Sexual Reproduction: Meiosis

Human Life Cycle

• 2 types of Nuclear Division

1. Mitosis

   • occurs during growth & repair, ensures every body cell has 23 pairs of chromosomes

2. Meiosis

   • involved in formation of gametes → spermatogenesis (males) & oogensis (females)

   • ensures gametes are haploid + have 23 chromosomes, 1 from each pairs of chromosomes

importance of meiosis

• reduce chromosomes number

• shuffle chromosomes and genes to produce genetically different gametes → sperm and eggs

Homologous chromosomes

• same size with the same centromere position → contain same genes

Diploid cells in humans have:

• 22 homologous pairs of autosomes

• 1 pair of sex chromosomes

• = 46 chromosomes

• Males → XY

• Females → XX

Homologous chromosomes (not identical)

• two homologs differ in combination of alleles or versions of the genes they carry

• each allele of a gene encodes a different version of the same protein

Meiosis Overview

1. Meiosis I → homologous chromosomes pair + separate

   • homologous pairs of chromosomes align during prophase I, line up at cell’s center during metaphase I, split apart during anaphase I, chromosomes arrive at poles in telophase I, cell divides (cytokinesis)

2. Meiosis II → sister chromatids of each duplicated chromosome separate

   • two products of meiosis I each divide → 4 haploid nuclei, then phases occur, then cytokinesis

3. Completion of Meiosis → 4 haploid daughter cells

Synapsis and Crossing Over

• In Meiosis I, homologous chromosomes of each pair (1 father & 1 mother) come together and line up side by side → synapsis

  • results in a tetrad, an association of four chromatids → 2 homologous chromosomes w/ 2 chromatids each

• when a tetrad forms during synapsis, non-sister chromatids may exchange genetic material → crossing-over

  • shuffles the alleles + serves as the way meiosis brings about genetic recombination

Genetic Recombination

• chiasma indicate where crossing-over has occured

• exchange of color = exchange of genetic material

• when chromatids separate during Meiosis II, some daughter cells receive daughter chromosomes w/ recombined alleles

Independent assortment of homologous chromosomes

• homologous chromosome pairs separate independently or randomly

• when homologues align at the metaphase plate, maternal or paternal homologue may be oriented toward either pole

Phases of Meiosis I

1. Prophase I

   • tetrads form, crossing-over occurs as chromosomes condense; the nuclear envelope fragments

   • pairing of homologous chromosomes; crossing-over

2. Metaphase I

   • tetrads align at spindle equator

3. Anaphase I

   • homologues separate, dyads move to poles

4. Telophase

   • daughter nuclei are haploid, having received one duplicated chromosome from each homologous pair

5. Interkinesis

   • short rest period prior to 2nd nuclear division → Meiosis II

   • similar to interphase between mitotic divisions except that DNA replication does not occur (chromosomes are already duplicated)

Phases of Meiosis II

1. Prophase II

   • chromosomes condense, nuclear envelope fragments

   • no pairing of chromosomes

2. Metaphase II

   • haploid no. of dyads at spindle equator

3. Anaphase II

   • sister chromatids separate, becoming daughter chromosomes that move to the poles

4. Telophase II

   • Four haploid cells are genetically different from each other and from parent cell

Changes in chromosomes number

• nondisjunction

• down-syndrome

• extra or missing sex-chromosomes: XXX, XXY, XYY and XO

Nondisjunction

• accounts for the inheritance of an abnormal number of chromosomes.

• a homologous pair of chromosomes fails to separate during meiosis I

Down syndrome

• Autosomal syndrome: inherits THREE copies of chromosome 21

Extra or missing chromosomes: xxx, xxy, xyy, and xo

• Nondisjunction can produce a gamete that contains two X or Y chromosomes instead of only one. Fertilization then produces a zygote with too many sex chromosomes: XXX, XXY, XYY

Sex chromosome abnormalities

• Triplo X (XXX)

• Klinefelter (XXY)

• Jacobs Syndrome (XYY)

• Turner Syndrom (XO)

Triplo X (XXX)

• females have 3 x chromosomes

• increased learning disabilities, delayed speech, weak muscle tone

Klinefelter (XXY)

• men who have an extra X chromosomes

• affects different stages of physical, language and social development

Jacobs Syndrome

• extra copy of the Y chromosome in each of male’s cells

• no unusual physical features

Turner Syndrome (XO)

• genetic disorder affecting girl’s development

• missing/incomplete X chromosome

• ovaries dont work properly, short in height