Nucleic acids are of two types, DNA and RNA.
Each nucleic acid is composed of repeated units called nucleotides.
- Each nucleotide contains:
- a phosphate group,
- a five-carbon (pentose) sugar, either ribose or deoxyribose, and
- a nitrogen-containing (nitrogenous) base, which is either:
- a two-ringed purine such as Adenine or
Guanine, or
- a one-ringed pyrimidine such as Cytosine,
Thymine, or Uracil
DeoxyriboNucleic Acid (DNA):
- Each DNA nucleotide contains phosphate, deoxyribose, and either A, G, C, or T.
- Each DNA molecule has two strands of nucleotides, going in opposite directions.
- Each strand is twisted into the shape of a helix (a corkscrew shape),
so the entire molecule is a double helix.
- The two strands are held together by numerous weak hydrogen bonds.
- Hydrogen bonding only allows certain base pairings:
- Adenine pairs only with Thymine (and vice versa) to make A-T pairs;
- Guanine pairs only with Cytosine (and vice versa) to make G-C pairs.
RiboNucleic Acid (RNA):
- Each RNA nucleotide contains phosphate, ribose, and either A, G, C, or U.
(U takes the place of T and pairs with A.)
- RNA molecules come in 3 types: mRNA, tRNA, and rRNA.
Most RNA is single-stranded (except in a few viruses).
- Ribosomal RNA (rRNA): occurs in ribosomes; synthesized in the nucleolus.
- Messenger RNA (mRNA): synthesized in the nucleus by transcription;
shaped as a single long strand.
It passes into the cytoplasm, where it acts with ribosomes to direct protein synthesis,
3 bases (one codon) at a time.
- Transfer RNA (tRNA): occurs in the cytoplasm; acts in protein synthesis,
Contains A, G, C, U, plus some unusual bases.
- Some parts of tRNA base-pair with other parts to form 3 "loops".
A flat drawing of this structure resembles a clover leaf.
- The actual 3-D shape is more complex and L-shaped.
- Over 40 types of tRNA are known. Each attaches at one end
to a specific amino acid and serves as an "adaptor."
Each also contains a unique 3-base anticodon sequence on
one of its loops.
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