DNA is a remarkably durable molecule, it can even stay in one piece for as long as 100,000 years under the right conditions. This durability is why scientists can recover DNA from 14,000 year old mammoths and larn that the mammoth is most closely related to today’s Asian elephants. The root of DNA’s extreme durability lies in its chemical and structural makeup.

Chemically, DNA is really simple. It’s made of three components: nitrogen-rich bases, deoxyribose sugars, phosphates. The three components combine to form a nucleotide. Thousands of nucleotides come together in pairs to form a single molecule of DNA.

Covering the bases

Each DNA molecule contains thousands of copies of four specific nitrogen-rich bases:

  • Adenine (A)
  • Guanine (G)
  • Cytosine (C)
  • Thymine (T)

These four bases come in two flavors:

  • Purines: The two purine bases in DNA are adenine and guanine. Purine actually mean a compound composed of two rings.
  • Pyrimidines: The two pyrimidine bases in DNA are cytosine and thymine. The term prymidine refer to chemicals that have a single six-sided ring structure.

Please check the figure below to see the details. Chemical Structure of DNA bases

The sequences of bases carries the message of DNA, which provides the information necessary to produce the corresponding protein. However, bonds can not bond together by themselves. Two more ingredients are needed: a special kind of sugar and phosphate. That is to say, to make a complete nucleotide the bases must attach to deoxyribose and a phosphate molecule. Deoxyribose is ribose sugar that has lost one of its oxygen atoms. When your body breaks down adenosine triphosphate (ATP), the molecule your body uses to power your cells, ribose is released with a phosphate molecule still attached to it.

The Structure of DNA: Double Helix

Nucleotides are the true building blocks of DNA. To make a complete DNA, single nucleotides join to make chains that come together as matched pairs and form long double strands. Nucleotides are a bit like coins in that they have two “sides” - phosphate side and a sugar side. Nucleotides can only make a connection by joining phosphates to sugars. After they formed, strands of DNA dont enjoy being single; they are always looking for a match. The arrangement in which strands of DNA match up is very important. A number of rules dictate how to single strands of DNA find their perfect matches to create a double helix.

A complete DNA molecule has

  • Two side-by-side polynucleotide strands twisted together
  • Bases attached in pairs in the center of the molecule
  • Sugars and phosphates are the outside, forming a backbone

If you were untwist a DNA double helix and lay it flat, it would look like a ladder. The DNA double helix

The molecule is guaranteed to be the same size all over because the matching bases complement each other, making whole pieces that are all the same size. Adenine complements thymine, and guanine complements cytosine. The bases always match up in this complementary fashion. Therefore, in every DNA molecule, the amount of one base is equal to the amount of its complementary base. This condition is known as Chargaff’s rules.

An important result of the bases’ complementary pairing is the way in which strands bond to each other. The number of bonds between the pairs differs; G-C pairs have three bonds, and A-T pairs have only two.

Diffirent Varieties of DNA

All DNA has the same four bases, obeys the same base pairing rules, and has the same double helix structure. No matter where it’s found or what function it’s carrying out, DNA is DNA. That said, different sets of DNA exist within a single organism. These sets carry out different genetic functions.

Nuclear DNA

Nuclear DNA is the DNA in cell nuclei, and it’s responsible for the majority of functions that cells carry out. Most of our genes are located in the nuclear DNA. In addition, the physical traits of an organism are generally the results of the genes located in the nuclear DNA. Nuclear DNA is packaged into chromosomes and passed from parent to offspring.

A genome is a full set of genetic instructions. The nuclear genome of humans is comprised of the DNA from all 22 pairs of autosomal chromosomes plus two sex chromosomes.

Mitochondrial DNA

Animals, plants and fungi all have mitochondria. These powerhouses of the cell come with their own DNA, which is quite diffirent in form from nuclear DNA. Each mitochondrian (singular of mitochondria) has many molecules of mitochondrial DNA, mtDNA, for short.

Human mtDNA is very short (slightly less than 17,000 base pairs) and has 37 genes. These genes control cellular metabolism - the processing of energy inside the cell.

Half of your nuclear DNA came from your mom, and the other half came from your dad. But all your mtDNA came from your mom. All mtDNA is passed from mother to child in the cytoplasm of the egg cell

Chloroplast DNA

Plants have three sets of DNA: nuclear in the form of chromosomes, mitochonrial, and chroloplast DNA (cpDNA). Chloroplats are organelles found only in plants, and there where photosynthesis (the conversion of light into chemical energy) occurs.

Chloroplast DNA molecules are circular and fairly large but only have about 120 genes. Most of those genes supply information use to carry out photosynthesis. Inheritence of cpDNA can be either maternal or paternal, and is transmitted to offspring in the cytoplasm of the seed.