DNA & RNA

Molecular Genetics

What is the genetic code and what are its main features?

The sequence of nucleotides in DNA forms a code that determines the code that determines the sequence of amino acids in proteins of an organism. 

In eukaryotes the DNA is largely confined in the nucleus, however, the synthesis of proteins takes place in the cytoplasm. So how is the DNA in the nucleus transferred to the cytoplasm where it is translated into proteins?  

Answer is that sections of the DNA code is transcribed onto single stranded molecule called Ribonucleic acid a.k.a RNA. 

There are a number of RNA the one that transfers the DNA code from the nucleus to the cytoplasm and acts as a messenger is called Messenger RNA or mRNA. mRNA is small enough to exit through the nuclear pores and the sequences of nucleotide bases on the mRNA are what we call the 'genetic code'. mRNA code is complimentary to the DNA code. The term codon refers to the 3 bases (triplet) on mRNA that codes for single amino acids. 

Main features of the genetic code:

• Amino acids in a protein is coded by a sequence of 3 nucleotide bases on mRNA (codon).
• A few amino acids have only one codon. 
• Codon = degenerate code. i.e most amino acids have more than one. 
• 3 codons don't code for anything called stop codons. Non-coding regions of DNA within a gene are called introns (for interruption sequences), while the coding parts of DNA are called exons (for expressed sequences). All eukaryotic genes have introns, and they are usually longer than the exons, so genes are often much longer than they really need to be! one knows what these introns are for, but they need to be removed before the mRNA can be translated into protein.

• It's non overlapping 
• Universal

What is the structure of Ribonucleic acid structure

RNA is usually single stranded and it is a polymer made up of repeating mononucleotide sub-units. It forms a single strand in which each nucleotide is made up of:

• Pentose sugar ribose
• Organic bases Adenine, guanine, cytosine and uracil. 
• Phosphate group.

The two types of RNA are:

• mRNA
• tRNA (Transcriptional)

Messenger RNA (mRNA)

mRNA is a long strand that is arranged into a single helix. It is made when DNA forms a mirror copy of part of one of its two strands. 

mRNA leaves the nucleus through the nuclear pores and associates with the ribosomes.

It then acts as a template onto which proteins are built. It's structure suits it's function, can also be easily broken.

Transfer RNA (tRNA)

tRNA is relatively small, it is made up of 80 nucleotides. It is single stranded and the chain is folded into a clover shape. One end of the chain extends beyond the other, allowing amino acids to easily attach. At the opposite end is a molecule called an 'anticodon' 

The anticodon will pair with the 3 bases on the mRNA molecule, there are different types of tRNA each with a different “anticodon”. 

Q&A

Q. Explain why the genetic code is described as:
a. Universal
b. Degenerate
c. non-overlapping. 

A. Universal - because it is the same in all organisms 
     Degenerate - Most amino acids have more than one codon.
     Non-overlapping - Because each base in the sequence is is read only once.

 Q. State four ways in which the molecular structure of RNA differs from DNA.
 A. 1. RNA is smaller than DNA
      2. RNA is usually single stranded whereas DNA is a double helix.
      3. The sugar in RNA is ribose while in DNA it's deoxyribose. 
      4. In RNA the base Uracil replaces Thymine in DNA. 

Q. Distinguish between a codon and an anticodon.
A. A codon is the triplet of bases on messenger RNA that codes for an amino acid.
     An anticodon is the triplet of bases on a transfer RNA molecule that is complimentary to a codon.