Genes

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Genes and Loci

DNA is the genetic blueprint which codes for, and determines, the characteristics of an organism

This includes the physical, behavioural and physiological features of the organism

DNA is packaged and organised into discrete structures called chromosomes

A gene is a sequence of DNA that encodes for a specific trait (traits may also be influenced by multiple genes)

The position of a gene on a particular chromosome is called the locus (plural = loci)

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Alleles

Alleles are alternative forms of a gene that code for the different variations of a specific trait

For example, the gene for eye colour has alleles that encode different shades / pigments

As alleles are alternative forms of the one gene, they possess very similar gene sequences

Alleles only differ from each other by one or a few bases

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Mutations

A gene mutation is a change in the nucleotide sequence of a section of DNA coding for a specific trait

New alleles are formed by mutation

Gene mutations can be beneficial, detrimental or neutral

Beneficial mutations change the gene sequence (missense mutations) to create new variations of a trait

Detrimental mutations truncate the gene sequence (nonsense mutations) to abrogate the normal function of a trait

Neutral mutations have no effect on the functioning of the specific feature (silent mutations)

Cause of Sickle Cell Anaemia

Sickle cell anaemia results from a change to the 6th codon for the beta chain of haemoglobin

DNA: The DNA sequence changes from GAG to GTG on the non-coding strand (CTC to CAC on the coding strand)

mRNA: The mRNA sequence changes from GAG to GUG at the 6th codon position

Polypeptide: The sixth amino acid for the beta chain of haemoglobin is changed from glutamic acid to valine (Glu to Val)

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Genomes

The genome is the totality of genetic information of a cell, organism or organelle

This includes all genes as well as non-coding DNA sequences (e.g. introns, promoters, short tandem repeats, etc.)

The human genome consists of:

46 chromosomes (barring aneuploidy)

~3 billion base pairs

~21,000 genes

The completion of the Human Genome Project in 2003 lead to many outcomes:

Mapping – The number, location, size and sequence of human genes is now established

Screening – This has allowed for the production of specific gene probes to detect sufferers and carriers of genetic diseases

Medicine – The discovery of new proteins have lead to improved treatments (pharmacogenetics and rational drug design)

Ancestry – Comparisons with other genomes have provided insight into the origins, evolution and migratory patterns of man

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Types of mutation

Point mutations are changes to one base in the DNA code and may involve either:

The substitution of a base (e.g. ATG becomes ACG)

The insertion of a base (e.g. ATG becomes ATCG)

The deletion of a base (e.g. ATG becomes AG)

The inversion of bases (e.g. ATG becomes AGT)

Base substitutions may create either silent, missense or nonsense mutations, while insertions and deletions cause frameshift mutations

Effects of Point Mutations

Silent mutations occur when the DNA change does not alter the amino acid sequence of the polypeptide

This is possible because the genetic code is degenerate and certain codons may code for the same amino acid

Missense mutations occur when the DNA change alters a single amino acid in the polypeptide chain

Sickle cell anaemia is an example of a disease caused by a single base substitution mutation (GAG → GTG ; Glu → Val)

Nonsense mutations occur when the DNA change creates a premature STOP codon which truncates the polypeptide

Frameshift mutations occur when the addition or removal of a base alters the reading frame of the gene

This change will affect every codon beyond the point of mutation and thus may dramatically change amino acid sequence

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