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DNA is a molecule that is found in every living organism and that holds the instructions for growth and development.
DNA had a double-stranded helical structure that is held together by complementary base units.
DNA is one of the most vital nucleic acids found in our body. The nucleic acid DNA refers to a polynucleotide which means that it is made up of several nucleotides that are bonded together in the form of a long chain.
It stores the information related to different processes performed by the cell. The crucial genetic information is also transferred to the next generation of cells. The latest advancements in technology have enabled us to employ DNA in different non-biological processes for the welfare of mankind.
In the next section of the article, we will discuss semi-conservative DNA replication.
Semi-Conservative DNA Replication
The semi-conservative DNA replication guarantees the genetic continuity between the generation of the cells. It takes place in preparation for mitosis. It implies that as the parent cell divides to produce two genetically identical daughter cells, each daughter cell has the same number of chromosomes as the parent cell. Hence, the number of DNA molecules should be doubled before the occurrence of mitosis.
The replication of DNA takes place during the S phase of the cell cycle. S phase is also referred to as the interphase and the cell does not divide during this phase. There is a breakdown of hydrogen bonds between the base pairs on the two antiparallel polynucleotide strands of DNA.
It disentangles the double helix DNA to create two single polynucleotide strands of DNA. Each of the single polynucleotide strands of DNA plays a role of a template for the creation of a new strand. The original and the new strands combine to create a new DNA molecule. This method of DNA replication is called semi-conservative replication because half of the original DNA molecule is preserved in each of the two new molecules of DNA.
Meselson and Stahl in 1958 showed semi-conservative replication as a method of replication. They demonstrated how the changes in DNA take place over a generation by using coli (a bacteria) and two nitrogen isotopes, a heavy form of 15N and the ‘normal’ form 14N.
DNA Polymerase
- There are free nucleotides in the nucleus to which two additional phosphates are added. These free nucleotides with three phosphate groups are referred to as nucleoside triphosphates or activated nucleotides.
- The additional phosphates activate the nucleotides, making them participate in the replication of DNA.
- On each of the template strands of DNA, free nucleoside triphosphates bases align with the complementary bases.
- The DNA polymerase enzyme synthesizes new strands of DNA from the two template strands
- It does so by speeding up the condensation reaction between the deoxyribose sugar and phosphate groups of adjoining nucleosides within the new strands to form the sugar-phosphate backbone of the new strands of DNA.
- The DNA polymerase splits up the two additional phosphates and employs the released energy to form the phosphodiester bonds between adjoining nucleotides.
- After that hydrogen bonds are formed between the template’s complementary base pairs and new strands of DNA.
What are Leading and Lagging Strands?
- DNA polymerase can form the new strands in a single direction only (5’ to 3’ direction)
- There is an unfastening of DNA from the 3’ towards the 5’ end. The DNA polymerase will get attached to the 3’ end of the original strand and travel towards the replication fork. The replication fork refers to the point at which the DNA molecule divides into two template strands.
- It implies that the DNA polymerase enzyme continuously synthesizes the leading strand
- This template strand that the DNA polymerase enzyme connects to is referred to as a leading strand.
- The other template strand that is formed during the replication of DNA is called the lagging strand.
- On this strand, the DNA polymerase shifts away from the replication fork (from the 5’ end to the 3’ end)
- It implies that the DNA polymerase enzyme synthesizes the lagging strand of DNA in tiny segments. These tiny segments are also referred to as Okazaki fragments.
- DNA ligase is the second enzyme that is required to connect these lagging strand segments to create a continuous complementary DNA strand.
- The DNA ligase enzyme does so by speeding up (catalyzing) the creation of phosphodiester bonds between the segments to form a continuous sugar-phosphate backbone.
In the next section of the article, we will discuss the structure of RNA. We will specifically compare the structure of RNA with that of DNA.
Structure of RNA
Similarities with DNA
- Similar to DNA, the nucleic acid RNA (ribonucleic acid) is a polynucleotide which means that it is composed of several nucleotides that are joined together in the form of a long chain.
- Like DNA, nitrogenous bases adenine (A), guanine (G) and cytosine (C) are present in the RNA nucleotides.
Differences Between DNA and RNA Structure
- Unlike DNA, the nitrogenous base thymine (T) is not present in RNA nucleotides. Instead of this, they have nitrogenous base uracil (U).
- As compared to DNA, RNA nucleotides have the pentose sugar ribose (in place of deoxyribose)
- Unlike DNA, RNA molecules are only composed of a single polynucleotide strand (it means that they are single-stranded)
- Each RNA polynucleotide is composed of alternating ribose sugars and phosphate groups. They are joined together with the nitrogenous bases of each nucleotide extending out sideways from the single-stranded RNA molecule
- The sugar-phosphate bonds between various nucleotides in the same strands are covalent. These covalent bonds are referred to as phosphodiester bonds.
- The phosphodiester bonds create the sugar-phosphate backbone of the RNA polynucleotide strand
- The phosphodiester bonds join the 5-carbon of a single ribose sugar molecule to the phosphate group from the same nucleotide. Another phosphodiester bond to the 3-carbon of the ribose sugar molecule of the next nucleotide in the strand links this nucleotide.
Example
mRNA (messenger RNA) is another example of an RNA molecule which is the transcript copy of the gene which encodes a particular polypeptide. The two examples include transfer RNA (tRNA) and ribosomal RNA (rRNA).
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