DNA and RNA: Structure, Packaging and Types
Nucleic Acids
Nucleic acids are formed by C, H, O, N and P. They are polymers of nucleotides.
Composition of a Nucleotide
- Pentose (ribose or deoxyribose)
- H3PO4 (phosphoric acid)
- Nitrogenous base (organic cyclic molecule with amino groups and basic character)
There are two types of nitrogenous bases:
- Purines (A, G): two rings, one hexagonal and one pentagonal.
- Pyrimidines (T, C, U): one hexagonal ring.
The N-glycosidic bond (between the OH of C1 of the pentose and the H of the amino group of the base) forms the nucleoside. The phosphoric acid then binds to the pentose at C5 through a phosphoester bond, forming the nucleotide.
Nucleotides join together through a phosphodiester bond, between the OH of C3 of one nucleotide and the phosphoric OH of another.
Structure of DNA
When the cell is in interphase, DNA forms chromatin, associated with proteins and organized in three structural levels. When the cell is in mitosis, DNA forms chromosomes, where it reaches its maximum degree of compaction.
Primary Structure
The sequence and order of nucleotides.
Secondary Structure
The spatial arrangement of the nucleotide chains. Two antiparallel chains face each other and coil into a double helix. The pentose and phosphoric acid are on the outside, while the bases are on the inside, linked by hydrogen bonds to the bases of the other chain. Denaturation and renaturation can occur.
Tertiary Structure
The arrangement of the DNA double helix and proteins, forming a supercoil (superhelix) or supercoiled DNA. It reduces space and facilitates DNA duplication. The supercoil has two structures: the DNA double helix and proteins (histones and protamines), which allow it to fold.
Levels of Packaging
100 Å Fiber or "Beads on a String"
Repetition of nucleosomes, formed by:
- A core, or octamer of histones.
- A DNA filament wrapped around the octamer, about 200 base pairs.
- The histone H1 molecule.
Each nucleosome is linked to two others.
300 Å Fiber or Solenoid
Nucleosomes coiled together. One turn equals six nucleosomes, with histones H1 in the central zone.
Higher Level
Formation of chromosomes.
Types of DNA
According to structure: single-stranded or double-stranded. According to shape: circular or linear.
RNA
In eukaryotic cells there are 5 to 10 times more RNA than DNA.
Functions of RNA
- Transporting genetic information from DNA to ribosomes.
- Contributing to the incorporation of amino acids to form proteins.
- Structural function, as a component of ribosomes.
- Storing the genetic information of the cell.
Types of RNA
Transfer RNA (tRNA)
Made of 70 to 90 nucleotides. Its function is to transport amino acids for protein synthesis. It has two lateral arms, the anticodon arm with a triplet of bases and the acceptor arm where the amino acid attaches. It has both double-stranded and single-stranded regions.
Messenger RNA (mRNA)
Single-stranded. Its function is to copy information from DNA and carry it to ribosomes. The pre-mRNA has exons and introns; the mature mRNA has the introns removed. The 5' end has a cap, and the 3' end has a poly-A tail (between 150 and 200 adenines).
Ribosomal RNA (rRNA)
Has single-stranded and double-stranded regions. It is formed in the nucleolus, from information in the nucleolar organizer region. When it leaves the nucleus into the cytoplasm, it forms ribosomal RNA as part of the ribosome.