Biology: 3.1 (Genes)

A mutation is a permanent, random change in genetic information. Most often, this mutation causes harm or has a neutral effect. Mutations that occur in the DNA of body cells are eliminated when that person dies. However, mutations that occur in the cells that develop into gametes can be passed on to offspring and cause genetic disease.

Mutation Definition.

The most significant of mutations is called a base substitution. In this case, one base in the sequence is swapped for a different base. A base substitution mutation leads to the expression of a different genetic sequence and therefore a different protein is produced.

Base Substitution Definition.

A gene is the basic unit of inheritance that controls a specific characteristic. Each gene determines one feature (one protein or one characteristic). A gene is found on a chromosome and is made up of a length of DNA with a specific sequence of bases.

Definition of a Gene

An allele is one specific form of a gene that differs from other alleles, but occupies the same locus as other alleles of the same gene. In this way, members of the same species possess the same genes, but not necessarily the same alleles for those genes. This variation in alleles is what gives rise to the differences in appearance and characteristics of members of the same species. The lengths of DNA that make up each gene vary slightly, which results in the different alleles for that gene. The sequence of bases on the DNA varies only by one or a few bases between alleles of that gene. New alleles are formed by mutation of a gene

Definition of an allele

The genome is the total genetic information of an organism and includes all of the DNA on all of the genes on all the chromosomes for that organism.

Definition of the Genome

Genes are found on chromosomes, with each gene having a specific and unique position on the chromosome called the locus. The locus of the gene is the same for every member of a particular species.

Definition of the Locus

Sickle cell anaemia is the most common genetic disease to affect humans. It occurs due to a base substitution mutation on chromosome 11 where the base sequence on the sixth codon (mRNA) of the gene, GAG is mutated to GTG. This causes the amino acid Glutamic Acid to be replaced by Valine and a new allele is formed. Most humans have the allele HbA, whilst those with the mutation have the allele HbS.

Genetic Disease: Sickle Cell Anaemia

In humans, the genome consists of the 46 molecules that form the chromosomes inside the nucleus, plus the DNA found within the mitochondria.

Genome in Humans

In plants, the genome includes the DNA found on the chromosomes in the nucleus, plus the DNA in the mitochondria and chloroplasts.

Genome in Plants

In prokaryotes, the genome includes the DNA in the circular chromosome, plus any plasmids.

Genome in Prokaryotes

The Human Genome Project began in 1990 and was completed in 2003. The aim of the project was to discover the base sequence of an entire human genome. The HGP has enabled researchers to use a massive database of information about the base sequences found within the human genome. It is hoped that the HGP will provide an understanding of genetic disorders and the production of medications to cure or treat them, determine the genetic conditions a person is prone to with genetic screening, and identify the gene/s involved in certain genetic conditions.

Human Genome Project

Animals (Fruit Fly): Larvae consume ripe fruit - 14,000. Animals (Caenorhabditis elegans): Small soil roundworm - 19,000. Animals (Humans): Large omnivorous biped - 23,000. Animals (Water Flea): Small pond crustacean - 31,000.

Number of genes in Animals

Fungi (Yeast): Unicellular Fungus - 6000

Number of genes in Fungi

Plants (Rice): Crop grown for food - 41,000. Plants (Thale Cress): Small annual weed - 26,000. Plants (Black cottonwood): Large Tree - 46,000.

Number of genes in Plants

Prokaryotes (Haemophilus Influenzae): Pathogenic Bacterium - 1700. Prokaryotes (Escherichia Coli): Gut bacterium - 3200

Number of genes in Prokaryotes

Protoctista (Trichomonas Vaginalis): Unicellular Parasite - 60,000

Number of genes in Protoctista

The Genbank database can be used to search for DNA base sequences. The cytochrome C gene sequence is available for many different organisms and is of particular interest because of its use in reclassifying organisms into three domains.

The Genbank Database

Sickle cell anaemia occurs due to the change in the codon of the mRNA. The tRNA with a different anticodon attaches and a different amino acid (valine) is joined to the peptide. This results in haemoglobin molecules that stick together, distorting the red blood cells into a sickle shape and reducing its ability to carry oxygen.

Why Sickle Cell Anaemia occurs?