Molecular genetics

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Genetics and Cell Biology

Molecular genetics:

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 isbuild 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 thenucleobases. 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 helixstructure). 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 thePrimer 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 Primerwith regular DNA-Nucleotides

5. We have now so called Okazaki-fragments on the lagging strand which are conntected by the Ligase in the end


1. The RNA-Polymerase binds to the Promoter. Then the double helix gets divided into the two strands.

2. The RNA-Polymerase works in the 5’ – 3’ direction and connects the RNA-Nucleotides. The RNA unbinds from the DNAstrand and the two DNA strands come together again.

3. RNA-Polymerase reaches the terminator and unbinds itself from the DNA strand. And a new m-RNA strand has been developed.

So what’s the big deal about transcription? Well you cannot build proteins out of the DNA. So in order to being able to translate your DNA code you have to transcribe it first into a format (m-RNA) from which you canbuild proteins.

Genetic Code
A gene is a combination of DNA-Sequences, which together code the information for a specific gene product (RNA, Polypeptid).
The structure and the function of a protein are defined by its amino acid sequence. So in order to being able to produce proteins you have to code this amino acid sequence in the genetic code. Three nucleobases together (codon) code one aminoacid. (This is necessary since there are 20 amino acids and only 4 nucleobases).
To know where a gene starts there is a starter sequence/promoter (AUG) which is recognized by the RNA-Polymerase. And there are also stop sequences/terminators (UGA, UAG, UAA).

Translation (prokaryotes)
The translation takes place in the ribosome.
t-RNA molecules are important for this process. t-RNA moleculesconsist of nucleotides and have 3 “arms”. On the arm in the middle one finds the anticodon. If this anticodon matches with the next codon of the m-RNA the t-RNA can bind to it and add his amino acid (bound to its “head”) to the newly formed protein. This takes place on the A-site. Afterwards the t-RNA shifts, together with the m-RNA, to the P-site. Another fitting t-RNA is approaching to the...