What's in this article?
What is DNA?
DNA or deoxyribonucleic acid is a long molecule that contains our unique genetic code. Like a recipe book it holds the instructions for making all the proteins in our bodies.
- Your genome is made of a chemical called deoxyribonucleic acid, or DNA for short.
- DNA contains four basic building blocks or bases: adenine(A), cytosine (C), guanine (G) and thymine (T).
- The order, or sequence, of these bases form the instructions in the genome.
- DNA is a two-stranded molecule.
- DNA has a unique ‘double helix’ shape, like a twisted ladder.
- Each strand is composed of long sequences of the four bases, A, C, G and T.
- The bases on one strand of the DNA molecule pair together with complementary bases on the opposite strand of DNA to form the ‘rungs’ of the DNA ‘ladder’.
- The bases always pair together in the same way, A with T, C with G.
- Each base pair is joined together by hydrogen bonds.
- Each strand of DNA has a beginning and an end, called 5’ (five prime) and 3’ (three prime) respectively.
- The two strands run in the opposite direction (antiparallel) to each other so that one runs 5’ to 3’ and one runs 3’ to 5’, they are called the sense strand and the antisense strand, respectively.
- The strands are separated during DNA replication.
- This double helix structure was first discovered by Francis Crick and James Watson with the help of Rosalind Franklin and Maurice Wilkins.
- The human genome is made of 3.2 billion bases of DNA but other organisms have different genome sizes.
What is DNA made of?
DNA contains four chemical bases:
- Adenine (A)
- Guanine (G)
- Cytosine (C)
- Thymine (T)
DNA base pairs
DNA bases pair up with each other, A with T and C with G, to form units called base pairs. Each base is also attached to a sugar molecule and a phosphate molecule.
DNA in humans contains around 3 billion bases and these are similar in two persons for about 99% of the total bases. These bases are sequenced differently for different information that needs to be transmitted. This is similar to the way that different sequences of letters form words and sequences of words form sentences.
Nucleotides and the double helix
A base, sugar, and phosphate in combination is called a nucleotide. Nucleotides are arranged in two long strands that form a spiral called a double helix. This looks like a twisted ladder and the base pairs form the rungs of the ladder and the sugar and phosphate molecules form the sides of the ladder.
How does DNA replicate itself?
The DNA can make copies of itself. Both the strands of the DNA open up and make a copy of each and become two DNA stands. Thus each new DNA has one copy of the old DNA from where the copy is made.
The mitochondria contain small amount of DNA. This genetic material is known as mitochondrial DNA or mtDNA.
Each cell contains hundreds to thousands of mitochondria that lie within the cytoplasm. Mitochondrial DNA contains 37 genes that help it to function normally. Thirteen of these genes provide instructions for making enzymes involved in energy production by oxidative phosphorylation. The rest of the genes help in making molecules called transfer RNAs (tRNAs) and ribosomal RNAs (rRNAs) that help in protein synthesis.
How did people find out that DNA is the hereditary material?
DNA was largely ignored for decades after a German chemist, Friedrich Miescher, first isolated the white, slightly acidic substance from the nucleus of cells in 1869. No one knew what DNA’s function was in fact, some doubted that it had a function at all so they pretty much left the stuff alone.
Very few people thought that DNA could be the hereditary material. Early studies of DNA suggested, erroneously, that the molecule was made up of the same sequence of four bases repeated over and over ACGTACGTACGT… for example. No one could imagine how such a monotonously simple molecule could contain the information necessary to build a living organism.
But during the 1930s and 1940s, new experiments began to suggest that DNA might, in fact, be important. It turned out that different strains of bacteria can exchange DNA and that when they do certain traits, such as the ability to cause disease in humans, can be passed from one strain of bacteria to another. Scientists also learned that when a virus infects a cell it injects its DNA into the cell, which then produces many copies of the virus, suggesting that DNA contains instructions for building viruses. And they found that different species of organisms have different proportions of bases in their DNA one species might have DNA that is 30 percent A, 20 percent C, 20 percent G, and 30 percent T, while another might have 20 percent A, 30 percent C, 30 percent G, and 20 percent T. People began to think that genetic information might be written in the differences between the DNA bases of different species.