Meiosis Process and Phases Quiz

Meiosis

• Meiosis is a cell division process that produces gametes (sperm and egg)
• It reduces the chromosome number by half, creating haploid cells from diploid cells.
• Haploid cells contain one set of chromosomes (n), while diploid cells contain two sets (2n).
• In humans, the haploid number is 23 (n = 23), and diploid cells have 46 chromosomes (2n = 46).
• Meiosis involves two consecutive cell divisions (meiosis I and meiosis II)

Learning Objectives

• Describe the organization of a eukaryotic genome in the human karyotype
• Define and compare haploid and diploid cells
• Define and compare autosomes and sex chromosomes
• Define chromatin, sister chromatids, chromosome, and homologous chromosomes
• Describe meiosis and its role in gamete production
• Compare mitosis to meiosis, highlighting their distinct purposes
• Discuss the mechanisms contributing to genetic variation

Overview: Reproduction and Chromosomal Inheritance

• Living organisms reproduce to create offspring of the same kind.
• Offspring inherit chromosomes, which contain genes from their parents, leading to unique combinations.

Comparison of Asexual and Sexual Reproduction

• Asexual reproduction involves a single individual passing genes to its offspring without gamete fusion, resulting in clones (genetically identical individuals).
• Sexual reproduction involves two parents, contributing unique gene combinations to their offspring.

Role of Meiosis in Sexual Reproduction

• Sexual reproduction relies on fertilization, the union of gametes (sperm and egg).
• Gametes are produced through meiosis.
• The fertilized egg (zygote) contains one set of chromosomes from each parent.
• The zygote develops into an adult organism via mitosis.

Sets of Chromosomes in Human Cells

• Somatic cells (all cells except gametes) have 23 pairs of chromosomes (2n or diploid) = 46 chromosomes
• Gametes (sperm and egg) have 23 chromosomes (n or haploid)
• Autosomes are non-sex chromosomes (22 pairs in humans).
• Sex chromosomes determine sex (1 pair in humans).
• Females have XX and males XY

Homologous Chromosomes

• Homologous chromosomes (homologs) are chromosome pairs with the same length and shape, carrying genes controlling the same inherited characteristics.
• Each pair contains one chromosome inherited from each parent.

Homologous Chromosomes (Homologs)

• The chromosome pairs during meiosis I.
• They have the same genes but potentially different alleles of those genes.
• One chromosome in each pair comes from the father (paternal homolog), and the other comes from the mother (maternal homolog)

Homologous Chromosomes - Detailed

• In a pair of homologous chromosomes, one is inherited from the male parent and the other from the female parent
• Alleles may be identical (e.g., AA or aa) or different (e.g., Cc)
• A locus specifies the location of a particular gene on a chromosome
• An individual has two alleles at each gene locus, one on each homologous chromosome.

Human Karyotype

• Karyotype is an ordered display of the pairs of chromosomes from a cell.
• Human somatic cells have 23 pairs of chromosomes. (22 pairs of autosomes and 1 pair of sex chromosomes)
• Sister chromatids are identical copies of a chromosome.

Chromosome Sets in Human Cells

• Each homologous chromosome pair contains one chromosome from each parent.
• A human somatic cell has 46 chromosomes.
• Each of these chromosomes consists of two sister chromatids (after DNA replication).

Chromosome Sets in the Human Life Cycle

• At sexual maturity, the gonads (testes and ovaries) produce haploid gametes (sperm and eggs).
• Each gamete contains a single set of chromosomes (n). In humans, n=23
• Each set of 23 chromosomes contains 22 autosomes and 1 sex chromosome. In eggs, the sex chromosome is always X. In sperm, it is either X or Y.

The Variety of Sexual Life Cycles

• Three main types of sexual life cycles in eukaryotes depend on the timing between meiosis and fertilization.
• Animal sexual life cycles—Gametes are the only haploid cells. They do not undergo further cell division before fertilization. Gametes fuse to form a diploid zygote which divides by mitosis to create a multicellular organism.
• In other organisms, both haploid and diploid cells divide by mitosis.

Meiosis reduces the number of chromosome sets from diploid to haploid

• Meiosis is preceded by the replication of chromosomes (DNA replication during S phase).
• Meiosis takes place in two sets of cell divisions called Meiosis I and Meiosis II.
• The two cell divisions result in four daughter cells. Each daughter cell has half as many chromosomes as the parent cell. (haploid)

The Stages of Meiosis

• Meiosis I and meiosis II are the two divisions of meiosis. Division in meiosis occurs in four phases.
• Prophase I, Metaphase I, Anaphase I, and Telophase I and Cytokinesis in meiosis I.
• Meiosis II follows with prophase II, metaphase II, anaphase II, and telophase II and cytokinesis
• These divisions result in the formation of four haploid daughter cells from the initial diploid cell.

Meiosis I: Separation of Homologous Chromosomes

• Homologous chromosomes pair up (synapsis).
• Nonsister chromatids exchange segments (crossing over)
• Homologous chromosomes separate.

Prophase I

• 90% of meiotic time is spent in prophase I.
• Chromosomes condense.
• Homologous chromosomes pair up (synapsis). Each pair of homologous chromosomes forms a tetrad (group of four chromatids).
• Chiasmata (X-shaped regions) form where crossing over occurs (exchange of DNA between non-sister chromatids)

Metaphase I

• Tetrads align at the metaphase plate.
• Microtubules attach to the kinetochores of each chromosome.

Anaphase I

• Homologous chromosomes separate.
• Sister chromatids remain attached.

Telophase I and Cytokinesis

• Each half of the cell has a haploid set of chromosomes that still contain two sister chromatids.
• Cytokinesis occurs and the cell divides into two haploid daughter cells.

Meiosis II: Separation of Sister Chromatids

• No chromosome replication occurs between meiosis I and meiosis II.
• Meiosis II is similar to mitosis.
• Sister chromatids separate.
• Results in four haploid daughter cells.

Prophase II

• Spindle apparatus forms.
• Chromosomes composed of two sister chromatids move to the metaphase plate.

Metaphase II

• Sister chromatids are arranged at the metaphase plate
• Kinetochores attach to microtubules extending from opposite poles.

Anaphase II

• Sister chromatids separate at the centromeres.
• Individual sister chromatids (now called chromosomes) move to opposite poles.

Telophase II and Cytokinesis

• Nuclei reform and the chromosomes decondense.
• Cytokinesis occurs, producing four haploid daughter cells, each with a single set of unreplicated chromosomes (1 chromatid per chromosome).

Chromosomal and DNA content during meiosis I and II

• Meiosis I results in two haploid daughter cells with replicated chromosomes (2 chromatids per chromosome). The cells are haploid in terms of chromosome number, but diploid in terms of DNA content.
• Meiosis II results in four haploid daughter cells with unreplicated chromosomes (1 chromatid per chromosome). The cells are haploid in terms of both chromosome number and DNA (chromatid) content.

Comparison of Mitosis and Meiosis

• Mitosis results in two genetically identical diploid cells from one diploid parent cell.
• Meiosis results in four genetically different haploid cells from one diploid parent cell.
• Crossing over, independent assortment, and random fertilization contribute to genetic variation in meiosis.

Origins of Genetic Variation Among Offspring

• Sexual reproduction contributes to evolution through genetic variation in offspring. Four sources create genetic variation:

1. Genetic Recombination (Crossing Over)

• Crossing over during prophase I generates recombinant chromosomes that combine DNA from each parent
• In crossing over, homologous portions of non-sister chromatids exchange places.

2. Independent Assortment of Chromosomes

• Homologous chromosome pairs orient randomly at metaphase I of meiosis.
• The number of combinations possible when chromosomes assort independently to form gametes is 2²³.

3. Random Fertilization

• Any sperm can fuse with any ovum, creating a vast array of possible combinations in the zygote.

4. Mutations

• Mutations introduce new alleles, creating the raw material for genetic diversity.
• Mutations are changes in the organism’s DNA.

The Evolutionary Significance of Genetic Variation Within Populations

• Sexual reproduction contributes to genetic variation in a population arising from mutations.
• Natural selection favors genetic variations that are environmentally beneficial.

Summary

• Meiosis is involved in the separation of homologous chromosomes during meiosis I, and the separation of sister chromatids during meiosis II.
• Four mechanisms contribute to genetic variation:
  • Genetic recombination (crossing over).
  • Independent assortment of chromosomes.
  • Random fertilization.
  • Mutations.