Fertilization and Meiosis (Lec 4)

Question 1

Gametes

Answer 1

Male: Spermatozoa produced in testes
Female: Ovum produved in ovaries

Question 2

Preparation for Meiosis

Answer 2

Replication of DNA in ‘S phase’ of Interphase

Question 3

Meiosis

Answer 3

Eukaryotic cell division that produces Haploid gametes and reshuffles genetic material. Introduces variation by:

• Crossing Over
• Independant Assortment

Question 4

Meiosis I: Reductional Division

Answer 4

Prophase I: condensed homologous pairs undergo synapsis, nuclear membrane dissolves, spindles form

Metaphase I: chromosomes align at equator, spindle fibres attach to centromeres

Anaphase I: chromosomes split, drawn to opposite poles

Telophase I: spindle disappears, nuclear membrane forms

Cytokinesis: cytoplasm separates, 2 separate cells formed

Question 5

Meiosis II: Equational Division

Answer 5

Prophase II: nuclear membrane dissolves, spindles form

Metaphase II: chromosomes align at the equator

Anaphase II: sister chromatids separate and are drawn to opposite poles by spindle fibres

Telophase II: spindle disappears, nuclear membrane forms

Cytokinesis: cytoplasm separates, 2 separate cells formed

Question 6

Introduction of Variation: Crossing Over

Answer 6

Occurs during Prophase I only. Crossing over of genetic information between 2 non-sister chromatids, several chiasmata depending on chromosome length

Question 7

Introduction of Variation: Independent Assortment

Answer 7

Distribution into daughter cells is random.

• Anaphase I: either homologous chromosome drawn to the poles
• Anaphase II: either sister chromatid drawn to the poles

Question 8

Oogenesis

Answer 8

Primary oocytes rest in Prophase of Meiosis I, after Mencarche FSH stimulates follicles to grow every cycle. Result: 2 cells (Secondary Oocyte and 1st Polar Body)

Question 9

Spermatogenesis

Answer 9

Spermatogonium -> Primary Spermatocyte

• > 1st Meiotic Divison = Secondary Spermatocyte
• > 2nd Meiotic Division = Spermatids
• > Maturation (Sertoli Cells) = Sperm

Question 10

Aneuploidy

Answer 10

Abnormal number of chromosomes in a cell

Question 11

Numerical Aberrations of Sex Chromosomes

- Monosomy X (Turner Syndrome)

Answer 11

Karyotype: 45, X (Genetically female)

Sterility, short stature, webbed neck

Question 12

Numerical Aberrations of Sex Chromosomes

- Trisomy X (Triple X)

Answer 12

Karyotype: 47, XXX (Genetically female)

Healthy and fertile, phenotypically normal

Question 13

Numerical Aberrations of Sex Chromosomes

- Klinefelter Syndrome

Answer 13

Karyotype: 47, XXY (Genetically male)

Feminine body characteristics, usually tall (long limbs), small testes and sterile

Question 14

Numerical Aberrations of Sex Chromosomes

- Double YY

Answer 14

Karyotype: 47, XYY (Genetically male)

Normal fertility and sexual development, often taller than average

Question 15

Numerical Aberrations of Autosomes

- Patau Syndrome (Trisomy 13)

Answer 15

Karyotype: 47, +13

Brain, eye, circulatory defects, cleft palate, rarely live more than a few months

Question 16

Numerical Aberrations of Autosomes

- Edward’s Syndrome (Trisomy 18)

Answer 16

Karyotype: 47, +18

Almost every organ system affected, rarely live longer than a few months

Question 17

Numerical Aberrations of Autosomes

- Down Syndrome (Trisomy 21)

Answer 17

Karyotype: 47, +21

Epicanthic folds, short stature, IQ mildly impaired, congenital heart abnormalities

Question 18

Polyploidy

Answer 18

More than 2 sets of comlpete chromosomes (e.g. Tetraploids)

Question 19

Fertilization

Answer 19

Egg must be fertilized within 12-24 hours

• Takes ovum 72 hours to reach uterus
• Sperm viable for up to 6 days

Question 20

Sperm Activation: Capacitation

Answer 20

Chemical changes of sperm after entering female reproductive tract
− Sperm head becomes penetrable to Ca2+ ions
− Hypermotility of sperm

Question 21

Sperm Activation: Acrosome Reaction

Answer 21

Acrosome reaction induced by Ca2+, progesterone + other factors
− Acrosome vesicle fuses with the Zona Pellucida and releases digestive enzymes which soften the glycoprotein matrix of the Zona Pellucida → sperm can penetrate

Question 22

Fertilization Process

Answer 22

(1) Sperm penetration of Cumulus cells
(2) Attachment to Zona Pellucida
(3) Exocytosis of acrosomal contents (enzymes digest the Zona Pellucida)
(4) Penetration of Zona Pellucida
(5) Entry into Perivitelline Space
(6) Membrane fusion of sperm and ovum → sperm nucleus enters cytoplasm
(7) Cortical reaction: enzymes released
(8) Block to polyspermy

Question 23

Preventing Polyspermy

Answer 23

Cortical Reaction:
− Triggered by Ca2+ release by the release of cortical granules from ovum
− Exocytosis of cortical granules from ovum modify extracellular matrix (Zona Pellucida) by serine proteases