Meiosis, Fertilisation, Early Embryonic Development Flashcards
What is sexual reproduction
- Reproduction: Biological process by which new individual organisms (offspring) are produced from their parents, two types (asexual and sexual), sexual cells
- Gametes: Spermatozoa (produced in male testes), ova (produced in female ovaries), haploid chromosome set, fertilisation leads to a zygote with diploid chromosome set
- DNA Replication: Preparation for meiosis, replication / duplication of DNA in S phase of interphase, four copies of each DNA molecule (two maternal and two paternal)
What is meiosis
- Eukaryotic cell division that produces haploid gametes and reshuffles genetic material
- Introduces variation in offspring by crossing over and independent assortment
- Number of chromosomes halved, conserves human chromosome number
- Two divisions, four daughter cells (n), not identical to mother, cells for reproduction, genetic variability
- Meiosis 1: Reduction division, separation of homologous chromosomes (diploid to haploid)
- Meiosis 2: Equational division, separation of sister chromatids (haploid to haploid), producing for spermatids (haploid n)
Describe stages of meiosis 1
- Interphase 1: DNA replication
- Prophase 1: Longer and more complex, chromosomes condense, nuclear membrane dissolves, spindle forms, homologous chromosomes pair and form synapsis, crossing over
- Metaphase 1: Homologous chromosomes align at equator, spindle fibres at centromeres
- Anaphase 1: Chiasmata, homologous chromosomes separate, drawn t opposite poles by spindle
- Telophase 1: Spindle disappears, nuclear membrane forms
- Cytokinesis: Division of cytoplasm (cleavage furrow), no further replication of chromosomes, 2 cells with half number of chromosomes
Describe stages of meiosis 2
- Prophase 2: Spindle assembly commences, nuclear membrane dissolves
- Metaphase 2: Chromosomes aligned on the metaphase plate, spindle fibres bind to both sides of centromere, each sister chromatid facing opposite poles
- Anaphase 2: Kinetochores unfuse, spindle fibres contract and sister chromatids move to opposite poles
- Telophase 2: Nuclear envelope re-forms, spindle disappears
- Cytokinesis: Cytoplasm separates, 4 daughter cells with haploid number of un-replicated chromosomes, 4 genetically different haploid cells
How is variation introduced
- Independent Assortment: Of homologous chromosomes (A1) and chromatids (AII), each segregates / is distributed independently
- Crossing Over: (P1), occurs when matching regions of non-sister chromatids on homologous chromosomes break off and reattach to chromosome (genetic recombination, mix from mother and father)
Describe the types of numerical chromosome aberrations
- Segregation: Homologous chromosomes get drawn to different poles in Anaphase I and sister chromatids get drawn to opposite poles in Anaphase II
- Non-Disjunction: Failure of chromosomes to segregate during meiosis (AI / AII) causing monosomy (45) and trisomy (47)
- Aneuploidy: Abnormal number of individual chromosomes in a cell, numerical aberration of sex chromosomes or autosomes
- Karyotype: Visualisation of chromosomes of a somatic cell, arrested in metaphase, ordered by length and location of centromere, female (46, XX) and male (46, XY)
- Visualisation of gross changes in chromosome number and structure
- Age Effect: Risk of aberration seems to be associated with delay in completion of prophase I in female gametes, paternal age is also important
Describe examples of numerical aberrations in sex chromosomes
Turner:
- Monosomy X (45, X), affects 1:2000 females
- Genetically female, only viable monosomy in humans
- No sexual maturation during puberty (sterile), short stature and normal intelligence
- Congenital abnormalities (webbed skin of neck)
Klinefelter:
- Aneuploidy X (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) and with normal intelligence
Triple X:
- Trisomy X (47, XXX), affects 1:1000 females
- Genetically female, healthy and fertile, phenotypically normal
Double Y:
- Aneuploidy Y (47, XYY), affects 1:1000 males
- Genetically male
- Nondisjunction in second meiotic division of father
- Normal fertility / sexual development, phenotypically normal, often taller
Describe examples of numerical aberrations in autosomes
Trisomy 13: - Patau syndrome - Affects 1:5000 live births - Serious eye, brain, circulatory defects as well as cleft palate - Rarely live more than a few months Trisomy 18: - Edwards syndrome - Affects 1:10,000 live births - Almost every organ system affected - Rarely live a few months Trisomy 21: - Down syndrome - Affects 1:800 live births - Altered phenotype - Characteristic facial features (round face), epicanthic folds, short stature, congenital abnormalities of heart and IQ mildly impaired - Correlated with mothers age and up to ¼ of cases originate from paternal nondisjunction
What is fertilisation
- Egg has to be fertilised within 12-24h after ovulation
- Takes ovum 72 hours to reach the uterus
- Sperm has to fertilise the egg in ampulla of the fallopian tube
- Sperm viable for up to 6 days
- Involves chemical changes in spermatozoa and oocyte
What is the process of fertilisation
- Many sperm cells attach to the corona radiata of a secondary oocyte
- Sperm attach to receptors on zona pellucida and acrosomal enzymes digest through zona pellucida
- Head of one sperm penetrates zona pellucida, membrane fusion of sperm and ovum, sperm nucleus enters cytoplasm, changes in zona pellucida, perivitelline space and prevent additional sperm cells from entering the oocyte
- The oocyte nucleus moves to one side, where it completes the second meiotic division and gives off a second polar body, oocyte nucleus now the female pronucleus moves back toward the centre
- The contents of the sperm head enlarge and become the male pronucleus.
- The two pronuclei fuse to form a single nucleus, fertilisation is complete and a zygote results
What is sperm migration and activation (capacitation / acrosome reaction)
Migration:
- Majority of sperm do not get to site of fertilisation
- Destroyed by vaginal acid, drain out of vagina, cannot penetrate cervical mucus, destroyed by leukocytes, half go in wrong fallopian tube
- Only ~2000-3000 spermatozoa reach ovum, takes 5-10min for spermatozoa to reach distal end of fallopian tube
Capacitation:
- Chemical changes of sperm after entering female reproductive tract, takes ~10h
- Fluid in female reproductive tract leach cholesterol from sperm plasma membrane
Acrosome Reaction
- Sperm head modified, releases enzymes which penetrate zona pellucida
- Induced by Ca and progesterone
- Acrosome membrane fuses with plasma membrane, releases digestive enzymes, soften glycoprotein matrix, penetrate pellucida
- Remodelling of protein complexes in sperm head (interact with pellucida)
- Hyper-motility of tail
How is polyspermy prevented
- Prevented by cortical reaction
- Sperm enters ova causing a rise in Ca in cytoplasm
- Causes cortical reaction in which granules secrete enzymes beneath zona pellucida (zonal inhibiting proteins)
- Enzymes destroy zone pellucide sperm binding receptors and removes any sperm still bound to membrane
- Exocytosis of cortical granules from ovum, modifies extracellular matrix (zona pellucida) by serine proteases
- Impenetrable to sperm entry (physical), down regulation of sperm receptors (chemical)
Provide an overview of fertilisation
- Fusion of sperm and oocyte plasma membranes
- Initiates Ca2+ influx and release of cortical granules = cortical reaction = block of polyspermy
- Stimulates completion of meiosis II in 2nd Oocyte = 2nd polar body extruded = ovum formed
- Results in one cell with haploid DNA from 1 sperm and haploid DNA from 1 ovum = cell is now diploid
- DNA is initially present in two separate areas within the cell (“head of sperm” & nucleus of oocyte), pronuclei fuse = zygote with diploid number of chromosomes
Provide an overview of female gametes
- Oogonia transform into primary oocytes - completed at birth
- Primary Oocytes rest in - Prophase I of meiosis I
- Meiosis I = Secondary Oocyte and 1st polar body
- Secondary Oocytes rest in metaphase II till after ovulation
- Only if fertilised completion of Meiosis II and 2nd polar body
Provide an overview of male gametes
- Primordial germ cells
- Spermatogonia = mitosis = more spermatogonia
- Primary Spermatocytes
- Meiosis I = Secondary Spermatocytes
- Meiosis II = Spermatids
- Spermiogenesis (takes ~74 days to complete) = maturation in epididymis = spermatozoa = sperm cells
