DNA is a biomolecule formed of the basic building blocks called nucleotides. Nucleotides, in turn, comprise three parts— a phosphate group, a sugar group, and a nitrogen base. To form a DNA strand, nucleotides link into chains with alternating phosphate and sugar groups.
The nitrogen bases found in nucleotides are of four types— adenine, thymine, guanine, and cytosine. The sequence of these bases in the DNA strand determines the biological instructions. Suppose the sequence ATC GTT might direct for blue eyes, while ATC GCT might direct the eyes to look brown.
The complete DNA instruction set or genome of a human contains nearly 3 billion bases and around 20,000 genes present on 23 pairs of chromosomes. DNA holds the instructions required for an organism to grow, survive and reproduce. The DNA sequences must be translated into instructions that can be used to generate proteins, the complex biomolecules responsible for doing most of the work in our bodies to perform these functions.
Where is DNA found?
In eukaryotes, DNA is present inside the cell’s nucleus. Each DNA molecule is tightly packaged because the cell is quite small, and there are many DNA molecules per cell. This tightly packed form of DNA is referred to as a chromosome. The DNA found inside the nucleus of the cell is called nuclear DNA. The complete set of nuclear DNA of an organism is called its genome. Apart from nuclear DNA, a small amount of DNA is present in the mitochondria of humans and some other complex organisms.
Where Does DNA Come From?
Half of the nuclear DNA is inherited from the male parent, while the other half from the female parent in organisms that undergo sexual reproduction. However, all of the mitochondrial DNA is inherited from the female parent. This is because organisms inherit all of their mitochondrial DNA from the female parent. This occurs because only the egg cells, that is, the female reproductive cells, retain their mitochondria during fertilisation.
How is DNA Copying Important?
DNA copying means the replication of DNA during the process of cell division. The transfer of genes from the parents to the next species is very important to carry on the progeny. The continuity of the species depends on this factor. For organisms that reproduce asexually, this process becomes specifically important. Only one parent is involved in reproduction in such organisms, so there is a single source to produce DNA variations in the progeny.
Example: Suppose a species can survive in water at a certain temperature. But the temperature of the water changes due to climatic changes. This would result in the species getting extinct as it wouldn’t be able to survive in the new water temperature. However, this does not happen as nature favours them by causing a change in their DNA. The DNA of some individuals of this species undergoes replication enabling them to survive in the water with a new temperature, either warmer or colder. This is how DNA replication favours the existence of species on Earth.
Steps of DNA Replication
DNA replication is when DNA produces a copy of itself during cell division.
- In the process of DNA replication, the first step is to ‘unzip’ or open the double helix structure of the DNA molecule. This is facilitated by an enzyme, helicase, which disintegrates the hydrogen bonds between the two strands holding the complementary bases of DNA together.
- This separation of the DNA strands creates a ‘Y’ shape referred to as the replication fork. These two separated strands play an important role in replication by acting as templates for making the new DNA strands.
- One strand lies in the 3′ to 5′ orientation, while the other lies in the 5′ to 3′ orientation. The strand that lies at an orientation of 3′ to 5′ is referred to is the leading strand, while the other is the lagging strand. As the two strands are differently oriented, they replicate differently.
- After this, a primer, a short piece of RNA produced by a primase enzyme, comes and attaches to the end of the leading strand. It acts as the starting point for the synthesis of DNA.
- Then DNA polymerase attaches to the leading strand and moves along with it while adding new complementary nucleotide bases like adenine, guanine, cytosine, and thymine to the DNA strand in the 5′ to 3′ direction. This type of replication is called continuous replication.
- The primase enzyme forms numerous RNA primers that bind at different points on the lagging strand. Afterward, collections of DNA known as Okazaki fragments are added to the lagging DNA strand again in a 5′ to 3′ direction. This sort of replication is referred to as discontinuous replication as the Okazaki fragments bind with it later.
- After all of the bases are paired up, A with T and C with G, an enzyme “exonuclease” strips away the primers, and more complementary nucleotides fill their gaps.
- The new DNA strand is proofread to ensure that the new DNA sequence has no mistakes. Finally, the DNA sequence is sealed into two uninterrupted double strands by an enzyme called DNA ligase.
- The outcome of this DNA replication is the formation of two DNA molecules that consist of one new and another old chain of nucleotides. For this reason, DNA replication is referred to as semiconservative because half of the chain consists of the old DNA molecule while the other half is brand new. After the replication process is over, the resultant DNA automatically binds into a double helix.
What is the Function of DNA Polymerase in the Process of Copying DNA?
The enzyme DNA polymerase is one of the key molecules in DNA replication. These enzymes are concerned with the synthesis of DNA by adding nucleotides to the growing DNA strand and incorporating them accurately by deciding those that complement the template. Here are some key points about DNA polymerases:
- They always require a template to work.
- They can only add nucleotides to the 3′ end of a DNA strand, not the other end.
- They can’t make up a DNA strand from scratch but need a pre-existing chain or a few nucleotides in the form of a primer.
- They proofread or scrutinise the final work, leaving out many “wrong” nucleotides that are accidentally attached to the chain.
- In prokaryotes like E. coli, two main DNA polymerases are involved in DNA replication, one is the major DNA-maker-DNA pol III, and the other is DNA pol I, which has a crucial role to play as a supporting role.
What is the Double Helix Model of DNA?
The term “double helix” is used to describe DNA’s winded, two-stranded structure. This model of the structure of DNA was introduced by James Watson and Francis Crick. Its shape, which looks more like a twisted ladder, enables the DNA to pass along biological instructions to future generations with great precision.
To understand the double-helix model of the DNA from a chemical standpoint, you can imagine the DNA strands composed of alternating sugar and phosphate groups as sides of a ladder. These DNA strands run in opposite directions— one in the 3′ to 5′ direction and another in the 5′ to 3′ direction. Now imagine that each “rung” of this ladder is composed of two nitrogen bases joined by hydrogen bonds. Because this type of chemical pairing is highly specific, the nitrogen base A always pairs with base T, and the base C with G. So, all you need to remember is the sequence of the bases on a DNA double helix strand, figure out the sequence of bases on the other strand is then a simple matter. Given below is an illustration of the double-helix model of DNA:
The unique structure of DNA enables it to copy itself or replicate during cell division. The DNA helix opens up in the middle to give two single strands when the cell begins to divide. The resultant single strands act as templates for forming two new DNA strands. The double-stranded DNA molecule thus formed is a replica of the original DNA molecule. After that, an A base is added wherever there is a T base and a C base where there is a G base, and so on until all of the nitrogen bases once again are paired up.
Additionally, when proteins are being produced, the double helix opens up to let a single strand of DNA to act as a template, which is then transcribed into mRNA. mRNA is a biomolecule that conveys important instructions to the cell’s protein-making components.
After going through the whole discussion, you know the importance of copying DNA during reproduction. Additionally, you are also acquainted with other facts about DNA, including its functions, composition, and structure. To score high marks in the exam, you need to understand every topic on a fundamental level. We hope that this discussion helped clear all your misconceptions and queries on the topic of DNA, and you will be able to answer all questions on it in the exam.
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