Molecular Biology: Proteins and Genetic Variation

Proteins and Amino Acids

Proteins are formed by a union of linear amino acids (NH2-COOH). There are 20 amino acids that are part of proteins. Nucleobases form a triplet or codon, which determines a particular amino acid. There are 4x4x4 = 64 different triplets; this implies that there are different triplets that encode the same amino acid. The code is degenerate because the combination of 3 letters adds up to 64 possible triplets.

The Genetic Code

The genetic code is the relationship between the sequence of bases in DNA and the ratio of amino acids in protein. There are three special triplets coding for a stop sign:

• TAA
• TAG
• TGA

There is also a triplet (ATG) determining a beginning.

Functions of Proteins

Proteins perform various essential functions:

• Transport of substances: Hemoglobin
• Defense mechanisms: Immunoglobulins
• Movement: Actin and myosin
• Hereditary packaging material: Histones
• Biochemical reactions: Enzymes

The Process of Protein Synthesis

During protein synthesis, DNA cannot leave the nucleus. To leave, a copy of the DNA is made through the transcription process that occurs in the nucleus, facilitated by RNA polymerases.

Mutation and Pathogenesis

A mutation is a change in the nucleotide sequence of DNA that can cause problems regarding the role and importance of the affected protein. If the mutation causes significant changes, it can lead to diseases (pathogenic mutations).

Understanding Genetic Polymorphism

Polymorphism occurs if a mutation happens within a gene but does not cause significant changes in the protein, or if it occurs in an area of DNA that does not encode an important protein. This is transmitted from parents to children and varies according to ethnic groups. It is used to identify individuals in forensics (humans share 99.7% of genetic information, while polymorphisms account for 0.3%).

Alleles and Genetic Drift

An allele represents each of the alternative forms of a gene that expresses a single character differently.

Genetic drift refers to changes in allele frequency. It occurs in two cases:

• When introducing a mutated gene into a population.
• The formation of small subpopulations isolated from a larger population.

Founder Effect and Gene Flow

The founder effect occurs if one of the founders of a new population is a carrier of a rare allele; this allele could become fixed in the population at a very high frequency (e.g., the Amish).

Gene flow is the slow spread of genes through population barriers. It is a process involving a large population and a gradual change in gene frequencies.