Molecular Genetics XIV

 

Central dogma of genetics

 

DNA ® RNA ® protein  That’s the central dogma of molecular genetics.

 

The nature of RNA

 

RNA is a polymer of ribonucleotides. It forms a single strand but may exhibit different conformations as regions may H-bond (hairpin loops are frequently encountered).  The sugar in RNA is ribose, and the base uracil is found instead of thymine.

For more info, check out

http://www.rothamsted.bbsrc.ac.uk/notebook/courses/guide/rnast.htm#Com

and

http://www.rothamsted.bbsrc.ac.uk/notebook/courses/guide/dnast.htm

 

There are four types of RNA. Each type of RNA is synthesized from the DNA template by RNA polymerase.

 

1. messenger RNA (mRNA). This RNA molecule carries information from the DNA to the ribosomes containing instructions to build polypeptides. In eukaryotes,

pre-RNA is the term used before processing to become mature mRNA.

 

2. ribosomal RNA (rRNA). This RNA molecule makes up a large part of the ribosomes which synthesize polypeptides.

 

3. transfer RNA (tRNA). This RNA molecule brings amino acids to the ribosomes to be incorporated into a growing polypeptide.

 

And unique to eukaryotes:

small nuclear RNA (snRNA). These RNA molecules together with small nuclear riboproteins (snRPs – pronounced “snurps”) are essential for RNA processing (splicing and editing)

 

Complementarity

 

In DNA replication, bases pair up always with A-T and C-G.  In DNA-RNA base pairing

DNA     A    C    T    G

RNA     U     G    A    C

Transcription v. Translation

 

Transcription is the synthesis of an RNA copy of a segment of DNA.  Translation is the synthesis of a polypeptide that is directed by the information coded on the mRNA.  The linear sequence on the mRNA is translated into the primary sequence of amino acids of a polypeptide.

 

Transcription involves RNA polymerase and the DNA template (one strand only is used as a template).  A DNA strand may be the coding strand for one gene and the non-coding strand for another gene. Translation involves ribosomes, mRNA, tRNA, and amino acids.

Both processes are anabolic and require cellular energy. Both require enzymes.

 

Try this link, please let me know if you can access it Thanks!

http://www.biology.com/learning/transcription/overview.html

 

Prokaryotes v. eukaryotes

 

In a prokaryotic cell, transcription and translation are coupled; that is, translation begins while the mRNA is still being synthesized. In a eukaryotic cell, transcription occurs in the nucleus and translation occurs in the cytoplasm. Transcription and translation are spatially and temporally separated in eukaryotic cells; that is, transcription occurs in the nucleus to produce a pre-mRNA molecule.

 

The genetic code

 

There are four bases in DNA and 20 amino acids. Code words consisting of two bases only could specify 16 amino acids only. Code words consisting of three bases could specify 64 amino acids. The code is degenerate and redundant but never ambiguous.  More than one codon may specify and amino acid (redundant) but no codon specifies more than one amino acid (not ambiguous). The genetic code (lots of interesting detective work figuring it out! – 1961 was a remarkable year of discovery in this field and a great year for wine and babies). In 1968, Marshall Nirenberg won the Nobel Prize in Physiology or Medicine (with Gobind Khorana and Robert Holley) for their interpretation of the genetic code and its function in protein synthesis.

 

IN CLASS: reading the genetic code

 

The three DNA bases are referred to as a triplet. The three RNA bases in mRNA are referred to as a codon. The three bases in tRNA are referred to as an anti-codon.