Ch. 13 Biology Meiosis and Sexual Life Cycles

Concept: GENES & ALLELES

• Genes: Small segments of DNA encoding proteins that could lead to expression of a trait (ex. gene for eye color)

  • Alleles: alternative versions of a same specific gene (ex. gene for blue eyes vs. gene for brown eyes)

  • Alleles are typically represented using capital/lower-case letters(ex. B=Blue eyes, b ==brown eyes).

Haploid Vs Diploid Cells

• Cell Ploidy: the number of copies/sets of specific genes/chromosomes found in a cell.

1. Haploid cells (n): contain one set of chromosomes(copy), typically seen in gametes (sperm and eggs).

2. Diploid cells (2n): contain two sets of chromosomes(copies), one inherited from each parent, found in somatic cells (body cells).

Concept: HOMOLOGOUS CHROMOSOMES

• Karyotype: an ordered display of all chromosomes in a cell.

• Human somatic cells contain 23 pairs of chromosomes (for a total of 46).

  • 22 pairs of autosomes: non-sex chromosomes that are found in both males & females.

  • 1 pair (X & Y chromosomes) are sex chromosomes that determines the sex of the organism.

  • Female: XX Male: XY

• Homologous Chromosomes: pairs similar in size/shape & carry the same genes but can have different alleles.

  • Within each homologous chromosome pair, one is paternally inherited & the other is maternally inherited.

Concept: LIFE CYCLE OF SEXUAL REPRODUCERS

• The life cycle for sexual reproducers includes both mitosis & meiosis.

• Meiosis: form of eukaryotic cell division that produces haploid(n) gametes (sperm or egg cells).

  • Fertilization: process of merging gametes (sperm & egg fusion) to create a zygote.

  • Zygote: the very first diploid (2n) cell of a new developing organism where Mitosis starts.

Concept: INTRODUCTION TO MEIOSIS

• Before meiosis, a diploid cell must replicate its DNA & make proteins for cell division in interphase.

• Meiosis starts with a diploid germ cell & ends with 4 genetically diverse haploid gametes.

  • Germ Cells: Diploid cells that are the precursor for making gametes(sperm or egg).

• Meiosis is broken down into 2 rounds of cell division:

• Meiosis 1 (Reductional Division): Reduces ploidy by separating homologous chromosomes.

  • Diploid (2n) germ cell divides into two haploid (n) daughter cells.

• Meiosis 2 (Equational Division): Maintains equal ploidy by separating sister chromatids.

  • Haploid (n) cells from meiosis 1 divide producing 4 genetically diverse haploid (n) gametes.

Mitosis is equational division.

Concept: MEIOSIS 1

• Meiosis 1 has similar steps to Mitosis, but differs significantly in metaphase 1 & anaphase 1:

  • In Metaphase 1, homologous chromosomes are paired & aligned in 2 rows in the middle of the cell.

  • In Anaphase 1, homologous chromosomes are separated while sister chromatids remain connected.

  • After Telophase 1, cytokinesis produces 2 haploid (n) daughter cells that can then begin Meiosis 2.

Concept: MEIOSIS 2

• In Meiosis 2, each haploid cell produced in Meiosis 1 divides, forming 4 genetically diverse, haploid gametes.

• In terms of the events that occur in each phase, Meiosis 2 is almost exactly the same as mitosis.

  • Similar to mitosis, chromosomes align in one row in metaphase 2.

  • Also, similar to Mitosis, sister chromatids are divided in Anaphase 2.

Concept: GENETIC VARIATION DURING MEIOSIS

• Meiosis creates genetic diversity via 2 events:

• Crossing Over: Process in which pairs of homologous chromosomes

  • Forms non-identical-sister chromatids during prophase 1 of meiosis 1.

  • Synapsis: process where homologous pairs align their DNA sequence at similar alleles.

  • Chiasma: sites of crossing over (attachment sites) between homologous chromosomes.

Independent Assortment

• During metaphase 1, pairs of homologous chromosomes are independently & randomly aligned.

  • Results in an enormous amount of possible genetic combination during meiosis.

  • To calculate # of combinations, use the equation 2ⁿ (n = haploid # of chromosomes).