In the human body, we have millions of eukaryotic cells. Amongst the organelles in the cell is the nucleus that contains the genetic information in the form of deoxyribonucleic acid (DNA) to control the cell. It is important to note that there are other types of eukaryotic cells besides the animal cell. These are plants, fungi and protists. The structure of a eukaryotic cell is shown below:
The nucleus consists of chromosomes. Chromosomes are synthesised from long DNA molecules. A gene is a short section of DNA. A gene occupies a fixed position, called a locus, on a particular DNA molecule. Each gene codes for a polypeptide (protein) by stating the order of the amino acids sequence. A polypeptide are lots of amino acids monomers bind together via peptide bonds to form protein polymers. This is done via a condensation reaction where water is also produced as a by-product. Therefore, the function of DNA is to produce proteins.
It is a nucleic acid polymer that consists of monomers called nucleotides that are bound together via a condensation reaction. Water is also formed in the reaction as a by-product.
A nucleotide is composed of the following elements:
A nucleotide consists of three components:
A) a phosphate group
B) a sugar
C) a nitrogenous base.
The phosphate group is negatively charged and this provides the nucleic acid with acidic properties. It is attached to the 5’ carbon of the sugar molecule
It is a 5-carbon sugar called a pentose. There are two types of pentose sugars: ribose and deoxyribose. Deoxyribose means that the ribose has lost an oxygen atom ‘de-oxy’.
There are four nucleobases: cytosine ( C), Guanine (G), Thymine (T) and Adenine (A). These are attached in the position where the hydroxyl group (-OH) is located in the sugar molecule which is 1’ carbon.
Nucleotides polymerisation does not involve the bases and is conducted by producing phosphodiester bonds between carbon 3' of the ribose sugar and an oxygen atom of the phosphate group. This is a form of a condensation reaction.
Two nucleotides: dinucleotides
Three nucleotides: trinucleotides
Few nucleotides: oligonucleotides.
Many nucleotides: polynucleotides
A polynucleotide has a free phosphate group at one end, called the 5' end. This is because the phosphate group is bound to carbon 5' of the ribose sugar. This forms the sugar phosphate backbone. There is a 3’ end because there is a hydroxyl (-OH) group on carbon 3’ of the ribose sugar.
- Double stranded as there are two nucleotide strands wounded together to form a double helix.
- The strands are wounded together by hydrogen bonding between the bases. This form of bonding provides stability and protection from damage.
- The bases form specific pairs: Adenine can only associate with Thymine whereas Cytosine can only associate with Guanine.This is known as complementary base pairing. This structure allows DNA to be adapted to its function as storying and expressing genetic material.
- These strands are anti-parallel, in other words they run opposite direction.
- The strands are complementary which provides a copy of genetic information. This is beneficial for repair, copy and checking errors.
Purines and Pyrimidines.
There are two forms of nitrogenous bases: purines and pyrimidines. Purines are double-ringed structures whereas pyrimidines have one ring.
- Adenine (A) - a purine
- Thymine (T) - a pyrimidine
- Cytosine(C) - a pyrimidine
- Guanine (G) - a purine
The genetic code is universal, non-overlapping and degenerate. There are 20 different amino acids and 4 different bases. Therefore, bases are read in groups of 3 called a triplet. Consequently, this provides 64 combinations that is more than sufficient enough to code for the 20 amino acids. A sequence of three DNA bases that code for an amino acid is called a codon. The 64 codons form the genetic code. Some codons symbolise the start of a gene whereas other codons symbolise the end of the gene.
- In a gene: non-coding DNA are called introns whereas the coding DNA molecules are called exons. All eukaryotic genes contain introns and their length is longer than exons. Introns are utilized for interruption sequences whereas eukaryotic genes are needed for expressed sequences.
- Between genes: non-coding DNA are called satellite DNA and are composed of simple repetitive DNA sequences that function in various means: DNA replication, gene regulation, aiding in coiling DNA into chromosomes, and some non-coding DNA are pseudogenes. Pseudogenes are untouched gene copies.
Alternatively, eukaryotic cells have a complex structure and it has a membrane bound nuclei. Examples of eukaryotic cells are plants and animal cells. The size of eukaryotes is 10-100um.
- Gene --> DNA --> Chromosomes.
- Adenosine always binds with Thymine
- Guanine always binds with Cysteine.
- Prokaryotes have DNA molecules are short, circular and not associated with histone proteins. On the other hand, eukaryotes DNA are long, linear and associated with histone proteins.
- Mitochondria and Chloroplasts contain DNA and are similar to prokaryotic DNA as they are short, circular and not linked with histone proteins.
- Most eukaryotic nuclear DNA do not code for polypeptides (proteins).
- Exons in a gene code for amino acids and are separated by non-coding sequences called introns.