Genes and Heredity



Genetics is the study of trait inheritance. A gene is the site on the chromosome that provides the DNA information for a particular trait. The genome is the sum of total genetic material.

Genetics is the branch of science concerned with the passing of genes from parent to daughter cells and the composition of genes in an organism. Microbial genetics is a field that studies the genetics of microorganisms, both single and multicellular, including bacteria, viruses, archaea, protozoa, and fungi.

A gene is a unit of heritable material that codes for a particular trait. Physical traits can be controlled by a single gene or many different genes. For example, human eye color is controlled by at least 10 different genes. In contrast, the bacterium Thermus aquaticus has a single gene that codes for a heat-resistant enzyme that allows the bacterium to replicate its DNA at very high temperatures. A chromosome is a structure that contains deoxyribonucleic acid (DNA), the genetic material that is passed from one generation to the next. Genes are segments of DNA that code for specific proteins that affect cellular and organism function. This information is written in the DNA in the form of nitrogenous bases: adenine, thymine, cytosine, and guanine. These bases are arranged in groups of three, called codons, and each codes for an amino acid. The amino acids, when linked together, form the protein for which the DNA codes.

The passing of genetic material to offspring requires a reproductive event. Reproductive events can be asexual, resulting in a duplicate of the parent cell, or sexual, resulting in each parent passing a random assemblage of genes to the offspring. A large percentage of microorganisms, the prokaryotes (organisms without a distinct nucleus), reproduce asexually. Eukaryotic microorganisms, however, can reproduce sexually, asexually, or both.

The genotype is the genetic makeup of an organism. The phenotype represents the observable characteristics of an organism that result from genetic and environmental influences. Phenotypes can change over the lifetime of an individual organism, but an organism’s genotype remains conserved. Phenotypic change, also called phenotypic plasticity, happens because of differential gene expression, which means certain genes can be turned on or off depending on age or environment. Phenotypic plasticity is more common in immobile organisms, such as plants, because mobile organisms can move away from hostile environments to more habitable ones. However, bacteria are known to change their shapes and sizes in response to influences in the environment. One such example is the bacterium Campylobacter jejuni, a species that causes food poisoning in humans. Normally spiral-shaped, C. jejuni may change to a spherical coccus shape when oxygen is present in higher concentrations.

Anatomy of a Gene

The structure and function of every cell in a living organism is determined by genetic information in the form of genes encoded in DNA. Genes are the inheritable traits contained within chromosomes.

DNA Structure

DNA is composed of three main parts: deoxyribose, a phosphate group, and a nitrogen base. The DNA of cellular organisms takes the form of a double helix, where each strand of DNA joins another by connecting nitrogen bases.

An organism's genotype is stored as deoxyribonucleic acid (DNA), an organic molecule containing coded instructions for the life processes of an organism. In some microorganisms, such as adenovirus or poxvirus, DNA takes the form of a double helix, where the nucleotides of two strands are bonded together. Some viruses, such as parvoviruses, contain only a single strand of DNA. Most eukaryotes have linear nuclear DNA, meaning the DNA strands have a definite beginning and an end. The DNA of prokaryotes exists in circular strands in the cytoplasm. Bacteria also have plasmids that contain genetic information apart from their chromosomes. Plasmids in most bacteria are tightly coiled, double-stranded DNA molecules. However, a few linear variants have been observed in the genus Borrelia.

A nucleotide is an organic compound consisting of a sugar, a phosphate, and a nitrogenous base. Nucleotides form the basis of a genetic sequence. The double-helix structure is formed by the joining of two single DNA strands. The sugar and phosphate form the backbone of each strand. Extending from each sugar is the nitrogenous base, and the bond between paired bases forms the center of the double helix. Nucleotides from each DNA strand bind together throughout the entire length of the double helix.

There are two types of nitrogenous bases in nucleic acids, purines and pyrimidines. A purine includes two joined rings containing carbon and nitrogen. Adenine and guanine are purines. A pyrimidine includes a single ring containing carbon and nitrogen. Thymine, uracil, and cytosine are pyrimidines. In DNA, adenine always pairs with thymine while cytosine always pairs with guanine. In ribonucleic acid (RNA), thymine is replaced with uracil. The overall length of the bonded nucleotides remains the same for the entire double helix by bonding a purine with a pyrimidine. Viral genomes can exist as single-stranded DNA (ssDNA), double-stranded DNA (dsDNA), single-stranded RNA (ssRNA), or double-stranded RNA (dsRNA), and may be linear or circular.

DNA Structure

The double-helical structure of DNA is accomplished by the joining of nitrogenous bases in between the two sugar phosphate backbones.

DNA Packaging

Packaging of DNA differs between prokaryotes and eukaryotes. Eukaryotes can have multiple chromosomes inside a membrane-bound nucleus. Prokaryotes have a single chromosome called a genophore packaged in an area called a nucleoid.
The genome, or the genetic material of an organism, contains hundreds to thousands of genes. Individual genes are composed of tens to tens of thousands of nucleotide base pairs. For example, the Escherichia coli (E.coli) genome contains over four million base pairs that encode over 4,000 genes. If the entire genome of E. coli were laid out linearly, it would be 100 times longer than the cell itself. Tight organization and compact packaging are required to ensure an organism's genome fits inside the cell. This process is handled differently between eukaryotes and prokaryotes.

Eukaryotic DNA Packaging

In eukaryotes, cells with internal organelles, chromosomes are stored in a double-membrane organelle called the nucleus. DNA is first coiled around proteins called histones to form a nucleosome, which is often described as a bead (histone) and string (DNA) configuration. These nucleosomes then coil together into fibers that are further twisted into even tighter fibers via supercoiling. Histone coiling affects gene expression because the coiling can control which parts of the DNA can be transcribed.

Chromosome Packaging

DNA packaging in a eukaryotic cell allows long DNA molecules to fit in the nucleus of the cell. DNA is first coiled around histone proteins. Histones are then further coiled tighter and tighter until the chromosome is formed.

Prokaryotic DNA Packaging

Prokaryotes do not possess proteins to assist with DNA packaging. Instead of a nucleus, prokaryotes have a nucleoid, which is the area inside a prokaryotic cell where genetic material (DNA) is found. The collective term for a prokaryote’s genetic material found in the nucleoid is genophore.
The genome of prokaryotes is not found within a membrane-bound nucleus. Instead, it is in the nucleoid (transmission electron microscope), which is an area in the central part of the cell where the genetic material is located.
Credit: Zhang et al.License: CC BY 4.0
The nucleoid typically consists of a single, circular chromosome. However, prokaryotes with two chromosomes have been discovered. This chromosome is supercoiled, meaning the DNA has been twisted over and over again until it is compact. Supercoiling can be mimicked when holding one end of a rubber band and twisting the other end repeatedly until the band is compact. DNA topoisomerases are enzymes that control DNA supercoiling by catalyzing the winding and unwinding of parts of the DNA strand. This must be done to allow access to certain genes for transcription. Topoisomerases make an incision in the DNA coil, unwind it, and then after transcription, rewind the DNA, sealing the breaks.

Supercoiling of DNA

Chromosomal DNA is maintained in a tightly packed supercoiled structure within the nucleus. During transcription, DNA topoisomerases uncoil the DNA, break the strands, and allow for transcription to occur. After transcription, the enzymes reseal and recoil the DNA.