Meiosis

1

• Chapter 12
• Meiosis

2
Outline

• Introductory material
• Overview of meiosis
• Phases of meiosis
• Heredity and genetic variation
• Role of sex
• Mistakes in meiosis

3
Early Research

• Hertwig and Fol (1876)
• observed fusion btwn sperm and egg in sea urchins
  
• followed embryonic development
• E. van Beneden (1883)
• observed 2 chromosomes in roundworm sperm and
  eggs and 4 chromosomes in somatic cells
• Weismann's hypothesis
• distinctive type of cell division used to make
  gametes reduces chromosome by ½
• Later known as meiosis

4
Early Research

• Sutton (1902)
• observed gamete formation and karyotype in lubber
  grasshoppers
• karyotype and type of chromosomes in a cell
• somatic cells 24 chromosomes w/ 12 distinct
  types and 2 copies of each type

5
Early Research

• Pair of chromosomes w/ same genes in same
  locations homologous chromosomes (homologs)
• carry different versions of same genes
• gene section of DNA that influences 1 traits
• different versions of specific gene alleles
• haploid / diploid / triploid / tetraploid /
  polyploid
• n of distinct types of chromosomes in a cell
• X and Y sex chromosomes, determine sex
• non-sex chromosomes autosomes

6
Meiosis

• Meiosis special type of cell division in
  sexually reproducing organisms that forms gametes
  (egg and sperm)
• reduces chromosome by ½, enabling sexual
  recombination
• meiosis of diploid cells produces haploid
  daughter cells
• during sexual reproduction, sperm egg unite to
  form new individual (fertilization)
• restores diploid chromosomes

7
Overview of Meiosis

• 2 divisions Meiosis I and Meiosis II
• ? chromosome by ½, enabling sexual
  recombination
• cells must be diploid or polyploid to undergo
  meiosis
• meiosis of 2n cell produces n daughter cells
  (gametes)
• prior to meiosis,
  
  each chromosome is
  replicated (like
  mitosis)
• following replication, each
  chromosome
  composed of
  2 sister chromatids
  attached
  at centromere (like mitosis)
• similar phases as mitosis

8
Two Parts to Meiosis

• Meiosis I
• replicated, homologous chromosomes undergo
  synapsis and crossing over
• homologous pairs align at cell center and pulled
  into 2 different daughter cells
• daughter cells are haploid w/ each chromosome
  having 2 identical sister chromatids
• Meiosis II
• sister chromatids separate and go to daughter
  cells
• at end of meiosis II, there are 4 haploid gametes
  (each w/ 1 copy of each chromosome)

9
(No Transcript)
10
(No Transcript)
11
Comparison of Mitosis and Meiosis

• Feature Mitosis Meiosis
• Cell divisions 1 2
• Chromosomes in daughter cells same half
  compared to parent cell
• Synapsis of homologs no yes
• Crossing-over events 0 1/pair of homologs
• Makeup of chromosomes in daughter
  identical either paternal or cells compared to
  parent cell maternal homolog present,
  not both segments mixed
• Role in life cycle reproduction gamete
  production in in unicellular, sexually
  reproducing tissue of multi- organisms
  cellular organisms

12
(No Transcript)
13
(No Transcript)
14
(No Transcript)
15
Prophase I

• Homologous chromosomes pair up
  (synapsis)
• one w/ paternal origin, other w/
  maternal origin
• each pair forms tetrad of 2 replicated homologous
  chromosomes (4 chromatids)
• Crossing over btwn homologous non-sister
  chromatids occurs
• chiasmata form btwn non-sister chromatids,
  1/homologous pair
• physical exchange of DNA
• produces chromosomes unlike either parent
• Non-sister chromatids begin to separate

16
(No Transcript)
17
Metaphase I

• Homologs migrate to center of cell along
  metaphase plate
• each homologous pair positioned randomly, w/o
  regard to which side of future line of cell
  division the paternal or maternal member of the
  pair is on

Anaphase I

• Homologs pulled to opposite poles of cell
• each chromosome moves independently of other
  chromosomes

18
Telophase I

• Homologs finish migrating to poles of cell
• At end of meiosis I, cell divides to form 2
  haploid daughter cells
• Cytoplasm separates into 2 daughter cells
• Random mixture of paternal versus maternal
  homologs of chromosomes ends up in each daughter
  cell (independent assortment)
• Each daughter cell is haploid, w/
  1 full set of
  chromosomes
  consisting of 2 chromatids

19
Prophase II

• At start of meiosis II, each daughter cell is
  haploid
• but each chromosome consists of 2 sister
  chromatids
• Spindle apparatus forms
• Chromosomes migrate to center of cell

Metaphase II

• Replicated chromosomes, consisting of 2 sister
  chromatids, line up at metaphase plate

20
Anaphase II

• Sister chromatids separate and pulled to opposite
  poles

Telophase II

• Nuclear envelope forms around each haploid set of
  chromosomes
• Cytoplasms separate
• At end of meiosis II, each cell divides
• thus 4 haploid daughter cells

21
Chromosomes and Heredity

• Chromosomes contain hereditary material and
  composed of genes
• gene chromosome segment containing instructions
  for inherited trait (i.e. eye color, gender,
  tongue rolling)
• every chromosome has multiple genes
• 3 key events ? genetic variation
• independent assortment or segregation of
  homologous pairs in metaphase I/anaphase I
• each homologous pair can orient in either of two
  ways at plane of cell division
• total of possible outcomes 2n where n
  haploid chromosomes
• if cell has 23 chromosomes in haploid state
• of possible combinations 223 8.4 M

22
OR
During meiosis I, tetrads line up two different
ways before homologs separate
Brown eyesBlack hair
Blue eyesRed hair
Blue eyesBlack hair
Brown eyesRed hair
23
Chromosomes and Heredity

• crossing over btwn homologous chromosomes in
  prophase I
• homologous chromosomes exchange DNA at chiasmata
• produces new combinations of alleles on same
  chromosome
• introduces TONS of genetic variation
• random fertilization btwn ? and ? gametes
• amnt of genetic variability introduced calculated
  by multiplying of possible gametes produced by
  ? by gametes produced by ?
• selfing - ? and ? gametes from same organism
• offspring genetically different from parents due
  to independent assortment and crossing over
• outcrossing - ? gametes from different individual
  than ? gamete
• maximizes genetic variability

24
Chromosomes and Heredity

• Independent assortment and crossing over ensure
  genetic variation even w/ self-fertilized sp.

25
Life Cycles of Sexually Reproducing Organisms

• Predominately diploid life cycles
• organism 2n, gametes n
• meiosis produces gametes, which undergo
  fertilization to form 2n zygote
• Predominately haploid life cycles
• organism n throughout all stages of life except
  zygote
• 2n zygote often dormant for survival of harsh
  conditions
• zygote undergoes meiosis when it matures and
  becomes active
• Alternate btwn haploid and diploid phases
  (alternation of generations)
• meiosis occurs in 2n adult phase, forms n spore
  cells
• spores germinate/grow into n phase, which
  produces n gametes by mitosis
• gametes fuse to form zygote that grows into 2n
  phase

26
(No Transcript)
27
(No Transcript)
28
Role of Meiosis and Sexual Reproduction in the
Evolution of Organisms

• Sexual reproduction common for multicellular
  organisms
• Most lineages undergo asexual reproduction
• bacteria, Archaea and few vertebrates reproduce
  asexually
• most algae, fungi, some land plants,
  and most vertebrates
  reproduce
  asexually and sexually
• sexual reproduction is major mode
  of reproduction in
  species-rich
  lineages (insects, vertebrates,
  
  flowering plants)

29
The Paradox of Sex

• Smith (1978)
• proposed asexually reproducing organisms should
  reproduce faster and outcompete sexual
  reproducing organisms
• asexual reproduction - no ? required, so
  population of all ? can produce more offspring
  than one in which some individuals are ?
• asexual individuals should ? in frequency and
  sexual individuals should ?
• Paradox - stable populations of sexually
  reproducing organisms exist

30
The Paradox of Sex

• If environment changes sexual reproduction ?
  diversity and thus species survival
• sexual reproduction may be adaptation that ?
  fitness of individuals in certain environments
• Hamilton et al. (1980) - sexual reproduction
  provides ? protection from diseases/pathogens

31
Mistakes in Meiosis

• Nondisjunctions homologous chromosomes fail to
  separate at meiosis I or when chromatids fail to
  separate at meiosis II
• both members of homologous chromosome pair
  migrate to same daughter cell
• 1 daughter cell will have 2 copies of chromosome
  and other will lack copy of chromosome after
  meiosis
• if daughter cells are fertilized by normal sperm,
  some zygotes will have 3 copies of chromosome
  (trisomy) and others will have only 1 copy of the
  chromosome

32
(No Transcript)
33
Mistakes in Meiosis

• Consequences of nondisjunctions
• nondisjunction may occur in as many as 10 of
  meiotic divisions
• aneuploid zygotes (zygotes w/ too few or too many
  chromosomes) typically do not survive
• some types of nondisjunctions allow survival,
  depending on chromosome involved (i.e., Down
  syndrome, trisomy 21)
• accidental and w/ no genetic predisposition
• sex chromosomes and chromosome 21 may be more
  prone to aneuploidy

34
Mistakes in Meiosis

• Polyploidy occurs when whole sets of chromosomes
  fail to separate
• 1 daughter cell receives 2 copies of every
  chromosome, other receives none and is not viable
• fertilization of 2n gamete by 1n sperm 3n
  zygote (triploid)
• organisms w/ odd chromosome sets cannot produce
  viable gametes (i.e., seedless fruits like
  watermelon and banana)