Genes

Introduction

Previous chapters looked at a cell's genetic material. Prokaryotes store their genetic information in their nucleoid and eukaryotes in their nucleus. A nucleus contains chromatin that forms chromosomes. Chromosomes are made of DNA molecules combined with different proteins. Long DNA molecules are divided into smaller sections called genes. This chapter looks at genes – the units of hereditary information.

History of genetics

Thousands of years ago people realised that children always resemble their parents or their other relatives. The set of characteristics passed from parents to their offspring is called genetic inheritance. Two individuals with similar features often produce offspring that have this same feature. This fact has been used in agriculture for centuries.

Genetics as a science was founded only in the 19th century by Austrian scientist Gregor Mendel. Mendel taught science and in his spare time studied the inheritance of different characteristics in plants. He cross-bred and studied thousands of pea plants recording all the characteristics of their offspring. He published the results of his research at the end of 19th century, but at that time the scientific world was not ready. Only a few decades later its importance was truly appreciated.

Genes: what are they?

Genes are found on chromosomes within the nucleus of every cell. During the process of protein synthesis, genes are 'read' by RNA molecules that later function as templates for protein structure. The type of protein determines the type of an organism. Proteins are polymers made of monomers called amino acids. A group of three nucleotides within DNA or RNA specifying a single amino acid is called codon. A larger sequence of nucleotides that specifies the sequence of amino acids in proteins is called a genetic code. Most living organisms use the same genetic code, which is called a standard genetic code.

The section of the gene that is not responsible for protein synthesis is called intron. The protein-coding section of the gene is called exon.

Genes can be defined as sections of DNA that determine an organism's characteristics and functions. Genes also control an organism's physical development and behaviour. See image 1.

Protein synthesis

All living organisms are made of proteins. Organisms grow, reproduce and function due to the process of protein synthesis. This process can be divided into the following parts. See image 2.

1. Transcription

Ribososmes are the organelles responsible for protein synthesis. They are in a cell's cytoplasm. The information about protein structure is encoded in DNA molecules which are only in a cell's nucleus. This genetic code needs some sort of messenger that is able to 'take a message' from the DNA and carry it into the cell's cytoplasm for the ribosomes so they can start assembling the protein molecules. The role of this messenger is played by the molecule of RNA. Before the process of protein synthesis begins, an enzyme called RNA polymerase synthesises a messenger RNA (mRNA) by 'reading' one strand of the DNA double helix in the cell's nucleus. Later, this mRNA moves to the cytoplasm. While mRNA moves, it goes through different stages. One of these stages is called splicing. During the process of splicing, mRNA eliminates the non-coding nucleotide sequences.

2. Translation

In the next stage, the actual protein synthesis begins. The ribosome binds to the mRNA spot called the start codon. After that ribosome starts synthesising complex protein molecules using amino acids delivered by transfer types of RNA (tRNA). The ribosome moves from codon to codon along the mRNA. Amino acids are added one by one, translated into sequences dictated by DNA and represented by mRNA. At the end of the synthesis the translation is terminated by the stop codon and a protein molecule is released from the ribosome.

Inheritance

Previous chapters have mentioned that an organism's genetic characteristics are called genotype. Genotype is encoded in DNA molecules and is often (but not always) expressed in an organism's phenotype. Phenotype is a set of visible characteristics of the organism. An organism's phenotype is always an expression of its genotype.

Some of an organism's inherited traits can be modified by the environment, while others cannot. For example, blood type can never be modified by environment while an organism's size can, because size depends on nutrition. Different types of inheritance are looked at in the next chapter.