Cell Division – Mitosis and Meiosis
 students will describe the processes of mitosis and meiosis
o define and explain the significance of chromosome number in
somatic and sex cells
o explain the events of the cell cycle
o compare the formation of identical and fraternal twins
Chromosome Structure and Vocab
Haploid (n)
 gametes (eggs and sperm)
 23 chromosomes in humans
Diploid (2n)
 Somatic cells (non gametes)
 46 chromosomes in humans
The Cell Cycle
 Growth and division of somatic cell
 How multicellular organisms grow in size
 Two phases: Interphase and Division Phase
Interphase
 Longest phase in the cell cycle
 Cell grows in size
 Doubles cytoplasmic organelles
 Duplicates chromosomes
 Three parts: G1, S, G2
o G1: cell growth
o S: synthesis of DNA (chromosome duplication
o G2: cell growth and preparation for division
Division Phase
 Mitosis and Cytokinesis
 Cell splits into two identical cells
Mitosis
 4 stages: Prophase, Metaphase, Anaphase, Telophase
 Prophase
o Chromosomes condense
o Centrioles form and move to opposite ends of cell
o Spindle fibers form and attach to centromeres
o Nuclear membrane starts to dissolve
 Metaphase
o Chromosomes (sister chromatids) move to center of cell
(equatorial plate)
o Nuclear membrane completely dissolved
o Chromosomes most visible at this stage
o Chromatids can become intertwined
 Anaphase
o Centromeres divide
o Sister chromatids (now separate chromosomes) move to
opposite ends of cell
o Should be identical chromosomes at each end
 Telophase
o Chromosomes lengthen (expand)
o Spindle fibers dissolve
o Nuclear membrane reforms around each group of
chromosomes
Cytokinesis
 Cytoplasm divides
 End with two identical cells
o Both still 2n
Mitosis and Aging
 Limited # of divisions
 Biological clock may regulate # of cell divisions allowed
 Study on heart cells
 Specialization affects mitosis rate
o More specialized cells divide fewer times
 Telomere (cap at end of chromosome) controls # of divisions
o In normal cells: telomeres shorten every time the cell
divides until it can’t shorten anymore without affecting
the chromosome. Cell is at max age and dies
o In abnormal cells: cell keeps dividing and chromosome
shortens = lose genetic information = abnormal cells
o In cancer cells: telomeres don’t shorten with each
division, so cell never stops dividing
 This is due to the presence of enzyme telomerase
in cancer cells that prevents shortening of
telomeres.
Practice Questions
Pg 564: #3, 5, 7, 8, 9, 15, 17
Cloning
 Forming identical offspring from a single cell or tissue in the
parent organ
 Clone and parent have identical DNA
 Happens naturally in nature (pg 565)
o Budding
o Runners
o Identical twins
 Allows for production of strains of plants with predictable
characteristics
Animal Cloning (1960s +)
 Frog cloning (pg 567)
o Blastocyst nucleus inserted into enucleated (no nucleus)
egg cell
o Egg cell divides mitotically to develop into normal
embryo and then frog
o New frog is genetically identical to original embryo
o Nucleus from older cells (gastrulation stage) did not
work as well – cells already too specialized to undergo
mitosis
 Mammal cloning
o Same process as frogs
o Only seemed to work with nucleus from blastocyst cells
o Then Dolly happened
o Took nucleus from adult cells (udder cells) of Finn
Dorsett and put in enucleated egg cell of Poll Dorsett
o New embryo (with transplanted nucleus) put into
surrogate sheep’s womb (Scottish Blackface)
o Lamb was born that was genetically identical to first
adult (Finn Dorsett)
Practice Questions
Pg 571: #1-4
 students will describe the processes of mitosis and meiosis
o describe the process of meiosis and the necessity for the reduction
of chromosome number
o compare the processes of mitosis and meiosis
o describe the processes of crossing over and nondisjunction
Meiosis
 cell division to form sex cells (aka gametes, sperm, eggs)
 happens only in testes and ovaries
 gametes are haploid (n)
 goes through 2 divisions similar to Mitosis
 starts with G1, S, G2
Meiosis I
 homologous pairs separate
 Prophase I
o Nuclear membrane dissolves
o Centrioles form and move to opposite ends
o Spindle fibers form and attach to centromeres
o Synapsis occurs: homologous pairs come together and
intertwine
o (tetrad = 4 chromatids)
o Crossing over may occur (pieces of chromatids break
off and exchange) = variability in offspring
 Metaphase I
o Tetrads line up on equatorial plate
 Anaphase I
o Homologous chromosomes separate to opposite ends
(segregation)
o Chromosome number has now been reduced to n
 Telophase I
o Nuclear membrane forms around each set
o These two new cells are not identical (unlike in mitosis)
because one has one set of the homologous pair and the
other has another set.
 Cytokinesis
o Cell membrane separates and you are left with two new
haploid cells
o Then both start Meiosis II without going through G1,
S, G2
 Meiosis II
 Sister chromatids separate (like in mitosis)
 Prophase II
o Nuclear membrane dissolves
o Centrioles form and move to opposite ends
o Spindle fibers form and attach to centromeres
 Metaphase II
o Sister chromatids line up on equatorial plate
 Anaphase II
o Sister chromatids separate to opposite ends
o Chromosomal number is still haploid
 Telophase II
o Nuclear membrane forms around each set
o These two new cells should be identical if there was no
crossing over previously.
 Cytokinesis
o Cell membrane separates and you are left with two new
haploid gametes
Practice Questions
Pg 575: #1-6
Pg 578: #7
Pg 581 #1,2,4,5,8,9,11
 students will describe the processes of mitosis and meiosis
o describe the diversity of reproductive strategies by comparing the
alternation of generations in a range of organisms
Key Terms
 Sporophyte: structure that releases spores - Diploid
 Spores: grow into gametophyte
 Gametophyte: structure that makes gametes - Haploid
Abnormal Meiosis
 Nondisjunction
o 2 homologous chromosomes fail to separate during
meiosis or mitosis
o 1 daughter cell has too many, 1 has too few
chromosomes
o Too much or too little, don’t function properly
o More consequences in meiosis = fetus that’s missing or
has too many chromosomes
 Polyploidy
o More than 2n (nondisjunction of all chromosomes)
o 3n triploidy
o 4n tetraploidy
 Failure of 2n zygote to divide after replicating (4n
embryo)
o Common and chemically encouraged in plants such as
wheat, oats, tobacco, and potatoes
 Humans and nondisjunction
o Split at meiosis II produces gametes of 22 and 24
chromosomes (instead of normal 23, 23)
o Therefore after fertilization with a normal gamete, you
get embryo of either 45 or 47
 1 chromosome where there should be 2 =
monosomy
 3 chromosomes where there should be 2 = trisomy
 As individual develops, every cell will either be 1
short or have 1 too many
Disorders (complete using your textbook)
 Down Syndrome
 Turner Syndrome
 Klinefelter syndrome
 Trisomic female
Karyotyping
- Read page584 and do the exercise on pg 585-586. Complete the
online activity (link on PowerSchool).
Ch. 17 Study Guide
Main Concepts
□ I can define and explain the significance of chromosome number in
somatic and sex cells (haploid, diploid, polyploidy)
□ I can explain, in general terms, the events of the cell cycle (interphase,
mitosis, and cytokinesis)
□ I can describe the process of meiosis (spermatogenesis and Oogenesis) and
the necessity for the reduction of chromosome number
□ I can compare the processes of mitosis and meiosis
□ I can describe the processes of crossing over and nondisjunction and
evaluate their significance to organism inheritance and development
□ I can compare the formation of fraternal and identical offspring
□ I can describe the diversity of reproductive strategies by comparing the
alternation of generations in a range of organisms
Review Questions: pg593 #1-3, 8-12, 15-17, 20-26
Vocabulary
Somatic Cell
Cell Cycle
Mitosis
Cytokinesis
Interphase
Chromatin
Centromere
Sister Chromatids
Centriole
Spindle Fibre
Enucleated
Stem Cell
Telomere
Meiosis
Haploid
Diploid
Homologous Chromosomes
Tetrad
Synapsis
Crossing Over
Gametogenesis
Ootid
Polar Body
Oocyte
Nondisjunction
Polyploidy
Trisomy
Monosomy
Karyotype Chart