Features of the Genetic Code (A-level Biology)

Features of the  Genetic Code

Genes and Alleles

Alleles

• Alleles are versions of genes. Every gene within a species is nearly identical, but not quite the same. For example, there are two alleles for eye colour. There are minor variations in the sequence which can result in different phenotypes of the gene.
• Alleles have different base sequences. Alleles have similar but marginally different base sequences, so therefore code for different versions of the same protein.

The Genetic Code

Understanding the Genetic Code

If a gene codes for a certain protein, it needs to be able to make all the amino acids which make up that protein. Therefore each section of the gene (called a codon) codes for a different amino acid.

To understand how a the base sequence of a gene links to the amino acid sequence of a protein, we need to understand the genetic code:

1. Every three bases on a gene are known as a codon.
2. Each codon encodes a specific amino acid.
3. A single gene is essentially a sequence of codons.
4. Codons are non-overlapping. This means that each codon codes only for its particular amino acid. It does not specify the positions of other amino acids on the final protein.
5. Codons are degenerate. This means that some of the twenty amino acids that our bodies use to make proteins can be specified by more than one codon. For example, the amino acid glycine can be made by one of four codons (GGU, GGC, GGA, GGG).
6. There is a total of 64 codons in most organisms. These codons are referred to as the genetic code.
7. Three of the 64 codons are STOP codons, which signals the end of a polypeptide chain.
8. One codon is called the START codon and has the sequence ATG. This signals the start of a polypeptide chain.
9. The remaining 60 codons encode for the 20 amino acids that our cells are able to use to make proteins.
10. The genetic code is universal. This means that it is the same in all living organisms. This provides indirect evidence for evolution.

Anatomy of a Gene

Introns and Exons

Before moving on, it is important that you clearly understand the structure of a gene. A gene is a particular section of DNA that codes for a specific protein or functional RNA.

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Every gene has the following structural features:

• Start codon: found at the 5’ end of the gene. It marks the beginning of the gene. The start codon has the sequence “ATG”.

• Exons: regions of the gene which are coding. They contain the codons that are later read to make proteins. Each codon of an exon specifies a particular amino acid. The sequence of the codons in the exons determines the sequence of the amino acids in a polypeptide, which is the primary structure that forms a protein.

• Introns: non-coding regions of the gene that do not contain codons needed to make the final protein. Introns are found in between exons.

• Stop codon: found at the 3’ end of the gene. It marks the end of coding sequence of the gene. It can have three sequences: “TAA”, “TAG”, or “TGA”.

Types of Introns

• Genes are regulated by non-coding regions of the DNA. One of the functions of non-coding regions of the DNA is to regulate gene expression. There are three major non-coding regulators of gene expression in eukaryotic cells:

• • Promoters: proteins known as transcription factors bind to the promoter of a particular gene in order to activate expression of the gene.

• • Enhancers: these are non-coding regions of DNA that are  usually found further away from a gene than promoters. Like promoters, transcription factors can bind to enhancers to ‘enhance’ the expression of a particular gene.
  • Insulators: these are non-coding regions of DNA that are usually found far away from genes. Inhibitors of a gene can bind to these regions in order to decrease, or completely stop the expression of a particular gene.