H Week 14: Meiosis

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

• 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

Lecture Goals:

Meiosis

Cell CycleReview

Meiosis

Types of Meiosis

Somatic & Gametic Cells

Presentation Links

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)

Cell Growth, Replication, and Division

Recall: Cell Cycle

Chromatid

Chromatid

Chromatid - tightly bundled DNA (bundles together right before a cell divides) Chromosome - chromatid that contains unique genetic information

Homologous Chromosomes vs Sister Chromatids

Recall: Nucleic Acids

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

• Five phases

• Followed by cytokinesis
• Forms two daughter cells

Somatic Cell Division

Recall: Mitosis (aka Karyokinesis)

• Draw the steps of mitosis. Try to draw it from memory. You will use your drawing when learning about meiosis so you can compare the two processes!

05:00

Check your notes

Gametic Cells

Somatic Cells

Somatic cells are body cells.

• Diploid (two sets of chromosomes)

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

What's the Difference?

Somatic & 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 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)

Fusion of Gametic Cells

Sexual Reproduction in Kingdom Animalia

• "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

Types of Gametic Cells (& Non-Gamete Alternative)

Gametic Cells

Submit your answers to the discussion post!

Define and explain:

• Haploid versus diploid cell
• Somatic versus gametic cell
• Sexual reproduction
• Zygote
• Oocytes, sperm, and spores

05:00

Discussion Post #1

• 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

Formation of Gametic Cells

Meiosis

4 haploid daughter cells

Diploid parent cell

Two rounds of cell division to produce haploid gametic cells

Overview

Meiosis

Meiosis I is the first round of cell division.

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

Overview

Meiosis 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!

Prophase I

Meiosis I

• 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

Prophase I: Crossing Over

Meiosis I

• Prometaphase I
• Mitotic 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

Prometaphase I and Metaphase I

Meiosis I

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

Anaphase I and Telophase I

Meiosis I

Consider: Why are the cells now considered haploid even though they still have the sister chromatids?

03:00

Check your notes

Meiosis II is the second round of cell division.

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

Overview

Meiosis II

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

Prophase II, Prometaphase II, and Metaphase II

Meiosis II

• Anaphase II
• Sister chromatids separate
• Telophase II and cytokinesis
• A total of four haploid cells form, each with a unique set of chromosomes

Anaphase II and Telophase II

Meiosis II

Submit your answers to the discussion post!

• Draw the steps of meiosis. Be sure to label each phase, homologous chromosomes, sister chromatids, mitotic spindles, and the nuclear envelope.

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

10:00

Discussion Post #2

Crossing over

1 round of cell division

2 rounds of cell division

2 diploid daughter cells

4 haploid daughter cells

One diploid parent cell

Differences and Similarities

Meiosis vs Mitosis

• Spermatogenesis - process of forming sperm

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

• Produces one gametic cell and three polar bodies

Formation of Different Gametic Cells

Meiosis in Mammals

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!

Non-Disjunction Errors

Meiosis 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

Crossing Over Errors

Meiosis Errors

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

OpenStax Biology 2e: Chapter 11

Recommended Textbook Reading