Q.20) Explain cell cycle with its different phases with neat and labelled diagram. Q.21 What are nucleic acids? Give a brief account of structure of DNA? Q.22) What is basic mechanism of enzyme action. Describe lock and key model with the help of diagram.
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A cell cycle is a series of events that takes place in a cell as it grows and divides. A cell spends most of its time in what is called interphase, and during this time it grows, replicates its chromosomes, and prepares for cell division. The cell then leaves interphase, undergoes mitosis, and completes its division.
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20)
A cell cycle is an ordered sequence of events by which a cell duplicates its genome, synthesizes the other constituents of the cell and eventually divides to form two daughter cells. It has four stages- G1, S, G2 and M phase. The interphase is the longest phase as it includes G1, S and G2 phases of the cell cycle.
21) Nucleic acids are long chainlike molecules composed of a series of nearly identical building blocks called nucleotides. Each nucleotide consists of a nitrogen-containing aromatic base attached to a pentose (five-carbon) sugar, which is in turn attached to a phosphate group.
22) The lock and key model also called Fisher's theory is one of two models which describe the enzyme-substrate interaction. The lock and key model assumes that the active site of the enzyme and the substrate are equal shaped. It supposes that the substrate fits perfectly into the active site of the enzyme.