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Nucleic Acid

Nucleic Acid

Nucleic acids are a group of molecules that include DNA (deoxyribonucleic acid) and RNA (ribonucleic acid)

Nucleotide Structure

  • A nucleotide is made up of 3 components:
  • Pentose Sugar
  • Phosphate group
  • A nitrogen containing organic base (Adenine, cytosine, thymine, guanine and uracil)
  • All 3 components are joined through a condensation reactions to form 1 nucleotide.
  • Two mononucleotides can join together as a result of a condensation reaction between one of the phosphate groups on one mononucleotides to the deoxyribose sugar on the other mononucleotides. The bond formed is a phosphodiester bond. The resulting product is a dinucleotide where if more nucleotides are added it then results in a polynucleotide

Ribonucleic Acid (RNA) Structure

  • Ribonucleic acid is a polymer made up of nucleotides
  • It is relatively short chain (especially when compared to DNA)
  • RNA is a polynucleotide chain where the pentose sugar is always ribose and the organic bases are Adenine, Guanine, Cytosine and Uracil.
  • One example of RNA transfers genetic information from the DNA to the ribosomes which has its own RNA strand to create proteins and other types of RNA. Other RNA can be found in the production of protein synthesis

Deoxyribonucleic Acid (DNA) Structure

  • Deoxyribonucleic acid is a polymer made up of nucleotides
  • It is long chain (especially when compared to RNA)
  • DNA is a polynucleotide chain where the pentose sugar is always ribose and the organic bases are Adenine, Guanine, Cytosine and Thymine.
  • DNA is made of 2 strands, with hydrogen bonds joining them together
  • The phosphate groups and the deoxyribose molecule alternate u[p the polynucleotide chain


Base Pairing

  • The bases on the two strand are joined together through hydrogen bond which holds the entire DNA molecule together
  • The base pairs are specific to each other:
  • Adenine and Thymine (uracil in bacteria)
  • Guanine and Cytosine

Guanine will always and only bond to cytosine, therefore guanine’s complimentary base pair is cytosine.  and adenine complimentary base pair is thymine (or uracil)

  • The amount of adenine and thymine, and guanine and cytosine are always the same however it is the ratio of the adenine and thymine to guanine and cytosine which varies from between different species


The stability of DNA

  • The phosphodiester backbone protects the more chemically reactive organic bases inside the double helix
  • The hydrogen bonds between the organic base pairs form bridges (rungs) between the phosphodiester uprights.
  • Between the cytosine and guanine are 3 hydrogen bonds, therefore increasing the stability of the molecule as a whole compared to the adenine and thymine which has 2 hydrogen bonds.
  • As the molecule is so stable, it is rare for mutations to occur which makes it ideal for carrying hereditary material that is passed on generation after generation

Function of DNA

  • DNA is a hereditary material responsible for passing genetic information from cell to cell and generation to generation
  • There are in total 3.2 billion base pairs in DNA for a typical mammalian cell
  • During DNA replication, the weak hydrogen bonds between the base pairs are able to separate to reveal their code
  • By having the actual genetic code embedded into the phosphate backbone, it protects the material from being corrupted by outside chemicals or physical forces
  • Base pairing allows for DNA to replicate and transfer information as mRNA

Prime Location

  • The carbon atom in the pentose sugar are numbered of particular importance as 3’ (3 prime) and 5’ (5 prime) carbon atoms.
  • 5’ has an attached phosphate group
  • 3’ has a hydroxyl group
  • When nucleotides are organised in to the double helix stand, one of the strands run in the 5’ to 3’ direction whilst the opposite strand runs in a 3’ to 5’ direction
  • The two strands are said to be antiparallel


DNA and Chromosomes

DNA and Chromosomes

Chromosome Structure

  • Chromosomes are only visible as distinct structures when a cell is dividing
  • Through interphase, the chromosomes are free to move in the nucleus
  • The first time they are visible is during cell division in which they appear as 2 threads (chromatids) joined at a single point (centromere)
  • Each chromatid has already replicated to give two identical DNA molecule
  • The DNA is wrapped around histones (protein) molecules which are then further wrapped to form chromosomes

Homologous Chromosomes

  • All chromosomes occur in pairs, one from the mother (maternal chromosome) and one from the father (paternal chromosome)
  • These are known as homologous chromosome pairs and that total number is reffered to as the diploid number
  • A homologous pair is always two chromosomes that correlate into the same genetic characteristic however determining the same genetic characteristic is not the same as being identical e.g. both homologous pairs have the allele for hair colour, but the maternal chromosome might code for blond hair whilst the paternal chromosome codes for brown hair.
  • Through meiosis, the division of the number of chromosomes is carried out to ensure that each daughter cell receives one chromosome from each homologous pair. Each cell therefore has one chromosome for each characteristic of the organism. When these haploid cells combine, the diploid state with paired homologous chromosome is restored


  • A small section of DNA is a gene. Each gene exists in 2 (and sometimes more) forms. Each form is called an allele
  • Each cell has one allele from its maternal and paternal cells. These two alleles may be the same or different. If they are different each allele will code for a different polypeptide