Meiosis and Sexual Life Cycles
I. Sexual vs asexual reproduction
A. asexual reproduction
1. asexual reproduction uses mitosis 
2. individual inherits all its genes from one parent
a. offspring genetically identical to parent
b. eg:
(1) one celled organisms divide (eg: yeast)
(2) hydra budding
(3) daphnia
(4) self pollinating flowers (eg: dandelions)
(5) some lizards (whiptails and racerunners)
(6) aphids
(7) aspens shoot up from roots
-the clone (the grove) is the genetic individual
(8) sharks
-shark story
-probably rare occurance
c. genetic variation comes only from mutations
d. greatly reduced ability to evolve
(1) very few "genetic possibilities" in the population
B. sexual reproduction
1. each new individual gets 1/2 genetic information
from one parent & 1/2 from other parent
2. offspring (genetically new individuals) are
genetically different from both parents
3. sexual reproduction produces much greater genetic variation
a. this genetic variation is critically important
to process of evolution
(1) evolution absolutely requires
pre-existing genetic variation
(2) sexual reproduction provides much of the
pre-existing genetic variation
II. Sexual cycles (humans)
A. human somatic cells = human body cells
1. 46 chromosomes in human body cells
-these 46 chromosomes can be either:
-unduplicated (eg: interphase G1)
-duplicated (eg: interphase G2)
2. all 46 chromosomes not alike
3. 23 "types" of chromosomes
4. 2 of each type (2 X 23 = 46)
a. get one of each type from father
b. get one of each type from mother
5. human somatic cells have 23 pairs of chromosomes
a. homologous chromosomes = two
chromosomes of a given matched pair
(same type)
-see cartoon of homologous pair
b. each chromosome has many genes
c. see a cartoon of the X chromosome
-light micrograph of X
-no labels
d. see a cartoon of the Y chromosome
e. see human transcripts web site
f. each gene occupies a specific "locus" on
a specific chromosome
(1) recall: chromosomes are long linear
strands DNA
g. homologous chromosomes have homologous
genes at homologous loci
(1) but, the two genes at the homologous loci
need not be identical
6. one pair of homologous chromosomes is different
a. human females
(1) true homologous pair XX
b. human males
(1) XY
(2) X & Y are different
(a) see a photo of the X and Y chromosomes
(b) compare the maps of the X and Y chromosomes
(c) Y has about 200 known genes
- cartoon of Y
(c) X has about 1000 known genes
- cartoon of X
(3) X & Y called sex chromosomes
(4) autosomes = all other non-sex
chromosomes
7. diploid = 2n = cells with two complete sets
of chromosomes = cells that have homologous pairs
-memorize this whole definition
-be able to apply it
a. almost all body cells of human are diploid
-eg: muscle cells, brain cells,
bone cells, skin cells, etc
b. diploid human cells have 46 chromosomes
c. diploid cells DO have pairs of
homologous chromosomes
d. NOTE: do not confuse "diploid cells"
with "duplicated chromosomes"
8. haploid = 1n = cells with one complete set of
chromosomes = haploid cells DO NOT have
homologous pairs of chromosomes
-memorize this whole definition and be able to apply it
a. unfertilized egg cells are haploid
b. sperm cells are haploid
c. haploid sperm fuses with haploid egg to get
diploid zygote (fertilized egg)
d. male parent provides 23 chromosomes (1 set)
e. female parent provides 23 chromosomes (1 set)
f. NOTE: do not confuse "haploid cells"
with "unduplicated chromosomes"
9. diploid zygote divides by mitosis to
yield new individual
10. How does the diploid individual make haploid egg or
sperm?
a. requires special kind cell & nuclear division
b. cell & nucleus divide such that number of
chromosomes is cut in half
c. special cell & nuclear division process = meiosis
d. meiosis = two stage cell division process that
results in the production of gametes with
1/2 chromosome number of the parent cells
III. Meiotic Cell Division
A. first there is chromosome replication during
S of interphase (see graphic)
B. then two consecutive cell divisions without a period
of chromosome replication
1. meiosis I = the first nuclear/cell division
2. meiosis II = the 2nd nuclear/cell division
C. meiosis = each parent cell with two complete
sets of duplicated chromosomes
produces 4 daughter cells each with 1/2 the
chromosomes of parent cell (one set) and the
chromosomes are in the unduplicated state
D. stages of Meiosis I
1. see Fig 15.15 and Fig 15.16 and Fig 15.17 in Brooker et al.
2. Interphase
a. unduplicated chromosomes in G1
b. DNA is replicated during S
become duplicated chromosomes
c. have two genetically identical chromatids in G2
d. see relative to cell cycle
3. Prophase I
a. graphic summary
b. lasts much longer than Prophase of mitosis
(1) can last for days
(2) accounts for 90% of time of meiosis
c. much more complex than Prophase
d. chromosomes condense
e. synapsis occurs
(1) homologous chromosomes come together
to form "tetrad"
(2) each gene brought into juxtaposition with
its homologue
(3) segments of nonsister chromatids
break/rejoin
f. crossing over = exchange of genes between
nonsister chromatids of homologous
chromosomes during synapsis
-number of crossover events varies with
length of the chromosomes
-might get 1 with shorter chromosome
-might get 2 with a longer chromosome
g. chromosomes thicken more but tetrad stays
together
h. spindle forms
i. chromosomes move toward metaphase I
plate
4. Metaphase I
a. graphic summary
b. align on metaphase plate
(NOTE the difference between
metaphase I and metaphase II)
c. both kinetochores of sister chromatids face
same pole
d. centromeres of the homologous
chromosomes face opposite poles
5. Anaphase I
a. graphic summary
b. attachment between the homologous
chromosomes breaks down
c. homologous chromosomes are separated
-Note: it is not the chromatids that separate
-it is the duplicated chromosomes of the
homologous pair that are separated
d. kinetochores pull duplicated chromosomes
toward poles
6. Telophase I
a. graphic summary
b. duplicated chromosomes reach the poles
(1) each pole now has a haploid set of
duplicated chromosomes
(2) each chromosome still has two chromatids
c. cytokinesis occurs
(1) two daughter cells form
(2) daughter cells are haploid with
duplicated chromosomes
(3) the two daughter cells are genetically different
d. may be an Interkinesis
(1) period of time between meiosis I and
meiosis II
(2) no further replication of genetic material
occurs
-recall: chromosomes still in duplicated condition
7. summary of meiosis I to memorize
-"diploid cells with duplicated chromosomes
exchange DNA between homologous strands
and then separate the homologous pairs such that
haploid cells with duplicated chromosomes
are produced"
-possible exam questions:
-Which of the statements below best describes meiosis I?
-Which of the statements below best describes meiosis II?
-Or, turn it around. The question gives the summary of
meiosis I and asks what is being described.
8. see summary cartoon of meiosis I & II
E. Stages of Meiosis II
1. see Fig 15.15 and Fig 15.16 and Fig 15.17 in Brooker et al.
2. Prophase II
a. graphic summary
b. if interkinesis-then NE breaks down
c. spindle reappears
d. duplicated chromosomes move toward
metaphase plate
(NOTE the difference between
prophase I and prophase II)
3. Metaphase II
a. graphic summary
b. duplicated chromosomes align at
metaphase plate just like mitosis
(NOTE the difference between
metaphase I and metaphase II)
c. kinetochores of sister chromatids directed to
opposite spindle poles
(NOTE the difference between
metaphase I and metaphase II)
4. Anaphase II
a. graphic summary
b. centromeres break down
c. kinetochores pull chromatids toward
opposite poles
(NOTE the difference between
anaphase I and anaphase II)
5. Telophase II
a. graphic summary
b. chromatids reach poles
c. NE reforms
d. cytokinesis occurs
6. now have:
a. 4 daughter cells
b. haploid
c. chromosomes are unduplicated
7. summary of meiosis II to memorize
-haploid cells with duplicated chromosomes
do a mitotic type nuclear/cell division such that
the daughter cells are haploid with
unduplicated chromosomes
8. see summary cartoon of meiosis I & II
9. do the formative assessment on
Mitosis and Meiosis
10. will be quiz/exam questions just like the
formative assessments on mitosis and meiosis
wednesday
V. Sexual Sources of Genetic Variation
A. pre-existing genetic variation is absolutely
required for evolution
B. sexual reproduction provides much variation
1. independent assortment of chromosomes
a. meiosis I
(1) homologous pairs (tetrads) align at metaphase plate
(2) one of pair from father and one of
pair from mother
(3) alignment of pairs is random
(4) chromosomes from father free to associate
with either pole
(5) chromosomes from mother free to associate
with either pole
(6) resulting daughter cells get mixing of
chromosomes of father and mother
during egg and sperm production
(7) amount mixing possible is very great
(a) humans
i) 23 pairs subjected to independent assortment
ii) 2 to 23rd power = 8 million combinations
of the 23 pairs of chromosomes
(8) besides producing variation, independent
assortment explains Mendel's Law of Segregation
2. random fertilization
a. egg combine with sperm
(1) combinations for egg
(a) 8 million
(2) combinations for sperm
(a) 8 million
b. combinations for zygote
(1) 8 million X 8 million = 64 trillion combinations
3. crossing over
a. genetic information (genes) exchanged
between the paired homologous
chromosomes during the tetrad stage
in Prophase I
(1) genes contributed by one parent end up on
chromosome contributed by other parent
(2) see cartoon of crossing over
(3) typical chromosomes might be 100
map units => 1 or 2 crossovers / chromosome
(4) crossovers occur within or between alleles
b. results in vastly greater mixing of genetic
information
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