Genome

What is the Genome?

Millions of cells, each with a complete set of different instructions called the genome, are found in every organism. The genome contains the complete set of instructions needed for an organism to develop and grow. We know that these sets of genetic instructions are made up of deoxyribonucleic acid (DNA). DNA is a double-stranded polymer with a double-helical conformation. DNA stores information as a code composed of monomers called nucleotides, each with one of four nitrogenous bases: Adenine (A), Guanine (G), Cytosine (C), and Thymine (T).

Genome examples

Even though the genome in all organisms is made up of DNA, they differ in the way the DNA molecule is organised. According to cell theory, all living organisms consist of cells that can be classified as either prokaryotic (bacterial) or eukaryotic.

Bacterial genome

The genome in bacterial cells, similarly to any other organism, is composed of double-stranded DNA. However, bacteria organise their DNA differently when compared to eukaryotic cells. Most bacterial DNA is confined in a single circular molecule known as the bacterial chromosome. This chromosome, when combined with many proteins and RNA molecules, creates the nucleoid. Bacterial cells do not have a nucleus inside, so the nucleoid is actually found in the bacterial cell's cytoplasm. The fact that the prokaryotic chromosome is located in the cytoplasm also means that transcription and translation can occur concurrently in the same location.

Fig. 2 - A circular chromosome and smaller circular DNA molecules called plasmids form the genetic material in bacterial cells

Eukaryotic genome

The primary distinction between prokaryotic and eukaryotic cells is that in eukaryotic cells, almost all of the chromosomes are contained within a membrane-bound nucleus (with a small portion present in membrane-bound organelles such as mitochondria or chloroplasts). Eukaryotic cells have also far more genetic material than prokaryotic cells.

Eukaryotic DNA is organised into separate chromosomes, each made up of a linear DNA molecule that is tightly wrapped around proteins called histones. These chromosomes are then highly compressed to fit inside the nucleus. All the chromosomes in the nucleus, in addition to the histone proteins bound to them, are known as chromatin. As opposed to prokaryotes, transcription needs to happen before translation in eukaryotic cells due to the confinement of the DNA in the nucleus. After transcription, the mRNA molecule is edited before it leaves the nucleus. Then, ribosomes in the cytoplasm carry out the translation process, making polypeptide chains based on the mRNA sequence.

Fig. 3 - In eukaryotes, the majority of DNA is confined in the nucleus in form of separate chromosomes

Genome project

In genome projects, a large number of DNA samples from various donors of the same species are gathered and sequenced to obtain a reference genome of that given species. For prokaryotes, the genome sequence is of their single circular chromosome. For eukaryotes, on the other hand, each chromosome needs to be sequenced separately.

Genome sequencing

The DNA sequence determines all aspects of the traits organisms inherit from their parents. Therefore, analysing the DNA sequence is a powerful tool for understanding genetics. DNA sequencing allows scientists to determine the base sequence of specific genes or even the whole genome of organisms. DNA sequencing methods are constantly evolving and becoming more efficient, but their principle is based on Sanger sequencing, an automated method invented by Fredrick Sanger in 1977. This method is also known as dideoxy sequencing.

Human genome

The human genome is a full collection of DNA nucleotide sequences for humans, contained in the cell nuclei within 23 pairs of chromosomes, 22 pairs of autosomes and 2 sex chromosomes.

The human genome includes sequences of non-coding and coding DNA. The coding regions are called genes, and they can encode either a protein or just an RNA molecule. Although the non-coding regions may not seem useful, they have critical roles, such as promoting or suppressing the expression of particular genes.

Fig. 5 - This is a karyotype of a male human. It shows the 22 pairs of numbered autosomal chromosomes as well as the X and Y sex chromosomes

Due to the large size of the human genome, sequencing was an enormous challenge. The Human Genome Project (HGP) was an international scientific collaboration that officially began in 1990 and finally was completed in 2003. The DNA used in this project was obtained from several anonymous volunteers. The HGP had many goals, such as obtaining a genetic map of the human genome and developing the technology needed for managing the human genome information. The main benefit of mapping the human genome is the ability to identify and study the sequences of genes in our body. We know mutations in various genes are responsible for certain human diseases, such as cystic fibrosis or even increased risk of cancer. Being able to identify mutant genes that drive genetic diseases was one of the main motivations for the HPG project.

A new and more grand undertaking called the 1000 Genomes Project was launched in 2008. This project aims to get a better understanding of human genetic variations on an international scale. This project came to fruition in 2012, and their findings were published in the journal Nature.

Genome testing

Because genetic abnormalities are responsible for heritable diseases, people are often looking to find ways to determine whether an individual carries certain types of genetic mutations. This is known as genome testing or genetic testing.

There are various methods that can be used for identifying genetic abnormalities. Some can be at the protein level testing for the presence of the mutant gene product. An alternative and more common type of testing method is performed at the DNA or chromosomal level, testing for the presence of nucleotide or chromosomal mutations. These testing strategies, however, require the scientists to have previously identified the mutant gene. Some human genes that have been identified include those involved in cystic fibrosis, Huntington's disease, and Duchenne muscular dystrophy diseases. As a result, it is possible to genetically test whether individuals have these conditions or are carriers of these mutations.