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Inheritance - Mitosis and Meiosis

Grade 12A LevelBiology

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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The Cell Cycle consists of Interphase (divided into G1G_1, SS, and G2G_2 phases) and the M-phase (Mitosis or Meiosis).

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Mitosis is nuclear division giving rise to two genetically identical daughter cells. It is essential for growth, tissue repair, and asexual reproduction.

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The stages of Mitosis are Prophase, Metaphase, Anaphase, and Telophase (PMATPMAT). During the SS phase, DNA replicates so each chromosome consists of two sister chromatids.

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Meiosis is a reduction division where the chromosome number is halved from diploid (2n2n) to haploid (nn), resulting in four genetically different daughter cells (gametes).

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Genetic variation in Meiosis is produced by 'Crossing Over' during Prophase I and 'Independent Assortment' of homologous chromosomes during Metaphase I.

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Homologous chromosomes are pairs of chromosomes that have the same genes at the same loci, but potentially different alleles.

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In humans, the diploid number is 2n=462n = 46 and the haploid number is n=23n = 23.

📐Formulae

2n→Mitosis2n2n \xrightarrow{\text{Mitosis}} 2n

2n→Meiosis In→Meiosis IIn2n \xrightarrow{\text{Meiosis I}} n \xrightarrow{\text{Meiosis II}} n

Number of possible chromosome combinations=2n\text{Number of possible chromosome combinations} = 2^n

DNA amount (c-value): 2c→S-phase4c→Mitosis2c\text{DNA amount (c-value): } 2c \xrightarrow{\text{S-phase}} 4c \xrightarrow{\text{Mitosis}} 2c

💡Examples

Problem 1:

A cell from an organism with a diploid number of 2n=122n = 12 undergoes meiosis. Calculate the number of chromosomes present in a cell at the end of Meiosis I and the total number of possible combinations of chromosomes in the gametes due to independent assortment.

Solution:

Chromosomes after Meiosis I: n=6n = 6. Combinations: 26=642^6 = 64.

Explanation:

Meiosis I is the reduction division where homologous pairs separate, halving the chromosome number from 1212 to 66. The formula for independent assortment is 2n2^n, where nn is the haploid number.

Problem 2:

Compare the mass of DNA in a nucleus during G1G_1 phase versus G2G_2 phase of the cell cycle.

Solution:

If G1=xG_1 = x, then G2=2xG_2 = 2x.

Explanation:

During the SS phase (Synthesis), which occurs between G1G_1 and G2G_2, DNA replication doubles the amount of DNA in the nucleus. However, the chromosome number (2n2n) remains the same as sister chromatids are still attached at the centromere.