Transcript 10.1 Meiosis Notes - Twanow

10.1 Meiosis
Learning Targets: Describe
chromosomes in the phases of
meiosis. Outline chiasmata in
crossing over. Explain how
meiosis results in genetic variety.
What are some advantages and
disadvantages of both asexual and sexual
reproduction?
 Asexual – only requires one parent
(advantage), but offspring are clones
(disadvantage for adaptation)
 Sexual – requires two parents
(disadvantage), produces unique
combinations and genetic diversity
(advantage for adaptation)
Define:
Diploid – a cell with two sets of
chromosomes – symbolized as 2n
 In humans, n=23, so diploid cells have
two sets of 23 chromosomes (2n=46)
 Examples: somatic (body) cells, a zygote
(fertilized egg)
Haploid – a cell with a single chromosome
set – symbolized as n
 In humans, haploid cells have one set of
23 chromosomes
 Examples: gametes (egg and sperm cells
formed by meiosis)
Homologous chromosomes – chromosomes carrying
genes for the same traits
 Examples: your diploid cells contain chromosome
1 from your mother, and chromosome 1 from
your father
Gene – a segment of DNA in which the sequence
carries the code for making an enzyme or protein
(in most cases)
Each chromosome carries a version of a gene
for the same trait (ex. blue vs. brown eyes)
Sister chromatids are identical;
Homologous chromosomes are not
Label:
Label:
Homologous
chromosomes
Sister chromatid
Chromosome
Karyotype
Autosomes
1-22
Sex
chromosomes
Review: In what phase of the cell cycle is
DNA synthesized to create identical sister
chromatids?
 Interphase (S phase of interphase)
BEFORE mitosis or meiosis
Why is it important that meiosis reduces a
cell’s chromosome number to produce
haploid gametes?
 Otherwise a fertilized egg would have
double the normal chromosomal material
– it would be 4n, the next generation
would be 8n, etc.
Meiosis I:
 Prophase I: Chromosomes
condense; homologous
chromosomes form tetrads;
crossing over occurs at chiasmata
 Metaphase I: Tetrads line up at
midline


Anaphase I: Homologous pairs
separate
Telophase I and cytokinesis: 2
haploid cells form (chromosomes
still consist of 2 non-identical sister
chromatids)
Meiosis II: similar to mitosis
 Prophase II, Metaphase II:
chromosomes line up at midline
 Anaphase II: sister chromatids
separate
 Telophase II and cytokinesis: 4
haploid cells form
Mitosis
All Cell Division
Meiosis
 Chromosomes
 One division
 Two divisions
 Forms 2 diploid
replicate once in  Forms 4 haploid
interphase before
daughter cells
cells
 Daughter cells are
 Crossing over
division
identical to parent  Prophase,
occurs in
metaphase,
cell
prophase I
 Functions in
anaphase,
 Cells are
growth, repair
telophase
genetically
unique
 Produces gametes
Sexual reproduction reshuffles genes to
increase variation (but mutation is the
original source of all variation)
Sources of genetic variation:
1. Independent assortment of homologous
chromosomes in Meiosis I, and nonidentical sister chromatids in Meiosis II

2n possibilities, so 223 options for each human
gamete

In example below, n=2, so 22= 4 different
possible gametes
2. Crossing over – genetic exchange creates
recombinant chromosomes with genes from
each parent
Tetrad
Chiasmata
Recombinant gametes
3. Random fertilization – creates unique
combinations of parental genes

223 x 223 options for each human zygote!