Meiosis and Mitosis: Chromosome Behavior, Genetic Variation, and Cell Division

Meiosis and Mitosis: An Overview

Learning Objectives

• Compare and contrast meiosis and mitosis.

• Explain the difference between diploid and haploid cells, and between somatic and germ line cells.

• Relate the concept of ploidy to chromosome numbers in specific organisms.

• Define allele, and explain how many different alleles for the same characteristic can be present in a population and in a diploid individual.

• Draw and differentiate between cells in metaphase of mitosis, metaphase I of meiosis, and metaphase II of meiosis.

• Explain the events of anaphase in mitosis, meiosis I, and meiosis II. Describe nondisjunction, and draw the outcome of various nondisjunction events.

• Describe crossing over.

• Explain independent assortment.

• Explain why self-fertilization does not result in offspring that are genetically identical to the parent or to each other.

Chromosome Structure and Vocabulary

Homologous Chromosomes

Homologous chromosomes are pairs of chromosomes that carry information for the same traits, but may have different alleles. They are not necessarily identical, but they align together during meiosis.

• Allele: A variant form of a gene at a particular locus.

• Sister chromatids: Identical copies of a chromosome connected by a centromere.

• Centromere: The region where sister chromatids are joined.

• Kinetochore: Protein structure on chromatids where spindle fibers attach during cell division.

Ploidy and Chromosome Number

Diploid vs. Haploid

Ploidy refers to the number of sets of chromosomes in a cell. Diploid (2n) cells have two sets, while haploid (n) cells have one set.

• Diploid (2n): Contains two sets of chromosomes, one from each parent.

• Haploid (n): Contains one set of chromosomes, typical of gametes.

For a species with eight distinct types of chromosomes, a haploid cell contains 8 chromosomes (n = 8).

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

Cell Division: Mitosis vs. Meiosis

Comparison Table

	Mitosis	Meiosis
# cell divisions	1	2
# chromosomes in daughter cell compared to parent cell	Same (2n → 2n)	Half (2n → n)
Cell type in which process occurs	Somatic cells	Germ cells
Makeup of chromosomes in daughter cells	Genetically identical	Genetically distinct

Key Differences

• Mitosis: Produces two genetically identical diploid daughter cells for growth and repair.

• Meiosis: Produces four genetically distinct haploid gametes for sexual reproduction.

Phases of Meiosis

Meiosis I

• Prophase I: Homologous chromosomes pair and exchange genetic material (crossing over).

• Metaphase I: Homologous pairs align at the metaphase plate.

• Anaphase I: Homologous chromosomes separate to opposite poles.

• Telophase I and Cytokinesis: Two haploid cells form.

Meiosis II

• Prophase II: Chromosomes condense in each haploid cell.

• Metaphase II: Chromosomes align at the metaphase plate.

• Anaphase II: Sister chromatids separate.

• Telophase II and Cytokinesis: Four haploid daughter cells result.

DNA Replication in Meiosis

DNA replication occurs once, before meiosis I begins, during the S phase of the cell cycle.

Genetic Variation in Meiosis

Crossing Over

During prophase I, homologous chromosomes exchange genetic material at chiasmata, resulting in new combinations of alleles.

• Bivalent: Structure formed by two homologous chromosomes (four chromatids).

• Chiasma: Site of crossing over.

Independent Assortment

During metaphase I, homologous chromosome pairs align randomly, leading to genetic variation in gametes.

Genetic Variation Table

Source of Variation	Description
Crossing Over	Exchange of genetic material between non-sister chromatids
Independent Assortment	Random alignment of homologous pairs
Random Fertilization	Random combination of gametes during fertilization

Nondisjunction and Aneuploidy

Nondisjunction

Nondisjunction is the failure of homologous chromosomes or sister chromatids to separate properly during meiosis, resulting in gametes with abnormal chromosome numbers.

• Aneuploidy: Condition of having too many or too few chromosomes in a cell.

Evolutionary Advantages of Sexual and Asexual Reproduction

Sexual Reproduction

• Produces genetically diverse offspring, increasing adaptability to changing environments.

• Genetic recombination through crossing over and independent assortment.

Asexual Reproduction

• Produces genetically identical offspring, allowing rapid population growth in stable environments.

• Requires only one parent.

Key Equations and Concepts

• Ploidy Equation: $2n \xrightarrow{\text{meiosis}} n$

• Chromosome Number in Gametes: $n = \text{number of distinct chromosome types}$

Summary Table: Mitosis vs. Meiosis

Feature	Mitosis	Meiosis
Number of Divisions	1	2
Number of Daughter Cells	2	4
Chromosome Number in Daughter Cells	Same as parent (2n)	Half of parent (n)
Genetic Identity	Identical	Distinct
Function	Growth, repair	Gamete production

Additional info:

• Self-fertilization does not produce offspring genetically identical to the parent due to genetic recombination during meiosis.

• Errors in chromosome separation (nondisjunction) can lead to genetic disorders such as Down syndrome (trisomy 21).