Meiosis

Uncontrolled Mitosis

  • If mitosis is not controlled unlimited cell division occurs causing cancerous tumors
  • Oncogenes are special proteins that increase the chance that a normal cell develops into a tumor cell

Meiosis

  • Preceded by interphase which includes chromosome replication
  • Two meiotic divisions --- Meiosis I and Meiosis II
  • Called Reduction - division
  • Original cell is diploid (2n)
  • Four daughter cells produced that are monoploid (1n)
  • Daughter cells contain half the number of chromosomes as the original cell
  • Produces gametes (eggs and sperm)
  • Occurs in the testes in males (spermatogenesis)
  • Occurs in the ovaries in females (Oogenesis)
  • Start with 46 double-stranded chromosomes (2n)
  • After 1 division -23 double-stranded chromosomes (n)
  • After 2nd division - 23 single-stranded chromosomes (n)
  • Occurs in our germ cells that produce gametes

Why do we need Meiosis?

  • It is the fundamental basis of sexual reproduction
  • two haploids (1n) gametes are brought together through fertilization to form a diploid (2n) zygote
  • Fertilization - Putting it all together

A Replicated Chromosome

Homologous - same genes different alleles

Meiosis Formes Haploid Gametes

  • Meiosis must reduce the chromosome number by half
  • Fertilization restores the 2n number
  • Meiosis reduces genetic content

Stages in Meiosis

Early prophase

  • Homologs pair

Crossing over occurs

Late prophase

  • Chromosomes condense
  • Spindle forms
  • Nuclear envelope fragments
  • Synapsis - when homologous pairs of chromosomes cone together
  • and form a tetrad
  • (means 4) Tetra= Homolog pair

Crossing over

  • Homologous chromosomes in a tetrad cross over each other
  • Pieces of chromosomes or genes are exchanged
  • Produces genetic recombination in the offspring
  • Crossing-over multiplies the already huge number of different gamete types produced by ind

Metaphase I

  • Homologous pairs of chromosomes align along the equator of the cell

Anaphase I

  • Homologs separate and move to opposite poles
  • Sister chromatids remain attached at their

Telophase I

  • Nuclear envelopes reassemble
  • Spindle disappears
  • Cytokinesis divides cells into two.

Meiosis II - Reducing Chromosome number

  • Only one homolog of each chromosome is present in the cell
  • Sister chromatids carry identical genetic information
  • Meiosis II produces gametes with one copy of each chromosome and thus one copy of each gene

Prophase II

Telophase II

  • Nuclear envelope assembles
  • Chromosomes decondense
  • Spindle disappears
  • Cytokinesis divides cells into two.

Results of Meiosis

  • Gametes (eggs and sperm) form
  • Four haploid cells with one copy of each chromosome
  • One allele of each gene
  • Different combinations of alleles for different genes along the chromosome

sponge notes:

  • Diploid cells are body cells
  • A gamete is a sex cell
  • Mitosis makes copies
  • Meiosis makes gametes
  • 2n = 4

}}→ Name the 3 types of variation that take place because of Meiosis.}}

crossing-over

Independent assortment happens in metaphase and anaphase (away the chromosomes are assigned

Random assortment fertilization of gametes no specific sperm cell fertilizes the egg

→

Parental recombinant is when things recombined

Reduction and Division mean going from diploid to haploid.

homologous means the same

Synopsis brings homologous pairs together

Forms a tetrad

the chromosome become recombinant (mom and dads chromosome mixing together)

Charismata- the location where home chromatids cross

homologous chromosomes- are similar genetic information size shape

Meiosis start w prophase 1 and metaphase 1 anaphase

both haploid cells - half the number of chromosomes both double stranded

Cytokinesis

single-stranded chromosomes are still haploid cells that are two chromatids connected to a chromatid

2n=2 diploid

end of meiosis 1 and end of meiosis 2 is when they are single stood chromosomes

meiosis produces gametes

Gametogenesis (producing gametes)

Oogenesis or spematogensis

Spermatogenesis
  • Occurs in the testes
  • Two divisions produce 4 spermatids
  • Spematids mature into sperm
  • Men produce about 250,000,000 sperm per day

Oogenesis

  • Occurs in the ovaries
  • Two divisions produce 3 polar bodies
  • that die and 1 egg
  • Polar bodies die because of unequal division of cytoplasm
  • An immature egg called an oocyte
  • Starting at puberty one oocyte matures into an ovum (egg) every 28 days

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prophase I:

  1. synapsis
  2. Chaismata - Location where homologin chromatids cross
  3. Crossing over
  4. Recombinant and parental

independent assortment

takes place in metaphase I and metaphase II

chromosomes line up in the tetrads metaphase plate

Questions:

sister chromatids are separated during anaphase 2

prophase one

Allele- different versions of a trait

Synapsis

Tetrads are formed

Mitosis asexually reproduction

Meiosis is sexual reproduction

Produces meiosis

a zygote is a fertilized egg

Alternative forms

homologous chromosome - same genetic information (one from mom one from dad)

chromatids - identical copies of DNA

Diploid cells have mom and dads chromosome

Crossing over the chromatids switch

Independent assortment