Introduction:
The number of chromosomes present in a pollen grain is determined by the number of chromosomes in the pollen mother cell (PMC). In this case, if the PMC has 20 chromosomes in the G2 phase, we need to understand the process of chromosome duplication and segregation during cell division to determine the number of chromosomes in each pollen grain.

Explanation:
The process of cell division in the PMC is known as meiosis, which consists of two rounds of division, called meiosis I and meiosis II. Meiosis I is preceded by DNA replication in the S phase of interphase, which results in the duplication of chromosomes.

Meiosis I:
1. Prophase I: During this phase, the replicated chromosomes condense and pair up with their homologous chromosome to form bivalents.
2. Metaphase I: The bivalents line up along the equator of the cell.
3. Anaphase I: The homologous chromosomes separate and move towards opposite poles of the cell.
4. Telophase I: The chromosomes reach the poles and the cell undergoes cytokinesis, resulting in the formation of two daughter cells, each with half the number of chromosomes as the parent cell.

Meiosis II:
5. Prophase II: The chromosomes condense again.
6. Metaphase II: The chromosomes line up along the equator of the cell.
7. Anaphase II: The sister chromatids separate and move towards opposite poles of the cell.
8. Telophase II: The chromosomes reach the poles and the cell undergoes cytokinesis again, resulting in the formation of four haploid daughter cells.

Number of chromosomes in each pollen grain:
Since the PMC had 20 chromosomes in the G2 phase, after meiosis I, the daughter cells will have 10 chromosomes each. These cells then undergo meiosis II, resulting in the formation of four haploid daughter cells, each with 10 chromosomes.

Therefore, each pollen grain will contain 10 chromosomes.

Summary:
In summary, if the pollen mother cell has 20 chromosomes in the G2 phase, after undergoing meiosis I and II, each pollen grain will contain 10 chromosomes. Meiosis is a crucial process in the formation of gametes, ensuring the correct distribution of genetic material and maintaining the correct chromosome number in the daughter cells.