DNA is a code which allows cells to produce specific proteins - every cell has the same DNA but they can produce different proteins.
Definitions
Histones are proteins found in chromatids, this forms chromosomes
Exons are the coding parts of DNA - they code for proteins
Introns are the non-coding regions of DNA
Degenerate means that most amino acids are coded for by more than one triplet
Non-overlapping means that each base of the sequence is only read once
Nucleotides
Both DNA and RNA are composed of the same monomer unit - nucleotides. These nucleotides are composed of three parts:
- A phosphate group
- A pentose sugar
- A nitrogenous base
This also happens to be the order of the nucleotide. However, there are differences between the nucleotides in DNA and RNA. The table below demonstrates this.
| DNA | RNA |
| Pentose sugar is deoxyribose | Pentose sugar is ribose |
| Nitrogenous bases: adenine, thymine, cytosine and guanine | Nitrogenous bases: adenine, uracil, cytosine and guanine |
Although deoxyribose and ribose are both pentose sugars, the key difference is that ribose has one more oxygen than deoxyribose.
Complementary base pairing
Only adenine and thymine can join together (thymine is replaces with uracil in RNA) - they have two hydrogen bonds.
Only cytosine and guanine can join together (in both DNA and RNA) - they have three hydrogen bonds.
Forming DNA and RNA
The nucleotides join together in condensation reactions, these take place between the phosphate group of one nucleotide and the pentose sugar of another nucleotide. A dinucleotide, water and the backbone of DNA and RNA is also formed.
To form the sugar-phosphate backbone, phosphodiester bonds form between adjacent DNA nucleotides.
Nitrogenous bases are the code and in DNA they connect to a second base to form a double strand by using hydrogen bonds.
DNA
DNA codes for amino acids - a triplet (composed of three base pairs) are needed to code for an amino acid. There are 20 regular amino acids which occur regularly in proteins - this means that there are 64 potential triplets and there is more than one triplet for each amino acid.
There are some codons which code for the cell to stop, however there is only one codon which tells a cell to start.
The second strand of DNA is upside down, therefore the DNA strands run anti-parallel. This forms a double helix and the DNA spirals upon itself.
DNA is also universal - therefore each triplet codes for the same amino acid in all organisms.
If there is a change in the sequence of DNA bases, it could result in a non-functional enzyme as the sequences are specific to the enzyme. This means that there would a be a change in the shape of the active site. As a result, the substrate will not be able to bind to the active site. Therefore, no enzyme-substrate complexes can form.
Types of RNA
- Messenger - mRNA (carries RNA out of the nucleus)
- Transfer - tRNA (collects free amino acids and transports them to ribosomes)
- Ribosomal - rRNA (these are a part of the ribosomes)