LECTURE 2-MEIOSIS, GAMETOGENESIS AND SOURCES OF GENETIC VARIATION

Quick Review of Main Concepts From Lecture 1-Chapter 16

What happens to the cleaved notch receptor?

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

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

• Parent and offspring

Sexual reproduction introduces genetic diversity by mixing parental DNA

• Diploid organisms contain 2 copies of each chromosome and gene

Development is a cyclical process

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

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

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

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

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)

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)

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

  

Secondary Oocytes are arrested at Metaphase II of meiosis

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

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)

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

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”

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Meiosis II: Separation of sister chromatids

Sexual Reproduction: independent assortment and crossing over

Abnormal chromosome segregation leads to aneuploid gametes

Abnormal chromosome segregation leads to aneuploid gametes

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

Pathogenic sources of genetic variation

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

• single gene mutations (RAS)

• copy number variations

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

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

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

MRI OF CDK5RAP2 patients and healthy control

• What type of genotype do these patients have?

• Why are there brains so small?

How do we identify mutations in humans?

• whole genome sequencing

How do we identify mutations in humans?

• SNP association

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

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

• identification of DNA variations associated with macular degeneration