H Week 14: Meiosis

Meiosis

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Lecture Goals:

• Explain the differences between somatic and gametic cells
• Understand the steps of meiosis and its role in adaptation
• Explain the differences between haploid and diploid cells
• Be able to compare and contrast mitosis and meiosis

Course Competencies: 16. Compare and contrast the biological processes of binary fission, mitosis and meiosis.(IX)

Presentation Links

Cell CycleReview

Somatic & Gametic Cells

Meiosis

Types of Meiosis

Recall: Cell Cycle

Cell Growth, Replication, and Division

The cell cycle is the process of a cell growing, performing required metabolic functions, replicating its DNA, and then dividing.

• Interphase
• Cell is growing, replicating its DNA, and performing its function
• Mitotic phase
• Cell is actively dividing
• Mitosis is used by somatic cells (body cells)
• Different form of cell division called meiosis is used by gametic cells (reproductive cells)

Recall: Nucleic Acids

Homologous Chromosomes vs Sister Chromatids

Chromatid - tightly bundled DNA (bundles together right before a cell divides) Chromosome - chromatid that contains unique genetic information Homologous chromosomes - chromosomes inherited from each parent that contain the same genes but may contain different alleles

Chromatid

Chromatid

Recall: Mitosis

Somatic Cell Division

Mitosis is the steps that a somatic cell takes to divide.

• Five subphases

• Followed by cytokinesis
• Forms two daughter cells

Somatic & Gametic Cells

What's the difference?

Somatic cells are body cells.

• Diploid (two sets of chromosomes)

Gametic cells are reproductive cells.

• Haploid (one set of chromosomes)
• Carry DNA for reproduction
• Used in animal reproduction
• Other domains/kingdoms vary in how they reproduce

Gametic Cells

Somatic Cells

Sexual Reproduction in Kingdom Animalia

Fusion of Gametic Cells

Sexual reproduction is an adaptation that leads to genetic variation.

• Many different adaptations/strategies that lead to genetic variation
• We call these strategies life cycles
• Organisms in the kingdom Animalia produce gametic cells
• Once mating, two haploid gametic cells fuse to form a diploid cell (called a zygote)
• Gametic cells will result in non-identical offspring (typically)

Gametic Cells

Types of Gametic Cells (& Non-Gamete Alternative)

• "High cost" gametes
• Larger
• Contain nutrients to develop after fusing
• Typically called oocytes (eggs)
• "Low cost" gametes
• Small
• Typically mobile
• Typically called sperm
• Spores
• Alternative to gametic cells
• A reproductive cell capable of developing into a new individual without fusing with another reproductive cell

Discussion Post #1

Define the following terms:

• Haploid cell and diploid cell

• Somatic cell and gametic cell

• Zygote
• Oocytes and sperm
• Spores

Submit your answers to the discussion post!

Meiosis

Formation of Gametic Cells

• Similar process to mitosis
• Series of steps where one diploid parent cell divides to make four haploid daughter cells
• Two main phases:
• Meiosis I
• Meiosis II

Meiosis

Overview

Diploid parent cell

4 haploid daughter cells

Two rounds of cell division to produce haploid gametic cells

Meiosis I

Overview

Meiosis I is the first round of cell division.

• Separation of homologous chromosomes
• Sister chromatids remain attached
• Creates two haploid cells

Meiosis I

Prophase I

• Like during mitosis, DNA is already replicated and sister chromatids are already attached
• Homologous chromosomes join at the centromeres
• Synaptonemal complex forms
• Lines up genes and "glues" the two homologous chromosomes together
• Crossing over
• Due to the overlap and close proximty of the homologous chromosomes, non-sister chromatids can swap genes
• Source of genetic variation!

Meiosis I

Prophase I: Crossing Over

• Increases genetic variation
• Can lead to new allele combinations
• Increases the likelihood that some offspring will have traits that allow them to better survive in the environment
• Sister chromatids are no longer identical

Meiosis I

Prometaphase I and Metaphase I

• Prometaphase I
• Spindles attach to homologous chromosome pairs
• Metaphase I
• Homologous chromosomes line up at the center of the cell (metaphase plate)
• Each homologous pair is next to each other

Meiosis I

Anaphase I, Telophase I, and Cytokinesis

• Anaphase I
• Homologous chromosomes separate
• Sister chromatids remain attached
• Telophase I and cytokinesis
• Cell divides into two haploid cells

Meiosis II

Overview

Meiosis II is the second round of cell division.

• Separation of sister chromatids
• Four haploid cells are formed (each daughter cell from meiosis I becomes two cells)

Meiosis II

Prophase II, Prometaphase II, and Metaphase II

• Prophase II
• Nuclear envelope breaks down again
• Spindles form
• Prometaphase II
• Spindles attach to the sister chromatids
• Metaphase II
• Sister chromatids line up at the center of the cell (metaphase plate)

Meiosis II

Anaphase II, Telophase II, and Cytokinesis

• Anaphase II
• Sister chromatids separate
• Telophase II and cytokinesis
• Each cell divides to form a total of four haploid cells (each with a unique set of chromosomes)

Discussion Post #2-3

• Draw the subphases of meiosis. Label the following: the name of each subphase, homologous chromosomes, sister chromatids, spindles, and the nuclear envelope.

• Think about mitosis versus meiosis. How are they similar? How are they different?

Submit your answers to the discussion post!

Meiosis vs Mitosis

Differences and Similarities

2 rounds of cell division

4 haploid daughter cells (gametic cells)

Crossing over

One diploid parent cell

2 diploid daughter cells

1 round of cell division

Meiosis in Mammals

Formation of Different Gametic Cells

• Spermatogenesis - process of forming sperm

• Produces four gametic cells
• Oogenesis - process of forming eggs (oocytes)

• Produces one gametic cell and three polar bodies

Meiosis Errors

Non-Disjunction Errors

Non-disjunction occurs when homologous chromosomes or sister chromatids don't separate.

• Trisomy (extra copy of a chromosome)
• Results in different genetic disorders
• Example: Down Syndrome
• Monosomy (missing chromosome)
• Typically results in zygote death (exception is a single X chromosome - Turner syndrome)
• Not universally harmful across all organisms
• Many plants have adaptations that allow them to deal with non-disjunctions and even benefit from non-disjunctions!

Meiosis Errors

Crossing Over Errors

• Types of crossing over errors include...

• Inversion - gene is inverted
• Deletion/duplication - genes don't swap, but instead, they are either deleted or duplicated
• Translocation - gene is moved to a new chromosome
• Effects of the errors vary
• Big source of mutation
• Sometimes beneficial and sometimes harmful

Recommended Textbook Reading

OpenStax Biology 2e: Chapter 11

Not required, but highly recommended (it can be very helpful to see the material again but phrased a little differently!)