The genetic material of a cell is its genome. Cells in which the genome is present in a single copy, such as prokaryotic cells and gametes of diploids, are called haploids. Cells in which the genome is present in two copies, such as the somatic cells of higher eukaryotic organisms, are called diploids. In eukaryotic cells, the genetic material is distributed in the nucleus, mitochondria, and chloroplasts. Usually, however, the term genome refers to the genetic material found in the nucleus.
To describe the length or sequence of nucleic acid is used the term number or base sequence respectively. We mean the number or sequence of nucleotides of nucleic acid. This simplification is done because the only part of the nucleotide that changes is the nitrogenous base. Thus, a DNA molecule is reported to be 2,000 base pairs long because it is double-stranded, while an mRNA molecule is 2,000 bases long because it is single-stranded.
What is Genetic Material: Definition, types, and functions of Genetic Material
What is Genetic Material?
DNA is the genetic material of all living organisms except RNA viruses present in the nucleus. According to the model of Watson and Crick, DNA consists of two polynucleotide chains, which form a double helix. The two strands of DNA are complementary to each other. The amount of DNA varies from organism to organism.
DNA contains all the genetic information of an organism that is necessary for the functions of the organism. The short sequence of nucleotides is called genes. Genes are the unit of information.
Prokaryotic cell genetic material has a major circular DNA molecule and sometimes several smaller circular molecules, called plasmids. In eukaryotic cells, DNA is distributed in the nucleus, mitochondria, and chloroplasts (plant cells). The DNA of the nucleus is distributed in humans into 23 pairs of chromosomes. Chromosomes show different degrees of aggregation depending on the stage of the cell cycle in which we observe them.
Chromosomes are visible under the optical microscope during metaphase, because at this stage of the cell cycle, they show the highest degree of aggregation. At this stage, each chromosome consists of two sister chromatids, which are retained in the centromere.
The representation of chromosomes in pairs of decreasing size is called a karyotype. Mitochondria (in most organisms) and chloroplasts have circular DNA molecules, which contain information about the function of these organelles.
Definition of genetic material:
Genetic material is defined as A material that carries the genetic information of organisms like Plants, Animals, and microorganisms and that passes it from one generation to the next generation is called genetic material. The information is contained in the form of a gene and it controls various metabolic reactions such as reproduction, growth and development, Photosynthesis, etc.
Types of Genetic Material:
There are two types of genetic material DNA and the second one is RNA.
RNA: RNA (Ribose nucleic acid)
- RNA is a type of nucleic acid-containing ribose sugar.
- Generally, RNA is single-stranded. In linear RNA, the nitrogen bases are unpaired but in the folded RNA nitrogen bases are paired in certain regions.
- Hence purine pyrimidine ratio may or may not be 1:1. It is formed by linking many smaller units called ribonucleotides linked with each other by a phosphodiester linkage.
- Each ribonucleotide consists of three chemicals ribose sugar, a phosphate group, and a type of nitrogen base (A, G, U, or C). RNA contains A, G as a purine and C, U(instead of T) as pyrimidine.
- There are two types of RNA: Genetic RNA. and Non-genetic RNA.
- Genetic RNA: The RNA which functions as genetic material is called genetic RNA. It stores and carries genetic information. It is present in some viruses (plant viruses).
- Non-genetic RNA: The RNA which does not function as genetic material is called non-genetic RNA. It is involved in protein synthesis and also carries out some other functions of the cell. it is present in all organisms except some viruses. Nongenetic RNA are of three types 1) Messenger RNA (mRNA), 2) Ribosomal RNA (rRNA), and 3) Transfer RNA (tRNA).
2. DNA: DNA (DeoxyRibose nucleic acid) :
DNA is the genetic material of all living organisms except RNA viruses. According to the model (1953) of Watson and Crick, DNA consists of two polynucleotide chains called strands, which form a double helix. The two strands of DNA are complementary to each other. The two strands are held together by weak hydrogen bonds present in between the nitrogen bases of the two strands. Each polynucleotide chain has 3′ and 5′ ends. 5′ end has a free phosphate group and the 3′ end has a free OH group. The amount of DNA varies from organism to organism. DNA contains all the genetic information of an organism that is necessary for the functions of the organism.
DNA is the genetic material:
The experiments, in brief, prove that DNA is the genetic material:
Griffith’s experiment (1928) :
- Frederick Griffith experimented on the pneumonia-causing bacteria Diplococcus pneumoniae.
- There are two strains of bacteria: a) Smooth strain (S strain) capsulated and pathogenic. b) Rough strain (R strain) non-capsulated and non-pathogenic.
- The smooth strain is capsulated and pathogenic or virulent while the rough strain is non-capsulated and non-pathogenic or virulent.
Griffith injected these bacteria in mice and found the following results :
- R-strain injected bacteria Survived
- S- strain injected bacteria died
- Heat killed S- strain injected bacteria survived
- Heat killed S- strain + living R Strain injected bacteria died.
- The bacteria isolated from the dead mice of the fourth set were found to contain cells of both strains.
- This indicates that in the fourth set of experiments heat-killed S strain caused the transformation of some R strain into S strain.
- From the above experiments, Griffith concluded that nonpathogenic K-strain bacteria are converted into pathogenic S- strain bacteria due to the active transforming principle i. e., genetic material.
- Griffith could not determine the chemical nature of the genetic material.
Experiment by Avery, Macleod and
- Avery, Macleod, and McCarty continued the experiment of bacterial transformation and identified DNA as genetic material.
- They separated carbohydrates, proteins, and DNA fractions from the pathogenic S-strain bacterium Diplococcus pneumonia. Each fraction was added to separate, non-pathogenic R-strain cell cultures as follows :
- i) R-cells +Carbohydrates of S-strain Rcells.
- R-cells + Protein of S-strain R-cells.
- R-cells + DNA of S-strain R-cells + S cells
- R- cells + DNA of S-strain + DNA-ase enzyme >R cells.
- Thus it was discovered that R-cells picked up characters only from DNA and get changed into S-cells. When DNA base (DNA destroying enzyme was added then transformation did not take place, hence DNA is a transforming substance i. e., genetic material.
Hershey and Chase show DNA as a genetic material :
- Hershey and Chase (1952) used a tadpole-shaped T, a bacteriophage that attacks the bacterium.
- They used radioactive S³⁵ and P³² (proteins get labeled with S³⁵ while DNA with p³²).
- Bacteria were grown in the medium containing these radioactive isotopes, S³⁵, and p³² separately.
- Then the bacteria were infected with phages as follows :
A) Bacteria were grown in S³⁵ medium :
- The bacteria incorporated S³⁵ into its proteins.
- The phages infecting these bacteria also became labeled with S³⁵.
- These S³⁵ labeled phages were grown on non-radioactive bacteria.
- After infection, the protein coats and the bacterial cells were separated by centrifugation and their radioactivity was measured.
- All the radioactivity was found to be associated with protein coats and none with the bacterial cell.
- This indicated that proteins of the phage had not entered the bacterial cell during infection.
B) Bacteria were grown in P³² medium:
- These bacteria incorporated P³² into their DNA.
- The phages after being grown on these bacteria also incorporated P³².
- After infection with non-radioactive bacteria, the protein coat and the bacteria were separated.
- In this case, all the radioactivity was found in the bacteria rather than the protein coat.
- These experiments proved that only the phage DNA entered the bacterium.
Prokaryotic cell Genetic Material:
The genetic material of prokaryotic cells is a double-stranded circular DNA molecule about 1 mm long. This circular DNA molecule is folded and packaged with the help of main proteins resulting in a final length in the cell of 1 μm. It contains a copy of the genome, so prokaryotic cells are haploid.
In many bacteria, in addition to the main circular DNA molecule, there are plasmids. Plasmids are double-stranded, circular DNA molecules of various sizes. They contain a small percentage of genetic information and make up 1-2% of bacterial DNA.
A bacterium may contain one or more plasmids, which replicate independently of the bacterium’s major DNA molecule. Among the genes contained in plasmids are antibiotic-resistance genes and genes involved in the transfer of genetic material from one bacterium to another.
Plasmids can exchange genetic material both with each other and with the bacterium’s major DNA molecule, as well as carry it from one bacterium to another. In this way, they transform the bacterium into which they enter and give it new properties. Plasmids are a valuable tool for Genetic Engineering techniques.
Eukaryotic cell Genetic material:
The genetic material of eukaryotic cells is longer than that of prokaryotes. The total DNA present in each eukaryotic cell is not a single molecule but consists of many linear molecules, the number, and length of which are characteristic of different species of organisms.
DNA molecules are packed with proteins to form chromatin fibrils. The total DNA in each human diploid cell is about 2 m long and clumps to such an extent that it fits in the nucleus, which is ten millionths of a meter in diameter!
In the electron microscope, after special treatment, the chromatin filaments look like bead rosaries. Each “bead” is called a nucleosome and is the basic unit of organization of chromatin. The nucleosome consists of DNA of 146 base pairs and eight molecules of proteins, called histones. The DNA is wrapped around the histone octamer.
The nucleosomes fold, resulting in the DNA being packaged to a greater extent, eventually forming the chromatin fibrils. Other types of proteins are involved in folding.
If we look at the genetic material of a eukaryotic cell, we see that it appears in different characteristic forms, depending on the stage of the cell cycle. During the mesophase, the genetic material has a low degree of aggregation and forms a network of chromatin fibrils. As a result, the chromatin fibers are not visible as individual structures under the microscope.
By the end of replication, each chromatin fibril has doubled. The two copies of each fibril are linked together by a structure called a centriole. The term sister chromatids are used to describe the duplicated chromosomes during the period attached to the centromere.
Mitochondria and chloroplasts have their genetic material
Mitochondria and chloroplasts have their DNA. The genetic material of mitochondria and chloroplasts contains information about their functions such as oxidative phosphorylation and photosynthesis respectively and encodes a small number of proteins.
But most proteins, which are necessary for the function of mitochondria and chloroplasts, are encoded by genes found in the DNA of the nucleus. This fact shows that these organelles are not independent of the cell nucleus and are therefore characterized as semi-autonomous.
The Mitochondrial DNA in most organisms is a circular molecule. Each mitochondrion contains two to ten copies of the circular DNA molecule. However, in some lower protozoa, it is linear. The zygote of higher organisms contains only the mitochondria that come from the egg. Therefore, the origin of the mitochondrial genes is maternal. Chloroplast DNA is a circular molecule and is larger than mitochondrial DNA.
Viruses have DNA or RNA genetic material
These viruses contain a single type of nucleic acid which can be DNA or RNA. Virus DNA can be single-stranded or double-stranded, linear, or circular. RNA viruses usually have linear RNA (in rare cases it is cyclic), which can be single-stranded or double-stranded. Viruses and their life cycle will be analyzed in the corresponding chapter.
Functions of Genetic Material:
- DNA is the genetic material of all cells and most viruses. Some viruses have RNA (RNA viruses) as their genetic material.
- The storage of genetic information. DNA (or RNA of viral RNAs) contains information that defines all the characteristics of an organism and which are organized into functional units, the genes.
- The preservation and transmission of genetic information from cell to cell and from organism to organism is ensured by DNA self-replication.
- The expression of genetic information is achieved by controlling the synthesis of proteins.
- The first step in expressing information in DNA is to transfer it to RNA through the transcription process. RNA, in turn, transmits information through the translation process to the proteins responsible for the structure and function of cells and, consequently, organisms.
- The DNA of an organism is the molecular “hard disk” that contains stored precise instructions, which determine the structure and function of the organism. At the same time, it contains the information for its self-replication, thus ensuring the transmission of genetic instructions from a cell to its subsidiaries and from an organism to its offspring.