Genetics and Cell Biology

Molecular genetics:

Nucleotides:
A Nucleotide consists of a nucleobase (Cytosine, Thymine, Adenine and Guanine), a sugar (desoxyribose) and a phosphate.

Molecular differences of RNA compared to DNA:
No Thymine appears in the RNA. Instead you find Uracil.
Instead of deoxyribose, ribose is used as a sugar for the RNA.

Structure of the DNA:
A DNA molecule is build out of millions of nucleotides. Those nucleotides are connected in a chain. The 3C-atom is connected with the 5C-atom of the next sugar molecule via a phosphate. As a result you get one strand of the DNA. Since the DNA consists out of two strands, there is a second strand (build according to the same principles). Those two strands are connected through the hydrogen bonds between the nucleobases. Each nucleobase builds a pair with another base from the other strand. Possible pairs: Adenine + Thymine and Cytosine + Guanine. One does also speak of complementary bases; a sting is also the building plan for the other strand. The two strands go in the opposite direction: one goes from 3’ to 5’ and the other from 5’ to 3’. The two strands rotate around a common axe (Double helix structure). The double helix is winded around histone proteins.

Replication of the DNA

1. Helicase (an enzyme) comes and cuts the two strands apart. In order to prevent the strands from fusing together afterwards there are also some stabilizing proteins.

2. The primase docks on the DNA and build the primer there (the primer is a little piece of RNA). The DNA-Polymerase III now can bind to the Primer and build a complementary strand to the preexistent.

3. The DNA-Polymerase can only work from the 3’ to the 5’ end. (Leading strand) On the other strand there are many primers built by the primase. The DNA-Polymerase III than can synthesize the areas between two primers in the 3’ to 5’ direction. (Lagging strand)

4. Now the DNA-Polymerase I comes and replaces the Primer