Quick DNA Overview

Double helix deoxyribosenucleic acid

Deoxyribosenucleic acid, as you know in humans and in other eukaryotic organisms codes our chromosomes and makes us the way we are!

Similarly, in prokaryotes, like bacteria, DNA makes up their genetic code or in other words what they are.  This includes their function and their appearance.

Universally, DNA contains purines (adenosine, guanine) and pyrimidines (cysteine, thymine, uracil (found in RNA)).  These guys are the bases, in which three of it together will code for a codon, that signifies an amino acid. A sequence of these codons assist with making up a peptide chain. As you know, peptides together, after having been rearranged into a 3D structure is a protein.

As you can see in the picture, DNA’s backbone is connected by phosphodiester bonds (I can get into this later and why its called what its called). These bonds help create the phosphate backbone of DNA. Since phosphate groups (PO4-) are negatively charged, a chunk of PO4- used as the backbone of the DNA will make the DNA a highly negative molecule.

As you already know, DNA is a double helix. The two strands are antiparallel. Basically, DNA strands are replicated from what is called the 5′ (five prime) end to the 3′ end (I’ll break it down in another post).



The bases (T, A, G, C) are connected to each other by weak hydrogen bonds.

Organization of DNA in Prokaryotes

DNA are supercoiled, and require ATP and an enzyme  topoisomerase to uncoil the DNA.

In prokaryotes, DNA are shown in plasmids and nucleoid.

Prokaryote cell replicates by binary fission.


Bacterial chromosomal DNA with plasmid DNA

Bacteria contains a chromosomal DNA as well as a plasmid DNA, independent from the chromosomal DNA and is capable of replicating itself without chromosomal DNA. Plasmids are also found in some eukaryotes. Plasmids codes for a gene that allows it to infect the host’s immune system.  The number of plasmids are unlimited in bacteria (more than thousands of its copy).

Benefit of Plasmid to Bacteria:

Protection against antibiotics

Chromosomal DNA

Chromosomes contain genes needed for the cell to survive.

Single copy genes – are very important and if they are damaged we would be in trouble since there is only ONE!

repetitive genes aka “junk DNA” – has unknown functionality


The genome of an organism is basically knowing every single gene.  So far, we know 100% of E. coli and thus, it is widely used in laboratories.

The human’s genome is only at about 90%. Even though our genotype is similar, our phenotype is vastly different due to limited or certain expression of certain genes.

Organization of DNA in Eukaryotes

Different than prokaryotes, in Eukaryotes:

  • Nucleus – containing chromosomes or DNA that are supercoiled
  • Nucleus – has histones - proteins covering DNA that allows our chromosome to condense.

Histones combined together–> chromatin fiber–> condensing of chromosome is done in prophase in mitosis and meosis.


The genetic of the cell (cytogenic) is a field that allows cytogenetic technlogist to advance as a genetic counselor who can get karyotyping done. Karyotype is done by amniocentesis which means removing some cells of the organism and inducing mitosis, which creates a mosaic of chromosome. This allows the reading of the genetic make up of an individual. In karyotyping, diseases such as trisonomy 21, which shows 3 copies instead of 2 in chromosome #21 and signifies Downe Syndrome.

DNA synthesis

semiconservative – only one strand is replicated at a time by DNA polymerase

DNA polymerase – the protein that brings the nucleosides (bases: ATCG)

Phosphodiester bonds – strong bond and is at the backbone of the DNA, connecting the phosphate groups together. Hydrogen bonds are weak and allows for the connection between the bases on opposing sides of the either strand of DNA causing the helical formation of the DNA (helicase, an enzyme breaks this bond)

  1. Histone and other proteins are removed.
  2. Helicase unwinds DNA by breaking the hydrogen bonds
  3. At the unwound site, called the replication fork, stabilizing proteins will come onto the unwounded DNA
  4. DNA polymerase using nculeoids (dATP, dGTP, dCTP, dTTP) to make the copied strand

Other DNA: Plasmids

Plasmids are used in biotech industry to implant a foreign gene to be replicated into the plasmid. Plasmids are vehicles to express foreign genes in bacteria like E. coli.

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