Each organism has a complete set of instructions in its cells called the genome. The genome identifies a species and each individual in that species. No two organisms have the same genome unless, of course, they're clones! Our genome is what explains inheritance and why we look like our parents because the information it holds is passed on between generations. It's also what makes us unique and what explains the degree of similarity between organisms.
Scientists have been trying to understand and decode the meaning and structure of the genome for several decades since the early works of Gregory Mendel, the father of genetics, and Darwin's work on evolution. Our genome holds what Mendel called the units of inheritance, even though he himself didn't know what it was! Check out the Genetics and Evolution article to learn more!
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).
The genome is all the genetic information of an organism, both coding and non-coding regions, and it is comprised of sequences of DNA nucleotides. The genome is made up of genes. A gene is a DNA sequence that encodes information to make up a protein.
Head to our DNA structure article to learn more about this amazing molecule that makes up our genome!
Although scientists had known about DNA since mid 19th century, many experts still believed that proteins contained genetic information and not DNA. DNA at the time was presumed to be an inert molecule locked away in the nucleus, and its main function was thought to be storing phosphorous. It was not until 1952 that the experiments performed by two scientists, Alfred Hershey and Martha Chase, helped confirm that DNA is a genetic molecule. They demonstrated that when bacterial cells are infected by bacteriophages, which are viruses made up of DNA and protein, the viral DNA enters the cells, but the majority of the viral protein does not. Hershey and Chase contributed to the conclusion that DNA is the genetic material!
Fig. 1 - This diagram shows an overview of Hershey and Chase experiment. They discovered that when bacterial cells are infected with bacteriophage viruses, the viral DNA enters the cells. Since the infected bacterial cell is then able to recreate many viruses, this showed that DNA is the genetic material
All cells within an organism have exactly the same DNA, but not all cells in an organism are the same! When cells express only specific parts of their genome, they become differentiated and adopt different shapes and functions. Read the Cell Specialisation article to learn more!
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
Transcription is the process through which a cell copies a portion of its DNA into RNA.
Translation is the process through which a cell produces proteins according to the genetic information contained in messenger RNA (mRNA).
Prokaryotic cells can divide quicker than eukaryotic cells because they have just one circular chromosome. Check out the Eukaryotes and Prokaryotes article to learn more about the differences between these two cell types!
In addition to their main chromosome, bacteria also contain smaller DNA molecules called plasmids. Bacteria are able to exchange plasmids with other bacteria or obtain them from their environment. Plasmids only contain a few genes that are not essential for the everyday life of the bacteria. Nonetheless, the plasmid genes help the bacterium deal with stressful conditions. For example, some plasmid genes, when expressed, make the host bacterium resistant to an antibiotic, so the bacterium would survive if treated with that antibiotic. Other plasmids carry genes that aid the bacteria in digesting uncommon substances or killing other bacteria. This highly promiscuous DNA transfer between bacteria can be very problematic!
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
Check out the Gene Expression and the Protein synthesis articles to learn more about this!
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.
DNA is a polynucleotide chain made up of many nucleotides. During DNA replication, each DNA strand is used as a template and nucleotides are bonded to each other to form a new DNA strand. The Sanger sequencing method uses synthetic nucleotides called dideoxyribonucleotide (DdNTPs), which are labelled with fluorescent tags. When a single DdNTPs is added to a strand, it prevents further nucleotides from being added to it. In other words, it stops DNA replication, so it is the last nucleotide added to a DNA strand. Now imagine you have thousands to millions of DNA strands that have various lengths, but all are terminated by DdNTPs. Scientists can then identify the sequence of the DNA by sorting these strands according to their lengths and finding out which type of DdNTPs is at the end of each strand.
Fig. 4 - This diagram shows the main steps involved in DNA sequencing
If you want to know more about this, make sure to check out our article on Genome projects! Beware that this might be slightly more advanced content.
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.
An autosome is one of the numbered chromosomes in eukaryotes that includes genes coding for anything other than sex determination.
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
In addition to the DNA content in the nucleus, human cells also contain small DNA molecules within the mitochondria. The DNA in the nucleus is known as the nuclear genome, and the DNA in the mitochondria is referred to as the mitochondrial genome.
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.
Genome - Key takeaways
- The genome is all the genetic information of an organism, both coding and non-coding regions, and it is comprised of sequences of DNA nucleotides.
- All cells within an organism have exactly the same genome.
- All living organisms consist of cells that can be classified as either prokaryotic (bacterial) or eukaryotic.
- Most bacterial DNA is confined in a single circular molecule known as the bacterial chromosome.
- Eukaryotic cells have far more genetic material than prokaryotic cells. Eukaryotic DNA is organised into separate chromosomes, each made up of a linear DNA molecule tightly wrapped around histone proteins.
- The information in the DNA is encoded into specific sequences of A, T, G and C.
- DNA sequencing allows scientists to determine the base sequence of specific genes or even the whole genome of organisms.
- The human genome is a full collection of DNA sequences in human cells. It consists of 46 chromosomes, 22 pairs of autosomes and 2 sex chromosomes.
- The use of testing tools to identify if an individual has a genetic anomaly or mutation is referred to as genetic testing or genome testing.
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