Welcome to this series looking at how the genetic code instructs cells to make proteins. We will be talking about DNA, RNA, and proteins a lot so head over to those articles if you are unsure about the basic structure or function of those molecules. Today, we’re going to be taking a look inside the nucleus to get an introduction to the genetic code.
DNA in the nucleus
In eukaryotic cells, DNA is found in the nucleus as chromosomes. In humans, there are 23 pairs of chromosomes. Each one is a single linear molecule of DNA which is wound up tightly around proteins called histones (the two things combined are called chromatin). This helps to make DNA compact to fit inside the nucleus. A pair of chromosomes with the same genes are called homologous chromosomes. They have the same genes at the same loci (positions), but they could have different alleles of the same gene. An allele is a different version of the same gene e.g. the brown eye allele and blue eye allele are different forms of the gene for eye colour.
Prokaryotic DNA
In prokaryotic cells, a single circular DNA molecule is found free in the cytoplasm. It is a much shorter molecule than eukaryotic DNA because there are fewer genes. Circular DNA is not associated with histone proteins, but it still needs to compact itself a bit so it ‘supercoils‘, rather like screwing up an elastic band. Interestingly, chloroplasts and mitochondria also contain their own circular DNA. This is because these two organelles were likely to have originally been prokaryotic organisms which eventually evolved to live inside another cell.
The genetic code
A gene is base sequence of DNA that codes for the amino acid sequence of a polypeptide. However, note that some genes code for a functional RNA – a bit of RNA that has a specific function e.g. tRNA or rRNA.
In the genetic code, a sequence of three bases codes for one amino acid. The three base sequence is called a triplet or a codon (but more often a codon refers to a three base sequence on mRNA).
The genetic code has three important features. It is:
- Universal – the same triplet codes for the same amino acid in all organisms.
- Non-overlapping – triplets do not overlap, and each base is only part of one triplet.
- Degenerate – there are more possible triplet base combinations than there are amino acids (remember there are 20 amino acids). This means that more than one triplet codes for each amino acid.
Non-coding DNA
In eukaryotic cells, not every part of a gene actually codes for amino acids. There are two types of base sequences in genes: introns and exons. Exons are the sequences which code for amino acids. Introns, however, do not code for amino acids and get chopped out of mRNA in a process called splicing. This leaves an mRNA containing just protein-coding exons to enter translation at the ribosome. Over the next couple of articles we will look at transcription and translation, and include how mRNA can be edited using splicing. In between genes, there are also other sections of DNA that do not code for amino acids. Sometimes they are repeats of the same short sequence of bases over and over which are called non-coding repeats. The function of non-coding DNA is still a bit of a mystery!
Summary
- In eukaryotic cells, DNA is found in the nucleus. It is linear and associated with histones.
- In prokaryotic cells, DNA is found in the cytoplasm as a single circular molecule.
- A three-base triplet codes for one amino acid.
- The genetic code is universal, non-overlapping and degenerate.
- Not all DNA codes for proteins.
