Deoxyribonucleic acid (DNA) is the hereditary material in most of the living organisms. It is a molecule comprising two helices that remain coiled around each other and have the genetic information to be transmitted to the RNA or ribonucleic acid through a process called transcription.
Human DNA is located in the nucleus in the form of a chromatin reticulum where DNA remains associated with histone and non-histone proteins. In prokaryotes like in bacteria, the DNA remains in the nucleoid region and does not remain associated with proteins. The DNA contains the code for protein synthesis in the form of nucleotides with four types of nitrogen bases, Adenine, Guanine, Cytosine and Thymine. The double-helical structure of DNA is typical for both eukaryotic and prokaryotic cells. However, DNA coiling is different.
Prokaryotic cells were formed before eukaryotic cells and have a much simpler structure than the latter.
Prokaryotic DNA: Prokaryotic DNA is double-stranded circular DNA which remains diffused in a dense region of cytoplasm called the nucleoid. There is no nuclear membrane surrounding the DNA in prokaryotes. The single circular DNA represents a single chromosome. This DNA is not a supercoiled structure as in eukaryotes as they do not have histone proteins in their structure. They form loops with the help of nucleoid-associated proteins. Due to the absence of the histone scaffolding, the prokaryotic DNA is often referred to as “naked DNA”.
The size of the DNA is around 160000 to 12.2 million base pairs. The genes present in this kind of DNA is less in number and is present in the form of operons. Operons are a group of genes with a common promoter and a common terminator sequence as a result of which these genes are expressed simultaneously. Since the number of base pairs it less, it does not keep much scope for “junk DNA”.
This means that most of the genes are without introns or non-functional DNA. There is a single origin of replication as a result of which the entire genome is replicated at once. During this type of replication, a single replication fork is formed. The rate of replication is close to 2000 nucleotides per second. Transposons or mobile genetic elements are small segments of DNA that can jump from one place to another in the genome. Such elements are present in prokaryotes and help in bringing in genetic variation. During transcription, the RNA formed is polycistronic, that contains information for more than one protein. When ribosomes attach to each of these cistrons, it forms a beaded structure called polyribosomes.
Apart from the DNA present in the nucleoid, prokaryotes also have an extranuclear, double-stranded, circular DNA in the cytoplasm. These are known as plasmids. The genes present in the plasmids help in the survival of the bacteria and contains antibiotic-resistant genes. They replicate autonomously and are transferred from one bacterium to another during conjugation.
Eukaryotic DNA: The DNA in eukaryotes is found in the nucleus enclosed in the nuclear membrane. It is linear in shape. This DNA is present in the form of chromatin reticulum when the cell is not dividing and condenses to form rod-shaped structures called chromosomes during cell division. The DNA molecule remains tightly coiled and supercoiled against basic proteins called histone proteins. Non-histone proteins are also present in this DNA. The DNA forms a coiled structure called nucleosome which supercoils many folds to form the condensed structure of the eukaryotic DNA. This coiling allows the large bulk of DNA to be incorporated into the nucleus.
The number of chromosomes varies from species to species and is unique for each one of them. The human genome consists of 23 pairs of chromosomes, consisting of 2.9 billion base pairs. There is a large amount of non-coding or junk DNA present in eukaryotes. Hence introns need to be cut and removed from the RNA during gene expression. This process is known as RNA splicing. There is more than one origin of replication allowing different genes to be transcribed separately. The rate of replication is 100 nucleotides per second and is hence much slower than that in prokaryotes. Transposons are inactive in eukaryotes. The mRNA formed during transcription are monocistronic, that is codes for only one protein. Polyribosomes are absent in eukaryotes.
1. State Five Important Differences Between Prokaryotic and Eukaryotic DNA.
Answer: Five Important Differences Between Prokaryotic and Eukaryotic DNA are as Follows:
They are present diffused in a central dense region of cytoplasm called a nucleoid.
The DNA is present in the nucleus surrounded by the nuclear membrane.
DNA is double-stranded circular with one origin of replication.
DNA is linear double-stranded with multiple origins of replication.
Introns or non-coding regions are absent
Introns are present in the hnRNA and need to be removed by RNA splicing to form functional mRNA
The DNA does not remain associated with proteins. Some nucleoid-associated proteins may be present forming loops in the DNA. This kind of DNA is known as naked DNA
The DNA is coiled against basic histone proteins to form nucleosomes. These are further coiled and supercoiled to form the chromatin threads that remain in the nucleus of a non-dividing cell. During cell division, the chromatin condenses further to form rod-shaped chromosomes.
The amount of DNA and genes is comparatively less. Some genes necessary for survival may be present in the extrachromosomal piece of DNA called a plasmid.
The amount of DNA and the number of genes is comparatively higher. Plasmids are absent. However, extra-chromosomal DNA is present in mitochondria and chloroplast.
2. What is a Prokaryotic Cell?
Answer: Prokaryotic cells are those which do not possess a well-defined nucleus. The genetic material remains diffused in a region of the cytoplasm called a nucleoid. The cell is much smaller in size, usually ranging between 1 to 5 micrometres. Membrane-bound cell organelles like mitochondria, endoplasmic reticulum, Golgi bodies are absent. Ribosomes are smaller in size with a sedimentation co-efficient 70S. The cell wall is simple and is made up of peptidoglycan, muramic acid, etc. During the process of DNA replication, the entire genome is replicated at once. Organisms which have prokaryotic cells are usually unicellular.
3. Is There Less Amount of DNA in Prokaryotes?
Ans: Unlike eukaryotic DNA which can also be found inside the chloroplast and mitochondria, prokaryotic DNA is found only in the central part of the cell called the nucleoid. The nucleoid is devoid of any membrane. Prokaryotic DNA is normally small with the size measuring not more than 0.1 pg.
4. What is Polyribosome?
Answer: The mRNA in prokaryotes is polycistronic. That is, they contain codes for more than one protein. Hence when several ribosomes attach to the cistrons simultaneously, they form a beaded structure, known as polyribosome.