Biochemistry Lesson Notes, Summaries of Law

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RNA structure and

genetic code

Medical Biochemistry

Aqaba Medical Sciences University جامعة العقبة للعلوم الطبية

Objectives

 Describe the primary, secondary, and tertiary structures of RNA.  Compare RNA and DNA structurally and functionally.  Identify and describe the major types of RNA and their functions.  Explain the genetic code, including its features and role in translation.  Understand the clinical significance of mutations related to the genetic code.

Structure of RNA  RNA is a single stranded polymer of ribonucleotides linked together by 3 ’- 5 ’ phosphodiester bond.  Each nucleotide is formed of: 1. Nitrogen containing bases

• Purine bases (Adenine [A], Guanine [G])
• Pyrimidine bases ( Uracil [U] , Cytosine [C]) 2. Pentose sugar: in the form of ribose. 3. Phosphate group
• Attached to the 5 ′ carbon of ribose.
• Links to the 3 ′ OH group of the previous nucleotide to form a phosphodiester bond.

B. Secondary structure of RNA

 RNA is usually a single-stranded molecule, but it can fold back on itself to form double-stranded regions.  The folding happens when certain bases in the RNA strand form hydrogen bonds with each other.  The most common secondary structure is the hairpin or stem-loop. It has a double-stranded stem and a loop of unpaired bases.  Other structures include bulges (extra bases on one side), internal loops (unpaired bases on both sides), and multi-loops (where several stems meet).

• In eukaryotes; mRNA underwent post-transcriptional modifications : 1. Splicing: Removal of the introns (non-coding sections of DNA), 2. A polyadenylate tail ( 20 – 250 base) is added at the 3 '-end that marks the mRNA for transportation to the cytoplasm and protects the mRNA from degradation by 3 ’- exonuclease. 3. 7 - methyl-guanosine triphosphate cap is added at the 5 '-end of mRNA. This cap directs the initiation of translation and protects the mRNA from degradation by 5 '-exonuclease.

Types of RNA

• In eukaryotes; tRNA underwent post-transcriptional

modifications :

1. Removal of the introns (non-coding sections of DNA),
2. Its bases undergo extensive number of modifications.
3. The 3 '-ACC amino acid acceptor sequence is added after transcription

Difference between DNA and RNA

Characters of the genetic code:

A. Specific or unambiguous: Each codon is specific for a single amino acid, e.g., UUU encodes phenylalanine only. B. Degenerate : There are 61 amino acid encoding codons, each amino acid may be encoded for by several codons. The degeneracy usually occurs at the third base of the codon (called the wobble position ). C. Universality : The genetic code is universal, i.e., specify the same amino acid in all living organisms from viruses, bacteria, plants, insects to mammals. D. Non-overlapping : The mRNA codons are read one after another in a continuous manner in the 5 ’- 3 ’ direction without interruption in three-base sequence E. Comma-less : Codons are a continuous structure without interruption.

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