Structure of DNA

The DNA (deoxyribonucleic acid) molecule is made up of two polynucleotide chains. The chains are antiparallel – to each other - one chain has the direction of phosphodiester bonds 5' » 3'  and the other 3' » 5' – we are talking about the 3' or 5' end. An -OH group is attached to the 3' end, while a phosphate group is attached to the 5' end.

General characteristics


The structure is made up of three components – sugar, phosphate, base. The sugar component consists of the five-carbon sugar 2-deoxy-D-ribose (oxygen is missing in the 2' position of DNA compared to normal ribose). Purine derivatives (Adenine, Guanine) and pyrimidine derivatives  (Cytosine, Thymine) are represented as nitrogenous bases in DNA. Bonding interactions occur between N-bases of opposite strands.

The law of complementarity
Only 2 specific N-bases are always bound together (always 1 pyrimidine base and 1 purine base), nameld: In addition,van der Waals forces (stabilization) act between neighboring bases.
 * A – T (connected by 2 hydrogen bonds);
 * C – G (connected by3 hydrogen bonds).

So the equation applied:
 * $$ \frac{\text{A + C}}{\text{T + G}} = 1 $$

Both polynucleotide strands (primary structure of DNA) reate the most common right-handed helix known as double helix (ssecondary structure of DNA) → the most frequently occurring form is the B-formof DNA = right-handed. DNA molecules can still occur in the right-handed form A and in theleft handed form Z. The transition between individual forms is possible based on a change in physical and chemical conditions.

Types of DNA


1. Nuclear (chromosomal)

From a functional point of view, these are: In eukaryotes, approximately 60% of DNA consists ofunique  (or low-repetition) sequences – this includes, for example, genesencoding polypeptides  or similar non-functional pseudogenesy. Others represented are repetitive sequences.
 * 1) DNA that encodes the sequence of amino acids in a polypeptide or some RNA.
 * DNA, which has a control and management function.
 * 1) Special types of DNA have specific functions in chromosomese.g., in the area of centromery and telomeres.
 * DNA, the function of which we do not yet know anything about.

We divide them into::
 * moderately repetitive sequence – number of copies in the genome 10–105 (this includes, for example, genes for rRNA and histone-type proteins);
 * highly repetitive  sequence – on the order of 106 copies / genome.

Repetitive sequences can be scattered throughout the genome. Long repetitive sequences are referred to as LINEs (Long Interspersed Nuclear Elements). Short repetitive sequences are referred to as SINEs (Short Interspersed Nuclear Elements). Most SINEs are derived from tRNA genes → their formation is explained by reposition (transcription) from RNA by reverse transcriptase. The so-called Alu-sequences, are specific for primates, where almost every 4kb section of human DNA contains this sequence - their origin is 7SL DNA. Another possibility is the so-called tandem repetitive sequences, where the individual repetitions are one after the other - e.g.: genes for rRNA or so-called satellite DNA.

2. Extrachromosomal


In humans, it is found in the  mitochondria.The arrangement of the mitochondrial genome is different from the nuclear genome of a eukaryotic cell, but it is similar to the arrangement of the genome in prokaryot. DNA has a circular  rrangement in mitochondria. In humans, it is 16.6 kb in size. In the human genome, a total of 37 genes code – of which 24 genes are involved in the proteosynthetic apparatus of mitochondria - 16S and 23S genes for rRNA, 22 genes for tRNA. The rest are involved in the enzymatic equipment of mitochondria. Most genes are coded on the H (heavy) strand of DNA. The information is quite strongly compressed, it does not contain introns! Other differences include that it has 4 triplets different  in meaning from those in the nuclear genome, there are also differences in initiation and termination.

related articles

 * DNA
 * DNA replication
 * Transcription
 * Posttranscriptional modifications
 * Transcription factors
 * Translation
 * Post-translational modifications


 * Repetitive sequences in the human genome