Fertilisation and Meiosis Flashcards
human sexual reproduction
- 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
preparation for meiosis (dna replication)
- 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)
meiosis
- 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
production of haploid gametes
- Meiosis I: Reductional Division
o Homologous chromosomes separate - Meiosis II: Equational Division
o Note: No DNA before Meiosis II
o Sister chromatids separate
Meiosis I - reductional division
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
meiosis II - equational division
= 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
introduction of variation via meiosis
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
Oogenesis
- 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)
Spermatogenesis
- Spermatogonium
- Primary Spermatocyte
- 1st meiotic division: –> Secondary spermatocyte
- 2nd meiotic division: –> Spermatids
- Maturation in epididymis nourished by sertoli cells –> sperm cells
numerical chromsome abberations
- 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
Karyotype
- 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
Aneuploidy
abnormal number of individual (ONE) chromosomes in a cell
- 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
Monosomy X (Turner’s Syndorme)
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
Trisomy X (Triple X Syndrome)
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)
Klinefelter’s Syndrome
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
Double Y Syndrome
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
Trisomy 13 (Patau Syndrome)
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
Trisomy 18 (Edward’s Syndorme)
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
Trisomy 21 (Down Syndrome)
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
effect of age on abberations
- 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
Polyploidy
- 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
Fertilisation
- 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
Sperm Migration
- 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)
sperm capacitation
- 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
fertilisation
(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
preventing polyspermy
- 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
