meiosis

human cells: 46 total chromosomes (23 pairs)

• 44 autosomes, 2 sex chromosomes

diploid: 2 sets of chromosomes (1 copy from mom and 1 copy from dad)

• called 2n (2 copies of each chromosome)
• somatic cells: all normal body cells
• humans
  • 2 sets of 23 chromosomes = 46 total
  • chromosomes pairs 1-22 are called autosomes
  • last pair of chromosomes (set 23) are called sex chromosomes
• new diploid cells formed by mitosis
  • when mitosis is used to form a new organism = asexual reproduction

haploid: 1 set of chromosomes (1/2 of total)

• called 1n (1 copy of each chromosome)
• also known as gametes (sex cells → egg/sperm)
• in humans: haploid cells have 23 chromosomes
• new haploid cells formed by meiosis

asexual reproduction: when one parent produces genetically identical offspring by budding, single cell division, or entire organism divides into 2 or more parts

• genome of offspring = exact copy of parent
  • no genetic variability
• evolutionary advantage: if parent can survive, then offspring will as well
• evolutionary disadvantage: if environment changes, there is little chance of survival due to lack of variability (cannot adapt)
• ex
  • binary fission: bacterial cell division
  • prokaryotes ONLY
  • cloning
  • producing genetically identical individuals
  • any organism / ex. dolly the sheep
  • budding
  • outgrowth from parent, pinches off / genetically identical
  • ex. hydra (sea anemone), fungi, yeast

sexual reproduction: involves the production and fusion of haploid gametes (sex cells → egg/sperm)

• 2 parents that each contribute 1/2 of offspring’s genes → genetic variability
• performed by mammals, amphibians, reptiles, aves/birds, fish… fungi, plants?
• meiosis (gamete production) → fertilization (union of gametes into zygote) → mitosis (multiplication into organism)
• evolutionary advantage: genetic variability → if environment changes, there is a greater chance of survival from adaptations as offspring are different from parents
• evolutionary disadvantage: requires mate, longer process

cell cycle of sex cells

• interphase: same as before
  • g1, s, g2
  • growth, copying DNA and organelles, basic cell activities (CR/P), protein synthesis
• meiosis: produces 4 haploid cells from a diploid parent cell
  • divided into 2 parts
  • meiosis 1: prophase 1, metaphase 1, anaphase 1, telophase 1 (+ cytokinesis)
  • meiosis 2: prophase 2, metaphase 2, anaphase 2, telophase 2 (+ cytokinesis) → interphase again

meiosis 1: 4 sets of chromosomes (2 sets bc diploid ; replicated)

• prophase 1 ==[diagram]==
  • chromatin coils into chromatids, sisters pair up
  • centrosomes move to poles
  • nuclear membrane/nucleolus disappear
  • synapsis occurs → tetrads are formed ==[diagram]==
  • tetrad: paired homologous chromosomes
  • homologous (nonsister) chromosomes: matching chromosomes (same chromosome set) from 2 parents with the same length and genes at the same locations but may have different alleles (not genetically identical)
  • crossing over: homologous chromosomes exchange genetic information
  • increases genetic diversity by creating a unique combination of genes in gametes
• metaphase 1 ==[diagram]==
  • tetrads line up on the equator/metaphase plate
  • spindle fibers from one pole attaches to one set of sister chromatids, spindle fibers from other pole attaches to other set of sister chromatids
• anaphase 1 ==[diagram]==
  • spindle fibers split the tetrad and pull sets of sister chromatids to opposite poles
• telophase 1 ==[diagram]==
  • sister chromatids reached opposite poles
  • nucleus forms
  • each end has two sets of each chromosome
  • cytokinesis into 2 independent cells → cleavage furrow forms
• straight to prophase 2 after this / no further DNA replication, interphase…

meiosis 2: now we have 2 cells each doing the following steps

• prophase 2 ==[diagram]==
  • spindle fibers appear
  • centrioles move to poles
  • nuclear membrane and nucleolus disappears again
  • sister chromatids begin to move to metaphase plate
  • tetrads do NOT form
• metaphase 2 ==[diagram]==
  • sister chromatids line up on the equator/metaphase plate
  • spindle fibers attach to centromeres
• anaphase 2 ==[diagram]==
  • sister chromatid pairs split
  • single stranded chromosomes pulled to opposite poles by spindle fibers
• telophase 2 ==[diagram]==
  • chromatid → chromatin
  • nucleolus/nuclear membrane reappear
  • cytokinesis → cleavage furrow forms
• each of the 4 cells end up with 1 copy of each chromosome → haploid
  • (at the end of meiosis 1, all cells are haploid → divide in meiosis 2 into haploid)

result of meiosis

• formation of 4 haploid gamete cells w/ 1 copy of each chromosome
  • egg/ovum: female gamete cell
  • sperm: male gamete cell
• when egg and sperm fuse, it forms a zygote
  • zygote: diploid cell formed by fertilization between two gametes / single fertilized egg
  • with meiosis, resulting zygote has the same number of chromosomes as regular body cells
  • gender is caused by the presence of X and Y chromosomes
  • eggs only give X, sperm gives X or Y
  • XX makes female, XY makes male

meiosis summary

diagrams / identifying specific stages

• meiosis 1: tetrads → sister chromatid pairs
• meiosis 2: sister chromatid pairs → single chromatids
  • identical to mitosis but may have 2 cells in picture

reasons for meiosis

• random assortment ==[diagram]==: chromosomes distributed at random during A1
  • sister chromosomes from mom and dad may end up in different cells → random genes in gametes
• crossing over ==[diagram]==: parts of homologous chromosomes are exchanged during P1
  • recombinant chromosomes with unique genes
• reduction of chromosome number: prevents cells in organism from having ever increasing chromosome number
  • zygotes have correct number of chromosomes (23+23=46)
  • prevents genetic disorders (ex. extra chromosome 21 = down syndrome / too many → death)

increasing genetic diversity

• random assortment, independent assortment
• crossing over
• random fertilization: do not know which egg will be fertilized by which sperm

mutations: changes in DNA sequence

• variety of causes
  • normal mistakes in DNA replication
  • chemical mutagens
  • UV radiation, ionizing radiation
• somatic cell mutations: occurs in body cells
  • not transmittable to offspring
  • cancer is a frequent result
• germ cell mutation: occurs in gametes
  • transmitted to offspring
  • result in birth defects and genetic disorders

chromosomal defects

• nondisjunction: failure of homologous pairs of chromosomes (tetrads) to separate properly during meiosis A1 ==[diagram]==
  • all 4 cells end up with the wrong number of chromosomes
  • some cells have extra chromosomes and some have missing chromosomes
  • trisomy: cells have 3 copies (extra) of a chromosome → n +1
    • ex. trisomy 21 = down syndrome
  • monosomy: cells have only 1 copy (missing) of a chromosome → n -1
    • ex. turner syndrome (x chromosome)
  • (polyploidy: multiple copies of ALL chromosomes → lethal in animals, common in plants for growth)
• problems with meiotic spindle cause errors in daughter cells
  • (nondisjunction) tetrads do not separate properly during meiosis 1 (all cells are defects)
  • sister chromatids fail to separate during meiosis 2 (1/2 of cells are defects)
  • too few or too many chromosomes
• karyotype: a person’s complete set of chromosomes
  • use to identify genetic disorders (chromosomal defects) and gender