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Repro > Fertilisation and Meiosis > Flashcards

Fertilisation and Meiosis Flashcards

1
Q

human sexual reproduction

A
  • Consists of gamete formation, fertilisation and development of the zygote

Gametes:

  • Spermatozoa: produced in male testes
  • Ova: produced in female ovaries
  • haploid chromosome set
  • Fertilization leads to a zygote with diploid chromosome set
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2
Q

preparation for meiosis (dna replication)

A
  • replication/duplication of DNA in S phase of interphase
  • 2n: homologous pairs of chromosomes (1x maternal, 1x paternal DNA copy)
  • 4 copies of each DNA molecule (2x maternal, 2x paternal DNA copies)
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3
Q

meiosis

A
  • Eukaryotic cell division that produces haploid gametes and reshuffles genetic material
  • Introduces variation in offspring by
    o Crossing over
    o Independent assortment
  • Meiosis I: Reductional division
  • Meiosis II: Equational division
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4
Q

production of haploid gametes

A
  • Meiosis I: Reductional Division
    o Homologous chromosomes separate
  • Meiosis II: Equational Division
    o Note: No DNA before Meiosis II
    o Sister chromatids separate
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5
Q

Meiosis I - reductional division

A

Prophase I: chromosomes condense, nuclear membrane dissolves, spindle forms, homologous chromosomes pair and form synapsis, crossing over

Metaphase I: homologous chromosomes align at equator, spindle fibers attach to centromers

Anaphase I: homologous chromosomes separate -> drawn to opposite poles by spindle

Telophase I: spindle disappears, nuclear membrane forms

Cytokinesis: cytoplasm separates, formation of two separate cells
–> two cells, each with half the number of chromosomes

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6
Q

meiosis II - equational division

A

= Mitotic division of haploid cells produced in Meiosis I

Prophase II: nuclear membrane dissolves, spindle forms

Metaphase II: chromosomes align at equator

Anaphase II: Sister chromatids separate and get drawn to opposite poles by spindle

Telophase II: spindle disappears, nuclear membrane forms

Cytokinesis: cytoplasm separates, formation of two separate cells
–> 4 genetically different haploid cells

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7
Q

introduction of variation via meiosis

A

crossing over

  • During Prophase I: Pairing structure of homologous chromosomes: Tetrad = Synapsis = Bivalent
  • Crossing over between non-sister chromatids to exchange of genetic material
  • Several chiasmata depending chromosome length

independent assortment

  • In Anaphase I: Either homologous chromosome drawn to the poles
  • In Anaphase II: Either chromatid drawn to the poles
  • -> distribution into the daughter cells is random
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8
Q

Oogenesis

A
  • Production of gametes in female
  • Transformation of oogonia into primary oocytes = oocytogenesis
  • Oocytogenesis is complete at birth of females
  • Primary Oocytes rest in Prophase I of meiosis I
  • After menarche FSH stimulates a few follicles to grow -> every menstrual cycle a few Oocytes complete meiosis I (including crossing over)
  • Result: 2 cells = Secondary Oocyte and 1st polar body
  • Secondary Oocyte develops to metaphase II, rests in this state till after ovulation
  • Only if fertilized completion of Meiosis II (casts off second polar body)
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9
Q

Spermatogenesis

A
  • Spermatogonium
  • Primary Spermatocyte
  • 1st meiotic division: –> Secondary spermatocyte
  • 2nd meiotic division: –> Spermatids
  • Maturation in epididymis nourished by sertoli cells –> sperm cells
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10
Q

numerical chromsome abberations

A
  • Non-disjunction (in Meiosis I or II) = failure of segregation
  • Errors in Meiosis are not rare but typically are lethal at the embryonic stage

Segregation:

  • Homologous chromosomes get drawn to different poles in Anaphase I
  • Sister chromatids get drawn to opposite poles in Anaphase II
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11
Q

Karyotype

A
  • Visualization of chromosomes of a somatic cell, arrested in metaphase
  • Chromosomes ordered by length and location of centromere
  • 22 Autosomes + 1 pair of sex chromosomes
  • Female karyotype: 46, XX
  • Male karyotype: 46, XY
  • Visualization of gross changes in chromosome number and structure
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12
Q

Aneuploidy

A

abnormal number of individual (ONE) chromosomes in a cell

  1. Numerical aberrations of the sex chromosomes
    o Turner syndrome, Klinefelter syndrome, Triple X Syndrome,Double Y syndrome
2. Numerical aberrations of the autosomes:
o Trisomies (one extra chromosome): possible for chromosomes 13, 18 or 21
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13
Q

Monosomy X (Turner’s Syndorme)

A

SEX CR ABBERATION

  • Karyotype: 45, X
  • Affects 1:2000 females
  • Individuals are genetically female
  • The only viable monosomy in humans
  • No sexual maturation during puberty -> sterile
  • Short stature and normal intelligence
  • Often congenital abnormalities –> webbed skin of neck
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14
Q

Trisomy X (Triple X Syndrome)

A

SEX CR ABBERATION

  • Karyotype: 47, XXX
  • Affects 1:1000 females
  • Individuals are genetically female
  • Healthy and fertile
  • Usually cannot be distinguished from normal female (phenotypically normal)
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15
Q

Klinefelter’s Syndrome

A

SEX CR ABBERATION

  • Karyotype: 47, XXY
  • Affects 1:1000 males
  • Genetically male with male sex organs
  • Unusually small testes & sterile
  • Breast enlargement and other feminine body characteristics
  • Usually tall (long limbs)
  • Normal intelligence
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16
Q

Double Y Syndrome

A

SEX CR ABBERATION

  • Karyotype: 47, XYY
  • Affects 1:1000 males
  • Genetically male
  • Nondisjunction in the second meiotic division of father
  • Normal fertility and sexual development
  • Individuals usually phenotypically normal but often taller than average
17
Q

Trisomy 13 (Patau Syndrome)

A

AUTOSOME CR ABBERATION

  • Karyotype: 47, +13
  • Affects 1:5000 live births
  • Serious eye, brain, circulatory defects as well as cleft palate
  • Children rarely live more than a few months
18
Q

Trisomy 18 (Edward’s Syndorme)

A

AUTOSOME CR ABBERATION

  • Karyotype: 47, +18
  • Affects 1:10,000 live births
  • Almost every organ system affected
  • Children rarely live more than a few months
19
Q

Trisomy 21 (Down Syndrome)

A 
AUTOSOME CR ABBERATION
- Karyotype: 47, +21 
- Affects 1:800 live births
- Altered phenotype:
o Characteristic facial features i.e. round face
o Epicanthic folds
o Short stature 
o Congenital abnormalities of heart
o IQ often mildly impaired
20
Q

effect of age on abberations

A
  • Risk seems to be associated with delay in completion of prophase I in female gametes
  • Down Syndrome is highly correlated with age of mother (risk at 40 years ~1%)
  • Newer studies – paternal age important also: up to ¼ of Down Syndrome cases found to originate from paternal non-disjunction
21
Q

Polyploidy

A
  • More than two complete sets of chromosomes, e.g. tetraploids (4n)
  • Often due to duplication of genome without subsequent mitosis
  • High frequency in flowering plants & some species of fish and frogs
  • Organisms with odd number of chromosome sets cannot produce viable gametes
  • Polyploid plants often larger and stronger than diploid plants
22
Q

Fertilisation

A
  • Egg has to be fertilized within 12-24h after ovulation
  • Takes ovum 72 hours to reach the uterus -> sperm has to fertilize the egg in ampulla of the fallopian tube
  • Sperm viable for up to six days
  • Fertilization involves chemical changes in the Spermatozoa and in the Oocyte
23
Q

Sperm Migration

A
  • Majority of sperm do not get to site of fertilization
  • destroyed by vaginal acid
  • drain out of vagina
  • cannot penetrate cervical mucus
  • get destroyed by leukocytes in uterus
  • Half go in the fallopian tube not containing an oocyte
    (only ~2000-3000 spermatozoa reach ovum)
  • Takes 5-10min for spermatozoa to reach distal end of fallopian tube
  • Spermatozoa have to undergo changes before fertilization can occur (capacitation)
24
Q

sperm capacitation

A
  • Chemical changes of sperm after entering female reproductive tract
  • Takes ~10h
  • Fluid in female reproductive tract leach cholesterol from sperm plasma membrane
  • Sperm head becomes penetrable to Ca2+ ions
  • Hypermotility of sperm
25
Q

fertilisation

A

(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 between sperm and ovum -> sperm nucleus enters cytoplasm
(7) cortical reaction: enzymes released
(8) block to polyspermy

26
Q

preventing polyspermy

A
  • Polyspermy prevented by cortical reaction:
  • Process initiated after fertilization triggered by Ca2+ release from ER by the release of cortical granules from ovum –> Calcium wave starts at site of sperm penetration
  • Permanent barrier to sperm entry gradually established = slow block of polyspermy in many animals
  • Exocytosis of cortical granules from ovum –> contents of the cortical granules released outside the cell –> modify extracellular matrix (zona pellucida) by serine proteases
    o becomes impenetrable to sperm entry = physical barrier and also receptors to bind sperm downregulated

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