Chapter 13: Meiosis Notes

Meiosis

This chapter explores meiosis, its role in promoting genetic diversity, and its significance in sexual reproduction.

13.1 How Does Meiosis Occur?

• Meiosis is compared to mitosis (Chapter 12).
• Key processes: Independent assortment, crossing over, and fertilization.

Introduction to Meiosis

• Sexual reproduction involves the union of reproductive cells called gametes to form a new individual.
• Fertilization: The process of gamete union.
• Gametes: Sperm and eggs in animals.
• Meiosis: A nuclear division that halves the chromosome number.
• Gametes must have half the chromosome number to restore the full number upon fertilization.

Chromosome Characteristics

• Every organism has a characteristic number of chromosomes.
• Sex chromosomes: Determine the sex of an individual.
  • Females (in many animals): Two X chromosomes.
  • Males (in many animals): One X and one Y chromosome.
• Autosomes: Non-sex chromosomes.

Homologous Chromosomes

• Homologous chromosomes (homologs): Chromosomes of the same type.
• Homologous pairs: Contain the same genes in the same position along the chromosome.
• Homologs are not necessarily identical.

Genes and Alleles

• Gene: A section of DNA that influences one or more hereditary traits.
• Alleles: Different versions of a specific gene.
• Homologs may contain different alleles.

The Concept of Ploidy

• Karyotype: Identifies the number and types of chromosomes in a species.
• Diploid: Organisms with 2 homologs of each chromosome and 2 alleles of each gene.
• Haploid: Organisms with only 1 of each type of chromosome and 1 allele of each gene.
• Haploid number (n): The number of distinct types of chromosomes present.
  • Sex chromosomes count as a single type.
  • In humans, n = 23.
• Ploidy (n, 2n, 3n…): Indicates the number of complete chromosome sets a cell contains.
• Diploid (2n) cells have paternal and maternal chromosomes.
  • Humans are diploid: 2n = 46.
• Polyploid organisms: Have 3 or more versions of each type of chromosome (3n, 4n, etc.).

An Overview of Meiosis

• Before meiosis, each chromosome in the diploid (2n) parent cell is replicated.
• Each replicated chromosome has two identical sister chromatids.
• Sister chromatids remain attached along most of their length and are considered a single replicated chromosome.

Meiosis Consists of Two Cell Divisions

• Meiosis I: The two homologs of each chromosome pair separate into two daughter cells.
  • Each daughter cell has one set of chromosomes.
  • A diploid parent produces two haploid daughter cells.
• Meiosis II: The sister chromatids of each chromosome separate into two daughter cells.
  • Each of the two daughter cells from meiosis I divides.
  • The result is four haploid cells.

Meiosis I is a Reduction Division

• Meiosis I reduces the chromosome number.
• In most plants and animals: One diploid cell produces 4 haploid daughter cells.
• In animals: Daughter cells become eggs or sperm through gametogenesis.
• Fertilization: Results in a diploid cell called a zygote, restoring the full diploid complement of chromosomes.
• Each diploid individual receives a haploid chromosome set from the mother and the father.

Meiosis I Phases

Meiosis I is a continuous process with five distinct phases:

• Interphase (before meiosis starts): Uncondensed chromosomes replicate in the parent cell.
• Early Prophase I:
  • The nuclear envelope begins to break down.
  • Chromosomes condense.
  • The spindle apparatus begins to form.
  • Homologous pairs come together in a process called synapsis, forming bivalents.
  • Bivalent: A set of paired homologous replicated chromosomes.
• Late Prophase I:
  • Homologs within each bivalent attach to microtubules from opposite poles of the spindle apparatus.
  • Homologous pairs begin to separate but remain attached at chiasmata.
  • Chiasmata are attachment points between non-sister chromatids and are sites for crossing over.
  • Crossing over: Exchange between homologous non-sister chromatids at chiasma.
  • Swapping of chromosome segments to produce chromosomes with a combination of maternal and paternal alleles.
  • Steps of homolog pairing and crossing over:
    • Sister chromatids are initially joined along their entire length by cohesions.
    • Homologs pair by synapsis and are held together by proteins called the synaptonemal complex.
    • Breaks are made in the DNA, and crossing over occurs between corresponding segments of non-sister chromatids.
    • The synaptonemal complex disassembles, and homologs are held together only at chiasmata.
• Metaphase I:
  • Kinetochore microtubules move the pairs of homologous chromosomes (bivalents) to the metaphase plate.
  • One homolog on one side, one on the other side.
  • The alignment of the homologs is random and not influenced by other homologous pairs.
  • Note: May not be an equal division of maternal and paternal chromosomes.
• Anaphase I:
  • Sister chromatids of each chromosome remain together.
  • The paired homologs separate and migrate to opposite ends of the cell.
• Telophase I:
  • The homologs finish migrating to the poles of the cell.
  • The cell divides in the process of cytokinesis.

Meiosis I Recap

• Meiosis I results in daughter cells with only one chromosome of each homologous pair, making them haploid.
• Cells still contain replicated chromosomes.
• Necessary genes should be represented, but daughters may have different alleles (mom vs. dad).
• Have a random assortment of maternal and paternal chromosomes/genes due to crossing over and the random distribution of homologs.

Phases of Meiosis II

• No chromosome replication occurs between meiosis I and meiosis II.
• The main task is to separate the sister chromatids.
• Meiosis II is a continuous process with four distinct phases:
  • Prophase II:
    • The spindle apparatus forms in each daughter cell.
    • One spindle fiber attaches to the centromere of each sister chromatid.
  • Metaphase II:
    • Replicated chromosomes line up at the metaphase plate.
    • Each chromosome is attached to microtubules from both poles.
    • Different from metaphase I, where chromosomes were attached to only one pole.
  • Anaphase II:
    • Sister chromatids separate.
    • The resulting daughter chromosomes begin moving to opposite sides of the cell.
  • Telophase II:
    • Chromosomes arrive at opposite sides of the cell.
    • A nuclear envelope forms around each haploid set of chromosomes.
    • Each cell then undergoes cytokinesis.

Recap: Meiosis II

• Separates sister chromatids.
• Results in four haploid daughter cells, each with one of each type of chromosome.
• One diploid cell with replicated chromosomes gives rise to 4 haploid cells with unreplicated chromosomes.

Mitosis versus Meiosis

• The key difference: Homologs pair in meiosis but not in mitosis.
• Homologs pair in prophase I and separate during anaphase I, resulting in reduction division.
• Mitosis produces two diploid daughter cells genetically identical to the parent cells.
• Meiosis produces four haploid daughter cells genetically distinct from each other and the parent cell.

More about Meiosis

• Meiosis results in 4 gametes with a chromosome composition different from each other and different from the parent cell.
• Independent shuffling of maternal and paternal chromosomes.
• Crossing over during meiosis I.
• Fertilization also introduces variation as haploid sets of chromosomes combine to make a unique offspring.

Meiosis Promotes Genetic Variation

• The changes in chromosomes produced by meiosis and fertilization are significant.
• Asexual reproduction produces clones genetically identical to one another and the parent.
• Sexual reproduction produces offspring with unique chromosome complements.
• Only sexual reproduction shuffles the alleles of the parents into the offspring.

Role of Independent Assortment

• Independent assortment: Due to the random separation of homologs during meiosis I.
• Each daughter cell gets a random assortment of maternal and paternal chromosomes and genes.
• Results in various combinations of maternal and paternal chromosomes and leads to genetic recombination.
• Genetic recombination: Creation of new combinations of alleles different from the parents.
• Generates a great deal of genetic diversity.
• A diploid organism can produce 2^n combinations if n is the haploid number.
  • For humans, with 23 pairs of chromosomes: 2^{23} = 8.4 million.

The Role of Crossing Over

• Crossing over produces new combinations of alleles within a chromosome; combinations that did not exist in each parent.
• Genetic recombination from crossing over and independent assortment:
  • Increases the genetic variability of gametes produced by meiosis beyond that produced by random assortment of chromosomes.
  • Increases number of unique gametes to a practically limitless number.

How Does Fertilization Affect Genetic Variation?

• Another source of variation comes from the random union of gametes at fertilization.
• Each gamete is genetically unique and combines with another unique gamete.
• Even in self-fertilization, offspring are genetically different from the parent.
• Outcrossing is more common in sexually reproducing species. It increases genetic diversity of offspring even further.
• In humans, just from independent assortment:
  • 8.4 \, \text{million} \times 8.4 \, \text{million} = 7.06 \times 10^{13} genetically distinct offspring are possible from two parents!

What Happens When Things Go Wrong in Meiosis?

• Errors in meiosis are common.
• At least 1/4th to 1/3rd of conceptions are spontaneously terminated because of problems in meiosis.
• Example: Down syndrome: 1/691 live births in the USA.
  • Caused by an extra copy of chromosome 21, called trisomy 21.
• Down syndrome is characterized by cognitive impairment, a high risk for heart disease and leukemia, and a degenerative brain disorder (dementia).

How Do Mistakes Occur?

• If both homologs or both sister chromatids move to the same pole of the parent cell, the products of meiosis will be abnormal.
• Nondisjunction: This sort of meiotic error where chromosomes fail to separate properly.
• Nondisjunction results in gametes that contain an extra chromosome (n + 1) or lack one chromosome (n − 1).
• Fertilization of an n + 1 gamete leads to trisomy.
• Fertilization of an n − 1 gamete leads to monosomy.
• Aneuploidy: Cells with too many or too few chromosomes.

Why Do Mistakes Occur?

• Meiotic errors are a result of random errors.
• Maternal age is an important factor in the frequency of trisomy.
• Primary oocytes:
  • Enter meiosis I during female embryonic development.
  • Arrest in prophase I until sexual maturity.
  • Don’t complete meiosis until ovulation, years later.

Why Does Meiosis Exist?

• Sexual reproduction is relatively uncommon among organisms.
• Most organisms undergo asexual reproduction.
• Asexual reproduction is much more efficient; it can produce 2x as many offspring in the same amount of time.
• Why sexual reproduction?

The Purifying Selection Hypothesis

• In asexual reproduction, a damaged gene will be inherited by all of that individual’s offspring.
• Sexually reproducing individuals are likely to have some offspring that lack deleterious alleles present in the parent.
• Natural selection against deleterious alleles is called purifying selection.
• Over time, purifying selection should steadily reduce the numerical advantage of asexual reproduction.

The Changing-Environment Hypothesis

• Offspring that are genetically different from their parents are more likely to survive and produce offspring if the environment changes.
• Offspring that are genetically identical to their parents are less likely to survive and produce offspring if the environment changes.
• Studies support the changing-environment hypothesis.
• Sexual reproduction may be an adaptation that increases the fitness of individuals in certain environments.
• Even though FEWER offspring may be produced, more VIABLE offspring result.

Chapter 13 Concept Check Questions

• During anaphase I, homologous chromosomes separate and move to opposite poles.
• Crossing over contributes genetic variability between homologous chromosomes.
• Statement A is incorrect concerning homologous chromosomes. Homologous chromosomes have identical alleles before crossing over.
• Independent assortment occurs during anaphase I.
• Meiosis of one diploid cell results in the production of 4 haploid cells.
• Muscle cells contain 2 copies of chromosome 14.
• Egg or sperm cells contain 1 copy of chromosome 14.
• The two copies of chromosome 14 in muscle cells come one from each parent.
• If you have an allele called MHY7 which codes for a muscle contraction protein, on your chromosome 14, then at least one of your parents must have the MHY7 allele on their chromosome 14.