Nucleic Acids And Protein Synthesis - Session 2

We already know that during S phase of cell cycle, cell undergoes DNA replication. This replication of DNA occurs before division of nucleus. But what exactly is DNA replication? Let us understand this process in detail. DNA replication is the process by which a cell duplicates its DNA to ensure that each daughter cell receives an identical copy of the genetic material. This process is vital for cell division, growth, and the transmission of genetic information from one generation to the next.

DNA replication begins at specific sites in the DNA molecule called replication origins. Replication origins are typically rich in specific DNA sequences. These sites serve as recognition sites for the initiation of replication. The replication origin is where the DNA strands separate. This results in the formation of a replication bubble.

Once the replication origin is activated, double stranded DNA molecule starts to unwind. The unwinding generates two separated strands. This creates replication fork at each end of the replication bubble. These replication forks serve as the sites where the actual DNA synthesis takes place.

Several enzymes have crucial roles in DNA replication. One of the key enzymes is helicase. Helicase unwinds the DNA double helix by breaking the hydrogen bonds between the base pairs. This activitcreates the open DNA strands required for replication.

DNA polymerases catalyze the synthesis of new DNA strands. It adds complementary nucleotides to the existing template strands. DNA polymerase moves along the template strand in a three to five direction. It synthesizes the complementary strand in a five to three direction. However, DNA polymerase can only add nucleotides to an existing strand. It requires a primer to initiate synthesis of nucleotides.

During DNA replication, there is an enzyme called primase. Primase makes small pieces of RNA called primers. Primers are like little placeholders made of RNA. DNA polymerase can grab onto primers and start making the new DNA strand. Primers equate the DNA template strand. They help DNA polymerase know where to begin. Once DNA polymerase starts constructing the new DNA strand, it replaces the RNA primers with DNA building-blocks. It is like swapping out the placeholders with the correct puzzle pieces.

During DNA replication, the two template strands of DNA are oriented in opposite directions. As a result, the DNA polymerase synthesizes two new strands differently. The strand which is synthesized continuously in the same direction as the replication fork movement is called leading strand. The leading strand is synthesized continuously because DNA polymerase can add nucleotides in a continuous manner as the replication fork opens up. Lagging strand is synthesized discontinuously in the opposite direction.

The lagging strand is synthesized in short fragments called Okazaki fragments. Okazaki fragments are short, newly synthesized DNA fragments. As the replication fork progresses, DNA polymerase synthesizes these fragments away from the replication fork. The length of Okazaki fragments typically ranges from a few hundred to a few thousand nucleotides.

DNA ligase enzyme is required to join Okazaki fragments together. DNA ligase catalyzes the formation of bonds between adjacent Okazaki fragments. It effectively seals the gaps between them and creates a continuous lagging strand.

DNA proofreading is like a built-in spell checker in our cells. When our cells make copies of DNA during replication, there can be mistakes or errors that occur. But DNA has a special enzyme called DNA polymerase that can check for these mistakes. DNA polymerase reads the DNA code and adds nucleotides to make a new DNA strand. But as it adds these nucleotides, it also checks if they are equal to the original DNA code. If it finds a mistake, it can remove the wrong nucleotide and replace it with the correct one. This helps ensure that the new DNA strand is an accurate copy of the original.

DNA polymerase has a built-in proofreading function called three to five exonuclease activity. This means that it can remove incorrectly paired nucleotides from the newly synthesized DNA strand. Once the incorrect base is removed, DNA polymerase then inserts the correct nucleotide and continues with DNA synthesis.

RNA is a molecule that has a crucial role in the cell. It is similar to DNA in some ways but differs in its structure and functions. Similar to DNA , RNA is made up of nucleotides. However, RNA consists of a single strand. DNA has a double helix structure.

RNA has various types. Each type has specific role in the cell. One essential type of RNA is messenger RNA. It is alco called mRNA. It acts as a messenger molecule. It carries genetic information from DNA to the cellular machinery responsible for protein synthesis. We know that DNA serves as the storage of genetic information. It contains a unique code that determines the sequence of amino acids in a protein. Proteins, in turn, have critical roles in the structure, function, and regulation of cells.

The genetic code is a set of rules that translates the information stored in DNA into specific proteins. The genetic code is carried out by mRNA. Genetic code consists of specific codons. Codons are specific sequences of three nucleotides. We already know that proteins are made up of amino acids. Codons code for individual amino acids.

For example, the codon AUG serves as the start codon. It initiates the protein synthesis. It also codes for the amino acid methionine. Codon, such as UAA, UAG, and UGA, are stop codons. They signal the end of protein synthesis.