Nucleotides, DNA, and RNA

Summary

The genetic information of an organism is stored in the form of nucleic acids. Nucleic acids, DNA (deoxyribonucleic acid) and RNA (ribonucleic acid), are long linear polymers composed of nucleotide building blocks. Each nucleotide is comprised of a sugar, a phosphate residue, and a nitrogenous bases (a purine or pyrimidine). DNA is longer than RNA and contains the entire genetic information of an organism encoded in the sequences of the bases. In contrast, RNA only contains a portion of the information and can have completely different functions in the cell.

DNA is structurally characterized by its double helix: two opposite, complementary, nucleic acids strands that spiral around one another. The DNA backbone, with alternatively linked sugar and phosphate residues, is located on the outside. The bases are located inside the helix and form the base pairs adenine and thymine or guanine and cytosine, which are linked by hydrogen bonds.

The human genome comprises 3.2 x 109 base pairs, which are distributed over 23 pairs of chromosomes. Each chromosome is a linear DNA molecule of a certain length. The chromosome is only well visualized under the light microscope during the metaphase of mitosis, as it is maximally condensed during this phase. Chromosomes are present as pairs in most cells of the body. One chromosome in each of the 23 pairs originates from the mother and the other from the father.

Both interrelated chromosomes are termed homologous because they each have a variant of the same gene. Alterations in the number or structure of the chromosomes lead to various conditions, e.g., developmental disorders. Chromosomal assessment with different molecular biology and cytogenetic methods often allows for a clear diagnosis.

Nucleotides

Nucleotides

Nucleobases

Rings Base Notable characteristics As a nucleoside unit in RNA As a nucleoside unit in DNA
Pyrimidines
  • 1 ring
Cytosine (C)
  • Deoxycytidine
Thymine (T)
  • Not present
  • Thymidine
Uracil (U)
  • Not present
Purines
  • 2 rings
Adenine (A)
  • Adenosine
  • Deoxyadenosine
Guanine (G)
  • Deoxyguanosine

CUT the PYrimidine; PURine As Gold.

Thymine is found in DNA; uracil is found in RNA.

A MEAN person GAGs a PURring cat!

Nucleic acid sugars

  • Structure: The sugar found in nucleic acids is a pentose, which has a five-atom ring. Specifically, the sugar in:
    • DNA = deoxyribose
    • RNA = ribose

  • Pentose binds:
    • Bases via N-glycosidic bonds
    • Phosphate residue via phosphodiester bonds

Phosphate group

Function of nucleotides and their derivates

Nucleotide and nucleotide derivatives have important functions in the body.

The energy carrier ATP contains ribose and not deoxyribose as a sugar, and therefore has a 2' OH group!

Overview of nucleic acids

Nucleic acids

  • Long, linear chains (polymers) of nucleotides
  • Alternating sugar and phosphate residues of individual nucleotides, linked by phosphodiester bonds, form the backbone
  • Phosphodiester bonds are negatively charged.
    • Negative charges stabilize the nucleic acids.
    • Phosphodiester bonds cannot be easily hydrolyzed like other esters.
  • The chemical composition of nucleic acids (DNA and RNA) and their structure of repetitive nucleotide units allow them to function as both information carrier and mediator.

Comparison of DNA and RNA

DNA RNA
Bases
Sugar
  • Deoxyribose
  • Ribose
Length
  • Depending on the organism
  • Ranging from several thousand to several millions of nucleotides
  • Varies considerably
Structure
  • Depends on the RNA type but is usually single-stranded
  • Various 3D structures are possible; e.g., loops through the formation of short sections with base pairing (double-stranded)
Function
  • Carries the hereditary information (collectively known as the genome) for the construction and function of the organism
  • Varies considerably depending on class, e.g., coding, regulatory, or enzymatic function (see table “Classification of RNA” below)

DNA structure and the human genome

Overview of DNA structure and packaging

Double-stranded DNA

  • DNA is primarily a double-stranded chain of deoxyribonucleotides in cells
  • Both strands are complementary to each other and run anti-parallel .
  • Double helix: 3D structure of DNA in which two polynucleotide strands are intertwined

Base pairs in DNA: guanine pairs with cytosine (3 H bonds), adenine pairs with thymine (2 H bonds)!

Chromatin

HeteroChromatin is HighlyCondensed while Euchromatin is Expressed!

Components of chromatin

Histones

Histone Methylation Mainly Mutes transcription. Histone Acetylation Activates transcription.

Nucleosome (nucleosome core particle)

Chromosomes

See the “Basics of human genetics” for more information.

Human genome

Nuclear genome

Mitochondrial genome (mitochondrial DNA, mtDNA)

  • Circular genome of ∼ 16,500 bp
  • Over 90% of mtDNA codes for structural genes, including for mRNA, tRNA, and rRNA.

RNA: Structure and characteristics

RNA classes and their structure

RNAs can be differentiated into various types, which differ in their length, structure, and function. Depending on the type, RNA can be a single-stranded or double-stranded segment.

Classification of RNA
Function Structure
mRNA (messenger RNA)
tRNA (transfer RNA)
rRNA (ribosomal RNA)
  • 5S, 5,8S, 18S, and 28S rRNA
    • 18S rRNA: component of the small subunit of ribosomes (40S)
    • 5S, 5,8S, and 28SrRNA: components of the large subunit of ribosomes (60S)
snRNA (small nuclear RNA
miRNA (microRNA)
  • Composed of ∼ 20–30 nucleotides
  • Formed from precursor molecules with a 5' cap and a poly(A) tail, but are then cleaved into smaller oligonucleotides
siRNA (small interfering RNA)
  • Class of regulatory, noncoding RNAs
  • Used experimentally or arise in viral infections, i.e., are introduced into the cell or organism and regulate gene expression (bind complementary mRNAs and ensure their degradation)
  • Composed of ∼ 20–30 nucleotides
  • Formed from double-stranded precursor molecules from a similar mechanism as for miRNA

last updated 11/26/2019
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