Genes and DNA

Gene

It is a unit of inheritance

It is the heredity factor, which transmits traits from parents to offspring.

Genes are located at fixed points on chromosomes; each point is called a locus (loci).

Gene is chemical in nature in the form of a nucleic acid molecule called Deoxyribonucleic acid (DNA).

A gene is a section of the DNA on the chromosome made of chains of bases along the DNA strand.

DNA

It is a complex molecule composed of three different components (that form a nucleotide);

• A five-carbon sugar – deoxyribose sugar
• A phosphate molecule
• A nitrogenous base

There are four types of nitrogenous bases; Adenine (A), Guanine (G), Thymine (T), and Cytosine (C).

DNA consists of several nucleotides joined together to form long chains called DNA strands.

Adenine (A) pairs with Thymine (T) while Guanine (G) pairs with Cytosine (C).

Nucleotide initiates and controls protein synthesis

Two parallel DNA strands twist on one another forming a double helix.

The Role of DNA

1. Stores genetic information in coded form.
2. Enables transfer of information unchanged to daughter cells through replication.
3. Translates the genetic information into the characteristics of an organism through protein synthesis.

DNA Replication

The DNA double helix consists of two long, separate strands joined together by the base pairs. When the molecule is due to replicate the double helix unwinds and the two stands unzip themselves. This is made possible by the presence of the weak hydrogen bonds that link the bases of the two strands. Unzipping is therefore as a result of breaking of the hydrogen bonds thus setting each strand free from the other. After unzipping, the information on the DNA strands, that is the base sequence, is copied out onto a new DNA structure using the parent DNA as the template and thus, the parent molecule is said to have  replicated itself.  The replication makes it possible to pass on the DNA molecule together with its exact genetic information to daughter cells during cell division in the organism.

As the cell prepares for division during interphase, genetic material (DNA) replicates i.e. doubles itself so that sufficient DNA is made available for each of the daughter strands.

Role of DNA in Protein Synthesis

The sequence of the bases along the DNA strands act as the alphabet or code that spells out the sequence of amino acids. Every set of three bases along the DNA strands is responsible for bringing into position a particular amino acid of a polypeptide chain. The set of the base triplet is known as a codon, and is said to code for a particular amino acid of a protein molecule.

Examples of such DNA codons are:     

• AAA coding for amino acid phenylalanine;
• TTT coding for amino acid lysine;
• CAA coding for amino acid valine;
• CTA coding for amino acid aspartic acid; etc.

Protein synthesis takes place in the ribosomes found in the cytoplasm. Since DNA molecules are confined to the cell nucleus, there has to means of communicating the DNA information to the ribosomes where protein synthesis occurs.

The cell has a special molecule that mediates between the DNA in the nucleus and the cytoplasm. This molecule is a nucleic acid called Ribonucleic acid (RNA). Because RNA carry genetic information from DNA to the site of protein synthesis in the cytoplasm, it is referred to as a messenger RNA (m-RNA) and is formed from the DNA strands.

 In the formation of m-RNA, an appropriate section of the DNA strand serves as a template.

The double helix of the DNA unzips and free nucleotides align themselves opposite the template.

The base sequence of the template is copied onto a new strand, which then becomes an RNA strand.

In the RNA, Thymine (T) is replaced by the base Uracil (U). The transfer of DNA base sequence on to the m-RNA leaves the nucleus with the full instructions from the DNA about the kind of protein to be synthesized by the cell.

This instruction is in the form of the base triplets or codons; which are the practically used to assemble the amino acids on the protein polypeptide chains.

Information on the RNA is translated by the ribosomes and is used to assemble amino acids into specific protein molecules.

Protein molecules determine the inherited characteristics in organisms.