LECTURE 2-MEIOSIS, GAMETOGENESIS AND SOURCES OF GENETIC VARIATION

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48 Terms

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Quick Review of Main Concepts From Lecture 1-Chapter 16

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What happens to the cleaved notch receptor?

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Lecture Outline: Meiosis, Gametogenesis, and sources of genetic variation

  • sexual reproduction: Fusing haploid gametes to establish diploid zygote

  • oocytes and sperm are produced in the ovary and the testis, respectively

  • Meiosis is a series of reductive cell divisions that yield haploid cells

  • human genetics of disease

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What do the hydra vulgaris and the osbournes have in common?

  • Parent and offspring

<ul><li><p>Parent and offspring </p></li></ul><p></p>
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Sexual reproduction introduces genetic diversity by mixing parental DNA

  • Diploid organisms contain 2 copies of each chromosome and gene

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Development is a cyclical process

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How does a single cell give rise to multiple cell types?

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Gametes are produced within the ovary and testis

Primordial germ cells (PGCs) are mitotic diploid germ cell precursor cells that commit to meiosis (diploid to haploid-reductive system) to become gametes

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BMP (important for differentiation) signaling in the posterior embryo induces PGC’s

  • Primordial germ cells (PGCs) are mitotic diploid germ cell precursor cells that commit to meiosis to become gametes

  • BMP4/BMP2 induce formation of PGCs in the embryo

  • Blimp1 is a transcriptional repressor important for PGC lineage specification

  • E=embryonic day

  • A=anterior

  • P=posterior

  • BMP=bone morphogenic protein

  • PGC=primordial germ cell

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Mitosis and Meiosis both begin with chromosome duplication

Mitosis-diploid cell to 2 diploid cells

Meiosis-diploid cell to 4 haploid gametes

  • all somatic cells are diploid

  • only gametes are haploid

  • gametes are generated from diploid cells by reductive cell division-meiosis

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Oogenesis in the Mammalian Ovary

  • female gametes are produced in the ovaries during Oogenesis

  • 1ry oocytes remains in Prophase I of meiosis (generated in the embryo) until puberty

  • 2ry oocyte remains in Metaphase II of meiosis

  • @ fertilization 2ry oocyte (n) finishes Meiosis II producing the mature egg cell

  • OH: oogenesis is the process of making oocytes (diploid before meiosis I and II and haploid after so genetically distinct)

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Prophase I has 4 key stages

  • Leptotene-duplicated chromosomes, each consisting of two sister chromatids condense

  • Zygotene-homologous chromosomes begin to pair up by formation of the synaptonemal complex-the bivalent is formed at this stage

  • Pachytene-homologous chromosomes are fully synapsed and crossing over (homologous recombination) occurs between non-sister chromatids

  • Diplotene-synaptonemal complex disassembles, homologous chromosomes begin to separate but remain connected at the chiasmata (points of crossing over)

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Primary oocytes are arrested at Prophase I of meiosis in the diplotene stage

  • Oocytes stay arrested in prophase I of meiosis at least until puberty

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Secondary Oocytes are arrested at Metaphase II of meiosis

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Major components of the ovarian follicle

  • oocyte=germ cell

  • somatic cells (granulosa cell and theca cell)—>estrogen

  • Ovarian follicle-fluid filled structure made of oocyte, theca, and granulosa cells

  • Theca and granulosa cells contribute to maturation of oocyte producing sex steroid hormones-estrogen

  • Ovulation-completing meiosis I

  • fertilization-completing meiosis II

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The seminiferous tubules of the adult mammalian testis contains true spermatogonial stem cells (SSCs)

  • spermatoza=germ cell

  • somatic cells (sertoli cells)—> leydig cells

  • Spermatogenesis-production of sperm cells within the seminiferous tubules of the male gonads (testes)

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Spermatogonial stem cells (SSCs) undergo meiosis to generate stem cells

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The seminiferous tubules contain support cells that also contribute to the differentiation of sperm cells

Leydig cells-release testosterone stimulating stem cells to differentiate into primary spermatocytes

Sertoli cells-provide nutrients to sperm cell + help reduce cytoplasm, sperm cells released from the seminiferous tubules mature and are stored in the “epididymus”

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Sperm cells have a very specialized structure

Head-contains nucleus and acrosome (cap like organelle derived from golgi contains enzymes important to get through the eggs outer layer)

Middle piece-contains a lot of mitochondria

Tail-composed of a flagellum (MT structure) helps in locomotion

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Sexual reproduction: Meiosis followed by Mitosis

  • Gametes are haploid (1n)

  • Males and females produce different gametes

  • Fertilized egg has homologous chromosomes from mother and father

  • Zygote produces individual with unique diploid (2n) set of chromosomes

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Differences between somatic and germ-line cells

  • Germ line cells are specified early in development and give rise to the haploid gametes by meiosis

  • Gametes propagate the genetic information to the next generation

  • Somatic cells form the body of the organism and are part of all our different tissues

  • Diploid germ line cells give rise to the haploid gametes

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Mitosis and Meiosis both begin with chromosome duplication

  • all somatic cells are diploid

  • 4N means 4 copies of each chromosome after replication

  • only gametes are haploid

  • gametes are generated from diploid cells by reductive cell division-meiosis

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Mitosis: DNA replication and cell division

  • start with 2n cell

  • duplication of the genetic material (4n stage)

  • During M phase genetic material is equally separated

  • Two identical daughter 2n cells

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Meiosis: DNA replication and two meiotic divisions

  • Start with 2n cell

  • duplication and recombination of genetic material (4n stage)

  • Meiosis I-produce 2n cells (reductionist division) that are genetically diverse

  • Meiosis II-reductionist division produces 1n cells that are genetically distinct from one another

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Mitosis vs Meiosis

  • Meiosis generates for 4 haploid cells that are not identical to each other (like why me and Mary look different!)

  • Mitosis generates 2 diploid cells that are genetically identically identical to each other

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Mitosis vs Meiosis: pairing of homologous chromosomes

Mitosis: duplicated chromosomes line up at the metaphase plate and the sister chromatids separate in anaphase generating 2 identical daughter cells

Meiosis-paternal and maternal homologous chromosomes align with one another and inheritance by the daughter cells is random-daughter cells are not identical

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Duplicated maternal and paternal chromosomes pair during Meiosis I to from bivalents

  • each bivalent contains 4 sister chromatids

  • duplicated maternal and paternal homologous chromosomes are joint at the centromeres

  • during meiosis the chromosome arms are also interacting with each other providing the basis for exchange of genetic material

  • “swaping” genetic material leads to genetic variability

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Chiasmata form between maternal and paternal chromosomes-crossing over between non-sister chromatids

  • Meiosis I-non-sister chromatids in each bivalent swap segments of DNA

  • exchange of material on inner chromosome arms is shown

  • region of crossing over is the chiasma

  • there are multiple sites of chiasma or crossing over

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Chiasmata formed between maternal and paternal chromosomes allow for recombination between non-sister chromosomes

  • micrograph of a human oocyte that was stained with fluorescent markers

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How does this “crossing over” happen?

  • recombination enzymes induce in either the maternal or paternal chromatid double strand breaks (DSB)

  • nucleases will digest the ends of strands leading to the formation of this overhangs

  • The template for homologous recombination repair will use the homologous chromatid from the other parent as a template for repair

  • Result-strand exchange where the maternal single strand will interact with the paternal homologous side

  • OH: in the end we repair the strand using enzymes and the template strand, so in this example the blue paternal chromatids count as the template strands

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Homologous recombination repair requires DNA synthesis and DNA ligation

  • uses paternal as template to fill in the gaps by DNA synthesis (green)

  • results in a structure with 2 cross overs and nicks (breaks) are joined by DNA ligase

  • Once meiosis I finishes they are going to separate out from each other

  • Each chromatid is going to contain a segment of DNA from the original parent and from the other parent

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Cohesin and the Synaptonemal Complex: Two protein complexes important for proper Meiosis

Cohesin-(green) protein rings that binds sister chromatids together

Synaptonemal complex-protein complex that binds homologous chromosomes

Cohesin are attached to axial core proteins that also bind to transverse filaments

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Cohesin and Synaptonemal Complex: Two protein complexes important for proper Meiosis

  • Transverse filaments form a zipper like structure that allows for inner arms of the chromosomes to interact with each other

  • Axial core and transverse filaments form the synaptonemal complex

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Meiosis I: Separation of homologous chromosomes

Metaphase of meiosis I: Chiasmata hold maternal and paternal homologs together, cohesin keeps sister chromatids joined together along the arms, kinetochores of sister chromatid function as a single unit

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Meiosis I: Separation of homologous chromosomes

  • Anaphase of meiosis I: Cohesins holding the arms together are degraded allowing homologs to separate, cohesins at the centromeres are not degraded

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Meiosis II: Separation of sister chromatids

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Sexual Reproduction: independent assortment and crossing over

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Abnormal chromosome segregation leads to aneuploid gametes

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Abnormal chromosome segregation leads to aneuploid gametes

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Genetics definitions (genotype, phenotype, wild-type, variants, mutants)

Genotype-specific set of alleles forming the genome of an individual

phenotype-visible trait, what we can see

wild-type-most common allelic form in a population

variants-alleles different in DNA sequence but showing wild type phenotype

Mutants-alleles with DNA sequence change that changes phenotype

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Pathogenic sources of genetic variation

  • chromosomal abnormalities (inversions, deletions, duplications, rearrangements)

  • single gene mutations (RAS)

  • copy number variations

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Types of alleles and what this means in terms of disease inheritance

  • homozygous-identical alleles

  • heterozygous-alleles are different from each other

  • hemizygous-single copy of gene present

  • some alleles can be either dominant or recessive

  • H-huntington’s allele

  • h-normal allele

  • possible gametes

  • possible combinations of alleles in offspring

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Mutations in protein coding genes can affect protein production in different manners

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Human genetics in the study of brain disorders CDK5RAP2 mutations lead to reduced brain size

  • a single mutation of a recessive allele does not cause the disease

  • consanguineous marriage results in offspring inherits both recessive alleles

  • CDK5RAP2 is a centrosomal protein

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MRI OF CDK5RAP2 patients and healthy control

  • What type of genotype do these patients have?

  • Why are there brains so small?

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How do we identify mutations in humans?

  • whole genome sequencing

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How do we identify mutations in humans?

  • SNP association

  • SNP-single nucleotide polymorphism (can lead to area with the gene associated with the disease!)

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Genome wide association studies in macular (membrane in the eye—>causes blindness) degeneration

  • identification of DNA variations associated with macular degeneration