DEV2011 Lecture Notes - Lecture 5: Spermatocyte, Oogenesis, Synapsis
25 May 2018
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Lecture 5 – Fertilisation and Embryo Cleavage
Meiosis
• Occurs within two cell types:
o Cells undergoing spermatogenesis
o Cells undergoing oogenesis
• Reduction division
• Start with parent cell → form cells with half genetic component
• Meosis in females begin during embryogenesis and stops at prophase I
o Reinitiated at puberty in specific oocytes → matures
o Rests at metaphase II
o Not completed in ovary until fertilisation has occurred
• Males: meiosis begins at puberty – doesn’t stop
o Testes: Spermatocyte → meiosis II: haploid (half genetic material) →
sperm cells
• Females
o Ovary: Meiosis II: 3 polar bodies
→
egg cell (haploid)
• Prophase (1)
o Duplicated chromosomes condense → synapsis occurs → crossing
over occurs → chiasma visible
▪ Chiasma lock together male and female homologues (pairs)
▪ Chromatids are glued together
▪ Arms then unglued but remain attached at centromeres
o Recombination occurs
▪ Paired homologous maternal and paternal derived
chromosomes cross over and exchange gene sequences
▪ Contributes to genetic diversity
• Metaphase (2)
o Chromosomes align at equatorial plane
o Spindle fibres attached to chromosomes
• Anaphase (3)
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o Chromosomes separate and move towards poles
• Telophase (4)
o Nuclear envelope reforms around chromosomes
o Cells begin to split, forming haploid daughter cells
• Prophase (II) → Metaphase (II) → Anaphase II → Telophase II →
Cytokinsesis
o M → A: microtubules of spindle attached to chromatids pull them in
opposite directions (poles of dividing oocyte or spermatocyte)
• Crossing Over
o Recombination during meiosis ensures shuffling of alleles
• Independent Assortment
o Homologues line up and cross randomly on metaphase plate in Meiosis
I
• Random Fertilization
o Ovum has 8 million possible chromosome combinations
o 8 million x 8 million = 64 trillion
• Non-Disjunction Events
o Can survive shortage of sex chromosomes
o Turner syndrome:
▪ One sex X chromosome (in woman)
▪ Shorter, retardation, limit of brain growth
o Kleinfelter
▪ Two X chromosomes and Y chromosome
▪ Taller and wider
o Autosomal chromosomals (non-sex chromsomes)
▪ Too much of product
▪ Aneuploidy – result in miscarriage
▪ Humans can survive extra chromosome of 21, 18 and 13
▪ Down syndrome: extra chromosome 21
▪ Edwards syndrome: chromosome 18
▪ Patau syndrome: chromosome 13: can only survive >1 year
How does Meiosis Differ in Oocytes and Sperm?
• 20-37% of oocytes are chromosomally abnormal
• 7-15% of sperm are chromosomally abnormal
• 40% of zygotes (Fertilised oocytes) are chromosomally abnormal
o Too abnormal – foetuses will undergo spontaneous abortion
(miscarriage) even before mother is aware
o Sperm have to wait for ovulation: 5-6 days before egg arrives
• Similarities between oogenesis and spermatogenesis
o Meiosis
o Extensive morphological differentiation
o Incapable of surviving very long if fertilisation doesn’t occur
• Differences between oogenesis and spermatogenesis
o Spermatogenesis has equivalent meiotic divisions → 4 equivalent
spermatids
o Oogenic meiosis is asymmetrical: only one egg formed together with
three polar bodies
o Different timing of maturation
find more resources at oneclass.com
find more resources at oneclass.com
