Question

The DNA replication occurs in
A. G1-phase
B. S-phase
C. G2-phase
D. M-phase

Answer 1

Hint: In cell division, there are some phases generally four phases which represent the developmental stage or phase of cell required for cell division. There are termed as G1, S, G2&M phase, in these phases the cell is preparing itself for the division like mitosis and meiosis. Below all the phases are described thoroughly.

Complete answer:
> Interphase-This is a phase between two successive M-phase. The interphase cell grows in size and prepares itself for the next division. The most active phase of the cell cycle is interphase. The interphase lasts more than ninety five percent of the duration of the cell cycle.
A series of metabolic changes occur during interphase in the cell. Some scientists termed interphase as a resting phase because in this phase change is not visible in the microscope. It is the time during which the cell is preparing for division by undergoing both cell growth and DNA replication in an orderly manner.
Howard and Pelc classified interphase into three sub-stages:

(I) G1-phase or Pre DNA synthesis phase (It Gap phase)
G1-phase corresponds to the interval between the initiation of DNA replication. The cell is metabolically active and continuously grows in G1-Stage.
During G1-most of cell organelles increase in cell and cell rapidly synthesizes different types of RNA and proteins. Synthesis of new protoplasm takes place in the cell and it starts growing in size due to availability of protein. The cell grows a maximum of only G1-stage.
(II) S-phase (DNA synthesis phase)
Replication of nuclear DNA and synthesis of histone protein takes place in s-phase. Replication of cytoplasmic DNA may occur in any stage of the cell cycle. During this time the amount of DNA per cell doubles generally. If the initial amount of DNA is denoted as 2C then it increases to 4C, But there is no increase in the chromosome number: if the cell had diploid or 2n number of chromosomes at G, also after phase the number of chromosomes remains the same. i.e. 2n.
S-phase marks the phase of DNA replication and chromosome duplication (DNA content in a.chromosome become double).
In animal cells, during the S phase. DNA replication begins in the nucleus. and the centriole duplicates in the cytoplasm.
G-phase (2 Gap phase) or Post DNA synthesis phase (Pre mitosis phase)-
Actual preparation (Final preparation) of M-phase occurs during this phase. Special materials required for M-phase are synthesized in G1-phase. eg. Tubulin protein-Required for the formation of(spindle fibres). Cell growth continues.

So B is the correct option.

Note: We remember that the replication of DNA occurs at any stage or phase of cell cycle.but the common stage means at s-phases the replication or duplication of DNA occurred, it is the general phase for replication of all cell, it is named as synthesis phase. So s-phase is the best option for DNA duplication.