Sexual Reproduction and Meiosis lecture 15

Lecture 15 Objectives

• Explain the sexual life cycle.
• Identify the structures of meiosis and summarize its events.
• Compare/contrast meiosis vs. mitosis.
• Outline how meiosis leads to gametic and zygotic diversity and importance of this diversity for evolution.

Clonal Reproduction

• Single-celled organisms reproduce by binary fission.
  • Amoeba, Paramecium, yeasts, and algae use binary fission.
• Multicellular organisms (plants and animals) use vegetative means.
  • Runners, bulbs, tubers, and rhizomes are examples of vegetative means.
  • Hydra, anemone, sponge, and starfish use vegetative means.
• Offspring are genetically identical to the parent in clonal reproduction.

Sexual Cycle

• The sexual cycle is found in almost all eukaryotes.
• Offspring are not genetically identical to their parents.
• Biological sex refers to gamete size, males produce small gametes
• Females produce large gametes
• In humans, gamete production usually correlates with chromosomal makeup:
  • Males have an X and Y chromosome
  • Females have two X chromosomes
• Gender is diverse, arising from the interaction of biology and societal factors.

Gamete Production

• Gametes produced by mitosis would lead to a doubling of chromosomes each generation.
  • 2n gametes would produce 4n embryos.
• A cell division process is required to halve the # of chromosomes in gametes to retain the diploid number in the zygote.
• Germline cells ensure each gamete receives one copy of each chromosome pair.

Meiosis

• Meiosis is cell division in sexually reproducing organisms.
• Consists of two rounds of cell division with one round of DNA replication.
• Results in four haploid, genetically distinct cells.

G2 of Interphase

• Nuclear envelope intact during G2
• The nucleolus is visible with one or more nucleoli.
• Two centrosomes have formed.
• Chromosomes are duplicated during the S phase but are not yet condensed.

Meiosis I

• Homologous pairs of chromosomes are separated.

Prophase I

• The nuclear envelope breaks down.
• Chromosomes condense, and the spindle forms.
• Crossing over occurs:
  • Exchange of DNA between nonsister chromatids which occurs at chiasmata.
• Each chromatid is a mix of DNA from each homologous chromosome.

Metaphase I

• Chromosomes attach to kinetochore microtubules at each centromere.
• Each pair is lined up independently
• Paired homologous chromosomes have moved to the metaphase plate.
• Chiasmata line up on the metaphase plate, not the centromeres as in mitosis.

Anaphase I

• Recombined homologous chromosomes separate (disjoin).
• Sister chromatids remain attached.
• The cell starts to elongate facilitated by nonkinetochore microtubules.
• Each duplicated chromosome moves to opposite ends of the cell.

Telophase I and Cytokinesis

• Duplicated chromosomes reach opposite poles.
• The spindle disappears, and the nuclear envelope reforms.
• In animal cells, cytokinesis forms a cleavage furrow, resulting in two haploid cells
• Happens as only half the genetic information is in each new cell.
• Crossing over generates genetically different cells/sister chromatids.

Meiosis II

• Sister chromatids are separated.

Prophase II

• The spindle forms as centrosomes duplicate and move to opposite poles.
• Kinetochore microtubules attach to each duplicated chromosome at the centromere via kinetochore proteins.
• Each duplicated chromosome is made of two chromatids attached at centromeres.

Metaphase II

• Duplicated chromosomes align at the metaphase plate.
• Centromeres lie on this metaphase plate.

Anaphase II

• Sister chromatids disjoin at the centromeres
• Each chromatid becomes an independent daughter chromosome.
• Daughter chromosomes move towards opposite poles as kinetochore microtubules shorten.
• Nonkinetochore microtubules lengthen, causing the cell to elongate.

Telophase II & Cytokinesis

• Two daughter nuclei with a nuclear envelope form in the cell.
• Four daughter cells are produced, each with an unduplicated set of chromosomes.
• The four daughter cells are genetically distinct from each other and from the parent cell.

Mitosis vs Meiosis

• MITOSIS
  • Chromosomes align independently
  • Absent chiasmata
  • Centromeres are on the metaphase plate
  • Chromatids disjoin
  • Daughter cells are genetically identical
  • Diploid to Diploid (2n -> 2n)
• MEIOSIS I
  • Homologous chromosomes synapse
  • Chiasmata are present
  • Chiasmata on metaphase plate
  • Chromosomes disjoin
  • Daughter cells are genetically different
  • Diploid to Haploid (2n -> n)
• MEIOSIS II
  • Chromosomes align independently
  • Absent chiasmata
  • Centromeres are on the metaphase plate
  • Chromatids disjoin
  • Daughter cells are genetically different
  • Haploid to Haploid (n -> n)

Sexual reproduction & genetic diversity

• Sexual reproduction produces genetic diversity through;
  • Independent assortment of chromosomes
  • Crossing over
  • Random fertilization of gametes
• Genetic diversity allows Selective responses to:
  • Spatially variable environmen
  • Changing environment
  • Sib-sib competition

Segregation & Independent Assortment

• Maternal & paternal sets of chromosomes
• Two arrangements of chromosomes are possible at metaphase I

Gamete Diversity & Chromosome No.

• Number of chromosome pairs determines number of possible gametes
  • 1 pair = 2 possible gametes
  • 2 pairs = 4 possible gametes
  • 3 pairs = 8 possible gametes
  • 4 pairs = 16 possible gametes
  • 5 pairs = 32 possible gametes
  • 23 pairs = 8,388,608 possible gametes
  • 'n' pairs = 2^n possible gametes

Lecture 15 Summary

• The sexual cycle combines genetic material from two parents alternating between meiosis and fertilization,
• Meiosis produces haploid gametes that join to make a diploid cell.
• In prophase I, homologous chromosomes undergo synapsis and crossing over.
• Homologous pairs align at the metaphase plate during metaphase I and are separated in anaphase I.
• After telophase I/cytokinesis, two genetically distinct haploid cells, with half the original chromosome number, are formed and chromosomes are still duplicated.
• In meiosis II, duplicated chromosomes align at the cell equator during metaphase II and sister chromatids are separated during anaphase II.
• Telophase II/cytokinesis yields four genetically distinct haploid cells, each a carrying single set of unduplicated chromosomes.
• Meiosis promotes genetic diversity by independent assortment of chromosomes, crossing over, and random fertilization
• This diversity is crucial for adapting to varying environments and changes and may equip individuals for survival/reproduction.