Meiosis Notes

Meiosis

Introduction to Meiosis

• During sexual reproduction, reproductive cells called gametes unite to form a new individual through fertilization.
• In animals, gametes are sperm and eggs.
• Meiosis is nuclear division that reduces the chromosome number by half.
• Gametes must contain half the chromosome number so that:
• At fertilization, the full chromosome number is restored.

Chromosome Characteristics

• Every organism has a characteristic number of chromosomes.
• Sex chromosomes determine the sex of the individual.
• In many animals:
  • Females have two X chromosomes.
  • Males have an X and a Y chromosome.
• Autosomes are non-sex chromosomes.

Homologous Chromosomes

• Chromosomes of the same type are called homologous chromosomes, or homologs.
• Homologous pairs contain the same genes in the same position along the chromosome.
• The two homologs are not necessarily identical.

Genes and Alleles

• A gene is a section of DNA that influences one or more hereditary traits.
• Different versions of a specific gene are called alleles.
• Homologs may contain different alleles.

The Concept of Ploidy

• Karyotype identifies the number and types of chromosomes in a species
• Many organisms are diploid, meaning they:
  • Have 2 homologs of each chromosome.
  • Have 2 alleles of each gene.
• Some organisms are haploid, meaning they:
  • Have only 1 of each type of chromosome.
  • Have just 1 allele of each gene.
• Each species has a haploid number (n), which indicates the number of distinct types of chromosomes present (i.e. #1, #2, #3…).
  • 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 chromosomes.
  • Maternal chromosomes.
  • Humans are diploid, where (2n = 46).
• Polyploid organisms have 3 or more versions of each type of chromosome (3n, 4n, etc.).

Terms for Describing Chromosomes

• Chromosome: Structure made up of DNA and proteins, carries the cell's hereditary information (genes)
• Sex chromosome: Chromosome associated with an individual's sex
  • X and Y chromosomes of humans (males are XY, females XX); Z and W chromosomes of birds and butterflies (males are ZZ, females ZW)
• Autosome: Any chromosome other than a sex chromosome
  • Chromosomes 1-22 in humans
• Unreplicated chromosome: A chromosome that consists of one double-helical molecule of DNA packaged with proteins for compactness.
• Replicated chromosome: A chromosome after DNA replication. Consists of two identical chromatids, each containing one double-helical DNA molecule packaged with proteins for compactness
• Sister chromatids: The two identical chromatid copies in a replicated chromosome
• Homologous chromosomes (homologs): Chromosomes that have the same genes in the same position and are the same size and shape. (Because the alleles of particular genes are often different between the homologs, homologs are not called identical chromosomes.)
• Non-sister chromatids: Chromatids on different members of a homologous chromosome pair. (To be non-sister chromatids, one of the chromatids is on one homolog and the other chromatid is on the other homolog.)
• Bivalent: Homologous replicated chromosomes that are joined together during prophase I and metaphase I of meiosis
• Haploid number: The number of different types of chromosomes in a cell; symbolized n
  • Humans have 23 different types of chromosomes (n = 23)
• Diploid number: The number of chromosomes present in a diploid cell; symbolized 2n
  • All human cells except gametes are diploid and contain 46 chromosomes (2n = 46)
• Ploidy: The number of each type of chromosome present; shown by the number in front of n (for example, 2n)
• Haploid: Having one of each type of chromosome (n)
  • Bacteria and archaea are haploid, as are many algae; most plant and animal gametes are haploid
• Diploid: Having two of each type of chromosome (2n)
  • Most familiar plants and animals are diploid
• Polyploid: Having more than two of each type of chromosome; may be triploid (3n), tetraploid (4n), hexaploid (6n), and so on
  • Seedless bananas are triploid; many ferns are tetraploid; bread wheat is hexaploid

An Overview of Meiosis

• Just before meiosis begins, each chromosome in the diploid (2n) parent cell is replicated. When replication is complete, each chromosome has two identical sister chromatids. Sister chromatids remain attached along most of their length and are still 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: Reduction Division

• Meiosis I reduces the chromosome number.
• In most plants and animals:
  • 1 diploid cell produces 4 haploid daughter cells.
• In animals:
  • Daughter cells become eggs or sperm by the process of gametogenesis.

Fertilization

• Results in a diploid cell called a zygote.
• Restores 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)
  • Early prophase I
  • Late prophase I
  • Metaphase I
  • Anaphase I
  • Telophase I

Interphase

• Uncondensed chromosomes replicate in parent cell.

Early Prophase I

• Nuclear envelope begins to break down.
• Chromosomes condense.
• Spindle apparatus begins to form.
• Homologous pairs come together in a process called synapsis.
• Form bivalents.
• Bivalent: set of paired homologous replicated chromosomes.

Late Prophase I

• Homologs within each bivalent become attached to microtubules from opposite poles of the spindle apparatus.
• Homologous pairs begin to separate, but remain attached at many points, called chiasmata.
• Attachment points are between non-sister chromatids, which are sites for crossing over.
• Exchange or crossing over between homologous non-sister chromatids occurs at chiasma (plural: chiasmata), which is the swapping of chromosome segments to produce chromosomes with a combination of maternal and paternal alleles.

Steps of homolog pairing and crossing over:

• As chromosomes condense, 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 is on one side, one on the other side.
• Alignment of the homologs is random and not influenced by other homologous pairs.
• Note: there may not be 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.
• Then the cell divides in the process of cytokinesis.

Meiosis I: A Recap

• Meiosis I results in daughter cells with only one chromosome of each homologous pair:
  • They are haploid (one copy of each type of chromosome) but still contain replicated chromosomes.
  • Note: all necessary genes should be represented, but different daughters may have different alleles (mom vs. dad).
  • Have a random assortment of maternal and paternal chromosomes/genes due to:
    • Crossing over
    • Random distribution of maternal and paternal homologs to daughter cells

Meiosis II

• No chromosome replication occurs between meiosis I and meiosis II.
• Main task is to separate the sister chromatids.
• Continuous process, with four distinct phases:
  • Prophase II
  • Metaphase II
  • Anaphase II
  • Telophase II

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.
• Note:
  • 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 has one of each type of chromosome.
• So ultimately…
  • 1 diploid cell with replicated chromosomes gives rise to 4 haploid cells with unreplicated chromosomes.

Mitosis versus Meiosis

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

Key Differences between Mitosis and Meiosis

More about Meiosis

• Meiosis results in 4 gametes with a chromosome composition different from each other and different from the parent cells due to:
  • 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 that are genetically identical to one another and to the parent.
• Sexual reproduction produces offspring with unique chromosome complements.
• Only sexual reproduction results in a shuffling of the alleles of the parents into the offspring.

Role of Independent Assortment

• Independent assortment:
  • Because of the random separation of homologs during meiosis I…
  • Each daughter cell gets a random assortment of maternal and paternal chromosomes and genes.
• Results in a variety of combinations of maternal and paternal chromosomes.
• Leads to genetic recombination, or the creation of new combinations of alleles that are different from the parents.
• Generates a great deal of genetic diversity.
• A diploid organism can produce 2^n combinations if n is the haploid #. For example, 23 pairs:
  • 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 from 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 important 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, where gametes from the same individual combine, offspring will be genetically different from the parent.
• Outcrossing is more common in species that reproduce sexually:
  • Gametes from two individuals combine.
  • Different individuals are likely to have different alleles.
  • Increases genetic diversity of offspring even further.
• In humans, just from independent assortment that’s…
  • 8.4 \, million \times 8.4 \, million = 7.06 \times 10^{13} genetically distinct offspring 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, high risk for heart disease and leukemia, degenerative brain disorder (dementia).

How Do Mistakes Occur?

• If both homologs of both sister chromatids move to the same pole of the parent cell, the products of meiosis will be abnormal.
• This sort of meiotic error is referred to as nondisjunction.
• 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, and can produce 2x as many offspring in the same amount of time.

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.
  • So even though FEWER offspring may be produced, there are more VIABLE offspring that result.

Concept Check Questions

1. During anaphase I, homologous chromosomes separate and move to opposite poles.
2. Crossing over contributes genetic variability between homologous chromosomes.
3. Homologous chromosomes have identical alleles before crossing over - this statement is incorrect.
4. Independent assortment occurs during anaphase I.
5. Meiosis of one diploid cell results in the production of 4 haploid cells.
6. Muscle cells contain 2 copies of chromosome 14.
7. Egg or sperm cells contain 1 copy of chromosome 14.
8. Two copies of chromosome 14 in muscle cells come one from the mother and one from the father.
9. If there is an allele called MHY7 on chromosome 14, then at least one of the parents must have the MHY7 allele on their chromosome 14.